Probing photoinduced intersystem crossing by two-color, double resonance single molecule spectroscopy

A photoinduced effect on the rate of reverse intersystem crossing (isc) in single sulforhodamine 101 (SR101) molecules has been observed using a novel two-color, multipulse, double resonance technique. The observed photoinduced intersystem crossing can be modeled by a four-electronic-state scheme (T-1 --> T-N --> S-1 --> S-0) in which one of the excitation colors is in resonance with the S-0 --> S-1 transition and the other is resonant with the T-1 --> T-N transition. Synchronous averaging over many cycles of the two-color pulsed excitation experiment provides transient data that can be analyzed in a straightforward manner that is analogous to a conventional chemical kinetics experiment. Separate single color pulsed experiments were used to evaluate whether single color excitation (543 nm) alone could be responsible for photoinduced reverse ise (as previously reported for other molecules), However, results from the single color, pulsed excitation experiments show no evidence of this effect for two dyes, DiI and SR101.