Donor–Pyrene–Acceptor Distance-Dependent Intramolecular Charge-Transfer Process: A State-Specific Solvation Preferred to the Linear-Response Approach

Abstract

Photoinduced intramolecular charge-transfer (ICT) molecules are important in various applications such as a probe for single-molecule spectroscopy, cell imaging, laser dyes, biomarkers, solar cells, in photosynthesis, etc. Here, we report a new set of substituted pyrene dye molecules, N,N-dimethylamino nitrilo pyrene and its higher analogues, containing pull–push donor (D)–chromophore (π)–acceptor (A) functional groups with enhanced photophysical characteristics like oscillator strength, light-harvesting, and ICT properties. The excited-state ICT process has been established by quantum chemical calculations using the density functional theory method in vacuo and in solvents of different polarity and hydrogen-bonding ability using linear-response (LR) and state-specific (SS) solvation approaches with gradually increasing the D–A distance. The studied molecules show solvent polarity-dependent larger Stokes’ shifts (3609–9016 cm–1, in acetonitrile), higher excited-state dipole moments (11.7–16.8 Debye, in acetonitrile), higher possibilities of highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) electronic transitions, etc., which support the occurrence of the excited-state ICT process. Here, we demonstrate how to increase the efficiency of the ICT process and also tune the ICT fluorescence maximum. We find that with a variation of the D–A distance, studied molecules show a noticeable effect on the spectroscopic and molecular properties such as the position of absorption and fluorescence band maxima, Stokes’ shift, dipole moment, light-harvesting, and ICT properties. We also show that the SS solvation approach is more supportive than the LR method to the ICT process.