Search results for "Intersystem crossing"
showing 10 items of 60 documents
Intersystem Crossing and Light-Induced Bistability in Iron(?) Spin-Crossover Compounds
1996
Abstract The dynamics of the high-spin→low-spin intersystem crossing process in iron(?) spin-crossover compounds are strongly influenced by cooperative effects of elastic origin which are due to the large difference in volume between high-spin and low-spin complexes. The deviation from first order kinetics is attributed to a build-up of an internal pressure as the relaxation proceeds, leading to a characteristic self-acceleration. The elastic interactions may lead to a light-induced bistability for systems which otherwise remain in the high-spin state down to cryogenic temperatures.
Corrigendum: An Unusually Small Singlet-Triplet Gap in a Quinoidal 1,6-Methano[10]annulene Resulting from Baird's 4n π-Electron Triplet Stabilization.
2016
Within the continuum of π-extended quinoidal electronic structures exist molecules that by design can support open-shell diradical structures. The prevailing molecular design criteria for such structures involve proaromatic nature that evolves aromaticity in open-shell diradical resonance structures. A new diradical species built upon a quinoidal methano[10]annulene unit is synthesized and spectroscopically evaluated. The requisite intersystem crossing in the open-shell structure is accompanied by structural reorganization from a contorted Mobius aromatic-like shape in S0 to a more planar shape in the Huckel aromatic-like T1. This stability was attributed to Baird’s Rule which dictates the …
A theoretical study on the thermal ring opening rearrangement of 1H-bicyclo[3.1.0]hexa-3,5-dien-2-one: a case of two state reactivity
2009
The molecular mechanism of the thermal rearrangement of singlet 1H-bicyclo[3.1.0]hexa-3,5-dien-2-one to give triplet 4-oxocyclohexa-2,5-dienylidene has been investigated using density functional theory (B3LYP and M05-2X functionals) as well as ab initio CASSCF and CASPT2 multiconfigurational methods. The reactant has a singlet ground state while the product can be found in three low lying electronic states P((3)B(1)), P((1)B(1)), and P((1)A'). Therefore, the molecular mechanism of this ring opening rearrangement may involve up to three different potential energy surfaces of two spin multiplicities: two singlet (closed shell, CS, and open shell, OS) and one triplet. The stationary points on …
Harvesting Fluorescence from Efficient Tk -> Sj (j, k > 1) Reverse Intersystem Crossing for ??* Emissive Transition-Metal Complexes
2013
Using a bimetallic Au(I) complex bearing alkynyl-(phenylene)3-diphosphine ligand (A-3), we demonstrate that the fluorescence can be exquisitely harvested upon T1 → Tk (k > 1) excitation followed by Tk → Sj (j, k > 1) intersystem crossing (ISC) back to the S1 state. Upon S0 → S1 355 nm excitation, the S1 → T1 intersystem crossing rate has been determined to be 8.9 × 108 s–1. Subsequently, in a two-step laser pump–probe experiment, following a 355 nm laser excitation, the 532 nm T1 → Tk probing gives the prominent blue 375 nm fluorescence, and this time-dependent pump–probe signal correlates well with the lifetime of the T1 state. Careful examination reveals the efficiency of Tk → Sj (j, k > …
Regiochemical memory in the adiabatic photolysis of thymine-derived oxetanes. A combined ultrafast spectroscopic and CASSCF/CASPT2 computational stud…
2020
[EN] The photoinduced cycloreversion of oxetanes has been thoroughly investigated in connection with the photorepair of the well-known DNA (6-4) photoproducts. In the present work, the direct photolysis of the two regioisomers arising from the irradiation of benzophenone (BP) and 1,3-dimethylthymine (DMT), namely the head-to-head (HH-1) and head-to-tail (HT-1) oxetane adducts, has been investigated by combining ultrafast spectroscopy and theoretical multiconfigurational quantum chemistry analysis. Both the experimental and computational results agree with the involvement of an excited triplet exciplex(3)[BPMIDLINE HORIZONTAL ELLIPSISDMT]* for the photoinduced oxetane cleavage to generate(3)…
Photophysical properties of the triangular [Au(HNCOH)]3 complex and its dimer
2020
Rate constants for radiative and non-radiative transitions of the [Au(HNCOH)]3 complex and its dimer were calculated within the Herzberg–Teller approximation based on quantum mechanical principles. A high triplet quantum yield was estimated for the monomer. Internal conversion (IC) was found to be the major competing process to the intersystem crossing (ISC) from the lowest excited singlet state (S1) to the lowest triplet state (T1). ISC and IC from the spin-mixed 1 state also dominate the triplet relaxation process resulting in a negligible phosphorescence quantum yield for the monomer. The IC and ISC rate constants of the dimer are considerably smaller due to much lower Franck–Condon fact…
All-organic fast intersystem crossing assisted exciplexes exhibiting sub-microsecond thermally activated delayed fluorescence
2021
A novel strategy is presented towards acquisition of exciplex systems exhibiting thermally activated delayed fluorescence (TADF) with a high reverse intersystem crossing (RISC) rate (exceeding 107 s−1). This approach involves constructing exciplex donor–acceptor molecular pairs, where the acceptor molecule possesses the ability to undergo fast and efficient intersystem crossing (ISC). With the use of 6-cyano-9-phenylpurine (PCP) acceptor and carbazole-based donor molecules, exciplexes were obtained, where the excitation is contained on PCP and undergoes fast ISC to form a local excited triplet state (3LEA). The controlled excitation transfer to the 3LEA level provides an optimal reverse int…
Thermally Activated Superradiance and Intersystem Crossing in the Water-Soluble Chlorophyll Binding Protein
2009
The crystal structure of the class IIb water-soluble chlorophyll binding protein (WSCP) from Lepidium virginicum is used to model linear absorption and circular dichroism spectra as well as excited state decay times of class IIa WSCP from cauliflower reconstituted with chlorophyll (Chl) a and Chl b. The close agreement between theory and experiment suggests that both types of WSCP share a common Chl binding motif, where the opening angle between pigment planes in class IIa WSCP should not differ by more than 10 degrees from that in class IIb. The experimentally observed (Schmitt et al. J. Phys. Chem. B 2008, 112, 13951) decrease in excited state lifetime of Chl a homodimers with increasing …
Photoinduced Formation Mechanism of the Thymine−Thymine (6−4) Adduct
2013
The photoinduced mechanism leading to the formation of the thymine-thymine (6-4) photolesion has been studied by using the CASPT2//CASSCF approach over a dinucleotide model in vacuo. Following light absorption, localization of the excitation on a single thymine leads to fast singlet-triplet crossing that populates the triplet (3)(nπ*) state of thymine. This state, displaying an elongated C(4)═O bond, triggers (6-4) dimer formation by reaction with the C(5)═C(6) double bond of the adjacent thymine, followed by a second intersystem crossing, which acts as a gate between the excited state of the reactant and the ground state of the photoproduct. The requirement of localized excitation on just …
Photoinduced intersystem crossing in DNA oxidative lesions and epigenetic intermediates
2020
[EN] The propensity of 5-formyluracil and 5-formylcytosine, i.e. oxidative lesions and epigenetic intermediates, in acting as intrinsic DNA photosensitizers is unraveled by using a combination of molecular modeling, simulation and spectroscopy. Exploration of potential energy surfaces and non-adiabatic dynamics confirm a higher intersystem crossing rate for 5-formyluracil, whereas the kinetic models evidence different equilibria in the excited states for both compounds.