Phenylpyran-fused coumarin novel derivatives: combined photophysical and theoretical study on structural modification for PET-inhibited ICT emission

Abstract

Synthesis and characterizations of fluorescent derivatives of 2-amino-5-oxo-4, 5-dihydropyrano[3,2-c] chromene-3-carbonitrile class are presented. Detailed photophysical characterizations were performed with different substituents, which enabled getting into the details of structural modulation to understand and tune photophysical properties in this biologically important class of dyes. One of the synthesized nitro-substituted dye (4a) is non-fluorescent due to the photoinduced electron transfer (PET) process. PET is quenched and fluorescence emission is achieved by (i) reduction of the –NO2 group to the –NH2 group, (ii) substitution of –NO2 by different substituents (−N(Me)2, −Cl, and –OH groups), and (iii) aggregate formation of 4a in the tetrahydrofuran–water system resultant from aggregation-induced enhanced emission (AIEE). Synthesized derivatives having substituents (−NH2, −N(Me)2, −Cl and –OH group) are highly fluorescent. Synthesized dyes showed positive solvatochromism and enhanced photophysical properties as compared to parent dye 7-(diethylamino)-4-hydroxy-2H-chromen-2-one. ICT characteristics of dyes are in good correlation with solvent polarity plots, multilinear regression analysis of solvent parameters, and Mulliken–Hush analysis. Mathematically deduced global and local descriptors and computationally obtained molecular orbital calculations are supporting well the PET process in 4a and ICT process in other dyes.