Photo-Induced Charge Separation vs. Degradation of a BODIPY-Based Photosensitizer Assessed by TDDFT and RASPT2

Abstract

A meso-mesityl-2,6-iodine substituted boron dipyrromethene (BODIPY) dye is investigated using a suite of computational methods addressing its functionality as photosensitizer, i.e., in the scope of light-driven hydrogen evolution in a two-component approach. Earlier reports on the performance of the present iodinated BODIPY dye proposed a significantly improved catalytic turn-over compared to its unsubstituted parent compound based on the population of long-lived charge-separated triplet states, accessible due to an enhanced spin-orbit coupling (SOC) introduced by the iodine atoms. The present quantum chemical study aims at elucidating the mechanisms of both the higher catalytic performance and the degradation pathways. Time-dependent density functional theory (TDDFT) and multi-state restricted active space perturbation theory through second-order (MS-RASPT2) simulations allowed identifying excited-state channels correlated to iodine dissociation. No evidence for an improved catalytic activity via enhanced SOCs among the low-lying states could be determined. However, the computational analysis reveals that the activation of the dye proceeds via pathways of the (prior chemically) singly-reduced species, featuring a pronounced stabilization of charge-separated species, while low barriers for carbon-iodine bond breaking determine the photostability of the BODIPY dye.

Photochemical Properties and Stability of BODIPY Dyes

“…The substituted compounds have found numerous potential applications, for instance, as a new agent in medical imagining [13,14] or as a photosensitizer for photodynamic therapy (PDT) [15,16]. Nevertheless, BODIPY dyes have so far been rarely applied in PDT, due to their weak absorption above 600 nm, poor water-solubility and photostability [17] as well as relatively inefficient intersystem crossing in the basic unsubstituted form [18,19]. However, due to their small internal conversion rate constant, extremely intensive fluorescence and easy chemical modifications that affect strongly their solubility and photophysical properties, they account for an attractive alternative as a PDT photosensitizer.…”

Intramolecular Hydrogen Bonds in Selected Aromatic Compounds: Recent Developments

“…The growing popularity of fluorescent dyes rests upon the possibility of chemical modification of both the chromophore core and its substituents, which can manifest in a variety of spectroscopic properties [70][71][72][73][74][75][76][77][78][79][80][81][82][83]. The main objective of fluorescent BODIPY (4,4′-difluoro-4-boron-3a,4a-diaza-s-indacene) dye studies has been to explore the influence of the tautomeric equilibrium sensor on the spectroscopic parameters of the dye [84][85][86][87][88].…”

Toward Prediction of Nonradiative Decay Pathways in Organic Compounds II: Two Internal Conversion Channels in BODIPYs

“…[127][128][129][130] For BODIPY we follow the experimental evidence that only one of the S 1 → S 0 and S 1 → S 2 (or L a → L b ) ICs is fast enough to be observed, being at least two orders of magnitude faster than any other nonradiative channel. 9,12, 22,24,26,27,29,30,35,42,46,47,50,51,62,64,66,67,[100][101][102]110,152,163 As a result,…”

“…Possible IC channels include the most common relaxation to the ground state (S 1 → S 0 ), the photoinduced isomerization (S 1 → S 1 /S 0 ), and the intramolecular charge transfer (ICT) reaction from a bright donating state to a dark accepting state (S 1 → S 2 or L a → L b ). 9,12, 22,24,26,27,29,30,35,42,46,47,50,51,62,64,66,67,[100][101][102]110,152,163 In the following sections, we will show that all three IC mechanisms are available in BODIPY, also depending on the substituents. Based on sufficient experimental data, our proposed model allows us to predict k fl , k nr , and Φ fl to the desired accuracy, validating its ability in the planned machine learning aided design of BODIPY.…”

“…Therefore an ideal model should be able to include an explicit treatment of relevant nuclear motions and an ergodic sampling of molecular configurations over relevant potential energy surfaces (PES), both of which are computationally infeasible if large scale design is necessary. 24,27,30,64,75,[81][82][83]90,94,96,97,100,102,103,112 As an approximate solution, researchers can also evaluate the key molecular configurations of BODIPY that are involved in the nonradiative mechanisms of interest, such as the Franck-Condon (FC) minimum, the minimum energy conical intersection (MECI), and the transition state, and constructed reduced dimensional PES in the vicinity of these regions. Unfortunately, this treatment is still expensive given the large size of BODIPY and the multireference character of the far-from-equilibrium molecular configurations.…”