Photophysics of BODIPY Dyes as Readily-Designable Photosensitisers in Light-Driven Proton Reduction

Abstract

A series of boron dipyrromethene (BODIPY) dyes was tested as photosensitisers for light-driven hydrogen evolution in combination with the complex [Pd(PPh 3 )Cl 2 ] 2 as a source for catalytically-active Pd nanoparticles and triethylamine as a sacrificial electron donor. In line with earlier reports, halogenated dyes showed significantly higher hydrogen production activity. All BODIPYs were fully characterised using stationary absorption and emission spectroscopy. Time-resolved spectroscopic investigations on meso-mesityl substituted compounds revealed that reduction of the photo-excited BODIPY by the sacrificial agent occurs from an excited singlet state, while, in halogenated species, long-lived triplet states are present, determining electron transfer processes from the sacrificial agent. Quantum chemical calculations performed at the time-dependent density functional level of theory indicate that the differences in the photocatalytic performance of the present series of dyes can be correlated to the varying efficiency of intersystem crossing in non-halogenated and halogenated species and not to alterations in the energy levels introduced upon substitution.

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

“…Subsequent intermolecular electron transfer among the PS and [Pd(PPh 3 )Cl 2 ] 2 generates the catalytically-active palladium species, which leads to the formation of molecular hydrogen. Experimental evidence for such a population transfer via ISC is given by the low emission quantum yields and low singlet excited-state lifetimes in comparison with the unsubstituted BODIPY dye, while the population of long-lived charge-separated states is suggested by transient absorption spectroscopy and time-dependent density functional theory (TDDFT) simulations [50]. This mechanism is denoted in the following as heavy atom mechanism.…”

“…The excitation of the reduced dye (with the excess charge on the BODIPY core) then leads to a charge transfer to the mesityl group and further-in accordance with the previously introduced heavy atom mechanism-to an Introduction of a mesityl group at the meso-position allowed enhancing the previously reported long-term stability of 20 h by a factor of ten. At the same time, the catalytic turn-over was increased by a factor of seven by virtue of iodine substituents at the positions 2 and 6 of the BODIPY (X 1 and X 2 ) compared to the unsubstituted parent compound (X 1 = X 2 = H) [50]. According to Beweries et al, the enhanced catalytic activity of the iodinated species, 1, is ascribed to an efficient intersystem crossing (ISC) between the initially populated excited singlet states and the nearby long-lived charge-separated triplet states by virtue of sizeable spin-orbit coupling (SOC) introduced by the iodine atoms.…”

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

“…or possesses a dimeric structure. 2,4,19,38,42,57,68,69,77,88,95,106,111 Therefore we expect an IC process to occur between two singlet states and to dominate the nonradiative mechanism. 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 ).…”

“…More specifically, organic syntheses can prepare BODIPY with particular combinations of substituents, 2,4,6,7,17,19,23,24,26,30, following which the photophysical properties of these molecules can be investigated via diverse spectroscopic tools. 1,9,11,12,18,20,28,29,31,32,41,[59][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74] However, the peaks that are observed in the electronic spectra can be so broad that the actual photophysical dynamics and structure-property relations are never precisely illustrated.…”

First-Principles View on Photoelectrochemistry: Water-Splitting as Case Study

“…For instance, more energy from sunlight strikes Earth in one hour than what society currently consumes in a year [13]; thus the Sun easily rises to the TW challenge. Solar energy can be harvested in many different ways [14][15][16][17]. For instance, photosynthesis, which play a crucial role for life on Earth, is able to transform sunlight to chemical energy in the form of biomass.…”