Authors: Zhou Lin | Alexander Kohn | Troy Van Voorhis
Source: ChemRxiv (article, 2019-09-03)
Field: Materials Science — Materials Chemistry

Toward Prediction of Nonradiative Decay Pathways in Organic Compounds II: Two Internal Conversion Channels in BODIPYs &lt;div&gt;Boron-dipyrromethene (BODIPY) molecules are widely used as laser dyes and have therefore become a popular research topic within recent decades. Numerous studies have been reported for the rational design of BODIPY derivatives based on their spectroscopic and photophysical properties, including absorption and fluorescence wavelengths (&lt;i&gt;λ&lt;/i&gt;&lt;sub&gt;abs&lt;/sub&gt; and &lt;i&gt;λ&lt;/i&gt;&lt;sub&gt;fl&lt;/sub&gt;), oscillator strength (&lt;i&gt;f&lt;/i&gt;), nonradiative pathways, and quantum yield (&lt;i&gt;ϕ&lt;/i&gt;). In the present work, we illustrate a theoretical, semi-empirical model that accurately predicts &lt;i&gt;ϕ&lt;/i&gt; for various BODIPY compounds based on inexpensive electronic structure calculations, following the data-driven algorithm proposed and tested on the naphthalene family by us [Kohn, Lin, and Van Voorhis, &lt;i&gt;J. Phys. Chem. C.&lt;/i&gt; &lt;b&gt;2019&lt;/b&gt;, &lt;i&gt;123&lt;/i&gt;, 15394]. The model allows us to identify the dominant nonradiative channel of any BODIPY molecule using its structure exclusively and to establish a correlation between the activation energy (&lt;i&gt;E&lt;/i&gt;&lt;sub&gt;a&lt;/sub&gt;) and the fluorescence quantum yield (&lt;i&gt;ϕ&lt;/i&gt;&lt;sub&gt;fl&lt;/sub&gt;). Based on our calculations, either the S&lt;sub&gt;1&lt;/sub&gt; → S&lt;sub&gt;0&lt;/sub&gt; or &lt;i&gt;L&lt;sub&gt;a&lt;/sub&gt;&lt;/i&gt; → &lt;i&gt;L&lt;sub&gt;b&lt;/sub&gt;&lt;/i&gt; internal conversion (IC) mechanism dominates in the majority of BODIPY derivatives, depending on the structural and electronic properties of the substituents. In both cases, the nonradiative rate (&lt;i&gt;k&lt;/i&gt;&lt;sub&gt;nr&lt;/sub&gt;) exhibits a straightforward Arrhenius-like relation with the associated &lt;i&gt;E&lt;/i&gt;&lt;sub&gt;a&lt;/sub&gt;. More interestingly, the S&lt;sub&gt;1&lt;/sub&gt; → S&lt;sub&gt;0&lt;/sub&gt; mechanism proceeds via a highly distorted intermediate structure in which the core BODIPY plane and the substituent at the 1-position are forced to bend, while the internal rotation of the very same substituent induces the &lt;i&gt;La &lt;/i&gt;→&lt;i&gt; Lb&lt;/i&gt; transition. Our model reproduces &lt;i&gt;k&lt;/i&gt;&lt;sub&gt;fl&lt;/sub&gt;, &lt;i&gt;k&lt;/i&gt;&lt;sub&gt;nr&lt;/sub&gt;, and &lt;i&gt;ϕ&lt;/i&gt;&lt;sub&gt;fl&lt;/sub&gt; to mean absolute errors (MAE) of 0.16 decades, 0.87 decades, and 0.26, when all outliers are considered. These results allow us to validate the predictive power of the proposed data-driven algorithm in &lt;i&gt;ϕ&lt;/i&gt;&lt;sub&gt;fl&lt;/sub&gt;. They also indicate that the model has a great potential to facilitate and accelerate the machine learning aided design of BODIPY dyes for imaging and sensing applications, given sufficient experimental data and appropriate molecular descriptors.&lt;/div&gt; Chemistry ; BODIPY ; Quantum yield ; Fluorescence ; Physics ; Quantum mechanics Luminescence and Fluorescent Materials ; Organic Light-Emitting Diodes Research ; Photochemistry and Electron Transfer Studies