Although acceptor–donor–acceptor (A–D–A)-type molecules offer advantages in constructing NIR absorbing photothermal agents (PTAs) due to their strong intramolecular charge transfer and molecular planarity, their applications in photothermal therapy (PTT) of tumors remain insufficiently explored. In particular, the influence of ESP distribution on the optical properties of A–D–A photosensitizers has not been investigated. Herein, we analyze and compare the difference in ESP distribution between A–D–A-type small molecules and polymers to construct NIR absorbing PTAs with a high extinction coefficient (ε) and high photothermal conversion efficiency (PCE). The calculation results of density functional theory (DFT) indicate that the large ESP difference makes A–D–A-type small molecules superior to their polymer counterparts in realizing tight molecular packing and strong NIR absorbance. Among the as-prepared nanoparticles (NPs), Y6 NPs exhibited an obvious bathochromic shift of absorption peak from 711 nm to 822 nm, with the NIR-II emission extended to 1400 nm. Moreover, a high ε value of 5.69 L g⁻¹ cm⁻¹ and a PCE of 66.3% were attained, making Y6 NPs suitable for PTT. With a concentration of 100 μg mL⁻¹, Y6 NPs in aqueous dispersion yielded a death rate of 93.4% for 4T1 cells upon 808 nm laser irradiation (1 W cm⁻²) for 10 min, which is comparable with the best results of recently reported PTT agents.