We report experimental evidence for the generation of ferroelectric polarization in an organic conductor -[bis(ethylenedithio)tetrathiafulvalene] 2 I 3 obtained by optical second-harmonic generation. The spontaneous polarization emerges along with a metal-to-insulator transition that is driven by the Wigner crystallization of electrons. The strong optical nonlinearity and its ultrafast photoresponse demonstrated by this study exemplify the nature of the ferroelectric polarization that originates from the electron ordering.

Ultrafast intersystem crossing of UVA-sensitive 4-thiothymidine in aqueous solution was investigated by pump-probe transient absorption. 4-Thiothymidine in the excited singlet state exhibited a distinctively different photophysical pathway from that of thymidine, with intersystem crossing to the triplet manifold being dominant and the triplet formation being completed within approximately 10 ps. This was further supported by quantum chemical calculations. We propose that the unique behavior of photoexcited 4-thiothymidine results from its distinctive molecular and electronic structures and that the triplet form is the initial toxic source to DNA and other biomolecules.

Articles you may be interested inCommunication: Ultrafast time-resolved ion photofragmentation spectroscopy of photoionization-induced proton transfer in phenol-ammonia complex Ultrafast nonradiative dynamics in electronically excited hexafluorobenzene by femtosecond time-resolved mass spectrometry Ultrafast processes in photoexcited N-salicylideneaniline have been investigated with femtosecond time-resolved resonance-enhanced multiphoton ionization spectroscopy. The ion signals via the S 1 (n,*) state of the enol form as well as the proton-transferred cis-keto form emerge within a few hundred femtoseconds after photoexcitation to the first S 1 (,*) state of the enol form. This reveals that two ultrafast processes, excited-state intramolecular proton transfer ͑ESIPT͒ reaction and an internal conversion ͑IC͒ to the S 1 (n,*) state, occur on a time scale less than a few hundred femtoseconds from the S 1 (,*) state of the enol form. The rise time of the transient corresponding to the production of the proton-transferred cis-keto form is within 750 fs when near the red edge of the absorption is excited, indicating that the ESIPT reaction occurs within 750 fs. The decay time of the S 1 (,*) state of the cis-keto form is 8.9 ps by exciting the enol form at 370 nm, but it dramatically decreases to be 1.5-1.6 ps for the excitation at 365-320 nm. The decrease in the decay time has been attributed to the opening of an efficient nonradiative channel; an IC from S 1 (,*) to S 1 (n,*) of the cis-keto form promotes the production of the trans-keto form as the final photochromic products. The two IC processes may provide opposite effect on the quantum yield of photochromic products: IC in the enol form may substantially reduce the quantum yield, but IC in the cis-keto form increase it.

The excited-state double-proton-transfer (ESDPT) reaction in the jet-cooled 7-azaindole dimer (7AI2) has been investigated with picosecond time-resolved resonance-enhanced multiphoton ionization spectroscopy. The observed decay profiles of 7AI2 by exciting the origin and the intermolecular stretch fundamental in the S1 state are well reproduced by single-exponential functions with time constants of 1.9 +/- 0.9 ps and 860 +/- 300 fs, respectively. This result provides clear evidence of the concerted mechanism of ESDPT in 7AI2.