Quantum Chemical Study of the Photocoloration Reaction in the Napthoxazine Series

Is this how bromine spiropyran salt is converted to merocyanine under UV irradiation? A look through the prism of quantum chemical calculations

The stability of merocyanine forms formed under UV irradiation of a solution of a spiropyran salt, in which an organic part acts as a cation and a compact bromide ion as an anion, their photophysical properties, and the formation mechanism are studied in this work using time-dependent density functional theory. Theoretical calculations show that TTC and CTT are the most stable open forms (the difference in stability energies is 10.5 and 12.0 kcal mol-1 relative to the formation energy of spiropyran, respectively). The simulated absorption bands in the UV spectrum of the merocyanine forms are observed both in the UV region at 308-366 nm and in the visible region at 544-757 nm due to n → π* and π → π* type transitions. We found that the isomerisation mechanism of spiropyran into merocyanine forms includes two key stages: the ring opening to form cisoid merocyanine forms (except unstable TCC) through conical intersection and their subsequent isomerisation to form stable transoid isomers. The length of the Cspiro-O bond is 1.97 Å and the C1'-C2'-C3'-C4' angle is 70° in the structure close to conical intersection. The stage that determines the rate of this process is the isomerisation between transoid forms, as in the case of transformation of open merocyanine forms into spiropyran.

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.

Rational molecular designs for drastic acceleration of the color-fading speed of photochromic naphthopyrans

We report rational molecular designs for acceleration of the color-fading speed of photochromic 3H-naphthopyrans. By using steric and electrostatic repulsions induced by substituents at the 2- and 10-positions of 3H-naphthopyrans, the color-fading speed accelerates from tens of minutes to microsecond time scales. The long-lived residual color, which is an important problem to be solved for industrial applications, can also be suppressed by these strategies.

Dye chemistry with time-dependent density functional theory

In this perspective, we present an overview of the determination of excited-state properties of "real-life" dyes, and notably of their optical absorption and emission spectra, performed during the last decade with time-dependent density functional theory (TD-DFT). We discuss the results obtained with both vertical and adiabatic (vibronic) approximations, choosing relevant examples for several series of dyes. These examples include reproducing absorption wavelengths of numerous families of coloured molecules, understanding the specific band shape of amino-anthraquinones, optimising the properties of dyes used in solar cells, mimicking the fluorescence wavelengths of fluorescent brighteners and BODIPY dyes, studying optically active biomolecules and photo-induced proton transfer, as well as improving the properties of photochromes.

Decoloration rates of a photomerocyanine dye as a visual probe into hydrogen bonding interactions

We have developed a visual marker for the investigation of hydrogen bonding (HB) effects. The decoloration rate of a photochromic dye that incorporates a latent intra-molecular HB feature can be linked to the HB character of the media. Kinetic and thermodynamic parameters of this simple decoloration approach for HB sensing are investigated both experimentally and by high level theoretical studies. This principle has been applied for the detection of changes in the HB character of stationary and fluidic systems. A major finding is the observation of a shear-related perturbation of the balance between intra- and inter-molecular HB within a dynamic thin film.

Rationalization and Design of Enhanced Photoinduced Cycloreversion in Photochromic Dimethyldihydropyrenes by Theoretical Calculations

This study presents a computational investigation of the initial step of the dimethyldihydropyrene (DHP) to cyclophanediene (CPD) photoinduced ring-opening reaction using time-dependent density functional theory (TD-DFT). In particular, the photochemical path corresponding to the formation of the CPD precursor (CPD*) on the zwitterionic state is scrutinized. The TD-DFT approach was first validated on the parent compound against accurate ab initio calculations. It confirms that CPD* formation is efficiently quenched in this system by an easily accessible S2/S1 conical intersection located in the vicinity of the CPD* minimum and leading to a locally excited state minimum responsible for DHP luminescence. Increased ring-opening quantum yields were observed in benzo[e]-fused-DHP (DHP-1), isobutenyl-DHP (DHP-2), and naphthoyl-DHP (DHP-3). The calculations show that CPD* formation is much more favorable in these systems, either due to an inversion of electronic states in DHP-1, suppressing the formation of the locally excited state, or due to efficient stabilization of CPD* on the S1 potential energy surface in DHP-2 and DHP-3. Both effects can be combined in a rationally designed benzo[e]-fused-naphthoyl-DHP (DHP-4) for which we anticipate an unprecedented efficiency.

Unravelling the S → O linkage photoisomerization mechanisms in cis- and trans-[Ru(bpy)2(DMSO)2](2+) using density functional theory

A mechanistic study of the intramolecular S → O linkage photoisomerization in the cis and trans isomers of [Ru(bpy)2(DMSO)2](2+) was performed using density functional theory. This study reveals that for the cis isomer the linkage photoisomerization of the two DMSO ligands occurs sequentially in the lowest triplet excited state and can either be achieved by a one-photon or by a two-photon mechanism. A mechanistic picture of the S → O photoisomerization of the trans isomer is also proposed. This work especially highlights that both adiabatic and nonadiabatic processes are involved in these mechanisms and that their coexistence is responsible for the rich photophysics and photochemical properties observed experimentally for the studied complexes. The different luminescent behavior experimentally observed at low temperature between the cis and trans isomers is rationalized based on the peculiarity of the topology of the triplet excited-state potential energy surfaces.

Computational study of the mechanism of the photochemical and thermal ring-opening/closure reactions and solvent dependence in spirooxazines

The spirooxazine/merocyanine couple constitutes a photochromic system that can change from the colorless spirooxazine to the intensely colored merocyanine by thermal or photochemical activation by a reaction that opens the spiro ring of the oxazine. The mechanisms of the ring-opening/closure reactions that interconnect these two isomers have been elucidated by means of a computational study. First, we have used the CASSCF/CASPT2 method to determine in detail these mechanisms in the gas phase for a small model that contains the photoactive part of the whole system. We have found that the state of spirooxazine excited by the initial absorption changes gradually to a lower excited state that is involved in a conical intersection that connects it with the ground state of merocyanine. The same conical intersection is involved in the backward photochemical reaction. Second, using a larger model that includes all the heteroatoms of the system and using the DFT (B3LYP) method, we have studied the influence of a solvent environment on the thermal equilibrium between the open and the closed species. It has been observed experimentally that the thermal equilibrium between these forms is practically unaltered by polar aprotic solvents, but it can be displaced toward the colored form in mixtures of polar protic and aprotic solvents, even if the first one is found in a very small proportion. To reproduce the experimental environments, we have taken into account the long-range effect of the polar aprotic solvent considering it a polarizable continuum, as done in the PCM method, and the short-range effect of the protic solvent including some explicit water molecules in the cluster studied at the atomic level. The results obtained are in good agreement with the experimental observations and explain the reason for this peculiar behavior.

Bridged photochromic diarylethenes investigated by ultrafast absorption spectroscopy: evidence for two distinct photocyclization pathways

Two photochromic diarylethenes blocked by alkyl bridges in an ideal conformation for photocyclization are studied by stationary and femtosecond transient spectroscopy in order to depict the photocyclization processes: the bistable 1,2-dicyano[2.n]metacyclophan-1-ene with n = 2, abbreviated as [2.2], and its non-bistable analogue with n = 4, abbreviated as [2.4]. The data are interpreted in the light of AM1-CIS calculations and state correlation diagrams based on conclusive TD-DFT calculations. For [2.2], a solvent-sensitive excitation wavelength threshold governing the photocyclization yield is clearly evidenced between the S(1) and S(2) singlet states. Excitation above and beyond this threshold induces two distinct photochemical pathways. The S(1) vertical excitation induces direct efficient (phi approximately = 0.9-1), and ultrafast (approximately 120 fs) photocylization from S(1) open form that leads to a ground-state transition structure, probably through a conical intersection, then to a hot cyclized ground state that relaxes by vibrational cooling. Upon higher excitation energy, the system undergoes internal conversion to the hot S(1) state, then evolves toward the cyclized S(1) state and relaxes by ultrafast S(1)-S(0) internal conversion. Alternatively, the possibility for a second conical intersection near hot S(1) state is discussed. This second photoclosure reaction is less efficient and both the photocylization yield and overall kinetics depend on solvent polarity (phi = 0.49, tau = 2.5 ps in nonpolar solvent; phi = 0.7, tau = 1.5 ps in polar solvent). In the case of [2.4], for which the distance between the two reactive carbons is larger, a unique photoclosure mechanism is found and a structural effect is reported. Indeed, this mechanim is similar to the above second reaction of [2.2] but characterized by much slower kinetics ranging from 12 to 20 ps (depending on the excitation wavelength and solvent polarity). All polarity effects are rationalized in terms of stabilization of the transient states of charge-transfer character involved in the photocyclization process.

Spectral properties of spirooxazine photochromes: TD-DFT insights

A large series of photochromes of the spirooxazine family has been investigated using density functional theory and time-dependent density functional theory, aiming at designing molecules with an open-ring merocyanine form absorbing at the longest possible wavelength. A complete methodological assessment (basis set, solvent effects, functionals) has been performed, allowing the design of efficient multilinear regressions for two solvents (cyclohexane and toluene). These regressions allow the estimate of the absorption wavelength of open spirooxazine with an error limited to about 5 nm. The thermodynamic and spectral properties of several isomers have been considered, and it turned out that only TTC and CTC structures may appear experimentally. These structures present similar stabilities and absorption wavelengths. The impact of the auxochromic groups on the UV/vis spectra was assessed and novel promising substitution patterns have been unravelled. It is shown that using a strong push moiety on the same side of the molecule as the pull group may be an effective procedure for tuning the visible spectra. In particular, several spirooxazines with absorption wavelength predicted to be close to or larger than 700 nm are proposed.

Theoretical Study of the Electronic and Optical Properties of Photochromic Dithienylethene Derivatives Connected to Small Gold Clusters

In the course of developing electronic devices on a molecular scale, dithienylethenes photochromic molecules constitute promising candidates for optoelectronic applications such as memories and switches. There is thus a great interest to understand and control the switching behavior of photochromic compounds deposited on metallic surfaces or nanoparticles. Within the framework of the density functional theory, we studied the effect of small gold clusters (Au3 and Au9) on the electronic structure and absorption spectrum of a model dithienylethene molecule. The molecular orbital interactions between the photochromic molecule and the gold cluster made it possible to rationalize some experimental findings (Dulic, D.; van der Molen, S. J.; Kudernac, T.; Jonkman, H. T.; de Jong, J. J. D.; Bowden, T. N.; van Esch, J.; Feringa, B. L.; van Wees, B. J. Phys. Rev. Lett. 2003, 91, 207402). For the closed-ring isomer, grafting a photochromic molecule on a small gold cluster does not change the characteristics of the electronic transition involved in the ring-opening reaction. On the opposite, the absorption spectrum of the photochromic open-ring isomer is strongly modified by the inclusion of the metallic cluster. In agreement with experimental results, our study thus showed that the cycloreversion reaction which involves the closed-ring isomer should be still possible, whereas the ring-closure reaction which involves the open-ring isomer should be inhibited. Connecting a dithienylethene molecule to a small gold cluster hence provides a qualitative comprehension of the photochromic activities of dithienylethenes connected to a gold surface.