Can Diarylethene Photochromism Be Explained by a Reaction Path Alone? A CASSCF Study with Model MMVB Dynamics

Predictive Guidelines for Electrocyclization of Dithienylethenes

Molecular photoswitches undergo a structural transformation upon photoexcitation to interconvert between two or more stable forms. In some cases, the structural rearrangement involves the transition between ring-open and ring-closed forms. In this work, we develop simple guidelines for the design of organic molecules able to undergo photochemical electrocyclization, using electronic structure calculations on dithienylethene (DTE)-based compounds. We conclude that the ability to photocyclize can be predicted from the localization and symmetry of frontier molecular orbitals. These ideas are based on the Woodward-Hoffmann rules but go beyond them, allowing us to address a more general family of chromophores. Our methodology has been experimentally validated in a novel series of quinoline-based DTEs, where the ability for photocyclization depends on specific chemical modifications. We use the DTE scaffold as the workhorse model in our study; however, these simple yet powerful guidelines are expected to be generally valid to guide the design of other diarylethenes.

A chiral photoswitch based on enantiospecific interconversion between binaphthyl and helicenoid skeletons

A novel chiral photoswitch composed of a binaphthyl unit and a hexafluorocyclopentene ring has been synthesized. This chiral photoswitch exhibited thermally reversible photochromism between the binaphthyl and helicenoid forms based on 6π-electrocyclization. The helicity of the binaphthyl moiety was reversed upon stereospecific photocyclization and reverted back during the thermal ring opening.

New insights into the photocyclization reaction of a popular diarylethene switch: a nonadiabatic molecular dynamics study

Diarylethene (DAE) molecular switches have continued to attract the attention of researchers for over 20 years. Their remarkable photophysical properties endow them with countless applications in photonics and molecular technologies. However, despite extensive experimental and theoretical research, the mechanism of DAE photoswitching is not yet fully rationalized. In this work, we investigate the ring closure dynamics of a popular DAE switch, 1,2-bis(3-methyl-5-phenyl-2 thienyl)perfluorocyclopentene (PT), using nonadiabatic molecular dynamics (NAMD) simulations. Employing the fewest switches surface hopping protocol, along with the semi-empirical multireference ODM2/MRCI-SD method, we investigate possible reaction pathways for this photoprocess, as well as their timescales and resulting photoproducts. Furthermore, using a dynamic configuration-space sampling procedure, we elucidate the role of triplet states in the photocyclization of PT, supporting available experimental data for the closely related DMPT molecule, which indicate an ultrafast intersystem crossing (ISC) transition competing with the singlet-driven photoswitching reaction. Our findings not only corroborate experimental studies on DAE switches, but also provide new mechanistic insights into the potential use in the rational design of DAE switches tailored for specific technological applications.

Tuning the photochemical ring-closing reaction efficiency in diarylethene-based photoswitches through engineering of internal charge transfer

The photochemical quantum yield is one of the key features for a photoswitch and its tuning is challenging. In an attempt to tackle this issue within the popular diarylethene-based switches, we have explored the potential to use internal charge transfer (CT), a readily controllable parameter, for an effective modulation of the photocyclization quantum yield. For this, a homogeneous family of terarylenes, a sub-class of diarylethenes, with different CT characters, but the same photochromic core was designed and its photochromic properties were fully investigated. A clear correlation was found between the cyclization quantum yield and the CT character of the switch. More precisely, almost linear relationships were established between the ring-closing quantum yield and (i) the electron density variation accompanying the S₀ → S₁ transition and (ii) the percentage of LUMO on the reactive carbon atoms. Such a correlation was rationalized by a joint spectroscopic analysis and theoretical modelling of both ground and first excited states, introducing the concept of "early" or "late" photochromes. Encouragingly, such a potentally predictive model also seemed relevant when applied to some other diarylethene-based switches reported in the literature.

Excited-state resonance Raman spectroscopy probes the sequential two-photon excitation mechanism of a photochromic molecular switch

Some diarylethene molecular switches have a low quantum yield for cycloreversion when excited by a single photon, but react more efficiently following sequential two-photon excitation. The increase in reaction efficiency depends on both the relative time delay and the wavelength of the second photon. This paper examines the wavelength-dependent mechanism for sequential excitation using excited-state resonance Raman spectroscopy to probe the ultrafast (sub-30 fs) dynamics on the upper electronic state following secondary excitation. The approach uses femtosecond stimulated Raman scattering (FSRS) to measure the time-gated, excited-state resonance Raman spectrum in resonance with two different excited-state absorption bands. The relative intensities of the Raman bands reveal the initial dynamics in the higher-lying states, S_n, by providing information on the relative gradients of the potential energy surfaces that are accessed via secondary excitation. The excited-state resonance Raman spectra reveal specific modes that become enhanced depending on the Raman excitation wavelength, 750 or 400 nm. Many of the modes that become enhanced in the 750 nm FSRS spectrum are assigned as vibrational motions localized on the central cyclohexadiene ring. Many of the modes that become enhanced in the 400 nm FSRS spectrum are assigned as motions along the conjugated backbone and peripheral phenyl rings. These observations are consistent with earlier measurements that showed higher efficiency following secondary excitation into the lower excited-state absorption band and illustrate a powerful new way to probe the ultrafast dynamics of higher-lying excited states immediately following sequential two-photon excitation.

Wide-range IR spectra of diarylethene derivatives and their simulation using the density functional theory

Diarylethenes (DAEs), promising photochromic molecular switches, undergo pericyclic reactions upon ultraviolet or visible light illumination. For this reason, most studies on DAEs employ UV–vis spectroscopies. However, also their infrared (IR) spectra are valuable, in particular, for understanding the vibrational dynamics which accompanies the relevant photoreactions. An accurate assignment of IR bands to molecular modes can be achieved through a comparison between experimental and computed theoretical spectra. Even though more sophisticated computational methods are available, the density functional theory (DFT) is usually employed for this task, because of its modest cost and versatility. Here, we have tested the ability of several DFT functionals to reproduce the wide-range, 400–3200 cm⁻¹, IR spectra of open and closed isomers of four representative DAE molecules. We find that global and range-separated, corrected for anharmonicity by scaling factors, hybrid DFT functionals are able to reproduce the IR spectra of DAEs, however, instead of the popular B3LYP functional we propose the use of the dispersion-corrected PBE0 functional. The paper also proposes a semi-automatic method of band assignment.

E/Z photoisomerization pathway in pristine and fluorinated di(3-furyl)ethenes

We report an XMCQDPT2 study of the E/Z photoisomerization in a series of fluorinated di(3-furyl)ethenes (3DFEs). Upon excitation, pristine and low-fluorinated 3DFE show conventional behavior of many diarylethenes: unhindered twisting motion toward the pyramidalized zwitterionic state where relaxation to the ground state occurs. However, deep fluorination of 3DFEs can hamper E-to-Z isomerization by giving rise to an alternative excited-state relaxation pathway: an out-of-plane motion of a ring fluorine atom. Importantly, the case of fluorinated 3DFEs reveals serious deficiencies of the popular TDDFT approach. With some commonly used exchange-correlation functionals, the alternative relaxation pathway is not reproduced and, moreover, an irrelevant ring rotation coordinate is predicted instead. Nevertheless, TDDFT remains qualitatively adequate for the E-to-Z twisting coordinate taken alone.

Ultrafast Dynamics of a Molecular Switch from Resonance Raman Spectroscopy: Comparing Visible and UV Excitation

Resonance Raman spectroscopy probes the ultrafast dynamics of a diarylethene (DAE) molecular switch following excitation into the first two optical absorption bands. Mode-specific resonance enhancements for Raman excitation at visible (750-560 nm) and near-UV (420-390 nm) wavelengths compared with the calculated and experimental off-resonance Raman spectrum at 785 nm reveal different Franck-Condon active vibrations for the two electronically excited states. The resonance enhancements at visible wavelengths are consistent with initial motion on the first excited-state that promotes the cycloreversion reaction, whereas the enhancements for excitation at near-UV wavelengths highlight motions involving conjugated backbone and phenyl ring stretching modes that are orthogonal to the reaction coordinate. The results support a mechanism involving rapid internal conversion from the higher-lying state followed by cycloreversion on the first excited state. These observations provide new information about the reactivity of DAE derivatives following excitation in the visible and near-UV.

Photo-induced 6π-electrocyclisation and cycloreversion of isolated dithienylethene anions

The diarylethene chromophore is commonly used in light-triggered molecular switches. The chromophore undergoes reversible 6π-electrocyclisation (ring closing) and cycloreversion (ring opening) reactions upon exposure to UV and visible light, respectively, providing bidirectional photoswitching. Here, we investigate the gas-phase photoisomerisation of meta- (m) and para- (p) substituted dithienylethene carboxylate anions (DTE^-) using tandem ion mobility mass spectrometry coupled with laser excitation. The ring-closed forms of p-DTE^- and m-DTE^- are found to undergo cycloreversion in the gas phase with maximum responses associated with bands in the visible (λ_max ≈ 600 nm) and the ultraviolet (λ_max ≈ 360 nm). The ring-open p-DTE^- isomer undergoes 6π-electrocyclisation in the ultraviolet region at wavelengths shorter than 350 nm, whereas no evidence is found for the corresponding electrocyclisation of ring-open m-DTE^-, a situation attributed to the fact that the antiparallel geometry required for electrocyclisation of m-DTE^- is energetically disfavoured. This highlights the influence of the carboxylate substitution position on the photochemical properties of DTE molecules. We find no evidence for the formation in the gas phase of the undesirable cyclic byproduct, which causes fatigue of DTE photoswitches in solution.

Providing theoretical insight into the role of symmetry in the photoisomerization mechanism of a non-symmetric dithienylethene photoswitch

Dithienylethene (DTE) molecular photoswitches have shown to be excellent candidates in the design of efficient optoelectronic devices, due to their high photoisomerization quantum yield (QY), for which symmetry is suggested to play a crucial role. Here, we present a theoretical study on the photochemistry of a non-symmetric dithienylethene photoswitch, with a special emphasis on the effect of asymmetric substitution on the photocyclization and photoreversion mechanisms. We used the Spin-Flip Time Dependent Density Functional Theory (SF-TDDFT) method to locate and characterize the main structures (conical intersections and minima) of the ground state and the first two excited states, S₁ and S₂, along the ring-opening/closure reaction coordinate of the photocyclization and photoreversion processes, and to identify the important coordinates governing the radiationless decay pathways. Our results suggest that while the main features that characterize the photoisomerization of symmetric DTEs are also present for the photoisomerization of the non-symmetric DTE, the lower energy barrier on S₁ along the cycloreversion reaction speaks in favor of a more efficient and therefore a higher cycloreversion QY for the non-symmetric DTEs, making them a better candidate for molecular optoelectronic devices than their symmetric counterparts.

Excited state dynamics of normal dithienylethene molecules either isolated or deposited on an argon cluster

Real-time dynamics of the electronically excited open-ring isomer of 1,2-bis(2-methylbenzo[b]thiophen-3-yl)perfluorocyclopentene (BTF6) and 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene (PTF6) molecules was investigated using a set-up that associates a molecular beam, femtosecond lasers and velocity map imaging. The molecules were either free in the gas phase or bound to an argon cluster. DFT and TDDFT calculations were performed on BTF6. The calculated vertical excitation energies indicate an excitation by the pump laser towards a superposition of S₅ and S₆ states. The free molecule dynamics was found to follow a three wavepacket model. One describes the parallel conformer (P) of these molecules. It is unreactive with respect to the ring closure reaction which is responsible for the photochromic property of these molecules. It has no observable decay at the experiment time scale (up to 350 ps). The other two wavepackets describe the reactive antiparallel conformer (AP). They are formed by an early splitting of the wavepacket that was launched initially by the pump laser. They can be considered as generated by excitation of different, essentially uncoupled, deformation modes. They subsequently evolve along independent pathways. One is directed ballistically towards a conical intersection (CI) and decays through the CI to a potential energy surface where it can no longer be detected. The other fraction of the wavepacket decays also towards undetected states but in this case the driving mechanism is a non-adiabatic electronic relaxation within a potential well of the energy surfaces where it was launched. When BTF6 and PTF6 molecules are bound to an argon cluster, the same three wavepacket model applies. The vibronic relaxation timespan is enhanced by a factor 5 and a larger fraction of AP conformers follows this pathway. In contrast, the time constant associated with the ballistic movement is enhanced by only a factor of 2.

Future-Oriented Advanced Diarylethene Photoswitches: From Molecular Design to Spontaneous Assembly Systems

Diarylethene (DAE) photoswitch is a new and promising family of photochromic molecules and has shown superior performance as a smart trigger in stimulus-responsive materials. During the past few decades, the DAE family has achieved a leap from simple molecules to functional molecules and developed toward validity as a universal switching building block. In recent years, the introduction of DAE into an assembly system has been an attractive strategy that enables the photochromic behavior of the building blocks to be manifested at the level of the entire system, beyond the DAE unit itself. This assembly-based strategy will bring many unexpected results that promote the design and manufacture of a new generation of advanced materials. Here, recent advances in the design and fabrication of diarylethene as a trigger in materials science, chemistry, and biomedicine are reviewed.

Suppression of reversible photocyclization reaction induced fluorescence enhancement: a theoretical study

The fluorescence intensity of SIP-2 and DPI under different environments are mainly related to the competition between the PC reaction and aggregation induced restriction of phenyl ring rotational motions.

Extending the Legible Time of Light-Responsive Rewritable Papers with a Tunable Photochromic Diarylethene Molecule

Inkless printing based on rewritable papers has recently made great progress because it can improve the utilization rate of papers, which is of great significance for saving resources and protecting the environment. Among them, light-responsive rewritable papers (LRPs) are a hot research topic because light is clean, easily available, wavelength and intensity adjustable, and noncontacting. However, the photochromic material, as the imaging substance of LRPs, is easily affected by environmental conditions, resulting in insufficient time to read the information. In view of this, we designed and constructed an acid/base tunable diarylethene molecular system that can effectively adjust the photochromic properties by reversibly changing the electron density of the diarylethene photoreaction center through protonation and demonstrated its potential as an imaging material with a longer legible time. What makes us more satisfied is that the acidification can not only extend the legible time of carrying information but also bring a clear and stable absorption/fluorescence imaging dual mode, which can better reflect details and improve contrast. Therefore, we believe that this tunable photochromic diarylethene molecule is a potential imaging material for the development of new LRPs.

"Click-switch" - one-step conversion of organic azides into photochromic diarylethenes for the generation of light-controlled systems

Diarylethenes (DAEs) are an established class of photochromic molecules, but their effective incorporation into pre-existing targets is synthetically difficult. Here we describe a new class of DAEs in which one of the aryl rings is a 1,2,3-triazole that is formed by “click” chemistry between an azide on the target and a matching alkyne–cyclopentene–thiophene component. This late-stage zero-length linking allows for tight integration of the DAE with the target, thereby increasing the chances for photomodulation of target functions. Nineteen different DAEs were synthesized and their properties investigated. All showed photochromism. Electron-withdrawing groups, and in particular −M-substituents at the triazole and/or thiophene moiety resulted in DAEs with high photo- and thermostability. Further, the chemical nature of the cyclopentene bridge had a strong influence on the behaviour upon UV light irradiation. Incorporation of perfluorinated cyclopentene led to compounds with high photo- and thermostability, but the reversible photochromic reaction was restricted to halogenated solvents. Compounds containing the perhydrogenated cyclopentene bridge, on the other hand, allowed the reversible photochromic reaction in a wide range of solvents, but had on average lower photo- and thermostabilities. The combination of the perhydrocyclopentene bridge and electron-withdrawing groups resulted in a DAE with improved photostability and no solvent restriction. Quantum chemical calculations helped to identify the photoproducts formed in halogenated as well as non-halogenated solvents. For two optimized DAE photoswitches, photostationary state composition and reaction quantum yields were determined. These data revealed efficient photochemical ring closure and opening. We envision applications of these new photochromic diarylethenes in photonics, nanotechnology, photobiology, photopharmacology and materials science.

New photochromic diarylethenes are reported in which one aryl ring is a 1,2,3-triazole that is formed by “click” chemistry between an azide on the target and a matching alkyne–cyclopentene–thiophene component.

Photo-cycloreversion mechanism in diarylethenes revisited: A multireference quantum-chemical study at the ODM2/MRCI level

Photoswitchable diarylethenes (DAEs), over years of intense fundamental and applied research, have been established among the most commonly chosen molecular photoswitches, often employed as controlling units in molecular devices and smart materials. At the same time, providing reliable explanation for their photophysical behavior, especially the mechanism of the photo-cycloreversion transformation, turned out to be a highly challenging task. Herein, we investigate this mechanism in detail by means of multireference semi-empirical quantum chemistry calculations, allowing, for the first time, for a balanced treatment of the static and dynamic correlation effects, both playing a crucial role in DAE photochemistry. In the course of our study, we find the second singlet excited state of double electronic-excitation character to be the key to understanding the nature of the photo-cycloreversion transformation in DAE molecular photoswitches.

"On-The-Fly" Non-Adiabatic Dynamics Simulations on Photoinduced Ring-Closing Reaction of a Nucleoside-Based Diarylethene Photoswitch

Nucleoside-based diarylethenes are emerging as an especial class of photochromic compounds that have potential applications in regulating biological systems using noninvasive light with high spatio-temporal resolution. However, relevant microscopic photochromic mechanisms at atomic level of these novel diarylethenes remain to be explored. Herein, we have employed static electronic structure calculations (MS-CASPT2//M06-2X, MS-CASPT2//SA-CASSCF) in combination with non-adiabatic dynamics simulations to explore the related photoinduced ring-closing reaction of a typical nucleoside-based diarylethene photoswitch, namely, PS-IV. Upon excitation with UV light, the open form PS-IV can be excited to a spectroscopically bright S₁ state. After that, the molecule relaxes to the conical intersection region within 150 fs according to the barrierless relaxed scan of the C1–C6 bond, which is followed by an immediate deactivation to the ground state. The conical intersection structure is very similar to the ground state transition state structure which connects the open and closed forms of PS-IV, and therefore plays a crucial role in the photochromism of PS-IV. Besides, after analyzing the hopping structures, we conclude that the ring closing reaction cannot complete in the S₁ state alone since all the C1–C6 distances of the hopping structures are larger than 2.00 Å. Once hopping to the ground state, the molecules either return to the original open form of PS-IV or produce the closed form of PS-IV within 100 fs, and the ring closing quantum yield is estimated to be 56%. Our present work not only elucidates the ultrafast photoinduced pericyclic reaction of the nucleoside-based diarylethene PS-IV, but can also be helpful for the future design of novel nucleoside-based diarylethenes with better performance.

The Mechanics of the Bicycle Pedal Photoisomerization in Crystalline cis,cis-1,4-Diphenyl-1,3-butadiene

Direct irradiation of crystalline cis,cis-1,4-diphenyl-1,3-butadiene (cc-DPB) forms trans,trans-1,4-diphenyl-1,3,-butadiene via a concerted two-bond isomerization called the bicycle pedal (BP) mechanism. However, little is known about photoisomerization pathways in the solid state and there has been much debate surrounding the interpretation of volume-conserving isomerization mechanisms. The bicycle pedal photoisomerization is investigated using the quantum mechanics/molecular mechanics complete active space self-consistent field/Amber force-field method. Important details about how the steric environment influences isomerization mechanisms are revealed including how the one-bond flip and hula-twist mechanisms are suppressed by the crystal cavity, the nature of the seam space in steric environments, and the features of the bicycle pedal mechanism. Specifically, in the bicycle pedal, the phenyl rings of cc-DPB are locked in place and the intermolecular packing allows a passageway for rotation of the central diene in a volume-conserving manner. In contrast, the bicycle pedal rotation in the gas phase is not a stable pathway, so single-bond rotation mechanisms become operative instead. Furthermore, the crystal BP mechanism is an activated process that occurs completely on the excited state; the photoproduct can decay to the ground state through radiative and non-radiative pathways. The present models, however, do not capture the quantitative activation barriers, and more work is needed to better model reactions in crystals. Last, the reaction barriers of the different crystalline conformations within the unit cell of cc-DPB are compared to investigate the possibility for conformation-dependent isomerization. Although some difference in reaction barriers is observed, the difference is most likely not responsible for the experimentally observed periods of fast and slow conversion.

A systematic model study quantifying how conical intersection topography modulates photochemical reactions

Despite their important role in photochemistry and expected presence in most polyatomic molecules, conical intersections have been thoroughly characterized in a comparatively small number of systems. Conical intersections can confer molecular photoreactivity or photostability, often with remarkable efficacy, due to their unique structure: at a conical intersection, the adiabatic potential energy surfaces of two or more electronic states are degenerate, enabling ultrafast decay from an excited state without radiative emission, known as nonadiabatic transfer. Furthermore, the precise conical intersection topography determines fundamental properties of photochemical processes, including excited-state decay rate, efficacy, and molecular products that are formed. However, these relationships have yet to be defined comprehensively. In this article, we use an adaptable computational model to investigate a variety of conical intersection topographies, simulate resulting nonadiabatic dynamics, and calculate key photochemical observables. We varied the vibrational mode frequencies to modify conical intersection topography systematically in four primary classes of conical intersections and quantified the resulting rate, total yield, and product yield of nonadiabatic decay. The results reveal that higher vibrational mode frequencies reduce nonadiabatic transfer, but increase the transfer rate and resulting photoproduct formation. These trends can inform progress toward experimental control of photochemical reactions or tuning of molecules' photochemical properties based on conical intersections and their topography.

Cyclization Dynamics and Competitive Processes of Photochromic Perfluorocyclopentene Dithienylethylene in Solution

Time-resolved absorption spectroscopy measurements were performed to study the dynamics of photochromic 1,2-Bis(2,4-dimethylthiophene-3-yl)perfluoro-cyclopentene (DMTPF) in chloroform, including antiparallel conformer ring-closure reaction and parallel conformer photophysics. All characteristic times are given, discussed and compared to a previous publication concerning the close molecule substituted with phenyl rings. (Hamdi et al., PCCP, 2016). Apart from the expected photocyclization process, condensed ring by-product formation is observed and hypotheses concerning the origin of this by-product are presented.

Spatial confinement alters the ultrafast photoisomerization dynamics of azobenzenes

Ultrafast transient absorption spectroscopy reveals new excited-state dynamics following excitation of trans-azobenzene (t-Az) and several alkyl-substituted t-Az derivatives encapsulated in a water-soluble supramolecular host-guest complex. Encapsulation increases the excited-state lifetimes and alters the yields of the trans → cis photoisomerization reaction compared with solution. Kinetic modeling of the transient spectra for unsubstituted t-Az following nπ* and ππ* excitation reveals steric trapping of excited-state species, as well as an adiabatic excited-state trans → cis isomerization pathway for confined molecules that is not observed in solution. Analysis of the transient spectra following ππ* excitation for a series of 4-alkyl and 4,4'-dialkyl substituted t-Az molecules suggests that additional crowding due to lengthening of the alkyl tails results in deeper trapping of the excited-state species, including distorted trans and cis structures. The variation of the dynamics due to crowding in the confined environment provides new evidence to explain the violation of Kasha's rule for nπ* and ππ* excitation of azobenzenes based on competition between in-plane inversion and out-of-plane rotation channels.

Photochromic organic solar cells based on diarylethenes

Photovoltaic devices that switch color depending on illumination conditions may find application in future smart window applications. Here a photochromic diarylethene molecule is used as sensitizer in a ternary bulk heterojunction blend, employing poly(4-butylphenyldiphenylamine) (poly-TPD) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) for the transport of holes and electrons, respectively. Sandwiched between two electrodes, the blend creates a photochromic photovoltaic device that changes color, light absorption, and photon-to-electron conversion efficiency in the visible spectral range after having been illuminated with UV light.

Geometrical Evolution and Formation of the Photoproduct in the Cycloreversion Reaction of a Diarylethene Derivative Probed by Vibrational Spectroscopy

The geometrical evolution of the reactant and formation of the photoproduct in the cycloreversion reaction of a diarylethene derivative were probed using time-resolved absorption spectroscopies in the visible to near-infrared and mid-infrared regions. The time-domain vibrational data in the visible region show that the initially formed Franck-Condon state is geometrically relaxed into the minimum in the excited state potential energy surface, concomitantly with the low-frequency coherent vibrations. Theoretical calculations indicate that the nuclear displacement in this coherent vibration is nearly parallel to that in the geometrical relaxation. Time-resolved mid-infrared spectroscopy directly detected the formation of the open-ring isomer with the same time constant as the decrease of the closed-ring isomer in the excited state minimum. This observation reveals that no detectable intermediate, in which the population is accumulated, is present between the excited closed-ring isomer and the open-ring isomer in the ground state.

Mechanistic Insights into the Triplet Sensitized Photochromism of Diarylethenes

Operating photoswitchable molecules repetitively and reliably is crucial for most of their applications, in particular in (opto)electronic devices, and related to reversibility and fatigue resistance, which both critically depend on the photoisomerization mechanism defined by the substitution pattern. Two diarylethene photoswitches bearing biacetyl triplet sensitizers either at the periphery or at the core were investigated using both stationary as well as transient UV/Vis absorption spectroscopy ranging from the femtosecond to the microsecond time scale. The diarylethene with two biacetyl moieties at the periphery is switching predominantly from the triplet excited state, giving rise to an enhanced fatigue resistance. In contrast, the diarylethene bearing one diketone at the photoreactive inner carbon atom cyclizes from the singlet excited state and shows significantly higher quantum yields for both cyclization and cycloreversion.

Restriction of the conrotatory motion in photo-induced 6π electrocyclic reaction: formation of the excited state of the closed-ring isomer in the cyclization

The electrocyclic reaction dynamics of a photochromic dithiazolylarylene derivative, 2,3-dithiazolylbenzothiophene (DTA) was investigated by using time-resolved transient absorption and fluorescence spectroscopies. The closed-ring isomer of DTA undergoes cycloreversion through the conical intersection mediating the potential energy surfaces of the excited and ground states, which is in agreement with the Woodward–Hoffmann rules for the electrocyclic reactions of 6π electron systems. On the other hand, a large portion of the open-ring isomer undergoes cyclization along the distinct reaction scheme, in which the cyclization takes place in the excited state manifold leading to the formation of the excited state of the closed-ring isomer. The suppression of the geometrical motion of DTA due to the intramolecular interaction could open a new efficient reaction pathway resulting in the formation of the electronically excited state of the product.

Restriction of the molecular geometry opens up a novel pathway in the cyclization reaction of a photochromic dithiazolylarylene derivative.

Electronic Excited States and UV-Vis Absorption Spectra of the Dihydropyrene/Cyclophanediene Photochromic Couple: a Theoretical Investigation

Dihydropyrene (DHP)/cyclophanediene (CPD) is a fascinating photoswitchable organic system displaying negative photochromism. Upon irradiation in the visible region, the colored DHP can be converted to its open-ring CPD colorless isomer, which can be converted back to DHP by UV light. DHP and CPD thus possess very different absorption spectra whose absorption bands have never been assigned in detail so far. In this work, we characterize the vertical electronic transitions of the first six and seven excited states of DHP and CPD, respectively, aiming for a realistic comparison with experiment. We used state-of-the-art electronic structure methods [e.g., complete active space second-order perturbation theory (CASPT2), n-electron valence-state perturbation theory (NEVPT2), extended multiconfigurational quasi-degenerate perturbation theory (XMCQDPT2), and third-order algebraic diagrammatic construction ADC(3)] capable of describing differential electron correlation. Vertical transition energies were also computed with time-dependent density functional theory (TD-DFT) and compared to these accurate methods. After the reliability of TD-DFT was validated for the main optical transitions, this efficient method was used to simulate the absorption spectra of DHP and CPD in the framework of the Franck-Condon Herzberg-Teller approximation and also using the nuclear ensemble approach. Overall, for both methods, the simulated absorption spectra reproduce nicely the main spectral features of the DHP and CPD isomers, that is, the main four absorption bands of increasing intensity of DHP and the absorption rise below 300 nm for CPD.

A dominant factor of the cycloreversion reactivity of diarylethene derivatives as revealed by femtosecond time-resolved absorption spectroscopy

Dynamics of the cycloreversion reaction of a photochromic diarylethene derivative with a small ring-opening reaction yield (∼1%) was investigated by using femtosecond transient absorption spectroscopy. The reaction rate constant and activation barrier on the reaction coordinate were quantitatively analyzed on the basis of the temperature and excitation wavelength dependencies of the reaction yield and excited state dynamics. From the comparison of the present results with those in a more reactive derivative, we concluded that a key factor regulating the overall reaction yield is the branching ratio at the conical intersection where the excited state population is split into the product and the initial reactant. The excitation wavelength dependence of the dynamics indicated that the geometrical relaxation and vibrational cooling proceed in a few picosecond time scale behind the cycloreversion process, and the vibrational excess energy assists the molecule to climb up the energy barrier.

Selective bond formation triggered by short optical pulses: quantum dynamics of a four-center ring closure

Making bonds with attopulses: quantum dynamics of the ring closure of norbornadiene to quadricyclane.

Core structure dependence of cycloreversion dynamics in diarylethene analogs

Increased core rigidity in diarylethene-type photoswitches results in shallower excited-state potential energy surfaces and faster funneling towards the conical intersections from which cycloreversion and nonreactive deactivation occur.

Energy- and conformer-dependent excited-state relaxation of an E/Z photoswitchable thienyl-ethene

Bis(bithienyl)-1,2-dicyanoethene (4TCE) is a photoswitch that operates via reversible E/Z photoisomerization following absorption of visible light. cis-to-trans photoisomerization of 4TCE requires excitation below 470 nm, is relatively inefficient (quantum yield < 5%) and occurs via the lowest-lying triplet. We present excitation-wavelength dependent (565-420 nm) transient absorption (TA) studies to probe the photophysics of cis-to-trans isomerization to identify sources of switching inefficiency. TA data reveals contributions from more than one switch conformer and relaxation cascades between multiple states. Fast (∼4 ps) and slow (∼40 ps) components of spectral dynamics observed at low excitation energies (>470 nm) are readily attributed to deactivation of two conformers; this assignment is supported by computed thermal populations and absorption strengths of two molecular geometries (P_A and P_B) characterized by roughly parallel dipoles for the thiophenes on opposite sides of the ethene bond. Only the P_B conformer is found to contribute to triplet population and the switching of cis-4TCE: high-energy excitation (<470 nm) of P_B involves direct excitation to S₂, relaxation from which prepares an ISC-active S₁ geometry (ISC QY 0.4-0.67, k_ISC∼ 1.6-2.6 × 10^-9 s^-1) that is the gateway to triplet population and isomerization. We ascribe low cis-to-trans isomerization yield to excitation of the nonreactive P_A conformer (75-85% loss) as well as loses along the P_B S₂→ S₁→ T₁ cascade (10-20% loss). In contrast, electrocyclization is inhibited by the electronic character of the excited states, as well as a non-existent thermal population of a reactive "antiparallel" ring conformation.

Ultrafast ring closing of a diarylethene-based photoswitchable nucleoside

Deoxyuridine nucleosides embodied into diarylethenes form an especial class of photoswitchable compounds that are designed to stack and pair with DNA bases. The molecular geometry can be switched between "open" and "closed" isomers by a pericyclic reaction that affects the stability of the surrounding double helix. This potentially enables light-induced control of DNA hybridization at microscopic resolution. Despite its importance for the optimization of DNA photoswitches, the ultrafast photoisomerization mechanism of these diarylethenes is still not well understood. In this work, femtosecond transient absorption spectroscopy is applied to study the ring closing reaction upon UV excitation with 45 fs pulses. Excited-state absorption decays rapidly and gives rise to the UV-Vis difference spectrum of the "closed" form within ≈15 ps. Time constants of 0.09, 0.49 and 6.6 ps characterize the multimodal dynamics, where a swift recurrence in the signal anisotropy indicates transient population of the intermediate 21A-like state.

Unraveling ultrafast dynamics of the photoswitchable bridged dithienylethenes under structural constraints

The excited state dynamics of constrained photochromic benzodithienylethenes were addressed by considering the bridging with polyether chains (from x = 4 to 6 units) at the ortho and meta positions of the aryl group, named DTE-o_x and DTE-m_x, via time-resolved absorption spectroscopy supported with (TD)-DFT calculations. The photochromic parameters and geometrical structures of these series are discussed. A novel photocyclization pathway via a triplet state, evidenced recently (Hamdi et al., Phys. Chem. Chem. Phys., 2016, 18, 28091-28100), is largely dependent on the length and the position of the polyether chain. For the first time, by comparing the two series, we revealed, for the DTE-o_x series, an interconversion not only in the ground state but also between the triplet states of the anti-parallel and parallel open form conformers.

Unveiling anharmonic coupling by means of excited state ab initio dynamics: application to diarylethene photoreactivity

In this work, excited state ab initio molecular dynamics together with a time resolved vibrational analysis is employed to shed light on the vibrational photoinduced dynamics of a well-known diarylethene molecule experiencing a ring opening reaction upon electronic excitation. The photoreactivity of diarylethenes is recognized to be controlled by a non-adiabatic intersection point between the ground and the first excited state surfaces. The computation of an energy scan, along a suitable reaction coordinate, allows us to identify the region of potential energy surfaces in which the ground (S0) and the first excited (S1) state are well separated. The adiabatic sampling of that region in S1 shows that in the first 3 picoseconds, the central CC bond, which is subject to break, oscillates in an antiphase with respect to the energy gap ΔE(S1 - S0). A multiresolution analysis based on the wavelet transform was then applied to the structural parameters extracted from the excited state dynamics. The wavelet maps show characteristic oscillations of the frequencies, mainly CC stretching and CCC bending localized on the central 4-ring moiety. Moreover, we have identified the main frequency (methyl wagging motion) involved in the modulation of these oscillations. The anharmonic coupling within a group of vibrational modes was therefore highlighted, in good agreement with experimental evidence. For the first time, a quantitative analysis of time resolved signals from a wavelet transform/ab initio molecular dynamics approach was performed.

Exploring the ultrafast dynamics of a diarylethene derivative using sub-10 fs laser pulses

The fast internal conversion S₁ → S₀ of a diarylethenes photoswitch, facilitated by two vibrational stretching modes, results in a low quantum yield of the ring-opening reaction.

Discovery of conical intersection mediated photochemistry with growing string methods

Conical intersections (CIs) are important features of photochemistry that determine yields and selectivity. Traditional CI optimizers require significant human effort and chemical intuition, which typically restricts searching to only a small region of the CI space. Herein, a systematic approach utilizing the growing string method is introduced to locate multiple CIs. Unintuitive MECI are found using driving coordinates that can be generated using a combinatorial search, and subsequent optimization allows reaction pathways, transition states, products, and seam-space pathways to be located. These capabilities are demonstrated by application to two prototypical photoisomerization reactions and the dimerization of butadiene. In total, many reaction pathways were uncovered, including the elusive stilbene hula-twist mechanism, and a previously unidentified product in butadiene dimerization. Overall, these results suggest that growing string methods provide a predictive strategy for exploring photochemistry.

Ultrafast Pump-Repump-Probe Photochemical Hole Burning as a Probe of Excited-State Reaction Pathway Branching

We demonstrate pump-repump-probe (PRP) transient hole burning as a spectroscopic tool for differentiating reactive from nonreactive deactivation of excited photochemical reactants observed by transient absorption spectroscopy (TAS). This method utilizes a time-delayed, wavelength-tunable ultrafast pulse to alter the excited reactant population, with the impact of "repumping" quantified through depletions in photoproduct absorption. We apply this approach to characterize dynamics affecting the nonadiabatic photocyclization efficiency to form S₀ dihydrotriphenylene (DHT) following 266 nm excitation of ortho-terphenyl (OTP). TAS studies revealed bimodal deactivation of OTP*, but neither relaxation time scale (700 fs and 3.0 ps) could be assigned unambiguously to DHT formation due to overlap of excited-state and product spectra. PRP studies reveal that S₁ OTP only cyclizes on the slower of these time scales, with the faster process attributable to nonreactive deactivation. We demonstrate that this method offers greater photochemical insights without assuming models to globally fit spectral transients collected by TAS.

Efficient Light-Induced pKa Modulation Coupled to Base-Catalyzed Photochromism

Photoswitchable acid-base pairs, whose pK_a values can be reversibly altered, are attractive molecular tools to control chemical and biological processes with light. A significant, light-induced pK_a change of three units in aqueous medium has been realized for two thermally stable states, which can be interconverted using UV and green light. The light-induced pK_a modulation is based on incorporating a 3-H-thiazol-2-one moiety into the framework of a diarylethene photoswitch, which loses the heteroaromatic stabilization of the negatively charged conjugate base upon photochemical ring closure, and hence becomes significantly less acidic. In addition, the efficiency of the photoreactions is drastically increased in the deprotonated state, giving rise to catalytically enhanced photochromism. It appears that protonation has a significant influence on the shape of the ground- and excited-state potential energy surfaces, as indicated by quantum-chemical calculations.

New insights into the photoswitching mechanisms of normal dithienylethenes

The photoswitching and competitive processes of the referent photochromic diarylethene derivative 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene (DTE) and a novel bridged analog DTE-m5 have been investigated by state-of-the-art TD-DFT calculations and ultrafast spectroscopy supported by advanced chemometric data treatments. Focusing on DTE, the overall deactivation pathway of both antiparallel (AP) and parallel (P) conformers of the open form (OF) (1 : 1 in solution) has been resolved and rationalized starting from the Franck-Condon (FC) region to the ground state recovery. For the photo-excited P conformer, after ultrafast relaxation (∼200 fs) towards the S₁ relaxed state, an expected ISC occurred (55 ps) to produce a triplet state, ³P, the latter relaxing within 2.5 μs. Concerning the AP conformer, the photocyclization reaction is reported to proceed immediately (100 fs) starting from the FC region while the relaxed singlet state is populated in parallel. For the first time, we discovered that the latter state evolves through an unexpected ISC process (1 ps) giving rise to a second triplet state,³AP. For DTE-m5, by slightly constraining the molecule with the bridge, this triplet becomes reactive and participates in the formation of 10% of closed form (CF) probably through an adiabatic mechanism. Concerning the photoreversion, in accordance with the literature, we report on a two-step process, a 190 fs vibrational relaxation followed by a 6 ps ring-opening reaction. For the overall species at the singlet or triplet manifold, the use of advanced MCR-ALS allows us to obtain specific spectral signatures. This study is therefore a new step within the comprehension of DTE photochemistry.

Ultrafast 25-fs relaxation in highly excited states of methyl azide mediated by strong nonadiabatic coupling

Highly excited electronic states are challenging to explore experimentally and theoretically-due to the large density of states and the fact that small structural changes lead to large changes in electronic character with associated strong nonadiabatic dynamics. They can play a key role in astrophysical and ionospheric chemistry, as well as the detonation chemistry of high-energy density materials. Here, we implement ultrafast vacuum-UV (VUV)-driven electron-ion coincidence imaging spectroscopy to directly probe the reaction pathways of highly excited states of energetic molecules-in this case, methyl azide. Our data, combined with advanced theoretical simulations, show that photoexcitation of methyl azide by a 10-fs UV pulse at 8 eV drives fast structural changes and strong nonadiabatic coupling that leads to relaxation to other excited states on a surprisingly fast timescale of 25 fs. This ultrafast relaxation differs from dynamics occurring on lower excited states, where the timescale required for the wavepacket to reach a region of strong nonadiabatic coupling is typically much longer. Moreover, our theoretical calculations show that ultrafast relaxation of the wavepacket to a lower excited state occurs along one of the conical intersection seams before reaching the minimum energy conical intersection. These findings are important for understanding the unique strongly coupled non-Born-Oppenheimer molecular dynamics of VUV-excited energetic molecules. Although such observations have been predicted for many years, this study represents one of the few where such strongly coupled non-Born-Oppenheimer molecular dynamics of VUV-excited energetic molecules have been conclusively observed directly, making it possible to identify the ultrafast reaction pathways.

A Theoretical Study of the N to O Linkage Photoisomerization Efficiency in a Series of Ruthenium Mononitrosyl Complexes

Ruthenium nitrosyl complexes are fascinating versatile photoactive molecules that can either undergo NO linkage photoisomerization or NO photorelease. The photochromic response of three ruthenium mononitrosyl complexes, trans-[RuCl(NO)(py)₄]2+, trans-[RuBr(NO)(py)₄]2+, and trans-(Cl,Cl)[RuCl₂(NO)(tpy)]⁺, has been investigated using density functional theory and time-dependent density functional theory. The N to O photoisomerization pathways and absorption properties of the various stable and metastable species have been computed, providing a simple rationalization of the photoconversion trend in this series of complexes. The dramatic decrease of the N to O photoisomerization efficiency going from the first to the last complex is mainly attributed to an increase of the photoproduct absorption at the irradiation wavelength, rather than a change in the photoisomerization pathways.

Temperature dependence of the conversion efficiency of photochromic perylene bisimide dithienylcyclopentene triads embedded in a polymer

Photochromic molecules that are covalently linked to a strong fluorophore combine the requirements of external control and strong fluorescence, which will become increasingly important for super-resolution microscopy techniques based on single molecules. However, given the bulky structure of such constructs, steric hindrance might affect their photoconversion efficiencies upon immobilising them for imaging purposes. In this study the efficiencies of the photochromic conversion processes of molecular triads that are embedded in a polymer have been studied as a function of temperature. The triads consist of two perylene bisimide dye molecules that are connected via a dithienylcyclopentene photochromic bridge that undergoes a cyclization/cycloreversion reaction upon appropriate illumination. It is found that photochromic switching remains active, even at 5 K, yet with reduced but finite efficiency for the cycloreversion reaction. This might even be an advantage for the achievement of high labelling densities in super-resolution microscopy.

Light penetration-coupled photoisomerization modeling for photodeformation of diarylethene single crystal: upscaling isomerization to macroscopic deformation

Diarylethene is one of the photo-responsive materials that show rapid and reversible changes in their color/electrochemical properties and macroscopic deformations in the crystalline phase by light irradiation. Photoisomerization is the main cause of the photo reactivity of diarylethene, and we established a statistical model based on the density matrix formalism, which predicts quantitative isomerization progress as a population term. The model reflects photo-switching properties of the target molecule, which were characterized by first principle calculations, and external stimulus factors (light irradiation conditions and temperature). By merging light penetration physics with the model, we derived light penetration depth dependent isomerization progress to theoretically investigate photodeformation of single crystal. The model well reproduced in-plane shear deformation under ultraviolet light irradiation which would provide guideline for photoactuator design. In addition, the statistical model addressed crucial findings (primary stimuli and molecular design parameter for increasing the isomerization rate, external stimuli enhancing fluorescence performance) itself.