Structure-Dependent Balance between Excited-State Deactivation Pathways in Cross-Conjugated Molecular Photoswitches.

Diaryl thieno-[3,4-]thiophenes (TT) are photoswitchable compounds that operate through reversible photoinduced cyclization/cycloreversion and have been designed specifically for integration within π-conjugated polymers to switchably manipulate polymer electronic properties. Here we report on how cross conjugating the central TT moiety impacts photocyclization dynamics as interrogated using transient absorption spectroscopy (TAS) for a series of switches built with electron-rich substituents that have various electronic interaction strengths with the TT core. For cross-conjugated structures exhibiting a propensity to switch in steady-state photoconversion experiments, ultrafast TAS reveals signatures of rapid dynamics (occurring within <1-10 ps) similar to those observed for unsubstituted switches and that are consistent with photocyclization.