Optical investigation of ZnPc and CuPc in formic acid

Abstract

Phthalocyanines are highly conjugated macrocyclic compounds known for their chemical stability and unique optical properties. With strong absorption in the visible region and fluorescence emission, they are widely studied for applications in optoelectronics, sensors, and photovoltaics. Among them, zinc phthalocyanine (ZnPc) and copper phthalocyanine (CuPc) are two of the most studied derivatives. This study presents a comparative analysis of the optical properties of ZnPc and CuPc dissolved in formic acid (FA), focusing on their UV-VIS absorption and fluorescence behavior in thin films. UV-VIS spectra revealed distinct Q and B absorption bands for each compound. ZnPc showed characteristic peaks at 627 nm, 657 nm, 690 nm, and 720 nm, while CuPc displayed maxima at 649 nm and 691 nm. Both compounds exhibited Soret bands around 295, 337, and 394 nm. Thin films prepared by two different methods, showed spectral changes, including bathochromic shifts and broadening due to molecular aggregation. Fluorescence measurements confirmed the UV-VIS behavior, with ZnPc displaying higher emission intensity than CuPc under identical conditions. Fluorescence spectra for both compounds were recorded at a concentration of 29 μM in (FA) using two excitation wavelengths (335 nm and 650 nm). ZnPc emitted strongly at 729 nm under both excitation conditions, while CuPc peaked at 695 nm and 726 nm, with lower intensity. At 335 nm excitation, ZnPc showed a red-shifted shoulder at 829 nm, while CuPc had a secondary peak at 726 nm. These results highlight the superior properties of ZnPc, suggesting its potential for fluorescence-based optoelectronic applications.