Ultrafast photoinduced intramolecular charge transfer in push–pull distyryl furan and benzofuran: solvent and molecular structure effect

Abstract

The excited state deactivation pathways of push-pull distyryl furan and benzofuran derivatives in several organic solvents were investigated in detail by using time-resolved transient absorption and fluorescence spectroscopies, with nano- and femto-second time resolution. Solvent polarity was found to play a key role in determining the efficiencies of fluorescence, intersystem crossing and internal conversion. The triplet yield gradually decreased, while the internal conversion increased upon increasing the solvent dielectric constant. However the fluorescence showed a different solvent polarity effect in the low and high solvent polarity region, with a reversal of the trend of fluorescence properties (quantum yield and lifetime). This fact points to an emitting state of a different nature (smaller and larger dipole moments) in the two cases, as also suggested by the huge fluorosolvatochromism. In fact the ultrafast spectroscopic investigation evidenced the presence of two transients characterized by peculiar spectral shapes assigned to a locally excited (LE) and a charge transfer (CT) state. In the more polar solvents the CT state was the longer lived, fluorescent one and an intramolecular charge transfer process was found to be operative and to become faster (up to ∼200-250 fs) in the higher polarity media. On the contrary, distyrylfuran, which exhibits the same molecular skeleton without the push-pull character showed a similar excited state dynamics in solvents of different polarities.

Comprehensive Photophysical Behaviour of Ethynyl Fluorenes and Ethynyl Anthracenes Investigated by Fast and Ultrafast Time‐Resolved Spectroscopy

“…The experimental setup for ultrafast spectroscopic and kinetic measurements has already been reported. [34,52,53] Briefly, the 400 nm excitation pulses of approximately 60 fs were generated by an amplified Ti:sapphire laser system (Spectra Physics, Mountain View, CA). The transient absorption setup (Helios, Ultrafast Systems) was characterised by temporal resolution of about 150 fs and spectral resolution of 1.5 nm.…”

“…This behaviour is analogous to what was already observed for other push-pull compounds. [34,35] Note that f T in different solvents was calculated with the e T measured in CH and assuming that the solvent can slightly shift the absorption spectrum but leaves unchanged the intensity of the bands.…”

Effect of the π Bridge and Acceptor on Intramolecular Charge Transfer in Push–Pull Cationic Chromophores: An Ultrafast Spectroscopic and TD‐DFT Computational Study

“…Our research has been recently devoted to the investigation of the solvent effect on the spectra, the photophysics and the excited‐state dynamics of push–pull molecules, both neutral12–16 and positively charged. Relevant results have been obtained for the latter,17, 18 which have shown a peculiar negative solvatochromic behavior and an interesting competition between the occurrence of solvation and photoinduced ICT during their excited‐state deactivation.…”

Synthesis, spectral properties and photobehaviour of push–pull distyrylbenzene nitro-derivatives

“…We reported recently the synthesis and photophysical study of conjugated systems bearing electron-donor/acceptor groups at the extreme ends, connected by a spacer [1][2][3][4][5][6][7]. The design and study of these push-pull (D--A) systems have attracted our attention because of their potential application as fluorescent probes, as dye laser systems and in optoelectronics [8,9], especially for their hyperpolarizability properties which make them candidates as components of non linear optical (NLO) materials [10][11][12][13].…”

“…Of particular interest was the study of 2-(p-nitrostyryl),5-styrylfuran and 2-(p-methylstyryl),3-(p-nitrostyryl)benzofuran [1,2,4,5], where the nitro group and the central -excessive furan ring are responsible for the introduction of electronic states with a strong CT character. Moreover, in the first molecule a peculiar, very interesting, dependence of the intersystem crossing kinetic constant (k ISC ) on the dielectric constant (ε) of the solvent was evidenced [14].…”