Relaxation pathways of photoexcited diaminostilbenes. The meta-amino effect

The spectroscopy and photochemistry of several 3-substituted trans-diaminostilbenes have been investigated and the results compared with those 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 "meta-amino 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 are similar to those of 4,4′-aminostilbenes; however, it has a lower torsional barrier, similar to that of the parent 4-aminostilbene.