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

Diaryl thieno-[3,4-b]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. In contrast, TAS reveals comparatively slower spectral dynamics (∼100 ps) that are not consistent with cyclization for switches that are cross-conjugated with substituents that have greater electronic interaction with the TT core and that exhibit no propensity to photoswitch in photoconversion experiments. Microsecond TAS confirms that photoinduced cyclization occurs for the former and that a metastable triplet state localized on the conjugated backbone is generated with the latter. We find that the balance of these two deactivation pathways is sensitive to the interaction strength of the conjugated substituents with the core, with select structures exhibiting signatures of both. These findings are consistent with prior work demonstrating that the LUMO character is delocalized over the switch backbone when there are strong interactions with cross-conjugating groups and reveal that the competition between deactivation pathways can be controlled structurally by weakening π conjugation across the backbone.