Unprecedented Improvement of Near-Infrared Photothermal Conversion Efficiency to 87.2% by Ultrafast Non-radiative Decay of Excited States of Self-Assembly Cocrystal

Near-infrared
(NIR) photothermal conversion is of great interest
in many fields. Here, a self-assembly organic cocrystal (N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) and
pyromellitic dianhydride (PMDA)) with strong absorption in NIR range
is constructed, with widespread absorption (200–1500 nm) and
very high NIR photothermal conversion efficiency (87.2%). Essentially,
in this cocrystal, a small HOMO–LUMO gap of donor–acceptor
pair boosts the absorption ability of this cocrystal in the NIR range.
The mixed stacking structure significantly enhances the intermolecular
interactions as well as the electron–hole delocalization, suppressing
the emission processes, leading to nonradiative decay processes from
excited states. Strong intermolecular interactions enable the cocrystal
to have dense electronic energy levels, leading to a high proportion
(94.4%) vibrational cooling and internal conversion processes with
ultrafast excited-state relaxation (0.12 ps), which contributes to
high NIR photothermal conversion efficiency. Furthermore, the cocrystal
has exhibited capable ability for being an excellent candidate for
a NIR photothermal therapy agent.