Planarity vs. Non-Planarity in the Electronic Communication of TCAQ-Based Push-Pull Chromophores

Abstract

Donor-acceptor alkynes, endowed with 11,11,12,12-tetracyano-9,10-anthraquinodimethane (TCAQ) and N,N-dimethylaniline (DMA) units, have been further functionalized by a [2+2] tetracyanoethylene (TCNE) cycloaddition followed by a subsequent retroelectrocyclization to form distorted non-planar molecular structures with 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) bridge ligands. Comprehensive spectroscopic, electrochemical, and computational studies have been carried out to compare the electronic communication in these planar (alkyne) and nonplanar (with TCBD units) TCAQ-based push-pull chromophores. Cyclic voltammetry and UV-Vis absorption measurements confirm a noticeable electronic communication between the TCAQ and DMA units regardless the quasi-orthogonal arrangement of the two dicyanovinyl halves of the TCBD groups, which partially hinder the electronic communication. The experimental trends are strongly supported by theoretical calculations performed at the density functional theory level, which further evidence an active electron-withdrawing role of the TCBD bridge both in the formation of the charged species and in the lowest-lying absorption features. The novel push-pull TCAQ-based derivatives including the TCBD bridge show a broad absorption in the whole visible range while having a structure highly distorted from planarity. These chromophores may therefore be viewed as appealing candidates to be exploited in photovoltaic devices with minimal aggregation phenomena.