Meso-N-linker engineering of benzo[cd]indole cyanines for mid-band (850–950 nm) near-infrared absorption and film implementation

Abstract

Near-infrared (NIR) absorbing films are essential components of CMOS image sensors, particularly for suppressing unwanted signals in the 750–1100 nm range. However, the mid-band NIR region (850–950 nm) remains challenging to cover due to the limited availability of suitable commercial cyanine dyes, necessitating new molecular design approaches. In this study, five benzo[cd]indolenyl heptamethine cyanine dyes were synthesized by introducing a para-aniline-based N-linker at the meso position bearing amine (Cy-5npnB), amide (Cy-5npaB), hydrogen (Cy-5npB), ester (Cy-5npeB), and cyano (Cy-5npcnB) substituents. Systematic modulation of the electron-donating and electron-withdrawing strengths of the N-linker enabled λ_max tuning over a wide range of approximately 116 nm, which correlated well with trends in LUMO stabilization. UV-Vis-NIR measurements in both solution and COP films confirmed that all synthesized dyes effectively covered the mid-band NIR region, demonstrating that meso-N-linker engineering alone enables precise tuning of the absorption profile within the mid-band NIR region. TD-DFT and RMSD analyses further revealed that the unusually low molar absorptivity of Cy-5npnB originates from its large S₀/S₁ geometric displacement, which reduces Franck–Condon overlap, and that the additional peak near 700 nm corresponds to an S₂ transition involving a HOMO–1 → LUMO excitation. To evaluate applicability in NIR cut-off filters, 3-mixed and 4-mixed films were fabricated. Cy-5npnB exhibited the largest absorbance loss upon film formation due to its strong electron-donating character, whereas Cy-5npaB effectively compensated for the residual 820 nm absorption in the prior 3-mixed system and provided an optimal balance between visible and NIR transmittance (76.8% visible, 2.3% NIR). Moreover, comparison of TGA results with film state thermal behavior revealed that, despite its high T_d in bulk, Cy-5npnB exhibited pronounced thermal degradation in COP films due to extensive S₀/S₁ structural reorganization, highlighting that bulk thermal stability does not necessarily translate to film-state robustness.