Triplet absorption in carbon nanotubes: a TD-DFT study

We predict properties of triplet excited states in single-walled carbon nanotubes (CNTs) using a time-dependent density-functional theory (TD-DFT). We show that the lowest triplet state energy in CNTs to be about 0.2-0.3 eV lower than the lowest singlet states. Like in $\pi$-conjugated polymers, the lowest CNT triplets are spatially localized. These states show strong optical absorption at about 0.5-0.6 eV to the higher lying delocalized triplet states. These results demonstrate striking similarity of the electronic features between CNTs and $\pi$-conjugated polymers and provide explicit guidelines for spectroscopic detection of CNT triplet states

Photoinduced Intra- and Intermolecular Energy Transfer in Chlorophyll a Dimer

Applying nonadiabatic excited-state molecular dynamics, we investigate excitation energy transfer and exciton localization dynamics in a chlorophyll a (Chla) dimer system at the interface of two monomers of light-harvesting complex II trimer. After its optical excitation at the red edge of the Soret (B) band, the Chla dimer experiences an ultrafast intra- and intermolecular nonradiative relaxation process to the lowest band (Qy). The energy relaxation is found to run faster in the Chla dimer than in the Chla monomer. Once the molecular system reaches the lowest Qy band composed of two lowest excited states S1 and S2, the concluding relaxation step involves the S2 → S1 population transfer, resulting in a relatively slower relaxation rate. The strength of thermal fluctuations exceeds intraband electronic coupling between the states belonging to a certain band (B, Qx, and Qy), producing localized states on individual chromophores. Therefore, time evolution of spatial electronic localization during internal conversion reveals transient trapping on one of the Chla monomers participating in the events of intermonomeric energy exchange. In the phase space domains where electronic states are strongly coupled, these states become nearly degenerate promoting Frenkel-exciton-like delocalization and interchromophore energy transfer. As energy relaxation occurs, redistribution of the transition density on two Chla monomers leads to nearly equal distribution of the exciton among the molecules. For a single Chla, our analysis of excitonic dynamics reveals wave function amplitude transfer from nitrogen and outer carbon atoms to inner carbon atoms during nonradiative relaxation.Fil: Zheng, Fulu. Nanyang Technological University; SingapurFil: Fernández Alberti, Sebastián. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes; ArgentinaFil: Tretiak, Sergei. Los Alamos National Laboratory; Estados UnidosFil: Zhao, Yang. Nanyang Technological University; Singapu

Nonadiabatic excited-state molecular dynamics: treatment of electronic Decoherence

Within the fewest switches surface hopping (FSSH) formulation, a swarm of independent trajectories is propagated and the equations of motion for the quantum coefficients are evolved coherently along each independent nuclear trajectory. That is, the phase factors, or quantum amplitudes, are retained. At a region of strong coupling, a trajectory can branch into multiple wavepackets. Directly following a hop, the two wavepackets remain in a region of nonadiabatic coupling and continue exchanging population. After these wavepackets have sufficiently separated in phase space, they should begin to evolve independently from one another, the process known as decoherence. Decoherence is not accounted for in the standard surface hopping algorithm and leads to internal inconsistency. FSSH is designed to ensure that at any time, the fraction of classical trajectories evolving on each quantum state is equal to the average quantum probability for that state. However, in many systems this internal consistency requirement is violated. Treating decoherence is an inherent problem that can be addressed by implementing some form of decoherence correction to the standard FSSH algorithm. In this study, we have implemented two forms of the instantaneous decoherence procedure where coefficients are reinitialized following hops. We also test the energy-based decoherence correction (EDC) scheme proposed by Granucci et al. and a related version where the form of the decoherence time is taken from Truhlar's Coherent Switching with Decay of Mixing method. The sensitivity of the EDC results to changes in parameters is also evaluated. The application of these computationally inexpensive ad hoc methods is demonstrated in the simulation of nonradiative relaxation in two conjugated oligomer systems, specifically poly-phenylene vinylene and poly-phenylene ethynylene. We find that methods that have been used successfully for treating small systems do not necessarily translate to large polyatomic systems and their success depends on the particular system under study.Fil: Nelson, Tammie. Los Alamos National Laboratory; Estados UnidosFil: Fernández Alberti, Sebastián. Universidad Nacional de Quilmes; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Roitberg, Adrián. University of Florida; Estados UnidosFil: Tretiak, Sergei. Los Alamos National Laboratory; Estados Unido

Two-photon transitions in triazole based quadrupolar and octupolar chromophores: a TD-DFT investigation

C. Katan present address: CNRS UMR6082 FOTON, INSA de Rennes, 20 avenue des Buttes de Coësmes, CS 70839, 35708 RENNES cedex 7, FranceInternational audienceSimultaneous absorption of two photons has gained increasing attention over recent years as it opens the way for improved and novel technological capabilities. In the search for adequate materials that combine large two-photon absorption (TPA) responses and attributes suitable for specific applications, the multibranch strategy has proved to be efficient. Such molecular engineering effort, based on the gathering of several molecular units, has benefited from various theoretical approaches. Among those, the Frenkel exciton model has been shown to often provide a valuable qualitative tool to connect the optical properties of a multibranched chromophore to those of its monomeric counterpart. In addition, recent extensions of time-dependent density functional theory (TD-DFT) based on hybrid functionals have shown excellent performance for the determination of nonlinear optical (NLO) responses of conjugated organic chromophores and various substituted branched structures. In this paper, we use these theoretical approaches to investigate the one- and two-photon properties of triazole-based chromophores. In fact, experimental data were shown to reveal quite different behaviors as compared to related quadrupolar and octupolar compounds. Our theoretical findings allow elucidating these differences and contribute to the general understanding of structure-property relations. This work opens new perspectives towards synergic TPA architectures

Quantum chemistry of the minimal CdSe clusters

Colloidal quantum dots are semiconductor nanocrystals (NCs) which have stimulated a great deal of research and have attracted technical interest in recent years due to their chemical stability and the tunability of photophysical properties. While internal structure of large quantum dots is similar to bulk, their surface structure and passivating role of capping ligands (surfactants) are not fully understood to date. We apply ab initio wavefunction methods, density functional theory, and semiempirical approaches to study the passivation effects of substituted phosphine and amine ligands on the minimal cluster Cd2Se2, which is also used to benchmark different computational methods versus high level ab initio techniques. Full geometry optimization of Cd2Se2 at different theory levels and ligand coverage is used to understand the affinities of various ligands and the impact of ligands on cluster structure. Most possible bonding patterns between ligands and surface Cd/Se atoms are considered, including a ligand coordinated to Se atoms. The degree of passivation of Cd and Se atoms (one or two ligands attached to one atom) is also studied. The results suggest that B3LYP/LANL2DZ level of theory is appropriate for the system modeling, whereas frequently used semiempirical methods (such as AM1 and PM3) produce unphysical results. The use of hydrogen atom for modeling of the cluster passivating ligands is found to yield unphysical results as well. Hence, the surface termination of II-VI semiconductor NCs with hydrogen atoms often used in computational models should probably be avoided. Basis set superposition error, zero-point energy, and thermal corrections, as well as solvent effects simulated with polarized continuum model are found to produce minor variations on the ligand binding energies. The effects of Cd-Se complex structure on both the electronic band gap (highest occupied molecular orbital-lowest unoccupied molecular orbital energy difference) and ligand binding energies are systematically examined. The role played by positive charges on ligand binding is also explored. The calculated binding energies for various ligands L are found to decrease in the order PMe3 \u3e OPH3 \u3e NH2Me \u3e = NH3 \u3e = NMe3 \u3e PMe3 \u3e PH3 for neutral clusters and OPMe3 \u3e OPH3 \u3e PMe3 \u3e = NMe3 \u3e = NH2Me \u3e = NH3 \u3e PH3 and OPMe3 \u3e OPH3 \u3e NH2Me \u3e = NMe3 \u3e = PMe3 \u3e = NH3 \u3e PH3 for single and double ligations of positively charged Cd2Se22+ cluster, respectively

A Diffusion Quantum Monte Carlo Approach to the Polaritonic Ground State

Making and using polaritonic states (i.e., hybrid electron-photon states) for chemical applications have recently become one of the most prominent and active fields that connects the communities of chemistry and quantum optics. Modeling of such polaritonic phenomena using ab initio approaches calls for new methodologies, leading to the reinvention of many commonly used electronic structure methods, such as Hartree-Fock, density functional, and coupled cluster theories. In this work, we explore the formally exact diffusion quantum Monte Carlo approach (DQMC) to obtain numerical solutions to the polaritonic ground state during the dissociation of the H$_2$ molecular system. We examine various electron-nuclear-photon properties throughout the dissociation, such as changes to the minimum of the cavity Born-Oppenheimer surface, the localization of the electronic wavefunction, and the average mode occupation. Finally, we directly compare our results to that obtained with state-of-the-art, yet approximate, polaritonic coupled cluster approaches

Position isomerism on one and two photon absorption in multibranched chromophores: a TDDFT investigation.

CK's Secondary address during the work : FOTON-INSA UMR6082 INSA de Rennes 20 Avenue des Buttes de Coësmes CS70839 F-35708 RENNES Cedex 7International audienceRecently, branching and click chemistry strategies have been combined to design a series of optically active chromophores built from triazole moieties. These triazole based multipolar chromophores have shown to be promising candidates for two-photon absorption (TPA) transparency optimization in perspective of optical limiting in the visible region. In this work, the nature of one- and two-photon absorption properties in a family of triazole based chromophores has been investigated using hybrid time dependent density functional theory (TD-DFT). We use recent extensions of TD-DFT to determine nonlinear optical responses, and Natural Transition Orbitals to analyze the underlying electronic processes. Our results are also interpreted in the framework of the Frenkel exciton model. In agreement with experimental data, we found that introducing triazole moiety into multibranched chromophores substantially modifies their optical behavior due to changes in electronic delocalization and charge-transfer properties between donating end-groups and branching center that can be controlled by the triazole ring. Structural conformations via modulation of the torsion between phenyl and triazole rings significantly alter excited state electronic structure. Moreover, isomer positioning also greatly influences both linear and nonlinear optical responses such as TPA. Our theoretical findings allow elucidating these differences and contribute to the general understanding of structure-property relations. Consequently, interplay of donor/acceptor strength, triazole regioisomerism, and branching are shown to provide flexible means allowing for precise tuning of both linear and nonlinear optical responses, thus opening new perspectives towards synergic TPA architectures