One nitrogen atom as electron source is enough for efficient intermolecular charge transfer in organic cocrystals toward NIR-photothermal conversion

In organic donor-acceptor (D-A) materials, a nitrogen atom can serve as a skeleton electron source to tune the inter-/intra-molecular charge transfer (CT) nature and photophysical/photochemical properties. Here, we employ the model molecule stilbene modified with non-/one-/two-nitrogen atoms in the benzene ring skeleton (TSB/TSP/DPE) as a donor and F4TCNQ as an acceptor to synthesize three cocrystals by a one-step mechanochemical method toward NIR photothermal conversion (NIR-PTC). The results show that "One Nitrogen Atom Is Good, Two Are Too Many." That is, one nitrogen atom in the donor (TSP) is enough to induce optimal performance of the CT effect with the degree of charge transfer of ∼1.16, redshift absorption of ∼2500 nm, and NIR-PTC efficiency of ∼89.4% @ 808 nm. Femtosecond transient absorption spectra exhibit that superior NIR-PTC of TSP-F4TCNQ cocrystal can be attributed to the first ultrafast internal conversion (IC) with a lifetime of 1.7 ± 0.4 ps and subsequent nonradiative decay (NR) via rotation/twisting of cyano-groups out of the plane of central ring of F4TCNQ with a lifetime of 17.1 ± 3.4 ps, followed by the third vibrational relaxation (VR) with a lifetime of 754 ± 220 ps. The individual partition contribution factor of IC/NR/VR channels is resolved to be about 0.33:1.51:1.12 based on a simple linear approximation treatment for the first time. This work provides a simple and potential approach for regulating photophysical properties of D-A cocrystals by introducing a skeleton electron source.