Benzimidazolylquinoxalines: novel fluorophores with tuneable sensitivity to solvent effects

Abstract

We report on the photophysical properties, conjugation, conformational behavior, intra- and intermolecular hydrogen bonds (HBs) of a series of novel fluorophores, consisting of 3-arylquinoxaline and benzimidazole moieties linked by a single CC bond. Computations employing density functional theory (DFT) reveal that conjugation between these moieties stabilizes syn-conformers with two HB centers located on the same side of the molecule. Anti-conformers form stronger intermolecular HBs with DMSO and DMF than syn-conformers, and this influences the energy gap between syn- and anti-forms, especially upon excitation of the molecules to the S state. Substituents introduced in various positions of the molecules modify their conformational behavior, and mutual disposition of excited singlet states relative to the ground states. Various substitution patterns produce very different effects on relative quantum yield of luminescence: from a moderate increase in polar DMSO and DMF relative to 1,2-dichloroethane solutions to complete quenching of emission which is observable in polar media. The observed behavior is understood with the aid of DFT and time-dependent DFT calculations. The tuneability of the spectroscopic range of the luminescence and especially of its sensitivity to environmental effects allows rational design of the novel fluorophores of this family for various applications.

A comparison of methods for theoretical photochemistry: Applications, successes and challenges

“…161 The study found that structural changes, for example, introduction of substituents to the para-positions of the aryl groups, or changing the substitution of the phosphorus atoms, can be used to tune the HOMO-LUMO gap of the molecules. TD-DFT and TDA-TD-DFT were also applied to the study of some fluorescent benzimidazolylquinoxaline derivatives, 162 finding good agreement between experiment and calculations using the PBE0 and B3LYP functionals.…”

Benzimidazole-Based N,O Boron Complexes as Deep Blue Solid-State Fluorophores

“…Benzimidazole and other azole derivatives raise much interest due to their luminescence properties, and as promising candidates for the development of sensors and solar cells [ 1 , 2 , 3 , 4 , 5 ]. They are receiving increasing attention due to their non-linear optical properties [ 6 , 7 ] and excellent thermal stability [ 8 , 9 ].…”

Characterization of Conjugation Effects in the Series of Quinoxaline-2-ones by Means of Vibrational Raman Spectroscopy

“…38 In a recent contribution we have shown that quinoxalinones/quinoxalines possess interesting properties in this regard 39−46 and can be proposed as π-bridging fragments of NLO-phores 39−42 and as novel luminophores. 45,46 Indeed, due to their highly π-deficient character, these units can play a dual role acting simultaneously as electron-withdrawing (A) and π-bridging motifs in D-π-A structures. The extent of the conjugation induced by the presence of the π-bridge is indeed fundamental in modulating not only the optical properties of such compounds but also their response to external stimuli and in many instances their photophysical properties.…”

“…The role of the π bridge is fundamental to ensure an optimal electron mobility through the system, and hence, to optimize its functions, a broad range of aromatic heterocyclic moieties have been considered as π-linkers including thiazole, − thiophene, − phenothiazine, pyrrole, , and various diazines . In a recent contribution we have shown that quinoxalinones/quinoxalines possess interesting properties in this regard − and can be proposed as π-bridging fragments of NLO-phores − and as novel luminophores. , Indeed, due to their highly π-deficient character, these units can play a dual role acting simultaneously as electron-withdrawing (A) and π-bridging motifs in D-π-A structures. The extent of the conjugation induced by the presence of the π-bridge is indeed fundamental in modulating not only the optical properties of such compounds but also their response to external stimuli and in many instances their photophysical properties.…”

“…Furthermore, π-conjugated D-π-A systems also experience an unusually strong coupling with the anharmonic vibrational degrees of freedom and a rather important solvatochromism that has strong consequences on their optical properties as was shown by linear absorption spectroscopy supported by quantum chemical calculations. Namely, this achievement required the use of molecular modeling and simulation, in the framework of density functional theory (DFT) and its time-dependent extension (TD-DFT) in combination with ab initio molecular dynamics (AIMD). , With the present contribution we aim at providing a useful way to characterize and quantify the conjugation length of π-bridges systems and their coupling with vibrational properties, making use of widespread techniques such as Raman spectroscopy in combination with a molecular modeling approach, and offering a suitable alternative to characterize such an important indicator. More specifically and as a most valuable example, we consider selected quinoxaline-2-one (Q) based systems and we rationalize their vibrational and structural properties.…”