Cyano substitution effect on the emission quantum efficiency in stilbene derivatives: A computational study

A series of cyano-substituted stilbene derivatives exhibit quenched fluorescence in solution, in sharp contrast to their highly emissive parent counterparts. By employing the polarizable continuum model (PCM) method coupled with thermal vibration correlation function (TVCF) formalism, we comparatively investigated the electronic structures and excited-state dynamics of cyano-substituted para-distyrylbenzene (CN-DSB), 1,2-bis(pyridylphenyl)ethane (CN-BPPE) and the corresponding non-substituted DSB and BPPE lack of the cyano groups at their vinylene moieties in solution. The results show that the non-radiative decay rate constants of the CN-substituents are greatly enhanced by more than 4 orders of magnitude, while the radiative decay rate constants experience slight reduction, resulting in the very tiny fluorescence quantum efficiency. By analyzing the reorganization energies projected onto both normal modes and internal coordinates, the non-radiative consumption ways via molecular torsional motions of the single even double bonds adjacent to the CN group are active upon CN-substitution. Our study presents a rational explanation for the experimental CN-substitution induced quenching phenomena in stilbene derivatives, which is beneficial for the design of new fluorescent emitters and probes.