Effects of Heterocyclic Ring Fusion and Chain Elongation on Chiroptical Properties of Polyaza[9]helicene: A Computational Study

Abstract

In the present work, the effect of lateral and helical extensions on the physical and chiroptical properties of azahelicenes is reported. Starting with the experimentally reported polyaza[9]helicene (9Ha), three derivatives, two with laterally fused electron-withdrawing rings and the third with larger helical length, are designed. For the excited-state properties such as UV− vis and CD spectra, performances of different DFT functionals are evaluated by comparing the energies and characters of the excited states against the ADC(2) results. CPL properties are calculated at DFT level. Among the three designed systems, pyrazine-based 9HaP shows an improved g CPL value compared to that for parent 9Ha. However, quinoxaline-based 9HaQ is found to be the worst CPL emitter with the lowest dissymmetry factor. The helically extended derivative, 11Ha, shows good CPL results, but g CPL remains smaller than that for the parent system. The CPL results are analyzed in terms of electric dipole transition moment (EDTM) and magnetic dipole transition moment (MDTM) vectors, and angles between these two vectors.

CPL calculations of [7]helicenes with alleged exceptional emission dissymmetry values

“…Our approach is based on time-dependent density functional theory (TD-DFT), 35,36 which in recent years has demonstrated good capability of reproducing CPL data of SOMs, including several helicenes and heterohelicenes. 14,[37][38][39][40][41][42][43][44][45][46][47][48][49][50][51] Conversely, CPL calculations of intramolecular excimers or exciplexes are still relatively limited in number. [52][53][54][55][56] Moreover, with respect to the most common calculation approach, we treated the influence of the solvent by means of the vertical excitation method (VEM), a state-specific (SS) approach to the polarizable continuum model (PCM) for excited states, for which the excited state energy gradients are available, 57,58 going well beyond other SS or the standard linear response (LR) schemes in the PCM.…”

Theoretical Investigation of the E/Z Photoisomerization Pathway in 1-(2-Pyrrolyl)-2-(2-thienyl)ethylene

“…Keeping this in mind, the main objective of our work is to illustrate the underlying excited-state reaction mechanisms of the vinylene-linked thiophene–pyrrole system by carrying out quantum-chemical calculations using wave-function-based methods. In our studies, we have employed both single and multireference methods, including the algebraic diagrammatic construction to second order (ADC(2)) method , due to its success in describing various excited-state properties − and the complete active space self-consistent field (CASSCF) method . Since the chosen systems have not been explored experimentally, our results were compared with the few literature reports on vinylene-linked systems such as ethylene and stilbene.…”

Modeling Absorption and Emission Spectroscopies of Symmetric and Asymmetric Azaoxahelicenes in Vacuum and Solution

“…These properties along with the semiconducting properties of helicenes make them especially interesting for applications in molecular electronics as chiral optoelectronic devices that can be used as either a detector, a source of circularly polarized emission, or an antenna that can induce a chiral signal from an achiral polymer or longer helicenes with interesting chiral properties. 3,4,10,11 Even though there have been many reports of synthesized helicenes of increasing length and complexity during the past few years, it remains a significant challenge to develop synthetic protocols for preparing new helicenes. 12−16 Moreover, there is no clear understanding of which structural features of helicenes give rise to enhancements of the chiral responses.…”

“…Over the last several years, helicenes have been widely studied for their impressive optical properties. − Helicenes are molecules comprising several angularly annulated aromatic rings that result in a stereogenic axis as the repulsion between different ends of the conjugated system results in a helical structure. − The large conjugated chiral system leads to high absorption and large chiral response in the UV–vis part of the spectrum. Furthermore, helicenes often also have significant fluorescence, giving rise to measurable circularly polarized luminescence (CPL).…”

“…Furthermore, helicenes often also have significant fluorescence, giving rise to measurable circularly polarized luminescence (CPL). These properties along with the semiconducting properties of helicenes make them especially interesting for applications in molecular electronics as chiral optoelectronic devices that can be used as either a detector, a source of circularly polarized emission, or an antenna that can induce a chiral signal from an achiral polymer or longer helicenes with interesting chiral properties. ,,, …”