Unusually high energy barriers for internal conversion in a {Ru(bpy)} chromophore

Cotic, Agustina; Veys, Koen; Escudero, Daniel; Cadranel, Alejandro

doi:10.1039/d2cp03160d

Cited by 4 publications

(9 citation statements)

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“…These k q are in agreement with diffusion limited processes for catalyst activation. (6) The nature of the activated catalyst models formed in the bimolecular electron transfer reactions, that is, the charge separated (CS) states, was revealed by nsTAS in the time range 1−400 μs. Upon quenching of 3 MLCT(L m ) and 3 MLCT(bpy) in the sub-μs timescale, CS species are essentially completely formed at a time delay of 1 μs, as evident in the kinetic traces at 14,900 cm −1 (Figure 7a).…”

Section: ■ Results and Discussionmentioning

confidence: 99%

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Anti-Dissipative Strategies toward More Efficient Solar Energy Conversion

et al. 2023

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In natural and artificial photosynthesis, light absorption and catalysis are separate processes linked together by exergonic electron transfer. This leads to free energy losses between the initial excited state, formed after light absorption, and the active catalyst formed after the electron transfer cascade. Additional deleterious processes, such as internal conversion (IC) and vibrational relaxation (VR), also dissipate as much as 20−30% of the absorbed photon energy. Minimization of these energy losses, a holy grail in solar energy conversion and solar fuel production, is a challenging task because excited states are usually strongly coupled which results in negligible kinetic barriers and very fast dissipation. Here, we show that topological control of oligomeric {Ru(bpy) 3 } chromophores resulted in small excited-state electronic couplings, leading to activation barriers for IC by means of inter-ligand electron transfer of around 2000 cm −1 and effectively slowing down dissipation. Two types of excited states are populated upon visible light excitation, that is, a bridging-ligand centered metal-to-ligand charge transfer [MLCT(L m )], and a 2,2′-bipyridine-centered MLCT [MLCT(bpy)], which lies 800−1400 cm −1 higher in energy. As a proof-of-concept, bimolecular electron transfer with tri-tolylamine (TTA) as electron donor was performed, which mimics catalyst activation by sacrificial electron donors in typical photocatalytic schemes. Both excited states were efficiently quenched by TTA. Hence, this novel strategy allows to trap higher energy excited states before IC and VR set in, saving between 100 and 170 meV. Furthermore, transient absorption spectroscopy suggests that electron transfer reactions with TTA produced the corresponding L m •− -centered and bpy •− -centered reduced photosensitizers, which involve different reducing abilities, that is, −0.79 and −0.93 V versus NHE for L m•− and bpy •− , respectively. Thus, this approach probably leads in fine to a 140 meV more potent reductant for energy conversion schemes and solar fuel production. These results lay the first stone for anti-dissipative energy conversion schemes which, in bimolecular electron transfer reactions, harness the excess energy saved by controlling dissipative conversion pathways.

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Section: ■ Results and Discussionmentioning

confidence: 99%

“…(IC: internal conversion, VR: vibrational relaxation, ISC: intersystem crossing, AD-Cat: anti-dissipative catalyst, Cat: traditional catalyst). Parts of this figure have been adapted from ref with permission from the PCCP Owner Societies.…”

Section: Introductionmentioning

confidence: 99%

Anti-Dissipative Strategies toward More Efficient Solar Energy Conversion

et al. 2023

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“…In the pursuit of finding new and efficient ways for solar energy conversion, , recent years have seen increasing interest in investigating reactivity from higher-lying excited states . To date, only a few systems can leverage higher-lying states to initiate photochemistry ,, because a fundamental statement is prohibiting access: Kasha’s rule states that relaxation to the lowest-energy state of a given multiplicity is ultrafast, and therefore, all photochemistry occurs from this state. − In this usual case, population from higher-energy states decays rapidly via IC to the lowest-energy state due to strong electronic coupling, resulting in a low barrier between the two states of interest (Figure , top left) . Weakening electronic coupling and raising this barrier (Figure , top right) leads to a deceleration of the IC rate so that photon emission or bimolecular reactivity (requiring ∼ns lifetimes) from that state can compete.…”

Section: Introductionmentioning

confidence: 99%

“…Most systems known to exhibit anti-Kasha behavior are organic chromophores, such as azulene, while transition-metal complexes remain underexplored. Previously studied systems include porphyrins, , as well as ruthenium or molybdenum polypyridine systems. ,,,, Despite recent advances, ,,, there is a growing need to explore relaxation dynamics initiated by higher excited states, also with computational protocols . Understanding and controlling the many photophysical phenomena that underpin vital natural processes hold the key to developing novel technologies. − While spectroscopy usually provides only a global response of the system to light, computer simulations help disentangle the multiple underlying spectroscopic signatures.…”

Section: Introductionmentioning

confidence: 99%

“…Previously studied systems include porphyrins, 23,24 as well as ruthenium or molybdenum polypyridine systems. 7,12,19,25,26 Despite recent advances, 11,25,27,28 there is a growing need to explore relaxation dynamics initiated by higher excited states, also with computational protocols. 28 Understanding and controlling the many photophysical phenomena that underpin vital natural processes hold the key to developing novel technologies.…”

Section: ■ Introductionmentioning

confidence: 99%

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Bifurcation of Excited-State Population Leads to Anti-Kasha Luminescence in a Disulfide-Decorated Organometallic Rhenium Photosensitizer

Franz,

Oelschlegel,

Zobel

et al. 2024

J. Am. Chem. Soc.

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We report a rhenium diimine photosensitizer equipped with a peripheral disulfide unit on one of the bipyridine ligands, [Re(CO) 3 (bpy)( S−S bpy 4,4 )] + (1 + , bpy = 2,2′-bipyridine, S−S bpy 4,4 = [1,2]dithiino[3,4-c:6,5-c′]dipyridine), showing anti-Kasha luminescence. Steady-state and ultrafast time-resolved spectroscopies complemented by nonadiabatic dynamics simulations are used to disclose its excited-state dynamics. The calculations show that after intersystem crossing the complex evolves to two different triplet minima: a ( S−S bpy 4,4 )-ligandcentered excited state ( 3 LC) lying at lower energy and a metal-to-(bpy)-ligand charge transfer ( 3 MLCT) state at higher energy, with relative yields of 90% and 10%, respectively. The 3 LC state involves local excitation of the disulfide group into the antibonding σ* orbital, leading to significant elongation of the S−S bond. Intriguingly, it is the higher-lying 3 MLCT state, which is assigned to display luminescence with a lifetime of 270 ns: a signature of anti-Kasha behavior. This assignment is consistent with an energy barrier ≥ 0.6 eV or negligible electronic coupling, preventing reaction toward the 3 LC state after the population is trapped in the 3 MLCT state. This study represents a striking example on how elusive excited-state dynamics of transition-metal photosensitizers can be deciphered by synergistic experiments and state-of-the-art calculations. Disulfide functionalization lays the foundation of a new design strategy toward harnessing excess energy in a system for possible bimolecular electron or energy transfer reactivity.

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Influence of Donor‐Acceptor Interactions on MLCT Hole Reconfiguration in {Ru(bpy)} Chromophores

Cotic,

Heinemann,

Slep

et al. 2024

ChemPhysChem

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In MLCT chromophores, internal conversion (IC) in the form of hole reconfiguration pathways (HR) is a major source of dissipation of the absorbed photon energy. Therefore, it is desirable to minimize their impact in energy conversion schemes by slowing them down. According to previous findings on {Ru(bpy)} chromophores, donor‐acceptor interactions between the Ru ion and the ligand scaffold might allow to control HR/IC rates. Here, a series of [Ru(tpm)(bpy)(R‐py)]2+ chromophores, where tpm is tris(1‐pyrazolyl)methane, bpy is 2,2’‐bipyridine and R‐py is a 4‐substituted pyridine, were prepared and fully characterized employing electrochemistry, spectroelectrochemistry, steady‐state absorption/emission spectroscopy and electronic structure computations based on DFT/TD‐DFT. Their excited‐state decay was monitored using nanosecond and femtosecond transient absorption spectroscopy. HR/IC lifetimes as slow as 568 ps were obtained in DMSO at room temperature, twice as slow as in the reference species [Ru(tpm)(bpy)(NCS)]+.

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Unusually high energy barriers for internal conversion in a {Ru(bpy)} chromophore

Abstract: Internal conversion (IC) coupled to vibrational relaxation (VR) in molecular chromophores is a source of major energy losses in natural and artificial solar-to-chemical energy conversion schemes. The development of anti-Kasha...

Cited by 4 publications

References 53 publications

Anti-Dissipative Strategies toward More Efficient Solar Energy Conversion

Anti-Dissipative Strategies toward More Efficient Solar Energy Conversion

Bifurcation of Excited-State Population Leads to Anti-Kasha Luminescence in a Disulfide-Decorated Organometallic Rhenium Photosensitizer

Influence of Donor‐Acceptor Interactions on MLCT Hole Reconfiguration in {Ru(bpy)} Chromophores

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