In silico strategy to design an efficient organic photoswitch based on excited-state cation transfer.

A new class of photoswitches and the corresponding elementary photoinduced reaction, the so-called Excited-State Cation Transfer (ESCT), are investigated. This reaction relies on an intramolecular photo-release/photo-complexation of cation: after irradiation, the cation is translocated from a complexation site 1 to a site 2 during the excited state lifetime. Our purpose is thus to develop a computational strategy based on Density Functional theory (DFT) and its time-dependent counterpart (TD-DFT) to improve the different properties of the ESCT photoswitches, namely (i) the ground state complexation constant K, (ii) the excited state complexation constant K*, (iii) the photoejection properties and (iv) the population of the triplet states from a singlet state via intersystem crossing to increase the lifetime of the excited state. In this work, we are interested in optimizing the ESCT properties of a betaine pyridinium chromophore substituted by a 15-aza-5-crown, that was previously shown to efficiently photoeject a Ca2+ cation from the site 1 but no photo-recapture was observed in the site 2 [Aloïse et al., Phys. Chem. Chem. Phys., 2016, 22, 15384]. To this purpose, we have investigated the impact of the modification of the site 1 on the ESCT properties by introducing different substituents (EDG groups, halogen atoms) at different positions. So far, promising systems have been identified but a simultaneous improvement of all the ESCT photoswitches properties has yet not been achieved.

Aggregation-Induced Emission: A Challenge for Computational Chemistry Taking TPA-BMO as an Example*.

A multi-environment computational approach is proposed to study the modulation of the emission behavior of the triphenylamine (Z)-4-benzylidene-2-methyloxazol-5(4H)-one (TPA-BMO) molecule [Tang et al., J. Phys. Chem. C 119, 21875 (2015)]. We aim at (1) proposing a realistic description of the molecule in several environments (solution, aggregate, polymer matrix), (2) modelling its absorption and emission properties, and (3) providing a qualitative understanding of the experimental observations by highlighting the photophysical phenomena leading to the emission modulation. To this purpose, we rely on (TD-)DFT calculations and classical Molecular Dynamics simulations, but also on the hybrid ONIOM QM/QM' approach and the in situ chemical polymerization methodology. In low-polar solvents, the investigation of the potential energy surfaces and the modulation of the emission quantum yield can be attributed to possible photophysical energy dissipation caused by low-frequency vibrational modes. In the aggregate and in the polymer matrix, the emission modulation can be qualitatively interpreted in terms of the possible restriction of the intramolecular vibrations. For these two systems, our study highlights that a careful modelling of the environment is far from trivial but is fundamental to model the optical properties of the fluorophore.