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

• Published in: Chemistry : a European journal Vol. 14; no. 23; pp. 6935 - 6947
• Main Authors: Zhao, Guang-Jiu, Chen, Rui-Kui, Sun, Meng-Tao, Liu, Jian-Yong, Li, Guang-Yue, Gao, Yun-Ling, Han, Ke-Li, Yang, Xi-Chuan, Sun, Licheng
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 08.08.2008; WILEY‐VCH Verlag; Wiley
• Subjects: charge transfer; Chemistry; Chemistry, Multidisciplinary; density functional calculations; donor-acceptor systems; fluorescence; nonlinear optics; Physical Sciences; Science & Technology; RECOMBINATION; AB-INITIO; PHOTOPHYSICAL PROPERTIES; charge transfer; DYNAMIC KINETIC RESOLUTION; EXCITED-STATES; TRANSITION; density functional calculations; nonlinear optics; TRANSFER MECHANISM; donor-acceptor systems; ELECTRON-TRANSFER; fluorescence; ENERGY-TRANSFER; BOND
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200701868

Experimental and theoretical methods were used to study newly synthesized thiophene‐π‐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 S2 fluorescence phenomenon for some of these compounds was also be observed. The fluorescence excitation spectra were used to confirm the S2 fluorescence. We demonstrate that S2 fluorescence can be explained by the calculated energy gap between the S2 and S1 states of these molecules. Furthermore, nonlinear optical behavior of the thiophene‐π‐conjugated compound with diethylcyanomethylphosphonate substituents was predicted in theory. Do the twist! Newly synthesized thiophene‐π‐conjugated donor–acceptor compounds with different electron‐withdrawing substituents exhibit efficient intramolecular charge‐transfer (ICT) emission in polar solvents with large Stokes shifts (>220 nm) and strong solvatochromism. A combination of TDDFT calculations on electronically excited states and spectroscopic studies unambiguously demonstrated a twisted ICT mechanism (see picture).