Addressing Charge‐Transfer and Locally‐Excited States in a Twisted Biphenyl Push‐Pull Chromophore

Abstract

We present the synthesis and spectroscopic characterization of a twisted push-pull biphenyl molecule undergoing photoinduced electron transfer. Steady-state and transient absorption spectra suggest, in this rigid molecular structure, a subtle interplay between locally-excited and charge-transfer states, whose equilibrium and dynamics is only driven by solvation. A theoretical model is presented for the solvation dynamics and, with the support of quantum chemical calculations, we demonstrate the existence of two sets of states, having either local or charge-transfer character, that only "communicate" thanks to solvation, which is the sole driving force for the charge-separation process.the molecule and hindering the rotation around the central CÀ C bond. Accordingly, conformational motion can be disregarded, leaving solvation as the only coordinate coupled to the electron transfer process. Upon photoexcitation, a longdistance and long-lived charge-transfer state is observed, whose photophysics is governed by polar solvation. In particular, we analyze the interplay between the LE and CT states in solution by means of steady-state and time-resolved optical spectroscopy, as well as by making resort to essential-state models and quantum chemical calculations. We do not limit the investigation to the first LE and CT states, but recognize and identify two distinct manifolds of LE and CT states that are not optically coupled and can interconvert only thanks to solvation after photoexcitation. 2 3 4 5 6 7 8

Selective Modification for Red‐Shifted Excitability: A Small Change in Structure, a Huge Change in Photochemistry

“…We also recorded steady‐state fluorescence emission spectra (Figure 4) to investigate the occurrence of dual fluorescence to probe for the ICT/‐H‐bonding hypothesis. We would expect fluorescence from the LE and CT states [26] . Experimentally, the following was observed: all derivatives showed weak fluorescence signals in general.…”

“…We would expect fluorescencef rom the LE and CT states. [26] Experimentally,t he following was observed:a ll derivatives showedw eak fluorescence signals in general.T his corresponds to our TDDFT calculations, which predicted low-energy transitions to the first excited states. Therefore, non-radiative decay is very likely.…”

Optical spectra of organic dyes in condensed phases: the role of the medium polarizability

“…Fast solvation then introduces a two-electron term in the electronic Hamiltonian, that however, in ESM acquires a very simple form with a clear physical meaning. Indeed for all CT dyes discussed here, with dipolar, quadrupolar or octupolar structure, one findsˆ µ 2 = µ 2 0ρ , so that H AA = H mol − elρ (9) where el = µ 2 0 r el /2 measures the amount of energy gained by the system in a zwitterionic state due the relaxation of the electronic clouds of the solvent molecule. In other terms, when going from gas phase to solution, the energy 2z required to separate a charge along a molecular arm is reduced by the solvent relaxation energy related to fast solvation, so that z → z − el /2.…”

“…The effective design of molecular materials for innovative applications requires the concurrent optimization of the active specie, the dye, and its matrix, a highly non trivial task, since the molecular properties depend, in an intrinsically non-linear way, from the properties of the local environment. Charge transfer (CT) dyes composed of electron-donor (D) and acceptor (A) moieties connected by π-conjugated bridges find applications in solar cells, 1 OLED, 2,3 non-linear optics, [4][5][6][7] and are interesting model systems for photoinduced CT. 8,9 The presence of low-lying excited states and of delocalized electrons makes these molecules extremely responsive to the local environment. 10 Intermolecular interactions in aggregates, supramolecular complexes and crystals have been discussed in different contexts, underlying how mutually interacting polarizable and/or polar molecules lead to specific spectroscopic features that cannot be reconciled with the standard exciton models.…”

Synergistic Approach of Ultrafast Spectroscopy and Molecular Simulations in the Characterization of Intramolecular Charge Transfer in Push-Pull Molecules

“…Recently theoretical and experimental studies have contributed to the description of the structure-function relationships in push-pull systems with respect to solvent effects on the ICT dynamics [7,29,31,33].…”