Helicity as a Steric Force: Stabilization and Helicity-Dependent Reversion of Colored o‑Quinonoid Intermediates of Helical Chromenes

Photolysis of regioisomeric helical chromenes 1 and 2 leads to colored reactive intermediates. While the latter generally decay quite rapidly, they are found to be longer lived in 1 and highly persistent in 2. The remarkable stability of the otherwise fleeting transient in 2 allowed isolation and structural characterization by X-ray crystallography. The structural analyses revealed that steric force inherent to the helical scaffold is the origin of stability as well as differentiation in the persistence of the intermediates of 1 and 2 (1Q and 2Q). The structure further shows that diphenylvinyl moiety in the TT isomer of 2Q gets splayed over the helical scaffold such that it is fraught with a huge steric strain to undergo required bond rotations to regenerate the precursor chromene. Otherwise, reversion of 2Q was found to occur at higher temperatures. Aazahelical chromenes 3 and 4 with varying magnitudes of helicity were designed in pursuit of o-quinonoid intermediates with graded activation barriers. Their photogenerated intermediates 3Q and 4Q were also isolated and structurally characterized. The activation barriers for thermal reversion of 2Q–4Q, as determined from Arrhenius and Eyring plots, are found to correlate nicely with the helical turn, which decisively determines the steric force. The exploitation of helicity is thus demonstrated to develop a novel set of photoresponsive helicenes 2–4 that lead to colored intermediates exhibiting graded stability. It is further shown that the photochromism of 2–4 in conjunction with response of 2Q–4Q to external stimuli (acid, heat, and visible radiation) permits development of molecular logic gates with INHIBIT function.