Optical properties of BPP and DPP spiro-cyclotriphosphazenes: experimental and DFT insights

Abstract

Spiro-cyclotriphosphazenes such as 2,2-dichloro-4,4,6,6-bis[spiro(2',2''-dioxy-1',1''-biphenyl)]cyclotriphosphazene (DPP) and 2,2,4,4-tetrachloro-6,6-[spiro(2',2''-dioxy-1',1''-biphenyl)]cyclotriphosphazene (BPP) are well-established intermediates for functional phosphazene derivatives. However, their intrinsic optical and electronic properties have not been systematically explored under varying experimental conditions. In this work, DPP and BPP were synthesized and characterized to elucidate the correlation between structural parameters and photophysical behavior. A complementary experimental approach was employed: DPP optical properties were examined in solution as a function of molarity, while BPP properties were investigated in the solid state as a function of film thickness. This strategy enables a comparative evaluation of how intermolecular interactions and structural topology influence optical behavior. UV-Vis measurements revealed band-gap narrowing from 3.904 to 3.805 eV for BPP and from 3.921 to 3.830 eV for DPP, accompanied by red shifts. DFT calculations (B3LYP/6-31G(d,p)) reproduced the relative trends, with HOMO-LUMO gaps of 1.91 eV (BPP) and 0.92 eV (DPP); although absolute values differ from experiment, the calculations correctly predict the relative absorption onsets and tunable optical behavior. Molecular electrostatic potential (MEP) maps and conceptual DFT descriptors indicate that BPP is harder and more electrophilic, while DPP is softer and more polarizable, consistent with red-shifted absorption. These findings highlight the intrinsic optical robustness of both compounds and demonstrate that parameters such as film thickness, solution molarity, and ring topology can be used to tailor optoelectronic properties. The study establishes BPP and DPP as promising candidates for UV-blue optoelectronic devices, photodetectors, and wide-bandgap photonic materials.