DFT Based Modeling of the Heavy Atom Effect in Triplet Photosensitizers: From Fundamental Mechanics to Rational Design

Author

Bret Andrew Steinkamp, Portland State UniversityFollow

Sponsor

Portland State University. Department of Chemistry

First Advisor

Theresa M. McCormick

Term of Graduation

Spring 2024

Date of Publication

6-3-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.) in Chemistry

Department

Chemistry

Language

English

Subjects

BODIPY, Heavy atom effect, Singlet oxygen quantum yield, Spin orbit coupling, Tellurorhodamine, Xanthene

DOI

10.15760/etd.3778

Physical Description

1 online resource (xx, 135 pages)

Abstract

Photochemistry is the study of how light interacts with molecules and includes both the absorption and release of light. This document focuses on triplet photosensitizers -- molecules that utilize the excited triplet state -- for a variety of applications including photodynamic therapy and photocatalysis. There are many factors that influence the design of triplet photosensitizers; in this work, we have developed and used a methodology for the rational design of triplet photosensitizers based on an approximation of the heavy atom effect (HAE) from density functional theory calculations and the experimentally measured singlet oxygen quantum yield (Ф_Δ).

As excited triplet states can be quenched by triplet oxygen to make singlet oxygen, the Ф_Δ is often used as a lower approximation for the triplet quantum yield. Unpredictable effects of even minor structural changes can drastically alter the Ф_Δ and complicate the design of new triplet photosensitizers. The most common strategy to increase Ф_Δ is to incorporate heavy atoms, promoting the HAE. However, the position and the identity of the heavy atom greatly influences the Ф_Δ. We have created a predictive model that correlates the ab initio calculated natural atomic orbital composition of the heavy atom(s) contributing to the frontier molecule orbitals of a photosensitizer with the experimentally measured Ф_Δ.

The initial model was made with data fit for xanthene type dyes. The xanthene model, utilizing a reciprocal trendline derived from several fluorescein derivatives, provides a calculated Ф_Δ in agreement with the experimental values for a variety of photosensitizers, including rhodamine dyes, fluorescein derivatives, and octahedral metal complexes.

Subsequently, we modified the original xanthene model to apply specifically to halogenated boron dipyrromethene (BODIPY) photosensitizers. In addition to developing a method to predict how changes in the structure of BODIPY affects the Ф_Δ, this model provides insight into why different structural changes have differing impacts to the Ф_Δ. 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 Ф_Δ with yields changing from 0.00 to 0.90 when replacing the two protons with iodides. Although the original model made using xanthene type chromophores provided reasonably good agreement with BODIPY type chromophores, the new reparametrized model allows for more accurate prediction of the Ф_Δ of BODIPY type chromophores and provides insight in the importance of substituent location to guide future chromophore design.

Tellurorhodamines are a class of triplet photosensitizer where the oxygen in the core of a rhodamine has been replaced by the heavy chalcogen, tellurium. This introduces the HAE, red shifting the chromophore and decreasing the fluorescent quantum yield (Ф_F) while increasing the Ф_Δ. However, the tellurium atom can be oxidized, going from a Te(II) to a Te(IV) state with reaction with oxygen or halogens. These oxidized tellurorhodamines have increased Ф_F with undetected Ф_Δ. In an effort to understand the disappearance of the HAE observed in these chromophores, we have investigated a mesityl tellurorhodamine, Te1, through both experimental and ab initio quantum calculations. Ab initio calculations show that the most common first excitation for both the reduced and oxidized tellurorhodamines is the HOMO to LUMO transition. Te1 has S₁ and T₂ states that are similar in energy along with high spin orbit coupling (SOC), resulting in high Ф_Δ. The telluroxides, excluding the iodinated variant, have low SOC along with a S₁ state lower in energy than the T₂ state. Additionally, the S₁ and T1 states have a large difference in energy. The large energy gap between singlet and triplet states (ΔE_ST) combined with low SOC result in little intersystem crossing to a triplet state and low Ф_Δ.

The initial xanthene model was designed as a fast predictive model for Ф_Δ; however, subsequent research has shown that this methodology is also useful for guiding photosensitizer design based on the HAE. It has been demonstrated that for the HAE to result in an increase in Ф_Δ, then the heavy atom’s orbitals must be interacting with the orbitals involved in the transition. While this interaction can be investigated with complex SOC calculations, the methodology described within this work reveals similar information without the need for expert SOC knowledge or a separate program. The xanthene and BODIPY models have shown the ability to predict Ф_Δ and approximate SOC of heavy atoms for a variety of chromophore scaffolds with the ability to be reparametrized to apply to any heavy atom containing chromophore scaffold.

Rights

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Persistent Identifier

https://archives.pdx.edu/ds/psu/42277

Recommended Citation

Steinkamp, Bret Andrew, "DFT Based Modeling of the Heavy Atom Effect in Triplet Photosensitizers: From Fundamental Mechanics to Rational Design" (2024). Dissertations and Theses. Paper 6646.
https://doi.org/10.15760/etd.3778