title: 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.
authors.affiliation: Department of Chemistry, Indian Institute of Technology, Kanpur, India. moorthy@iitk.ac.in
authors.forename: Jarugu Narasimha
authors.initials: JN
authors.lastname: Moorthy
authors.forename: Susovan
authors.initials: S
authors.lastname: Mandal
authors.forename: Arindam
authors.initials: A
authors.lastname: Mukhopadhyay
authors.forename: Subhas
authors.initials: S
authors.lastname: Samanta
mesh_terms: D001578:Benzopyrans / Q000138:chemical synthesis / Q000737:chemistry*; D003116:Color; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D015394:Molecular Structure; D010782:Photolysis; D011809:Quinones / Q000737:chemistry*
chemical_list: D001578:Benzopyrans; D011809:Quinones