Arindam Mukhopadhyay

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.

Orthogonal self-assembly of a trigonal triptycene triacid: signaling of exfoliation of porous 2D metal–organic layers by fluorescence and selective CO2 capture by the hydrogen-bonded MOF

Trigonal 3-connecting imidazole-annulated triptycene triacid (H3TPA) is a molecular module that is programmed for orthogonal self-assembly. Upon treatment with metal salts such as CoCl2, Mn(NO3)2, Zn(NO3)2 and Cd(NO3)2, highly porous isostructural MOFs are obtained in which the TPA linker undergoes metal–ligand coordination and hydrogen bonding through carboxylate groups and imidazole moieties, respectively. The MOFs are constructed by hydrogen bond-mediated offset stacking of porous (3,3) honeycomb layers formed by the self-assembly of TPA with 3-connecting triangular bimetallic SBUs. We show that the interlayer hydrogen bonds can be disrupted by solvents such as DMSO, leading to solvent-induced delamination of 2D metal–organic nanosheets. The delamination is signaled by turn-on of fluorescence, which is suppressed in the bulk state. Indeed, the extent of exfoliation with different solvents – as reflected from fluorescence quantum yields as well as solvent-induced shifts in emission maxima – can be nicely correlated with Gutmanns solvent DN numbers, which are a measure of the ability of solvents to accept hydrogens in hydrogen bonds. The results demonstrate how the bulk materials with layered structures can be (i) engineered in a ‘bottom-up’ approach by orthogonal self-assembly of an organic linker created by de novo design and (ii) in turn be exfoliated in a ‘top-down’ approach by solvent-induced ultrasonication. As bulk materials, the hydrogen-bonded MOFs lend themselves to selective as well as high adsorption of CO2 under ambient conditions as a result of the nitrogenous environment of pores conferred by the benzimidazole moieties.

Influence of silyloxy substitution on the photochromic properties of diarylbenzo- and naphthopyrans\(^{\S }\)

Diarylbenzopyrans and their annulated derivatives are a class of photochromic compounds, which have been extensively investigated for photochromism. In fact, some naphthopyrans are employed industrially in ophthalmic lens applications. Photoirradiation of 2,2-diphenylbenzopyran leads to the formation of colored o-quinonoid intermediates. The latter revert too rapidly to allow their observation only at low temperatures (173–263 K). Annulation and other strategies are exploited to observe the colored o-quinonoid intermediates that persist for a few minutes at room temperature. We have examined photochromism of a set of silyloxy-substituted 2,2-diphenylbenzo- and naphthopyrans to explore how the mesomeric effects transmitted by the strongly e-donating silyloxy group substituted in the ring and at phenyl groups manifest in the spectrokinetic properties of their photogenerated colored o-quinonoid intermediates. We show that silyloxy substitution in the benzopyran ring leads to remarkable stabilization of the colored intermediates to enable their persistence at 283 K for a few minutes. In contrast, similar substitution in the C2-phenyl rings destabilizes the colored intermediates.