Femtosecond Spectroscopic Study on Photochromic Diarylethenes with Terthiophene

Femtosecond dynamics of the ring closing process of diarylethene: a case study of electrocyclic reactions in photochromic single crystals

The cyclization reaction of the photochromic diarylethene derivative 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene was studied in its single crystal phase with femtosecond transient absorption spectroscopy. The transient absorption measurements were performed with a robust acquisition scheme that explicitly exploits the photoreversibility of the molecular system and monitors the reversibility conditions. The crystalline system demonstrated 3 × 10(4) repeatable cycles before significant degradation was observed. Immediately following photoexcitation, the excited state absorption associated with the open-ring conformation undergoes a large spectral shift with a time constant of approximately 200 fs. Following this evolution on the excited state potential energy surface, the ring closure occurs with a time constant of 5.3 ps, which is significantly slower than previously reported measurements for similar derivatives in the solution phase. Time resolved electron diffraction studies were used to further demonstrate the assignment of the transient absorption dynamics to the ring closing reaction. The mechanistic details of the ring closing are discussed in the context of prior computational work along with a vibrational mode analysis using density functional theory to give some insight into the primary motions involved in the ring closing reaction.

Ring closure dynamics of BTE-based photochromic switches: perfluoro- versus perhydrocyclopentene derivatives

The switching behavior of 1,2-bis(5-phenyl-2-methylthien-3-yl)perfluorocyclopentene and its nonfluorinated (perhydro) analogue are compared. For both molecules, the dynamics after optical excitation can be separated into three regimes: preswiching due to excited state mixing; the ring closure itself; postswitching related to vibrational cooling. The fluorinated version switches faster than its nonfluorinated analogue by about a factor of 4.7. This is explained by electronic level shifts near the crossing region between the S(1) and S(0) potential energy surfaces. In the nonfluorinated molecule the various levels involved in the switching have well-separated transition frequencies, which allow for a clear interpretation of experimental data. Thus, the fluorinated molecule makes a better (more efficient and faster) switch, but the nonfluorinated molecule provides a better model system for fundamental studies.