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.