Photoinduced Intramolecular Charge Transfer and S₂ Fluorescence in Thiophene‐π‐Conjugated Donor–Acceptor Systems: Experimental and TDDFT Studies

Abstract

Experimental and theoretical methods were used to study newly synthesized thiophene-pi-conjugated donor-acceptor compounds, which were found to exhibit efficient intramolecular charge-transfer emission in polar solvents with relatively large Stokes shifts and strong solvatochromism. To gain insight into the solvatochromic behavior of these compounds, the dependence of the spectra on solvent polarity was studied on the basis of Lippert-Mataga models. We found that intramolecular charge transfer in these donor-acceptor systems is significantly dependent on the electron-withdrawing substituents at the thienyl 2-position. The dependence of the absorption and emission spectra of these compounds in methanol on the concentration of trifluoroacetic acid was used to confirm intramolecular charge-transfer emission. Moreover, the calculated absorption and emission energies, which are in accordance with the experimental values, suggested that fluorescence can be emitted from different geometric conformations. In addition, a novel S(2) fluorescence phenomenon for some of these compounds was also be observed. The fluorescence excitation spectra were used to confirm the S(2) fluorescence. We demonstrate that S(2) fluorescence can be explained by the calculated energy gap between the S(2) and S(1) states of these molecules. Furthermore, nonlinear optical behavior of the thiophene-pi-conjugated compound with diethylcyanomethylphosphonate substituents was predicted in theory.

Molecular Rotors as Fluorescent Viscosity Sensors: Molecular Design, Polarity Sensitivity, Dipole Moments Changes, Screening Solvents, and Deactivation Channel of the Excited States

“…This means that the emission of 1 and 5 is due to the LE excited state, not the TICT state, otherwise a blue-shifted emission would be observed. [20,34,42,43] Minor changes were observed for the TFA titration of other compounds (see the Supporting Information).…”

“…The emission of 6 was sensitive to solvent polarity and the fluorescence was completely quenched in polar solvents, such as methanol. [31,34] However, by decreasing the ICT effect with oxidation of the sulfur atom, the emission of 7 was no longer sensitive to the solvent polarity (Figure 2, e). Alternatively, with carbazole as the fluorophore, which is a weaker electron donor than phenothiazine, compound 8 shows emission that is not sensitive to the solvent polarity (see the Supporting Information).…”

“…[20,34,42,43] The TFA titration experiment showed that, with protonation of 1 and its analogue 5, the emission intensity was enhanced, but no blueshifting of the emission band was observed (see the Supporting Information). This means that the emission of 1 and 5 is due to the LE excited state, not the TICT state, otherwise a blue-shifted emission would be observed.…”

Hydrogen-bonding study of photoexcited 4-nitro-1,8-naphthalimide in hydrogen-donating solvents

“…Experiments performed using the femtosecond time-resolved vibrational spectroscopy have shown the potential to monitor hydrogen-bonding dynamics. Hydrogen-bonding dynamics always occurs on ultrafast timescales, which plays an increasingly important role in many photophysical processes and photochemical reactions [41][42][43][44][45][46][47][48][49][50][51][52][53]. It has been confirmed by Zhao et al that the intermolecular hydrogen bond in many molecular systems is greatly strengthened in the electronically excited state.…”

Insight from Singlet into Triplet Excited‐State Hydrogen Bonding Dynamics in Solution

“…[52][53][54][55][56][57][58][59]. The ground state and electronic excited states were investigated using the density functional theory (DFT) and the time-dependent density functional theory (TDDFT) methods respectively.…”