DFT based predictive model for BODIPY triplet quantum yields

Triplet photosensitizers have been developed for use in a variety of applications such as photodynamic therapy, photocatalysis, and organic LEDs. All of these require a high triplet excited-state quantum yield, which is often measured as the singlet oxygen quantum yield (ФΔ). A common method to increase ФΔ is through the heavy-atom-effect achieved by the addition of heavy-atoms to the chromophore – such as the heavy halogens bromine and iodine. Previously, our group developed an empirical model using DFT to predict the ФΔ of heavy-atom containing chromophores. The model correlates a natural atomic orbital composition calculation of the heavy-atoms to the ФΔ of a chromophore. Herein, we modify the original model to apply specifically to halogenated boron dipyrromethene (BODIPY) dyes. In addition to developing a method to predict how changes in the structure of BODIPY dyes affects the ФΔ, this model provides insight into why different structural changes have differing impacts to the ФΔ. The BODIPY core has several unique substitution positions that can be halogenated; however, the model indicates that the 2- and 6-positions on BODIPY (IUPAC numbering) have the greatest impact on the ФΔ with yields changing from 0.00 to 0.90 when replacing the two protons with iodides. Although the original model made using xanthene type dyes provided reasonably good agreement with BODIPY dyes, the new reparametrized model allows for more accurate prediction of the ФΔ of BODIPY type chromophores and provides insight in the importance of substituent location to guide future chromophore design

Empirical DFT Model to Predict Triplet Quantum Yield Through Singlet Oxygen Yields

Triplet photosensitizers can be used for a variety of applications, including photocatalysis, OLEDs, and photodynamic therapy. Excited triplet states can be quenched by triplet oxygen to make singlet oxygen. Often the singlet oxygen quantum yield (Φ▵) is used as a lower approximation for the triplet yield. Unpredictable effects of even minor structural changes can drastically alter the Φ▵ and complicate the design of new triplet photosensitizers. The most common strategy to increase Φ▵ is to incorporate heavy atoms, promoting the “heavy atom effect”. However, the position and the identity of the heavy atom greatly influences the Φ▵. We have created a predictive model that correlates calculated natural atomic orbital composition of the heavy atom(s) contributing to the frontier molecule orbitals of a photosensitizer with the experimental Φ▵. The model, derived from several fluorescein derivatives, provides a calculated Φ▵ in agreement with the experimental values for a variety of well-known photosensitizers, including rhodamine dyes, fluorescein derivatives, and octahedral metal complexes

Simplification of the Potassium Ferrioxalate Actinometer Through Carbon Dioxide Monitoring

Abstract Chemical actinometry can be used to determine photons absorbed for a photochemical reaction, which is required to calculate the quantum yield. A photochemical reaction with a known quantum yield can be used as a relative standard for the determination of an unknown quantum yield for a light-driven reaction. Herein, we have developed a simplified approach to using the popular potassium ferrioxalate actinometer. Traditionally, the photoreduction of Fe(III) to Fe(II) is monitored by following the absorbance of Fe(II) by reacting aliquots of the actinometry solution with 9,10-phenanthroline to form a red colored complex. The multiple steps for this method make it tedious and vulnerable to errors, especially inadvertent light exposure. In lieu of spectroscopic measurements of the Fe(II) concentration, the production of CO2 was measured to determine the number of photons absorbed over time. CO2 production was measured in two different ways: by the pressure increase in a sealed system and the volume change by trapping the CO2. Both methods were considerably less laborious and showed agreeable results compared with the traditional spectroscopic method