Low-Bandgap, Donor-Acceptor Conjugates Based on Tetracyanobutadiene (TCBD) and Dicyanoquinodimethane (DCNQ): Synthesis, Structure and Photochemical Properties

Abstract

Ground and excited state charge transfer interactions in donor-acceptor conjugates has been one of the most widely studied topics by the scientific community over the last two decades as its significance is profound in solar energy conversion, and building optoelectronic devices. Among these, p-conjugated donor-acceptor conjugates are appealing because of their applications in the field of non-linear optics and electroluminescent devices. The photonic properties of p-conjugated donor-acceptor conjugates can be tuned by altering the strength of the donor or acceptor units, and by varying the connecting p-linker. Photosensitizer, electron donors such as porphyrins, phthalocyanines and BODIPYs have been traditionally used as building blocks in the construction of such light-powered ensembles due to their easily modifiable spectral and redox properties.In the present contribution, we report development of a new class of intramolecular charge transfer complexes derived from nonplanar, push-pull substituted buta-1,3-dienes by [2+2] cycloaddition of either tetracyanoethene (TCNE) or tetracyano-p-quinodimethane (TCNQ) with a variety of photosensitizer (electron donor) functionalized alkynes. This reaction efficiently converted the TCNE and TCNQ entities to highly electron-deficient, tetracyanobutadiene (TCBD) and dicyanodiquinodimethane (DCNQ) entities at the buta-1,3-dienes site.1-3 These charge transfer complexes are engineered to have small bandgap to extend the absorption well into the near-IR region; that is, to provide a better optical window for optoelectronic applications, especially in organic photovoltaics that use the majority of available sunlight. The newly synthesized complexes have been fully characterized using spectral, electrochemical, spectroelectrochemical and computational methods, and occurrence of ground state charge transfer interactions have been established. Further, excited state charge transfer in the complexes has been confirmed by photochemical studies performed using time-resolved emission and femtosecond transient absorption studies in solvents of varying polarity. Ultrafast charge separation has been witnessed in these closely spaced, strongly interacting triads.1-3 1. Sharma, M. B. Thomas, R. Misra, F. D’Souza, ‘Strong Ground and Excited State Charge Transfer in C3‐Symmetric Truxene Derived, Low‐Band Gap, Phenothiazine‐Tetracyanobutadine (TCBD) and Expanded TCBD Conjugates’, Angew. Chem. Int. Ed. 2019, 58, 4350-4355.2. Y. Rout, Y. Jang, H. B. Gobeze, R. Misra, F. D’Souza, ‘Conversion of Large Bandgap Triphenylamine-Benzothiadiazole to Low Bandgap, Wide-Band Capturing Donor-Acceptor Systems by Tetracyanobutadiene and/or Dicyanoquinodimethane Insertion for Ultrafast Charge Separation’ J. Phys. Chem. C. 2019, 123, 13382-23389.3. P. Gautam, R. Misra, M. B. Thomas, F. D’Souza, Ultrafast Charge-Separation in Triphenylamine-BODIPY-Derived Triads Carrying Centrally Positioned, Highly Electron- Deficient, Dicyanoquinodimethane or Tetracyanobutadiene Electron-Acceptors’ Chem. Eur. J. 2017, 23, 9192-9200.