Photophysics of charge transfer cocrystals composed of fluorene and its heterocyclic analogues as donors and TCNQ as an acceptor

Abstract

Charge transfer cocrystals offer an opportunity to alter the photophysical characteristics of organic chromophores. Modulation of charge transfer interactions of cocrystals to attain tunable optical properties is of great importance for the construction of solid-state luminescent materials. Herein, the charge transfer interactions and photophysics of four cocrystals (C1, C2, C3 and C4) composed of fluorine and its heterocyclic analogues, including carbazole, dibenzofuran, and dibenzothiophene, as π-donors (D) and TCNQ as a π-acceptor (A) were investigated. The single-crystal structures reveal that the donor and acceptor of each cocrystal are combined at a 1 : 1 stoichiometric ratio and adopt a ⋯DADADA⋯ mixed stack arrangement, implying the existence of charge transfer interactions. Cocrystals C1, C3 and C4 have charge transfer degrees of 0.03, 0.07 and 0.13, respectively, and show a macroscopic red-shift of 250–350 nm for both the absorption and emission (671 nm, 627 nm and 694 nm) spectra relative to their pristine donors. In contrast, the absorption band of C2 is further red-shifted to 840 nm with a much larger charge transfer degree of 0.25. However, no significant photon emission was observed for C2, demonstrating non-radiative decay of the excited state, and it agrees well with the very small band gap of this cocrystal. Cocrystals C1, C3 and C4 can be used to develop deep red fluorescent materials, while C2 illustrates photothermal conversion characteristics. The totally different photophysics of these cocrystals depends on their different charge transfer degrees. Therefore, it is necessary to modulate the charge transfer degree to an appropriate extent by the elaborate design of donor–acceptor pairs to construct cocrystal materials with specific functions.