Controlling the Charge Transfer in D–A–D Chromophores Based on Pyrazine Derivatives

A series
of symmetrical donor–acceptor–donor (D–A–D)
chromophores bearing various electron-withdrawing groups, such as
quinoxa­line (Qx), benzo­[g]­quinoxaline
(BQ), phenazine (Pz), benzo­[b]­phenazine (BP), thieno­[3,4-b]­pyrazine
(TP), and thieno­[3,4-b]­quinoxaline (TQ), has been designed and synthesized. Intramolecular charge
transfer (ICT) interactions can be found for all the chromophores
due to the electron-withdrawing properties of the two imine nitrogens
in the pyrazine ring and the electron-donating properties of the other
two amine nitrogens in the two tri­phenyl­amines. Upon the
fusion of either benzene or thiophene ring on the pyrazine acceptor
unit, the ICT interactions are strengthened, which results in the
batho­chromically shifted ICT band. Moreover, the thiophene ring
is superior to the benzene ring in enlarging the ICT interaction and
expanding the absorption spectrum. Typically, when a thiophene ring
is fused on the Qx unit in DQxD, a near-infrared
dye is realized in simple chromophore DTQD, which displays
the maximum absorption wavelength at 716 nm with the threshold over
900 nm. This is probably due to the enhanced charge density on the
acceptor moiety and better orbital overlap, as revealed by theoretical
calculation. These results suggest that extending the conjugation
of a pyrazine acceptor in an orthogonal direction to the D–A–D
backbone can dramatically improve the ICT interactions.