Origins of Large Stokes Shifts in a Pyrene–Styrene-Based Push–Pull Organic Molecular Dyad in Polar Solvents and Large Electron Mobility in the Crystalline State: A Theoretical Perspective

Push–Pull molecular chromophores are of increased research interest because of their potential as sensors in bioimaging and biomedicine as well as their promise as components in photonic and photovoltaic applications. Herein, the photophysical properties of a recently synthesized modular fluorescent probe, which was made of a pyrene- and styrene-based donor–acceptor dyad (StyPy), in different polar solvents and the charge transport in the crystalline state are investigated using first-principles density functional theory (DFT) and time-dependent DFT employing an optimally tuned range-separated hybrid functional. In agreement with the experimental observations, the calculated low-energy optical absorption of StyPy is only marginally affected by the solvent polarity. In contrast, the emission is largely red-shifted with increasing solvent polarity. Importantly, it is unveiled that the partial charge-transfer-assisted significant excited-state relaxation produces an almost planarized StyPy from the pretwisted ground-state structure, causing the observed Stokes shift. A greater frontier orbital delocalization leads to an increased spectral intensity and also to an excited-state structural rigidity as the planar StyPy suppresses the nonradiative decay, supporting an extraordinarily high fluorescence quantum yield. Furthermore, the crystalline StyPy exhibits a greater (by 2 orders of magnitude) electron mobility compared to that of holes, which is attributed to the smaller reorganization energy and larger electronic coupling for the transport of electrons than the holes. This study provides a fundamental understanding of the origins of an experimentally observed large Stokes shift for StyPy in a polar medium and also suggests crystalline StyPy as a potential n-type transport material, showing its great promise toward bioimaging and electronic applications.