Tuning the optical properties of dithienylethenes: Theoretical insights

Theoretical Study on Second-Order Nonlinear Optical Responses of Pyrazine-carbazole Derivatives in Gas, Solution, and Crystal States

The packing fashion of an organic molecule in the crystal plays a critical role in the global nonlinear optical (NLO) responses under ambient conditions. To better understand how the crystal packing affects the first hyperpolarizability (β) and achieve efficient NLO material, herein, the three positional isomers (regioisomers) through changing the substituted position of 3-carbazole-pyrazine-based isomers were performed. The phenyl groups with different positions (ortho-, meta-, and para-) of pyrazine, named 23-B3C, 25-B3C, and 26-B3C, are theoretically studied in gas, solvent, and solid states by using the polarizable continuum model and the combined quantum mechanics and molecular mechanics method, respectively. These two regioisomers (23-B3C and 25-B3C) exhibit totally different aggregation behaviors. Through density functional theory (DFT) and time-dependent DFT (TDDFT) calculations, it has been concluded that the geometric changes of 23-B3C and 25-B3C are mainly contributed by the bond length and dihedral angle from gas to the solid phase. Herein, a lower HOMO-LUMO energy gap and a better intramolecular charge-transfer absorption are found for 25-B3C owing to a more sizable π-conjugation effect with smaller bond length alternation. Importantly, the β value of 25-B3C in the crystal with the π-π stacking with the same CT direction between the whole backbones (H-aggregate) and the end-groups (J-aggregate) commonly can substantially increase compared to that of the monomer. However, the obtained reduction of β value for 23-B3C is mainly because of V-aggregate (α > 90°) and reverse J-aggregate in crystal. Thus, our work showcases a profound understanding of the relationship between solid-state packing and macroscopic second-order NLO properties and provides a feasible molecule strategy for the development of high-efficiency NLO materials.

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.

Unsymmetrical benzothiazole-based dithienylethene photoswitches

Herein, we investigate the structure-property relationships in a new series of benzothiazole based unsymmetrical hexafluorocyclopentene dithienylethenes (DTEs) and compare the results with the known facts for symmetric diarylethenes (DAEs). We reveal high photocyclization efficiency resulting from a significant shift of ground state equilibrium to the antiparallel conformation and a barrierless excited state pathway to conical intersection, which remains unperturbed even in polar solvents for most of the prepared DTEs. Furthermore, we uncover that the rate of back thermal cycloreversion correlates clearly more with the central C-C bond-length in the transition state than with the central C-C bond-length in the ground state of the cyclic form. Finally, our detailed vibrational spectral analysis of studied DTEs points out significant changes in Raman and infrared spectra during photoswitching cycles which pave the way for a non-destructive readout of stored information.

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.

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.

Dithienylethene-Based Photochromic Siloles: A Straightforward and Divergent Synthetic Strategy

A straightforward synthetic methodology for the preparation of photochromic siloles based on the dithienylethene motif is developed. It relies upon an efficient palladium-catalyzed annulation reaction of a 2,3-bis(3-thienyl)-silirene with terminal alkynes in mild conditions. The reaction is functional group-tolerant and can be performed in high yields with a variety of functional terminal alkynes. It can even be extended to a polymeric polypropargylmethacrylamide (PPMA) substrate affording the corresponding photochromic polymer with different degree of photochromic unit incorporation by simply adjusting the polymer/ silirene stoichiometric ratio.

Photoinduced Changes in Aromaticity Facilitate Electrocyclization of Dithienylbenzene Switches

The concepts of excited-state aromaticity and antiaromaticity have in recent years with increasing frequency been invoked to rationalize the photochemistry of cyclic conjugated organic compounds, with the long-term goal of using these concepts to improve the reactivities of such compounds toward different photochemical transformations. In this regard, it is of particular interest to assess how the presence of a benzene motif affects photochemical reactivity, as benzene is well-known to completely change its aromatic character in its lowest excited states. Here, we investigate how a benzene motif influences the photoinduced electrocyclization of dithienylethenes, a major class of molecular switches. Specifically, we report on the synthesis of a dithienylbenzene switch where the typical nonaromatic, ethene-like motif bridging the two thienyl units is replaced by a benzene motif, and show that this compound undergoes electrocyclization upon irradiation with UV-light. Furthermore, through a detailed quantum chemical analysis, we demonstrate that the electrocyclization is driven jointly and synergistically by the loss of aromaticity in this motif from the formation of a reactive, antiaromatic excited state during the initial photoexcitation, and by the subsequent relief of this antiaromaticity as the reaction progresses from the Franck–Condon region. Overall, we conclude that photoinduced changes in aromaticity facilitate the electrocyclization of dithienylbenzene switches.

Is energy transfer limiting multiphotochromism? answers from ab initio quantifications

Dithienylethenes (DTEs) can be assembled to form supramolecular multiphotochromic systems that are highly functional molecular architectures of potential interest for building complex optoelectronic devices. Yet even simple DTE dimers relying on an organic linker may suffer from a partial photoactivity, i.e., only one of the two switches does isomerise. Experimentally, this limited photochromism has been attributed to an excited state energy transfer (EET) between the two DTEs of the multimer; this EET taking place instead of the desired photoinduced cyclisation of the DTE. However, no clear evidences of this phenomenon have been provided so far. In this work we propose the first rationalisation of this potential parasite photoinduced event using a computational approach based on Time-Dependent Density Functional Theory (TD-DFT) for the calculation of the electronic coupling in DTE dimers. Besides quantifying EET in several systems, we dissect the role of through-bond and through-space mechanisms on this process and clarify their dependence on both the nature and length of the bridge separating the two photochromes. The theoretical data obtained in this framework are in full agreement with the experimental outcomes and pave the way toward a molecular design of coupled, yet fully functionals, DTE-based multiswitches.

Theoretical investigation on second-order nonlinear optical properties of ruthenium alkynyl-dihydroazulene/vinylheptafulvene complexes

Ru metal acetylide electron donor-acceptor complexes have important applications in the field of nonlinear optics. Herein, in this work, a series of half-sandwich ruthenium-based Cp*(dpe)Ru ([Ru*]) metal complexes with the dihydroazulene/vinylheptafulvene (DHA/VHF) have been investigated by density functional theory (DFT) calculations. The results showed that the position of the [Ru*] acetylide functionality, either para or meta on the phenylene ring to the DHA/VHF core (1c/1o and 2c/2o), and additional a p-phenylene spacer (3c/3o) had a great influence on the second-order nonlinear optical (NLO) responses. The systems 1 and 3 can significantly increased second-order NLO responses compared with system 2. It was attributed to the more obvious charge transfer along y-axis, which is from [Ru*] acetylide functionality to DHA, accompanied by a significant decrease of the transition energy according electron density difference maps and time-dependent DFT calculations. The β_vec values of the open-ring complexes were larger than the corresponding closed-ring complexes owing to the smaller HOMO-LUMO gap in the open-ring complexes. It was also because of the smaller BLA values in open-ring complexes, which had stronger π-conjugation. Especially, the change ratio of β_vec value of system 2 was the largest due to the fact that their charge transfers degree varied greatly. In addition, the frequency-dependent NLO properties of the studied complexes were evaluated at 0.0239 a.u. and 0.0340 a.u. The calculation results demonstrated that the magnitude of the frequency-dependent first hyperpolarizability increased with the increasing frequency. We believe that our present work will be beneficial for further theoretical and experimental studies on large second-order NLO responses of metal complexes.

A curve-crossing model to rationalize and optimize diarylethene dyads

Extra crossing points play a key role in the photochemistry of diarylethene dyads.

Going from photochromic compounds presenting a single switchable function to multi-addressable photochromic multimers remains an extremely difficult task notably because the interactions of several photochromic units through a linker generally result in a substantial loss of photoactivity. Due to their size and the intrinsic complexity of their electronic structure, coupled photochromes also constitute a fundamental challenge for theoretical chemistry. We present here an effective curve-crossing model that, used in connection with easily accessible ab initio data, allows a first understanding of the difficulty to obtain efficient multiphotochromes. Indeed, we demonstrate that extra crossing points, specific to multiphotochromes, have to be passed to ensure reactivity. In addition, the proposed approach allows the definition of an intuitive tilt criterion that can be used to screen a large number of substitution patterns and hence help in the design of new compounds, an aspect that is also developed here. The compatibility of this tilt criterion with previously proposed static Franck–Condon parameters is discussed as well.

Designing efficient photochromic dithienylethene dyads

Aiming at designing more efficient multiphotochromes, we investigate with the help of ab initio tools the impact of the substitution on a series of dimers constituted of two dithienylethene (DTE) moieties, strongly coupled to each other through an ethynyl linker. The electronic structure and the optical properties of a large panel of compounds, substituted on different positions by various types of electroactive groups, have been compared with the aim of designing a dyad in which the three possible isomers (open-open, closed-open, closed-closed) can be reached. We show that appending the reactive carbons atoms of the DTE core with electroactive groups on one of the two photochromes allows cyclisation to be induced on a specific moiety, which leads to the formation of the desired closed-open isomer. Substituting the lateral positions of the thiophene rings provides further control of the topology of the frontier molecular orbitals, so that the electronic transition inducing the second ring closure stands out in the spectrum of the intermediate isomer.

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.

Multiphotochromic molecular systems

We review molecular compounds encompassing several photochromic units with a focus on their functionalities.

Density functional theory study of the conformation and optical properties of hybrid Au(n)-dithienylethene systems (n = 3, 19, 25)

We present a theoretical study of Aun-dithienylethene hybrid systems (n = 3, 19, 25), where the organic molecule is covalently linked to a nanometer-scaled gold nanoparticle (NP). We aim at gaining insights on the optical properties of such photochromic devices and proposing a size-limited gold aggregate model able to recover the optical properties of the experimental system. We thus present a DFT-based calculation scheme to model the ground-state (conformation, energetic parameters) and excited-state properties (UV-visible absorption spectra) of this type of hybrid systems. Within this framework, the structural parameters (adsorption site, orientation, and internal structure of the photochrome) are found to be slightly dependent on the size/shape of the gold aggregate. The influence of the gold fragment on the optical properties of the resulting hybrid system is then discussed with the help of TD-DFT combined with an analysis of the virtual orbitals involved in the photochromic transitions. We show that, for the open hybrid isomer, the number of gold atoms is the key parameter to recover the photoactive properties that are experimentally observed. On the contrary, for hybrid closed systems, the three-dimensional structure of the metallic aggregate is of high impact. We thus conclude that Au25 corresponds to the most appropriate fragment to model nanometer-sized NP-DTE hybrid device.

Competitive direct vs. indirect photochromism dynamics of constrained inverse dithienylethene molecules

Relaxation observed through several parallel pathways from the first excited state to the ground-state in inverse dithienylethene molecules.

A DFT study of a new class of gold nanocluster-photochrome multi-functional switches

The structural and electronic properties of dithienylethene photochromic molecules grafted onto a Au₂₅ nanocluster are reviewed and electron/energy transfers are discussed with the help of (TD-)DFT calculations.