Vibronic coupling in the ground and excited states of the naphthalene cation

Delayed Relaxation of Highly Excited Cationic States in Naphthalene

Rapid energy transfer from electronic to nuclear degrees of freedom underlies many biological processes and astrophysical observations. The efficiency of this energy transfer depends strongly on the complex interplay between electronic and nuclear motions. In this study, we report two-color pump-probe experiments that probe the relaxation dynamics of highly excited cationic states of naphthalene, a prototypical polycyclic aromatic hydrocarbon molecule, which are produced using wavelength-selected, ultrashort extreme ultraviolet pulses. Surprisingly, the relaxation lifetimes increase with the cationic excitation energy. We postulate that the observed effect is the result of a population trapping that leads to delayed relaxation.

Non-adiabatic molecular dynamics with ΔSCF excited states

Accurate modelling of nonadiabatic transitions and electron-phonon interactions in extended systems is essential for understanding the charge and energy transfer in photovoltaic and photocatalytic materials. The extensive computational costs of the advanced excited state methods have stimulated the development of many approximations to study the nonadiabatic molecular dynamics (NA-MD) in solid-state and molecular materials. In this work, we present a novel ▵SCF-NA-MD methodology that aims to account for electron-hole interactions and electron-phonon back-reaction critical in modelling photoinduced nuclear dynamics. The excited states dynamics is described using the delta self-consistent field (▵SCF) technique within the density functional formalism and the trajectory surface hopping. The technique is implemented in the open-source Libra-X package freely available on the Internet (https://github.com/Quantum-Dynamics-Hub/Libra-X). This work illustrates the general utility of the developed ▵SCF-NA-MD methodology by characterizing the excited state energies and lifetimes, reorganization energies, photoisomerization quantum yields, and by providing the mechanistic details of reactive processes in a number of organic molecules.

Efficient Exciton Diffusion and Resonance-Energy Transfer in Multilayered Organic Epitaxial Nanofibers

Multilayered epitaxial nanofibers are exemplary model systems for the study of exciton dynamics and lasing in organic materials because of their well-defined morphology, high luminescence efficiencies, and color tunability. We use temperature-dependent continuous wave and picosecond photoluminescence (PL) spectroscopy to quantify exciton diffusion and resonance-energy transfer (RET) processes in multilayered nanofibers consisting of alternating layers of para-hexaphenyl (p6P) and α-sexithiophene (6T) serving as exciton donor and acceptor material, respectively. The high probability for RET processes is confirmed by quantum chemical calculations. The activation energy for exciton diffusion in p6P is determined to be as low as 19 meV, proving p6P epitaxial layers also as a very suitable donor material system. The small activation energy for exciton diffusion of the p6P donor material, the inferred high p6P-to-6T resonance-energy-transfer efficiency, and the observed weak PL temperature dependence of the 6T acceptor material together result in an exceptionally high optical emission performance of this all-organic material system, thus making it well suited, for example, for organic light-emitting devices.

A multilayer MCTDH study on the full dimensional vibronic dynamics of naphthalene and anthracene cations

Employing the multilayer multiconfiguration time-dependent Hartree (ML-MCTDH) method in conjunction with the multistate multimode vibronic coupling Hamiltonian model, we perform a full dimensional quantum dynamical study on the naphthalene (48D) and anthracene (66D) radical cations in their six lowest-lying doublet electronic states. For easily comparing results of full and reduced dimensionalities, MCTDH simulations based on larger sizes of primitive basis functions and single-particle functions than the previous ones [S. Ghanta, V. S. Reddy, and S. Mahapatra, Phys. Chem. Chem. Phys. 13, 14531 (2011)], are also performed. Extensive ML-MCTDH test calculations are performed to find appropriate ML separations of the wave functions (so-called ML-trees), and the convergence of the dynamical calculations are carefully checked. The ML-MCTDH method was developed for efficiently simulating quantum dynamics of large systems, and in fact the full dimensional ML-MCTDH calculations save a considerable amount of CPU-time in comparison with corresponding reduced dimensional MCTDH simulations. On basis of the present full and reduced dimensional simulations, the photoelectron (PE) spectra of these two cations are simulated and compared with corresponding experimental spectra. The agreement between theoretical and experimental PE spectra is good. Both full and reduced dimensional simulations give all main bands in the PE spectra. The vibronic energy-level positions from both ML-MCTDH and MCTDH calculations agree with corresponding experimental results. These quantum dynamical studies also complement the observations on diffuse interstellar bands with the wavelength of ~7088, ~6707, ~6490, ~6120, and ~5959 Å measured by astronomers as well as laboratory experimentalists.

First principles quantum dynamical investigation provides evidence for the role of polycyclic aromatic hydrocarbon radical cations in interstellar physics

Inspired by the recent astronomical discovery of new diffused interstellar bands (DIBs) assigned to the electronic transitions in the naphthalene radical cation based on complementary laboratory measurements, we attempt here an ab initio quantum dynamical study to validate this assignment. In addition, the existence and mechanistic details of nonradiative deactivation of electronically excited polycyclic aromatic hydrocarbon (PAH) radical cations in the interstellar medium and their identity as carriers of DIBs are established here focusing on the prototypical naphthalene and anthracene radical cations of the PAH family.

Hole-vibronic coupling in oligothiophenes: impact of backbone torsional flexibility on relaxation energies

Density functional theory calculations together with highly resolved gas-phase ultraviolet photoelectron spectroscopy have been applied to oligothiophene chains with up to eight thiophene rings. One of the important parameters governing the charge transport properties in the condensed phase is the amount of energy relaxation upon ionization. Here, we investigate the impact on this parameter of the backbone flexibility present in oligothiophenes as a result of inter-ring torsional motions. With respect to oligoacenes that are characterized by a coplanar and rigid backbone, the torsional flexibility in oligothiophenes adds to the relaxation energy and leads to the broadening of the first ionization peak, making its analysis more complex.

Vibronic Coupling in the Ground and Excited States of Oligoacene Cations

The vibrational coupling in the ground and excited states of positively charged naphthalene, anthracene, tetracene, and pentacene molecules is studied on the basis of a joint experimental and theoretical study of ionization spectra using high-resolution gas-phase photoelectron spectroscopy and first-principles correlated quantum-mechanical calculations. Our theoretical and experimental results reveal that, while the main contribution to relaxation energy in the ground state of oligoacene systems comes from high-energy vibrations, the excited-state relaxation energies show a significant redistribution toward lower-frequency vibrations. A direct correlation is found between the nature of the vibronic interaction and the pattern of the electronic state structure.

Assessment of Recently Developed Multicoefficient Strategies for the Treatment of π-Conjugated Molecules

Newly developed hybrid functionals (MPW1k and BB1k) have been systematically applied for the description of conjugation effects in organic molecules. These functionals are also used as part of the recently developed general-purpose multicoefficient methods MC3MPW and MC3BB. The performance of the various approaches is compared not only for relative energies but also through the calculation of torsion energy profiles for critical comparison with available reference data; thus, a numerical criterion depending on local behavior could be correspondingly defined. The results show that MC3-based methods are very accurate when faced to other approaches having comparable computational cost; thus, paving the way toward new applications and achievements in the field of conjugated materials.