The effect of solvent polarity on the photophysical properties of 4-cyano-(4′-methylthio)diphenylacetylene: A prototypic donor–acceptor system

Abstract

The photophysical properties of the target compound are extremely sensitive to changes in solvent polarity since the lowest-energy excited states possess considerable charge-transfer character. Excitation results in a greatly increased dipole moment, with the resultant excited singlet state retaining a lifetime of ca. 1 ns in all solvents. Radiative decay involves coupling between the lowest-energy excited singlet state and both the ground state and an upper excited singlet state. The level of coupling to the upper singlet decreases in non-polar solvents, presumably due to symmetry factors. The radiative rate constant decreases smoothly with increasing solvent polarity function as the molecule acquires an ever increasing dipolar character. Non-radiative decay includes both intersystem crossing and internal conversion, but the former process dominates in polar solvents. The excited singlet state lifetime is very weakly dependent upon temperature in the solid state. However, in polar solutions where the Stokes' shift decreases with decreasing temperature, there is clear evidence for an activated process. This is believed to involve coupling to the upper-lying singlet excited state.

Photophysics of two Prototypical Molecular‐Wire Building Blocks: Solvent‐Induced Conformational Dynamics?

“…Additionally the absorption spectra become apparently more symmetric in polar media. The fluorescence spectra of 2 , on the other hand, change from a strongly vibronically structured band shape to totally unstructured emission, which can be well described by an asymmetric Gaussian and resembles the emission spectra of donor–acceptor‐substituted PPE analogues 26…”

“…Instead of following the solvatochromism of the transition’s maximum or center of gravity, we chose to extract the lowest energetic vibronic absorption transition ${\tilde \nu _{\rm{a}} }$ and the highest energy vibronic emission transition ${\tilde \nu _{\rm{f}} }$ . The simplest and most straightforward approach to this end consists of fitting multiple Gaussians to the experimental spectra 26. The spectra could be fitted by using multiple Gaussians with only one bandwidth σ , a common progression per solvent ${{\rm{\Delta }}_{\tilde \nu } }$ , and varying relative Gaussian amplitudes A i :49 where ${F\left( {\tilde \nu } \right)}$ is ${\varepsilon {{\left( {\tilde \nu } \right)} \mathord{\bigg/ {\vphantom {{\left( {\tilde \nu } \right)} {\tilde \nu }}} \kern-\nulldelimiterspace} {\tilde \nu }}}$ for absorption and ${I{{\left( {\tilde \nu } \right)} \mathord{\bigg/ {\vphantom {{\left( {\tilde \nu } \right)} {\tilde \nu ^3 }}} \kern-\nulldelimiterspace} {\tilde \nu ^3 }}}$ for emission,50 ${\tilde \nu _0 }$ denotes either ${\tilde \nu _{\rm{a}} }$ or ${\tilde \nu _{\rm{f}} }$ , and + is used for absorption and − for emission spectra.…”

Oligo(p‐phenyleneethynylene) (OPE) Molecular Wires: Synthesis and Length Dependence of Photoinduced Charge Transfer in OPEs with Triarylamine and Diaryloxadiazole End Groups

“…This supports the notion that the significantly red-shifted emission in polar media for the shorter compounds 1-3 with Stokes shifts of 6970-8680 cm -1 can be ascribed to ICT states. [33,34,35] on changing from cyclohexane to dichloromethane in a reported [36] D--A OPE system with dialkylamine as the donor group, 6-methylpyran-2-one as the acceptor group and -para-C 6 H 4 CCC 6 H 4 CC-as the OPE bridge. This molecule has a highly polar excited state with ICT properties.…”

Asymmetric Color‐Changeable Luminophore with Donor–Acceptor–Donor Structure for Solvent and Temperature Sensitive Properties

“…CT luminophores comprise electron‐donor (D) and electron‐acceptor (A) units to separate the highest occupied and lowest unoccupied molecular orbitals . An excited CT state induces strong polarization in luminophores, which shows solvent‐sensitive luminescence properties . A strong CT state also exhibits a thermally assisted delayed fluorescence (TADF), that gives a temperature‐sensitive luminescence with a long lifetime (μs order) .…”

“…[3] An excited CT state induces strong polarization in luminophores, which shows solvent-sensitive luminescence properties. [4] A strong CT state also exhibits a thermally assisted delayed fluorescence (TADF), that gives a temperature-sensitive luminescence with a long lifetime (μs order). [5] To date, many environment-sensitive luminescent materials with various electron donor and acceptor structures (D À A) have been reported.…”