Asymmetrical organic D–π–A conjugate with ‘V’-shaped crystal packing: quest to transcend the limits of photophysical properties and applications

Abstract

Despite many reports on organic D–π–A fluorescent materials, the ardent realization of seven different photophysical features in a single material remains obstinate to attain six multitudinous real-world applications. In this present report on indole-anthracene-π-pyridine (IAPY), novelty aligns with the judicious design where each molecular part is in a different plane with a well-organized ‘V’ shape herringbone crystal packing with a centrosymmetric C2/c space group. Only one molecule exists in the asymmetric unit where all the (hetero)aryl centroids are equally separated (8.663 Å) and little translated with an equal number of two-fold rotation and two-one screw axes. The molecules are well-packed in the crystal lattice with 22 non-covalent interactions, including weak intramolecular N⋯H bonding from the pyridinyl N-atom. Even the design of IAPY meets the conditions of photophysical quantum chemical descriptors. Hence, relinquishing closer π–π stacking, IAPY emits brightly in solids (λ_max = 517 nm, Φ_f = 26.64%), solvents, aggregates (λ_max = 530 nm, I/I₀ = 17), and in a viscous environment (λ_max = 524 nm, I/I₀ = 23), indicating an ever-present emission. The existence of C⋯H (29.9%) interactions with 8.9% void space further contributes to the remarkable mechano-force-induced enhanced emission (Φ_f 26.54% to 53.32%). Again, the presence of pyridinyl ‘N’-center with 1.6% solvent accessible void space enables IAPY to display reversible 75 nm red-shifted emission upon exposure to acid vapor. The variable contribution from LE and CT state by tuning solvent polarity leads to whitish light emission, a rare event for DSEgens. All the solid-state photophysical phenomena are explained from detailed crystal packing analysis, void-space calculation, PXRD patterns, DSC profiles, and Hirshfeld surface analysis. The dual state emission is realized by in-depth TD-DFT investigations considering root-mean-square-displacement (RMSD), oscillator strengths, excited state planarity, and natural transition orbitals (NTOs). The high photostability, emission tunability, and biocompatibility of IAPY comprehend numerous real-world applications that include hidden stimuli-responsive QR-code-based device fabrication and security stamps as data storage for anticounterfeiting applications, forensic utility to detect blood-smeared fingerprints on different surfaces, food additive detection, data encryption, and fluorescent detection of hospital disposed-of biohazards. Besides, effective wash-free bioimaging of FaDu cancer cells at lower probe concentrations is noteworthy.