Diazo-Chalcone Enhanced Cyclotriphosphazene Photodiode Systems: Their Structure– Property Relationships, Photophysical and Electrical Performance

In this study, a series of novel dispiro-cyclotriphosphazene (CRG) derivatives were synthesized by functionalizing a phosphazene core with diazo-chalcone ligands bearing electron-donating groups such as methoxy, phenyl, and methyl. The molecular structures of the synthesized compounds were confirmed through FT-IR, ¹H, ¹³C, and ³¹P NMR spectroscopy, elemental analysis, and MALDI-TOF-MS. Thermal properties, evaluated using TGA/DTG, revealed significantly enhanced thermal stability in the modified systems compared to the CRG, with decomposition temperatures exceeding 450°C. UV-Vis spectral analyses indicated strong π–π* transitions in the 340–370 nm range, with substituent-dependent shifts and absorbance intensities. Optical band gaps determined using Tauc plots demonstrated that CRG-OMe exhibited the lowest Eg (3.08 eV), owing to its strong resonance donor capacity. Theoretical calculations employing DFT and TD-DFT at the B3LYP/6-311G(d,p) level supported these results, with CRG-OMe showing a HOMO–LUMO gap of 3.24 eV, in strong correlation with experimental findings. Additionally, global reactivity descriptors, electrostatic potential surfaces (MEP), and average local ionization energy (ALIE) maps provided insight into the charge distribution and reactivity of the molecules. Electrical measurements confirmed the semiconducting nature and rectification behavior of the compounds. The CRG-OMe-based diode displayed a high rectification ratio (RR = 3643), low reverse saturation current, and notable photoconductivity under various light intensities, highlighting its suitability for optoelectronic applications. The combined experimental and theoretical results point to these materials as promising candidates for organic photodiodes, photovoltaic devices, and next-generation optoelectronic platforms.