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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]

Thiazolo-[4,5-d]-thiazo-frame (tztz) compounds are important heteroaromatic organic systems, which recently became a subject of several studies in the field of organic electronics and organic photovoltaics. The most important physical nature of these systems is reported to be an equilibrium between enol and keto forms following excited-state proton transfer. This process originates from a flat trend of the S1 PE (potential energy) profile along the proton transfer coordinate. In the present work, we determined and interpreted the excited-state proton transfer and photophysical nature of these systems extensively by means of the MP2/CC2 and CASSCF theoretical approaches. Also, the effects of amine (-NH2) and cyano (-CN) substitutions were taken into account comprehensively by considering the transition energies and proton transfer pathways of the resulting tztz derivatives. It has been predicted that the physical nature of the excited-state intramolecular proton transfer, as the main character of these systems, is being affected significantly by substitutions. For all of the considered tztz derivatives, a conical intersection (CI) between ground and the S1 excited state was predicted. This CI makes the considered species capable to be responsible for photochromism and photoswitching as well. [ABSTRACT FROM AUTHOR]

The geometry, electronic structures and potential energy profiles of protonated furan and thiophene have been extensively investigated, using the RI-MP2 and RI-CC2 methods. According to RI-CC2 calculated results, the adiabatic S1(1ππ*)–S0 transition energies of protonated furan and thiophene, have been predicted to be 4.41 eV and 3.70 eV respectively. Thus, protonation is accompanied by a large red shift effect on the first 1ππ* transition of the title systems (ΔE ＞ 1.5 eV). The significant spectral-movements, predicted based on the calculated results of this work, indicate an essential effect of protonation on the geometry, electronic structures and optical characters of the five membered heterocyclic systems. In addition, it has been found that excitation of protonated furan and thiophene, with sufficient excess energy above the band origin of S1(1ππ*)–S0 transition, is accompanied by the S–C or O–C bond breaking. This mechanism is mostly governed by a dissociative 1πσ* PE profile in both protonated systems. [ABSTRACT FROM AUTHOR]

TheRI-MP2 and RI-CC2 methods have been employed to determine thepotential energy profiles of neutral and protonated α-naphthol,in their individual forms and microhydrated with 1 and 3 water molecules,at different electronic states. According to calculated results, ithas been predicted that dynamics of nonradiative processes in protonatedα-naphthol is essentially different from that of its neutralhomologue. In protonated α-naphthol, the calculations revealthat 1σπ* state, is the most important photophysicalstate, having a bound nature with a broad potential curve along theOH coordinate of isolated system, while it is dissociative in monohydratedhomologue. In neutral system, similar to phenol, the 1πσ*state, plays the fundamental relaxation role along the O–Hstretching coordinate. Moreover, microhydration strongly affects thephotophysical properties of α-naphthol, mostly by alterationof the 1ππ* PE profile, from a bound statein an isolated analogue to a dissociative state in hydrated systems.Furthermore, it has been found that three water molecules are necessaryfor ground state proton transfer between protonated α-naphtholand water; with a small barrier; (ΔE< 0.1eV). [ABSTRACT FROM AUTHOR]

TheCC2 (second-order approximate coupled cluster method) has been employedto investigate microhydration effect on electronic properties of protonatedphenol (PhH+) According to the CC2 calculation resultson electronic excited states of microhydrated PhH+, forthe S1and S2electronic states, which are of 1ππ* nature and belong to the A′ representationof molecular Cspointgroup, a significant blue shift effect on the S1 and S2 electronicstates, which are of 1ππ* nature and belong to the A′representation of molecular Cspoint group,in comparison to corresponding transitions on bare cation (PhH+), has been predicted. Nevertheless, for the S3–S0(1A′′, 1σπ*) transition,a large red shift effect has been predicted. Furthermore, it has beenfound that the lowest 1σπ* state plays a prominentrole in the photochemistry of these systems. In the bare protonatedphenol, the 1σπ* state is a bound state witha broad potential curve along the OH stretching coordinate, whileit is dissociative in microhydrated species. This indicates to a predissociationof the S1(1ππ*) state by a low-lying 1σπ* state, which leads the excited system to aconcerted proton-transfer reaction from protonated chromophore tothe solvent. The dissociative 1σπ* state inmonohydrated PhH+has small barrier, while increasing thesolvent molecules up to three removes the barrier and consequentlyexpedites the proton-transfer reaction dynamics. [ABSTRACT FROM AUTHOR]

Abstract: The low-lying electronic excited states of protonated phenol and para-Fluorophenol have been investigated extensively by RI-MP2/RI-CC2 methods. Although, protonation of phenol leads to a small red-shift-effect on the S1–S0 (ππ∗) electronic transition in respect to its neutral homologue, a large red-shift-effect, on the same electronic transitions of para-substituted phenol has been predicted. The ππ∗ excited state of protonated phenol stays in the UV range (4.34eV), while its πσ∗ state lies in the VUV region (8.3eV). The S1 excited-state geometry optimization of protonated phenol predicted unstable S1 state owing to the strong out-of-plane deformation in the benzene ring. [Copyright &y& Elsevier]

Abstract: The excitation spectrum of protonated salicylaldehyde has been recorded in the 20,800–22,400cm−1 region (480–450nm). The first excited state of protonated salicylaldehyde is a ππ∗ state, largely red shifted as compared to the ππ∗ transition of its neutral analogue. Like protonated benzaldehyde and in contrast to some other protonated aromatic molecules such as benzene or tryptophan in which the excited state dynamics is so fast that no vibrational structure can be observed, the vibrational bands are well resolved and assigned. This molecule has many low energy isomers and the simulations of the electronic spectrum via ab initio excited state optimizations and Franck–Condon calculations are precise enough to assign the observed electronic spectrum to one of the isomers. [Copyright &y& Elsevier]

Abstract: The low lying singlet and triplet electronic excited states of neutral and protonated Acenaphthylene (C12H8, ACYN) and Acenaphthene (C12H10, ACN) have been investigated extensively by RI–MP2 and RI–CC2 methods. The first and second electronic excited tates (S1,S2) of protonated ACYN and ACN have ππ∗ nature and lie in the visible or UV region. Similar to naphthalene, anthracene, and other linear PAHs, the protonation of ACYN and ACN leads to a strong red shift of the electronic transition as compared to the neutral molecule. The calculations indicate a charged transfer character of S1–S0 transition in protonated ACYN and ACN as well as protonated naphthalene. [Copyright &y& Elsevier]

Abstract: Spectral shifts upon core protonation of a series of electron-rich and electron-deficient porphyrins with five and six membered rings at the meso-positions have been studied by UV–vis and 1H NMR spectroscopy. The direction of the shift of the Q(0,0) band of different porphyrins depends on the electron-donating or electron-withdrawing character of the meso-substituent and therefore it may be used to evaluate the relative electron donor ability of the substituents. Also, the shift of the Soret bands was found to be influenced to a lesser extent by the electron donor ability of substituted groups at the porphyrin periphery. Although, large red shifts of the Soret band of H4T(o-NO2)P2+, H4T(p-SCH3P)P2+ and H4T(p-NO2P)P2+ indicate a significant out-of-plane deformation of porphyrin core, negligible upfield shifts of the β protons in their 1H NMR spectra show that the practice of using the change in the chemical shift of the β protons as an indicator of structurally induced changes in the porphyrin ring current should be approached with more caution. Interestingly, high level ab initio and DFT calculations demonstrate that in spite of the better electron donating ability of –OCH3 to benzene ring compared to that of –SCH3, meso-(p-SCH3)phenyl group is a clearly better electron donor to the porphyrin core than meso-(p-OCH3)phenyl one. [Copyright &y& Elsevier]

The excitation spectrum of the protonated benzene dimer has been recorded in the 415-600 nm wavelength range. In contrast to the neutral iso-electronic benzene dimer, its absorption spectrum extends in the visible spectral region. This huge spectral shift has been interpreted with ab initio calculations, which indicate that the first excited states should be charge transfer states. [ABSTRACT FROM AUTHOR]