ABSTRACT
A set of three donor-acceptor azahelical coumarins (DA-AHCs), namely, H-AHC, Me-AHC, and Ph-AHC, were rationally designed and synthesized, and their excited-state properties were comprehensively investigated. All three DA-AHCs are shown to display very high fluorosolvatochromic shifts as a result of significant intramolecular charge transfer in their excited states. The para-quinoidal forms of the latter apparently contribute predominantly to large dipole moments in their excited states. By virtue of the fact that these helical systems structurally incorporate a highly fluorescent coumarin dye, they exhibit high quantum yields in both solution and solid states. Indeed, their emission behaviors in the crystalline media are shown to be remarkably correlated with their respective crystal packings. Incisive analyses demonstrate (i) strengthening of hydrogen bonding in the excited state promotes quenching (H-AHC), (ii) efficient crystal packing promotes high emission (Me-AHC) by precluding deactivations via vibrational motions, and (iii) loose crystal packing contributes to excited-state deactivation to account for low quantum yields of emission (Ph-AHC).
ABSTRACT
Regioisomeric naphthopyrans annulated with triptycene, i. e., Prox-NP and Dist-NP, display divergent photochromic behaviors. While steady-state photolysis of Dist-NP led to a very labile colored intermediate that is not observable at room temperature, Prox-NP yielded a remarkably stable species characterized by X-ray crystallography as the TT isomer of o-quinonoid intermediate (Prox-NPQ) with t1/2 ca. 0.18â years at 298â K. The kinetic analysis of thermal reversion reveals that the bleaching of Prox-NPQ is entropically controlled; the steric effect due to the rigid triptycene scaffold renders Prox-NP a highly constrained system such that the photogenerated colored o-quinonoid form is more entropically relaxed. This constitutes the first instance of an entropically-controlled thermal reversion for the celebrated class of photochromic naphthopyrans. Based on the response of Prox-NP and its colored intermediate Prox-NPQ to different stimuli, namely, light, heat, and acid, the molecular system can be likened to a logic gate with the 'INHIBIT' function.
ABSTRACT
One-pot synthesis has been developed for 4-carboethoxybenzo[h]coumarins starting from α-/ß-naphthols. Accordingly, diverse 4-carboethoxybenzocoumarins can be synthesized in moderate-to-excellent (31-75%) isolated yields. The synthesis involves initial oxidation of naphthols to the intermediary 1,2-naphthoquinones with 2-iodoxybenzoic acid followed by a cascade of reactions, namely, Wittig olefination, Michael addition, ß-elimination, and cyclization. Furthermore, we have comprehensively investigated the excited-state properties of differently substituted 4-carboalkoxybenzo[h]coumarins. It is shown that they exhibit low to high fluorescence quantum yields (1-36%) and excited-state lifetimes (ca. 1-7 ns) depending on the substitution pattern and solvent employed.
ABSTRACT
Two regioisomeric pairs of heptahelical mono- and biscoumarins that are differentiated by "inward" and "outward" disposition of the pyran-2-one moiety have been synthesized and investigated to understand the influence of helicity on excited-state and chiroptical properties. A slight variation in the helicities is found to manifest in contrasting excited-state properties of coumarin-annelated heptahelicenes; in addition to the intramolecular charge transfer, structural relaxation in the excited state is shown from theoretical calculations to cause decrease in the fluorescence quantum yield for a system with higher helicity. The optically pure enantiomers of heptahelical coumarins exhibit helicity-dependent chiroptical properties, namely, specific rotations, molar ellipticities, Cotton effects, and anisotropic dissymmetry factors. Theoretical calculations point to factors that are not readily explicable.
