Excited-state intramolecular proton transfer and photoswitching in hydroxyphenyl-imidazopyridine derivatives: A theoretical study

Designing Multifunctional AIEgens by Molecular Engineering of Imidazo[1,2-a]pyridine For Color Tunable Molecular Salts, Anti-Counterfeit Applications and Sensing of Mn2+, Ag+, and Fe3

Mechanochromic materials, known for their ability to change color in response to mechanical stimuli such as pressure, stretching, grinding, or rubbing, hold significant importance due to their diverse applications. In this study, we synthesized and characterized two novel pyridine-tethered imidazo[1,2-a]pyridine mechanoresponsive luminogens with appended tetraphenylethene, named GBY-10 and GBY-11. GBY-10 exhibited reversible mechanofluorochromism, while GBY-11 did not revert to its original color after solvent fuming. The photophysical properties of these luminogens were significantly influenced by the position of the terminal pyridine. Additionally, we created color-tunable mechanoresponsive molecular salts by co-grinding GBY-11 with various aryl acid derivatives. Co-grinding GBY-11 with pentafluorobenzoic acid resulted in a significant bathochromic shift of the emission maxima by 66 nm, compared to 37 nm and 12 nm shift for benzoic acid and para-nitrobenzoic acid, respectively. This counter-ion-dependent luminescence suggests strong electronic interactions between the counter ions. These luminogens also demonstrated reversible pH-responsive behaviour, making them suitable for anti-counterfeiting applications. Furthermore, the pyridine-functionalized luminogen, GBY-10, showed metal ion detection (Mn2+, Ag+, Fe3+) ability in water, with detection limits as low as 0.0043, 0.015, and 0.0029 mM, respectively. This report opens new avenues for designing promising AIE-active materials for potential applications in anti-counterfeiting, sensing, optoelectronics, and biomedicine. A de-novo approach of engineering imidazo[1,2-a]pyridine scaffold to mechanoresponsive AIE-active molecules for anti-counterfeiting and metal sensing applications. By co-grinding the designed luminogens with various aryl acid derivatives, color tuneable mechanoresponsive molecular salts can be further developed.

Unveiling the Reaction Mechanisms of the Synthesis and the Excited State Intramolecular Proton Transfer of 2-(2'-hydroxyphenyl)imidazo[1,2-a]pyridine

Given its wide variety of applications in the pharmaceutical industry, the synthesis of imidazo[1,2-a]pyridines has been extensively studied since the beginning of the last century. Here, we disclose the mechanism for the synthesis of imidazo[1,2-a]pyridines by means of the Ortoleva-King reaction. We also reveal the reaction pathway leading to the formation of a iodinated byproduct, demonstrating the challenge of preventing the formation of such a byproduct because of the low energy barrier to access it. Moreover, quantum chemistry tools were employed to investigate the mechanism of intramolecular proton transfer in the excited state, and connections with aromaticity were explored.

Azaindolizine proton cranes attached to 7-hydroxyquinoline and 3-hydroxypyridine: a comparative theoretical study

Theoretical design of several proton cranes, based on 7-hydroxyquinoline and 3-hydroxypyridine as proton-transfer frames, has been attempted using ground and excited-state density functional theory (DFT) calculations in various environments. Imidazo[1,2-a]pyridine, pyrazolo[1,5-a]pyridine and benzimidazole were considered as proton crane units. The proton crane action requires the existence of a single enol-like form in the ground state, which under excitation goes to the end keto-like one through a series of consecutive excited-state intramolecular proton transfers (ESIPT) and twisting steps with the participation of a crane unit, resulting in a long-range intramolecular proton transfer. The results suggest that 3-hydroxypyridine is not suitable for a proton-transfer frame and 8-(imidazo[1,2-a]pyridin-2-yl)quinolin-7-ol and 8-(pyrazolo[1,5-a]pyridin-2-yl)quinolin-7-ol behave as non-conjugated proton cranes, instead of tautomeric re-arrangement in the latter, which was thought to be possible.

Electronic Excited States and UV-Vis Absorption Spectra of the Dihydropyrene/Cyclophanediene Photochromic Couple: a Theoretical Investigation

Dihydropyrene (DHP)/cyclophanediene (CPD) is a fascinating photoswitchable organic system displaying negative photochromism. Upon irradiation in the visible region, the colored DHP can be converted to its open-ring CPD colorless isomer, which can be converted back to DHP by UV light. DHP and CPD thus possess very different absorption spectra whose absorption bands have never been assigned in detail so far. In this work, we characterize the vertical electronic transitions of the first six and seven excited states of DHP and CPD, respectively, aiming for a realistic comparison with experiment. We used state-of-the-art electronic structure methods [e.g., complete active space second-order perturbation theory (CASPT2), n-electron valence-state perturbation theory (NEVPT2), extended multiconfigurational quasi-degenerate perturbation theory (XMCQDPT2), and third-order algebraic diagrammatic construction ADC(3)] capable of describing differential electron correlation. Vertical transition energies were also computed with time-dependent density functional theory (TD-DFT) and compared to these accurate methods. After the reliability of TD-DFT was validated for the main optical transitions, this efficient method was used to simulate the absorption spectra of DHP and CPD in the framework of the Franck-Condon Herzberg-Teller approximation and also using the nuclear ensemble approach. Overall, for both methods, the simulated absorption spectra reproduce nicely the main spectral features of the DHP and CPD isomers, that is, the main four absorption bands of increasing intensity of DHP and the absorption rise below 300 nm for CPD.

Excited-State Proton Transfer in Thiazolo-[4, 5-d]thiazo Heterocyclic Systems and the Geometry Alterations' Effect on Photophysical Characters: A Theoretical Study

Thiazolo-[4,5- d]-thiazo-frame (tztz) compounds are important heteroaromatic organic systems, which recently became a subject of several studies in the field of organic electronics and organic photovoltaics. The most important physical nature of these systems is reported to be an equilibrium between enol and keto forms following excited-state proton transfer. This process originates from a flat trend of the S₁ PE (potential energy) profile along the proton transfer coordinate. In the present work, we determined and interpreted the excited-state proton transfer and photophysical nature of these systems extensively by means of the MP2/CC2 and CASSCF theoretical approaches. Also, the effects of amine (-NH₂) and cyano (-CN) substitutions were taken into account comprehensively by considering the transition energies and proton transfer pathways of the resulting tztz derivatives. It has been predicted that the physical nature of the excited-state intramolecular proton transfer, as the main character of these systems, is being affected significantly by substitutions. For all of the considered tztz derivatives, a conical intersection (CI) between ground and the S₁ excited state was predicted. This CI makes the considered species capable to be responsible for photochromism and photoswitching as well.