Search results for "Conical intersection"
showing 10 items of 44 documents
Essential on the Photophysics and Photochemistry of the Indole Chromophore by Using a Totally Unconstrained Theoretical Approach
2015
Indole is a chromophore present in many different molecules of biological interest, such as the essential amino acid tryptophan and the neurotransmitter serotonin. On the basis of CASPT2//CASSCF quantum chemical calculations, the photophysical properties of the system after UV irradiation have been studied through the exploration of the potential energy hypersurfaces of the singlet and triplet low-lying valence excited states. In contrast to previous studies, the present work has been carried out without imposing any restriction to the geometry of the molecule (C1 symmetry) and by performing minimum energy path calculations, which is the only instrument able to provide the lowest-energy evo…
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 …
Inverted energy gap law for the nonradiative decay in fluorescent floppy molecules: larger fluorescence quantum yields for smaller energy gaps
2019
A data survey on experimental fluorescence quantum yields of (multi)substituted dicyano-distyrylbenzenes in fluid solution evidences that non-radiative decay increases with the Franck-Condon energy (E-FC), being opposite to the conventional energy gap law. Quantum-chemistry indicates that this is controlled by access to the conical intersection (CI) following the Bell-Evans-Polanyi principle as a first-step approximation for this family of molecules; the variations in E-FC among the different compounds are found to be decisive, while those of E-CI are estimated to be weaker or even enhancing the effect. The current findings may have significant consequences for the design of molecules for o…
Excited-state non-radiative decay in stilbenoid compounds: An: Ab initio quantum-chemistry study on size and substituent effects
2019
In the framework of optoelectronic luminescent materials, non-radiative decay mechanisms are relevant to interpret efficiency losses. These radiationless processes are herein studied theoretically for a series of stilbenoid derivatives, including distyrylbenzene (DSB) and cyano-substituted distyrylbenzene (DCS) molecules in vacuo. Given the difficulties of excited-state reaction path determinations, a simplified computational strategy is defined based on the exploration of the potential energy surfaces (PES) along the elongation, twisting, and pyramidalization of the vinyl bonds. For such exploration, density functional theory (DFT), time-dependent (TD)DFT, and complete-active-space self-co…
Modelling Photoionisation in Isocytosine: Potential Formation of Longer‐Lived Excited State Cations in its Keto Form
2021
Abstract Studying the effects of UV and VUV radiation on non‐canonical DNA/RNA nucleobases allows us to compare how they release excess energy following absorption with respect to their canonical counterparts. This has attracted much research attention in recent years because of its likely influence on the origin of our genetic lexicon in prebiotic times. Here we present a CASSCF and XMS‐CASPT2 theoretical study of the photoionisation of non‐canonical pyrimidine nucleobase isocytosine in both its keto and enol tautomeric forms. We analyse their lowest energy cationic excited states including 2π+ , 2nO+ and 2nN+ and compare these to the corresponding electronic states in cytosine. Investigat…
Photoreactivity of Furocoumarins and DNA in PUVA Therapy: Formation of Psoralen−Thymine Adducts
2008
The mechanism of the [2 + 2] cycloaddition photoreaction of psoralen and a DNA nucleobase, thymine, cornerstone of the furocoumarin-based PUVA (psoralen + UVA radiation) phototherapy, has been studied by the quantum-chemical multiconfigurational CASPT2 method. Triplet- and singlet-mediated mono- and diadduct formations have been determined to take place via singlet-triplet crossings and conical intersections, correlated with the initially promoted triplet or singlet states in different possible reactive orientations. Pyroneside monoadducts are suggested to be formed in the triplet manifold of the system, and to be less prone to yield diadducts because of the properties of the monoadduct low…
The Role of Adenine Excimers in the Photophysics of Oligonucleotides
2009
Energies and structures of different arrangements of the stacked adenine homodimer have been computed at the ab initio CASPT2 level of theory in isolation and in an aqueous environment. Adenine dimers are shown to form excimer singlet states with different degrees of stacking and interaction. A model for a 2-fold decay dynamics of adenine oligomers can be supported in which, after initial excitation in the middle UV range, unstacked or slightly stacked pairs of nucleobases will relax by an ultrafast internal conversion to the ground state, localizing the excitation in the monomer and through the corresponding conical intersection with the ground state. On the other hand, long-lifetime intra…
Theoretical Insight into the Spectroscopy and Photochemistry of Isoalloxazine, the Flavin Core Ring
2006
The electronic singlet-singlet and singlet-triplet electronic transitions of the isoalloxazine ring of the flavin core are studied using second-order perturbation theory within the framework of the CASPT2//CASSCF protocol. The main features of the absorption spectrum are computed at 3.09, 4.28, 4.69, 5.00, and 5.37 eV. The lowest singlet (S1) and triplet (T1) excited states are found to be both of pi character with a singlet-triplet splitting of 0.57 eV. On the basis of the analysis of the computed spin-orbit couplings and the potential energy hypersurfaces built for the relevant excited states, the intrinsic mechanism for photoinduced population of T1 is discussed. Upon light absorption, e…
Multiple Decay Mechanisms and 2D-UV Spectroscopic Fingerprints of Singlet Excited Solvated Adenine-Uracil Monophosphate
2016
The decay channels of singlet excited adenine uracil monophosphate (ApU) in water are studied with CASPT2//CASSCF:MM potential energy calculations and simulation of the 2D-UV spectroscopic fingerprints with the aim of elucidating the role of the different electronic states of the stacked conformer in the excited state dynamics. The adenine 1La state can decay without a barrier to a conical intersection with the ground state. In contrast, the adenine 1Lb and uracil S(U) states have minima that are separated from the intersections by sizeable barriers. Depending on the backbone conformation, the CT state can undergo inter-base hydrogen transfer and decay to the ground state through a conical …
On the FCNS⇆FC(NS) reaction: A matrix isolation and theoretical study
2015
Abstract The FCNS ⇆ FC(NS) photoisomerization process is a simple model system for molecular switches. Here, we examined the switching processes by experimental and theoretical methods. Prior matrix-isolation IR spectroscopic studies were complemented by matrix-isolation UV spectroscopic measurements to assist the interpretation of the mechanism of the ring closure and opening processes and to verify the accuracy of the computations on the vertical excitation energies. Vertical excitation energies were computed by the EOMEE-CCSD, MCSCF, and MR-CISD methods. Conical intersections were also searched for and three conical intersections along the reaction path FCNS → FC(NS) were located, one co…