Effects of Heavy Iodine Atoms and π-Expanded Conjugation on Charge Transfer Dynamics in Carboxylic Acid BODIPY Derivatives as Triplet Photosensitizers

Borondipyrromethene (BODIPY) chromophores are composed of a functional-COOH group at meso position with or without a biphenyl ring, and their compounds with heavy iodine atoms at -2, -6 positions of the BODIPY indacene core were synthesized. The photophysical properties of the compounds were studied with steady-state absorption and fluorescence measurements. It was observed that the absorption band is significantly red-shifted, and fluorescence signals are quenched in the presence of iodine atoms. In addition to that, it was indicated that the biphenyl ring does not affect the spectral shifting in the absorption as well as fluorescence spectra. In an attempt to investigate the effect of pi-expanded biphenyl moieties and heavy iodine atoms on charge transfer dynamics, femtosecond transient absorption spectroscopy measurements were carried out in the environment of the tetrahydrofuran (THF) solution. Based on the performed ultrafast pump-probe spectroscopy, BODIPY compounds with iodine atoms lead to intersystem crossing (ISC) and ISC rates were determined as 150 ps and 180 ps for iodine BODIPY compounds with and without pi-expanded biphenyl moieties, respectively. According to the theoretical results, the charge transfer in the investigated compounds mostly appears to be intrinsic local excitations, corresponding to high photoluminescence efficiency. These experimental findings are useful for the design and study of the fundamental photochemistry of organic triplet photosensitizers.

Automated summary

In this study, the photophysical properties of BODIPY compounds were investigated. It was found that the presence of iodine atoms caused a significant red-shift in the absorption band and quenched the fluorescence signals. The introduction of biphenyl rings did not affect the spectral shifting. Femtosecond transient absorption spectroscopy measurements revealed that BODIPY compounds with iodine atoms exhibited intersystem crossing, with ISC rates of 150 ps and 180 ps for compounds with and without pi-expanded biphenyl rings, respectively. The charge transfer in the investigated compounds mostly appeared as intrinsic local excitations, indicating high photoluminescence efficiency. These experimental findings have important implications for the design and study of organic triplet photosensitizers.