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We present a comprehensive computational study describing the excited state dynamics and consequent photostability of amino-cyclohexenone (ACyO), the central template of mycosporine systems, widely recognized for their photoprotection of aquatic species. Photoexcitation to the first excited electronic state (S1, 1nπ*) of ACyO is considered an optically dark transition, while photoexcitation to the second excited electronic state (S21ππ*) is an optically bright 1ππ* transition and largely responsible for UV absorption properties of this molecule. We show that following initial photoexcitation to S2, ACyO relaxes via two competing deactivation mechanisms, each mediated by an S1/S0 conical intersection, which directs the excited state population to the electronic ground state (S0). Our ab initio computational results are supported with nonadiabatic dynamics simulation results, yielding an excited state lifetime of ∼280 fs for this system in vacuo. These results explain the inherent photostability of this core structure, commonplace in a wide range of microorganisms. [ABSTRACT FROM AUTHOR]

The MP2/CC2 and CASSCF theoretical approaches have been employed to determine the excited state proton transfer and photophysical nature of the four organic compounds, having the main frame of hydroxyphenyl-imidzaopyridine (HPIP). The nitrogen insertion effect, in addition to amine (-NH2) substitution has been investigated extensively by following the transition energies and deactivation pathways of resulted HPIP derivatives. It has been predicted that the excited state intramolecular proton transfer with or without small barrier is the most important feature of these compounds. Also, for all of the considered HPIP derivatives, a conical intersection (CI) between ground and the S1 excited state has been predicted. The strong non-adiabatic coupling in the CI (S1/S0), drives the system back to the ground state in which the proton may either return to the phenoxy unit and thus close the photocycle, or the system can continue the twisting motion that results in formation of a γ-photochromic species. This latter species can be responsible for photochromism of HPIP derivative systems. [ABSTRACT FROM AUTHOR]

Vibrational and electronic photodissociation spectra of mass-selected protonated benzaldehyde- (water)n clusters, [BZ-(H2O)n]H+ with n ≤ 5, are analyzed by quantum chemical calculations to determine the protonation site in the ground electronic state (S0) and ππ ∗ excited state (S1) as a function of microhydration. IR spectra of [BZ-(H2O)n]H+ with n ≤ 2 are consistent with BZH+-(H2O)n type structures, in which the excess proton is localized on benzaldehyde. IR spectra of clusters with n ≥ 3 are assigned to structures, in which the excess proton is located on the (H2O)n solvent moiety, BZ-(H2O)nH+. Quantum chemical calculations at the B3LYP, MP2, and ri-CC2 levels support the conclusion of proton transfer from BZH+ to the solvent moiety in the S0 state for hydration sizes larger than the critical value nc = 3. The vibronic spectrum of the S1 ← S0 transition (ππ ∗) of the n = 1 cluster is consistent with a cis-BZH+-H2O structure in both electronic states. The large blueshift of the S1 origin by 2106 cm−1 upon hydration with a single H2O ligand indicates that the proton affinity of BZ is substantially increased upon S1 excitation, thus strongly destabilizing the hydrogen bond to the solvent. The adiabatic S1 excitation energy and vibronic structure calculated at the ri-CC2/aug-cc-pVDZ level agrees well with the measured spectrum, supporting the notion of a cis-BZH+-H2O geometry. The doubly hydrated species, cis-BZH+-(H2O)2, does not absorb in the spectral range of 23 000–27 400 cm−1, because of the additional large blueshift of the ππ ∗ transition upon attachment of the second H2O molecule. Calculations predict roughly linear and large incremental blueshifts for the ππ ∗ transition in [BZ-(H2O)n]H+ as a function of n. In the size range n ≥ 3, the calculations predict a proton transfer from the (H2O)nH+ solvent back to the BZ solute upon electronic ππ ∗ excitation. [ABSTRACT FROM AUTHOR]

The CC2 (second order approximate coupled cluster method) has been applied to investigate protonation effect on electronic transition energies of 2-pyridone (2PY), 2-pyridone dimer, and microsolvated 2-pyridone (0-2 water molecules). The PE profiles of protonated 2-pyridone (2PYH+) as well as monohydrated 2PYH+ at the different electronic states have been investigated. The ¹πσ* state in protonated species (2PYH+) is a barrier free and dissociative state along the O-H stretching coordinate. In this reaction coordinate, the lowest lying ¹πσ* predissociates the bound S1(¹ππ*) state, connecting the latter to a conical intersection with the S0 state. These conical intersections lead the ¹ππ* state to proceed as predissociative state and finally direct the excited system to the ground state. Furthermore, in presence of water molecule, the ¹πσ* state still remains dissociative but the conical intersection between ¹πσ* and ground state disappears. In addition, according to the CC2 calculation results, it has been predicted that protonation significantly blue shifts the S1-S0 electronic transition of monomer, dimer, and microhydrated 2-pyridone. [ABSTRACT FROM AUTHOR]

An experimental and ab initio study on the excitation spectrum of the protonated benzene dimer and the neutral iso-electronic benzene dimer is reported. The absorption spectrum of protonated benzene dimer which extended in the visible spectral region indicated that the first excited state is charge transfer state.

The interaction of a water molecule with ferric heme—iron protoporphyrin ([PP FeIII]+) has been investigated in the gas phase in an ion trap and studied theoretically by density functional theory. It is found that the interaction of water with ferric heme leads to a stable [PP-FeIII–H2O]+ complex in the intermediate spin state (S = 3/2), in the same state as its unligated [PP-FeIII]+ homologue, without spin crossing during water attachment. Using the Van't Hoff equation, the reaction enthalpy for the formation of a Fe–OH2 bond has been determined for [PP-FeIII–H2O]+ and [PP-FeIII–(H2O)2]+. The corrected binding energy for a single Fe–H2O bond is −12.2 ± 0.6 kcal mol−1, while DFT calculations at the OPBE level yield −11.7 kcal mol−1. The binding energy of the second ligation yielding a six coordinated FeIII atom is decreased with a bond energy of −9 ± 0.9 kcal mol−1, well reproduced by calculations as −7.1 kcal mol−1. However, calculations reveal features of a weaker bond type, such as a rather long Fe–O bond with 2.28 Å for the [PP-FeIII–H2O]+ complex and the absence of a spin change by complexation. Thus despite a strong bond with H2O, the FeIII atom does not show, through theoretical modelling, a strong acceptor character in its half filled 3dz2 orbital. It is also observed that the binding properties of H2O to hemes seem strikingly specific to ferric heme and we have shown, experimentally and theoretically, that the affinity of H2O for protonated heme [H PP-Fe]+, an intermediate between FeIII and FeII, is strongly reduced compared to that for ferric heme. [ABSTRACT FROM AUTHOR]