Does Bimolecular Charge Recombination in Highly Exergonic Electron Transfer Afford the Triplet Excited State or the Ground State of a Photosensitizer?

2008 ◽  
Vol 112 (4) ◽  
pp. 635-642 ◽  
Author(s):  
Motonobu Murakami ◽  
Kei Ohkubo ◽  
Paulami Mandal ◽  
Tapan Ganguly ◽  
Shunichi Fukuzumi
Keyword(s):  
Electron Transfer ◽  
Excited State ◽  
Ground State ◽  
Charge Recombination ◽  
Triplet Excited State

Long-lived charge-separated states of simple electron donor-acceptor dyads using porphyrins and phthalocyanines

2008 ◽  
Vol 12 (09) ◽  
pp. 993-1004 ◽  
Author(s):  
Kei Ohkubo ◽  
Shunichi Fukuzumi
Keyword(s):  
Electron Transfer ◽  
Excited State ◽  
Electron Donor ◽  
Driving Force ◽  
Reorganization Energy ◽  
Charge Recombination ◽  
Zinc Phthalocyanine ◽  
Intramolecular Electron Transfer ◽  
Triplet Excited State ◽  
Donor Acceptor

Control of electron-transfer processes is described for a number of electron donor-acceptor dyads containing porphyrins or phthalocyanines as models for the photosynthetic reaction center. The rates for intramolecular electron transfer in the dyads are controlled by the driving force and reorganization energy of electron transfer. The small reorganization energy of electron transfer reactions and large driving force of charge recombination are required to form long-lived charge-separated states. A directly linked zinc chlorin-fullerene dyad, especially, has the longest lifetime of charge-separated state at 120 s at -150 °C, which is a much longer lifetime and higher energy than those of natural photosynthetic reaction centers. On the other hand, the charge-separated states of the phthalocyanine-based donor-acceptor dyads (silicon phthalocyanine-fullerene, and zinc phthalocyanine-perylenebisimide) are short-lived since charge recombination forms the low-lying triplet excited state of the chromophore. The energy of the charge-separated state of a zinc phthalocyanine-perylenebisimide dyad is decreased by binding of metal ions to the radical anion moiety in order to be lower than the triplet excited state. This results in formation of a long-lived charge-separated state. The mechanistic viability of formation of long-lived charge-separated states is demonstrated by a variety of examples based on the Marcus theory of electron transfer.

scholarly journals Electron Transfer Initiated Reactions Photoinduced by Pterins

Pteridines ◽  
2011 ◽  
Vol 22 (1) ◽  
pp. 111-119 ◽  
Author(s):  
Carolina Lorente ◽  
Gabriela Petroselli ◽  
M. Laura Dántola ◽  
Esther Oliveros ◽  
Andrés H. Thomas
Keyword(s):  
Electron Transfer ◽  
Excited State ◽  
Redox Reactions ◽  
Type I ◽  
Biological Processes ◽  
Type Ii ◽  
Triplet Excited State ◽  
Oxygen Production ◽  
Production Type ◽  
Transfer Mechanisms

Abstract Interest in the photochemistry and photophysics of pterins has increased since the participation of this family of compounds in different photobiological processes has been suggested or demonstrated in recent decades. Pterins participate in relevant biological processes, such as metabolic redox reactions, and can photoinduce the oxidation of biomolecules through both electron transfer mechanisms (Type I) and singlet oxygen production (Type II). This article describes recent findings on electron transfer-initiated reactions photoinduced by the triplet excited state of pterins and connects them in the context of photosensitized processes of biological relevance.

scholarly journals Halogen-Bond Assisted Photoinduced Electron Transfer

Molecules ◽  
2019 ◽  
Vol 24 (23) ◽  
pp. 4361
Author(s):  
Bogdan Dereka ◽  
Ina Fureraj ◽  
Arnulf Rosspeintner ◽  
Eric Vauthey
Keyword(s):  
Electron Transfer ◽  
Excited State ◽  
Ground State ◽  
Halogen Bond ◽  
Quenching Rate ◽  
Excited Complex ◽  
Diffusion Controlled ◽  
Donor Acceptor ◽  
Quenching Rate Constant ◽  
Excited State Population

The formation of a halogen-bond (XB) complex in the excited state was recently reported with a quadrupolar acceptor–donor–acceptor dye in two iodine-based liquids (J. Phys. Chem. Lett. 2017, 8, 3927–3932). The ultrafast decay of this excited complex to the ground state was ascribed to an electron transfer quenching by the XB donors. We examined the mechanism of this process by investigating the quenching dynamics of the dye in the S1 state using the same two iodo-compounds diluted in inert solvents. The results were compared with those obtained with a non-halogenated electron acceptor, fumaronitrile. Whereas quenching by fumaronitrile was found to be diffusion controlled, that by the two XB compounds is slower, despite a larger driving force for electron transfer. A Smoluchowski–Collins–Kimball analysis of the excited-state population decays reveals that both the intrinsic quenching rate constant and the quenching radius are significantly smaller with the XB compounds. These results point to much stronger orientational constraint for quenching with the XB compounds, indicating that electron transfer occurs upon formation of the halogen bond.

The mechanism of the photochemical cycloaddition reaction of N-benzoylindole with cyclopentene

1990 ◽  
Vol 68 (10) ◽  
pp. 1685-1692 ◽  
Author(s):  
Bimsara W. Disanayaka ◽  
Alan C. Weedon
Keyword(s):  
Excited State ◽  
Quantum Yield ◽  
Ground State ◽  
Mechanistic Model ◽  
Cycloaddition Reaction ◽  
Intersystem Crossing ◽  
Triplet Excited State ◽  
Excited State Lifetime ◽  
Yield Data ◽  
Counting Procedure

The mechanism of the photochemical cycloaddition reaction between N-benzoylindole, 1, and cyclopentene to give cyclobutane adducts 2 and 3 has been examined. The triplet excited state lifetime and quantum yield of intersystem crossing were determined for 1 as (2.8 ± 0.3) × 10−8 s and 0.39 ± 0.01, respectively, using the triplet counting procedure. In addition, the dependence of the quantum yield of cycloadduct formation upon the concentration of cyclopentene and upon the concentration of excited state quenchers has been determined. The results are used to propose a mechanistic model in which the triplet excited state of 1 reacts with cyclopentene to give a triplet 1,4-biradical intermediate. Following spin inversion the biradical intermediate reverts to the ground state starting materials or proceeds to the products 2 and 3; this partitioning, along with the quantum yield of intersystem crossing, gives rise to a limiting quantum yield of cycloaddition at infinite alkene concentration of 0.061. It is calculated that 84% of the biradical intermediates revert to the starting materials and 16% proceed to cycloadducts. The quantum yield data are also used to calculate two independent values of the rate constant for reaction of the triplet excited 1 with alkene; the values are (1.8 ± 0.1) × 107M−1 s−1 and (4.0 ± 0.8) × 106 M−1 s−1'. Some evidence for self quenching of the triplet excited state of 1 by ground state 1 was also observed. The quantum yield of intersystem crossing and the triplet excited state lifetime of 1 were found to vary with the solvent used; this is discussed in terms of the possible existence of a charge transfer triplet excited state. Keywords: indole, photocycloaddition, mechanism.

scholarly journals Role of active site conformational changes in photocycle activation of the AppA BLUF photoreceptor

2017 ◽  
Vol 114 (7) ◽  
pp. 1480-1485 ◽  
Author(s):  
Puja Goyal ◽  
Sharon Hammes-Schiffer
Keyword(s):  
Electron Transfer ◽  
Free Energy ◽  
Excited State ◽  
Charge Transfer ◽  
Conformational Changes ◽  
Ground State ◽  
Active Site ◽  
Locally Excited ◽  
Proton Relay ◽  
Locally Excited State

Blue light using flavin adenine dinucleotide (BLUF) proteins are essential for the light regulation of a variety of physiologically important processes and serve as a prototype for photoinduced proton-coupled electron transfer (PCET). Free-energy simulations elucidate the active site conformations in the AppA (activation of photopigment and puc expression) BLUF domain before and following photoexcitation. The free-energy profile for interconversion between conformations with either Trp104 or Met106 closer to the flavin, denoted Trpin/Metout and Trpout/Metin, reveals that both conformations are sampled on the ground state, with the former thermodynamically favorable by ∼3 kcal/mol. These results are consistent with the experimental observation of both conformations. To analyze the proton relay from Tyr21 to the flavin via Gln63, the free-energy profiles for Gln63 rotation were calculated on the ground state, the locally excited state of the flavin, and the charge-transfer state associated with electron transfer from Tyr21 to the flavin. For the Trpin/Metout conformation, the hydrogen-bonding pattern conducive to the proton relay is not thermodynamically favorable on the ground state but becomes more favorable, corresponding to approximately half of the configurations sampled, on the locally excited state. The calculated energy gaps between the locally excited and charge-transfer states suggest that electron transfer from Tyr21 to the flavin is more facile for configurations conducive to proton transfer. When the active site conformation is not conducive to PCET from Tyr21, Trp104 can directly compete with Tyr21 for electron transfer to the flavin through a nonproductive pathway, impeding the signaling efficiency.

Forbidden Electronic Transitions between the Singlet Ground State and the Triplet Excited State of Pt(II) Complexes

1998 ◽  
Vol 37 (14) ◽  
pp. 3588-3592 ◽  
Author(s):  
Greg Y. Zheng ◽  
D. Paul Rillema ◽  
Jeff DePriest ◽  
Clifton Woods
Keyword(s):  
Excited State ◽  
Ground State ◽  
Electronic Transitions ◽  
Singlet Ground State ◽  
Triplet Excited State
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