Abstract
We have studied the trans-4-(N-arylamino)stilbenes (p1H, p1CN, and p1OM) and 4-((E)-2-(dimethyl(4-((E)-styryl)phenyl)silyl)vinyl)-N,N-dimethylaniline by using time-resolved spectra and performed calculations based on density functional theory. The arylaminostilbenes relaxed to the equilibrium position of the planar intramolecular charge transfer (PICT) state from the Franck-Condon region in 0.5-1.8 ps. Then, p1H emitted fluorescence or underwent isomerization back to the ground state. The time constant of the isomerization process is about 10-120 ps varied with solvents viscosity and polarity. In polar solvent, p1CN and p1OM underwent twisting a single bond to reach the TICT surface; the time constant of this conversion is 5-30 ps for p1CN and 4-30 ps for p1OM. The third temporal component shows abnormal elongation lifetime in ether solvents. We propose that the PICT and the TICT states are close in energy and electronically coupled resulting in mixing the TICT character in PICT state. Molecule p1CN exhibits the hydrogen-bond effect according to the observed short lifetimes in alcohols. Moreover, p1OM may undergo isomerization in the TICT surface because the measured lifetime is five-times shorter than that in p1CN. For 4-((E)-2-(dimethyl(4-((E)-styryl)phenyl)silyl)vinyl)-N,N-dimethylaniline, the energy was transferred rapidly ~1 ps from the stilbenyl moiety to the aminostryenyl upon photoexcitation at 266 nm. Then the excited aminostryenyl in either 21ππ* state or 11ππ* relaxed to the ground state in 65 ps and 2 ns, respectively. In the polar environment, the charge transfer process occurred; the charge migrated from aminostryenyl to stilbenyl in 0.75 ps, then relaxed to the ground state by the charge recombination. In polar solvents, the transient absorption band of the charge transfer state peaked at 530 nm shows blue shift; the time constant of the blue shift becomes increased with solvent viscosity indicating structural variation in this process. We propose that the molecule in the charge-transfer surface changed the conformation from the anti to the syn form.