Structure-Dependent\nBalance between Excited-State\nDeactivation Pathways in Cross-Conjugated Molecular Photoswitches

Diaryl thieno-[3,4-<i>b</i>]thiophenes (TT)\nare photoswitchable\ncompounds that operate through reversible photoinduced cyclization/cycloreversion\nand have been designed specifically for integration within π-conjugated\npolymers to switchably manipulate polymer electronic properties. Here\nwe report on how cross conjugating the central TT moiety impacts photocyclization\ndynamics as interrogated using transient absorption spectroscopy (TAS)\nfor a series of switches built with electron-rich substituents that\nhave various electronic interaction strengths with the TT core. For\ncross-conjugated structures exhibiting a propensity to switch in steady-state\nphotoconversion experiments, ultrafast TAS reveals signatures of rapid\ndynamics (occurring within <1–10 ps) similar to those observed\nfor unsubstituted switches and that are consistent with photocyclization.\nIn contrast, TAS reveals comparatively slower spectral dynamics (∼100\nps) that are not consistent with cyclization for switches that are\ncross-conjugated with substituents that have greater electronic interaction\nwith the TT core and that exhibit no propensity to photoswitch in\nphotoconversion experiments. Microsecond TAS confirms that photoinduced\ncyclization occurs for the former and that a metastable triplet state\nlocalized on the conjugated backbone is generated with the latter.\nWe find that the balance of these two deactivation pathways is sensitive\nto the interaction strength of the conjugated substituents with the\ncore, with select structures exhibiting signatures of both. These\nfindings are consistent with prior work demonstrating that the LUMO\ncharacter is delocalized over the switch backbone when there are strong\ninteractions with cross-conjugating groups and reveal that the competition\nbetween deactivation pathways can be controlled structurally by weakening\nπ conjugation across the backbone.