Structural Effects on Nonadiabatic Photocyclization in ortho-Arenes

Nonadiabatic photocyclization is the chemical dynamic relevant to the function of many photoswitchable materials as well as photochemical synthesis of polyaromatic hydrocarbons by cyclodehydrogenation. ortho-arenes are an under-studied class of molecular photoswitch owing to their low cyclized product stabilities that otherwise provide a unique opportunity for the experimentalist to study photocyclization mechanisms in detail. Through the use of time-resolved absorption spectroscopy on femtosecond to microsecond timescales the entire photocyclization process can be monitored from “birth” to “death,” i.e. ring-fusion to ring-fission. Following ultraviolet photoexcitation OTP undergoes cyclization to form DHT. Although global spectral analysis with simple kinetic models adequately fits spectral dynamics, signatures of DHT formation are obscured by spectral overlap with the excited-state that has a lifetime of ~3 picoseconds. Thermal ring-reopening of DHT to regenerate OTP occurs with a 38 nanosecond lifetime and an activation energy of 0.27 eV. Following the study of OTP a variety of other ortho-arenes were examined by systematic substitution, including phenyl substituted analogs, 1,2,3-triphenylbenzene, ortho-quaterphenyl and hexaphenylbenzene, as well as boron-nitrogen substituted analogs, including hexaphenylborazine and 1,2:3,4:5,6 tris(o,o’-biphenylylene) borazine. Generally these substitutions increased the excited-state lifetime relative to OTP due to an increase in either electronic delocalization or structural hindrance within excited-state geometries while the stability of the corresponding photoproducts decreased relative to DHT due to entropic effects. A notable exception is hexaphenylbenzene, which exhibits a 2 microsecond lifetime for ring-reopening of the photoproduct tetraphenyl-DHT that is a consequence of entropic stabilization due to increased phenyl-phenyl steric interactions that constrain thermally activated relaxation to the transition state. Furthermore, excited hexaphenylborazine decays within 3 picoseconds due to the localized electronic character of the borazine ring. No direct spectroscopic observation of cyclization was observed for any boron-nitrogen substituted system due to the increase in charge localization that reduces the stability of the conjugated DHT photoproduct. More recent experiments utilizing pump-repump-probe spectroscopy have determined that the observed excited-state decay is kinetically decoupled from photocyclization, which occurs in less than 200 femtoseconds. The results presented in this thesis provide insight for the improvement of photoswitch and photosynthetic efficiency through the generalization of these structure-dynamics relationships