Spectral and Photophysical Properties of Ethylene-Bridged Side-to-Side Porphyrin Dimers. 2. Femtosecond Transient Absorption and Picosecond Fluorescence Study of trans-1,2-Bis(meso-octaethylporphyrinyl)ethene

• Published in: Journal of physical chemistry (1952) 1996-08, Vol.100 (32), p.13867-13873
• Main Authors: Chachisvilis, Mirianas, Chirvony, Vladimir S, Shulga, Alexander M, Källebring, Bruno, Larsson, Sven, Sundström, Villy
• Format: Article
• Language: English
• ISSN: 0022-3654; 1541-5740
• DOI: 10.1021/jp960738w

Femtosecond transient absorption and picosecond fluorescence spectra and kinetics have been measured for an ethylene-bridged porphyrin dimer molecule, trans-1,2-bis(meso-octaethylporphyrinyl)ethene (tbisOEP). In part 1 of this work it was shown with the help of absorption and fluorescence spectroscopy that in solution the molecule exists in at least two different conformers, the P conformer with porphyrin-type spectral properties and the U conformer with spectral properties atypical for porphyrin molecules. As our quantum chemical calculations showed, the U conformer is characterized by a common π-conjugation through the double-bond bridge. In the present work we have found that the lifetimes of the lowest excited singlet states of the P and U conformers are extraordinary short, ∼6 ps for P and ∼7−9 ps for U in toluene at room temperature. Singlet-state lifetimes of both conformers were found to be strongly dependent on solvent viscosity with that of the U-form exhibiting the most pronounced dependence; a fluorescence lifetime of approximately 460 ps was measured for the U conformer in a frozen toluene solution at 77 K. Photoisomerization-like conformational relaxation in the S1 state, leading the system to the “point of funnel” characterized by decreased ΔE(S1−S0) energy gap, is proposed as a mechanism to explain the photophysics of the P and U conformers.