Organic small-molecule photothermal agents operating in the second near-infrared window (NIR-II, 1000-1700 nm) are attractive candidates for tumor photothermal therapy owing to their well-defined structures, tunable optical properties, and favorable biocompatibility. However, for dyes that combine deep-near-infrared (deep-NIR) absorption with NIR-II emission, establishing a coherent relationship linking molecular structure, spectral response, photothermal conversion efficiency (PCE), and biological performance under comparable experimental conditions remains a challenge. Here, we report ZZ, a bis(triphenylamine)-conjugated hemicyanine dye featuring a double-donor-π-bridge-acceptor (D-D'-π-A) framework. ZZ exhibited a deep-NIR absorption maximum at approximately 850 nm and NIR-II emission spanning 950-1300 nm. Systematic steady-state absorption and fluorescence measurements in the film state and in solvents of different polarity revealed pronounced positive solvatochromism. Specifically, from 1,4-dioxane to DMSO, the absorption peak red-shifted by only about 11 nm (from 840 to 851 nm), whereas the emission peak red-shifted by 91 nm (from 973 to 1064 nm), accompanied by an increase in the Stokes shift from 1623 to 2352 cm-1 and a decrease in the emission energy of about 0.109 eV. This behavior is consistent with intramolecular charge transfer (ICT) character, in which the more polar solvent preferentially stabilizes the charge-separated excited state and enhances nonradiative decay. Under 808 nm laser irradiation, ZZ in DMSO (100 μM) produced a maximum temperature increase of 19.5 ℃ and a photothermal conversion efficiency of 76.1%, markedly higher than that of indocyanine green (ICG, 32.6%) measured under matched solvent, absorbance at 808 nm, and irradiation conditions. ZZ also retained stable photothermal performance over five laser on/off cycles and induced concentration-dependent, light-triggered cytotoxicity against 4T1 cells (cell viability decreased to 13.2% at 100 μM). In 4T1 tumor-bearing mice, intratumoral injection of ZZ followed by 808 nm irradiation generated localized heating and significantly inhibited tumor growth, while body-weight monitoring and H&E staining of major organs revealed no obvious short-term abnormalities. Because no nanodelivery system was employed, these findings establish the local photothermal potential of ZZ rather than its systemic therapeutic performance. ZZ therefore provides a structurally defined molecular scaffold for deciphering the structure-photophysics-photothermal relationship and for the further optimization of hemicyanine-based photothermal agents and their delivery strategies.