Relaxation Pathways of Photoexcited Diaminostilbenes. The meta-Amino Effect

摘要

The spectroscopy and photochemistry of several 3-substituted trans-diaminostilbenes have been investigated and the results compared withthose from previous studies of 4,4'-diaminostilbenes. The excited-state properties of 3,5-diaminostilbene and 3,3'-diaminostilbene are similar to those for the parent 3-aminostilbene. These amines display long, temperature-independent singlet lifetimes and large fluorescence quantum yields. The long singlet lifetimes are a consequence of large barriers for torsion about the central double bond on the singlet state surface and low fluorescence rate constants. Thus, these diaminostilbenes display the "metaamino effect" previously identified for 3-aminostilbene and for some amine-donor-acceptor substituted stilbenes. The 4,4'-diaminostilbenes also have larger torsional barriers in nonpolar solvents but shorter singlet lifetimes as a consequence of larger fluorescence and intersystem crossing rate constants. The photophysical properties of 3,4'-dimethylaminostilbene ar similar to those of 4,4'-aminostilbenes; however, it has a lower torsional barrier, similar to that of the parent 4-aminostilbene.