Protonated Benzene Dimer: An Experimental and Ab Initio Study

Intramolecular hole-transfer in protonated anthracene

Excitation spectra of protonated and deuteronated anthracene are obtained by triple-resonance dissociation spectroscopy. Very cold cations, protonated/deuteronated exclusively at the 9-position, are generated from two-colour two-photon threshold ionisation of 9-dihydroanthracenyl radicals (C₁₄H₁₁). The excitation spectra reveal rich structure, not resolved in previous studies, that is assigned based on anharmonic and Herzberg-Teller coupling calculations. This work reveals that the excitation of protonated anthracene induces a symmetry-breaking intramolecular charge-transfer process along a Marcus-Hush coordinate, where the positively charged hole hops from the central bridging sp² carbon, onto one of the aromatic rings. Signatures of this charge-transfer event are observed in the excitation spectrum, through active Herzberg-Teller progressions.

Multi-stimuli responsive properties switch by intra- and inter-molecular charge transfer constructed from triphenylamine derivative

The compound TPA-BI exhibited multi-responsive fluorescence behaviors caused by inter-molecular charge transfer (CT) and intra-molecular CT formation.

Photophysics of Protonated and Microhydrated 2-Aminobenzaldehyde: Theoretical Insights into Photoswitchability of Protonated Systems

The photoswitchability of a protonated aromatic compound (2-aminobenzaldehyde, 2ABZH⁺) in its individual and microhydrated states has been addressed based on the RI-MP2/RI-CC2 theoretical methods. Our calculated results give insight into the ultrafast nonradiative deactivation mechanism of the 2ABZH⁺, driven by a conical intersection between the S₁/ S₀ potential energy surfaces. Also, it has been predicted that protonation accompanies a significant blue shift effect on the first ¹ππ* excited state of 2ABZ by 0.87 eV (at least 50 nm).

Influence of hydrogen bonds on edge-to-face interactions between pyridine molecules

Edge-to-face interactions between two pyridine molecules and the influence of simultaneous hydrogen bonding of one or both of the pyridines to water on those interactions were studied by analyzing data from ab initio calculations. The results show that the edge-to-face interactions of pyridine dimers that are hydrogen bonded to water are generally stronger than those of non-H-bonded pyridine dimers, especially when the donor pyridine forms a hydrogen bond. The binding energy of the most stable edge-to-face interacting H-bonded pyridine dimer is -5.05 kcal/mol, while that for the most stable edge-to-face interacting non-H-bonded pyridine dimer is -3.64 kcal/mol. The interaction energy data obtained in this study cannot be explained solely by the differences in electrostatic potential between pyridine and the pyridine-water dimer. However, the calculated cooperative effect can be predicted using electrostatic potential maps.

Understanding proton capture and cation-induced dimerization of [Ag29(BDT)12]3−clusters by ion mobility mass spectrometry

We present a unique reaction of [Ag₂₉(BDT)₁₂]³⁻cluster with protons and dimerization of the cluster induced by alkali metal ions.

Stereochemistry-dependent structure of hydrogen-bonded protonated dimers: the case of 1-amino-2-indanol

Stereochemistry effects on the structure of molecular aggregates are studied in the prototypical 1-amino-2-indanol. Conformer-selective IR-UV double resonance spectroscopy reveals how stereochemistry shapes its dimers.

Protonated serotonin: Geometry, electronic structures and photophysical properties

The geometry and electronic structures of protonated serotonin have been investigated by the aim of MP2 and CC2 methods. The relative stabilities, transition energies and geometry of sixteen different protonated isomers of serotonin have been presented. It has been predicted that protonation does not exhibit essential alteration on the S₁←S₀ electronic transition energy of serotonin. Instead, more complicated photophysical nature in respect to its neutral analogue is suggested for protonated system owing to radiative and non-radiative deactivation pathways. In addition to hydrogen detachment (HD), hydrogen/proton transfer (H/PT) processes from ammonium to indole ring along the NH⁺⋯π hydrogen bond have been predicted as the most important photophysical consequences of SERH⁺ at S₁ excited state. The PT processes is suggested to be responsible for fluorescence of SERH⁺ while the HD driving coordinate is proposed for elucidation of its nonradiative deactivation mechanism.

Electronically excited state of neutral/protonated, indole/5-hydroxyinodole–water clusters: a theoretical study

Simulated IR-spectra of neutral, protonated indole–water clusters calculated at the RI-MP2, RI-CC2/aug-cc-pVDZ level of theory at the ground and S₁ excited states.

A theoretical exploration on electronically excited states of protonated furan and thiophene

MPE profiles of the ground and S1 excited states of protonated furan along C–O bond-stretching reaction coordinate, computed at the RI-CC2/aug-cc-pVDZ level of theory.

Ultraviolet photodissociation action spectroscopy of gas-phase protonated quinoline and isoquinoline cations

Gas-phase two-photon UV action spectra of protonated quinoline and isoquinoline cations are reported revealing two broad, vibrationally-structured electronic bands and multiple photoproduct channels.

Excited states of proton-bound DNA/RNA base homodimers: pyrimidines

We are presenting the electronic photofragment spectra of the protonated pyrimidine DNA base homodimers. Only the thymine dimer exhibits a well structured vibrational progression, while the protonated monomer shows broad vibrational bands. This shows that proton bonding can block some nonradiative processes present in the monomer.

Excited state deactivation pathways of neutral/protonated anisole and p-fluoroanisole: a theoretical study

The potential energy profiles of neutral and protonated anisole and p-fluoroanisole at different electronic states have been investigated extensively by the RI-MP2 and RI-CC2 methods. The calculations reveal that the relaxation dynamics in protonated anisole and p-fluoroanisole are essentially different from those of the neutral analogues. In neutral anisole/p-fluoroanisole, the (1)πσ* state plays a vital relaxation role along the O-CH3 coordinate, yielding the CH3 radical. For both of these molecules, the calculations indicate conical intersections (CIs) between the ground and excited state potential energy (PE) curves, hindered by a small barrier, and providing non-adiabatic gates for radiation-less deactivation to the ground state. Nevertheless, for the protonated cases, besides the prefulvenic deformation of the benzene ring, it has been predicted that the lowest (1)(σ,n)π* state along the C-O-C bond angle plays an important role in photochemistry and the relaxation dynamics. The S1, S0 PE profiles of protonated anisole along with the former reaction coordinate (out-of-plane deformation) show a barrierless relaxation pathway, which can be responsible for the ultrafast deactivation of excited systems to the ground state via the low-lying S1/S0 conical intersection. Moreover, the later reaction coordinate in protonated species (C-O-C angle from 120°-180°) is consequently accompanied with the bond cleavage of C-OCH3 at the (1)(σ,n)π* state, hindered by a barrier of ∼0.51 eV, and can be responsible for the relaxation of excited systems with significant excess energy (hν≥ 5 eV). Furthermore, according to the RI-CC2 calculated results, different effects on the S1-S0 electronic transition energy of anisole and p-fluoroanisole upon protonation have been predicted. The first electronic transitions of anisole and p-fluoroanisole shift by ∼0.3 and 1.3 eV to the red respectively due to protonation.

Hydrogen bond strengthening of cis-trans glyoxal dimers in electronic excited states: a theoretical study

The second-order approximate coupled-cluster (CC2) method was performed to investigate the excited state hydrogen-bonding properties of Glyoxal (C2H2O2, Gl) dimers. Since the strengthening and weakening of hydrogen bonds can be investigated by monitoring the vibrational absorption spectra of some hydrogen-bonded groups in different electronic states, the infrared spectra of the hydrogen-bonded Gl-Gl complexes in both of the ground state and the S1 electronically excited state are calculated using the MP2/CC2 methods respectively. We demonstrated that the intermolecular hydrogen bond C=O⋯H-C between two glyoxal molecules is significantly strengthened in the electronically excited S1 state upon photoexcitation of the hydrogen-bonded Gl-Gl complexes.

Protonation effect on the electronic properties of 2-pyridone monomer, dimer and its water clusters: a theoretical study

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 micro-solvated 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 (1)πσ∗ 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 (1)πσ∗ predissociates the bound S1((1)ππ∗) state, connecting the latter to a conical intersection with the S0 state. These conical intersections lead the (1)ππ∗ state to proceed as predissociative state and finally direct the excited system to the ground state. Furthermore, in presence of water molecule, the (1)πσ∗ state still remains dissociative but the conical intersection between (1)πσ∗ 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.

Microhydration effects on the electronic properties of protonated phenol: a theoretical study

The CC2 (second-order approximate coupled cluster method) has been employed to investigate microhydration effect on electronic properties of protonated phenol (PhH(+)) According to the CC2 calculation results on electronic excited states of microhydrated PhH(+), for the S1 and S2 electronic states, which are of (1)ππ* nature and belong to the A' representation of molecular Cs point group, a significant blue shift effect on the S1 and S2 electronic states, which are of 1ππ* nature and belong to the A' representation of molecular Cs point 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 been found that the lowest (1)σπ* state plays a prominent role in the photochemistry of these systems. In the bare protonated phenol, the (1)σπ* state is a bound state with a broad potential curve along the OH stretching coordinate, while it is dissociative in microhydrated species. This indicates to a predissociation of the S1((1)ππ*) state by a low-lying (1)σπ* state, which leads the excited system to a concerted proton-transfer reaction from protonated chromophore to the solvent. The dissociative (1)σπ* state in monohydrated PhH(+) has small barrier, while increasing the solvent molecules up to three removes the barrier and consequently expedites the proton-transfer reaction dynamics.