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

Abstract

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.

P‐Type Photochromism of New Helical Naphthopyrans: Synthesis and Photochemical, Photophysical and Theoretical Study

“…Values for the t 2 of as imilar order of magnitude have also been found by Moorthy et al, [38] who estimated, for the TT obtained from ar elatedh elical naphthopyran, al ifetime of 46 years in am ixed mesitylene/1,2-dichlorobenzene solvent. In view of the data listed in Ta ble 5, it is likely that, in toluene, the interaction between the solventm olecules and the TT form changes to some extent the conformation of the latter and the activation barrier of the TT-to-TC process.…”

Diastereo‐ and Enantioselective Synthesis of Organometallic Bis(helicene)s by a Combination of CH Activation and Dynamic Isomerization

“…The 1 H NMR spectra showed characteristic platinum satellites ( 195 Pt, S = 0.5, natural abundance: 33.8 %); 3 JA C H T U N G T R E N N U N G (Pt,H) coupling constants were observed. 195 Pt, 1 H, and 13 C NMR chemical shifts are reported in parts per million (ppm) relative to Na 2 PtCl 4 or Me 4 Si as an external standard. Spectroscopic assignment of the hydrogen atoms was based on COSY experiments.…”

“…[1] In this context, helicenes show great potential because, owing to their extended p-conjugated twisted backbone, they combine high optical activity with other properties, such as absorption, emission, and redox activity. [2][3][4] Recently, organometallic helicenes in which a transition metal (Pt, Ir) is included within the helical p-framework have emerged as highly promising candidates for optoelectronic applications (OLEDs, switches, sensors, etc. ), owing to the presence of the metal center.…”

Synthetische flüssige Kohlenwasserstoffe aus erneuerbaren Energien – Ergebnisse der Helmholtz Energieallianz

“…have been shown to influence the photochromic phenomenon of chromenes. We recently showed that helicity, as a steric force, allows unprecedented stabilization of the otherwise fleeting photogenerated colored o ‐quinonoid intermediates 7h. The influence of through‐space electronic effects has also been demonstrated in chromenes with cofacially oriented aryl rings7f and also in paracyclophanes grafted with chromenes 7g,7i.…”