Excited State Energy Transfer Processes Research Articles

Ultrafast Excited-State Energy Transfer in Phenylene Ethynylene Dendrimer: Quantum Dynamics with the Tensor Network Method.

Photoinduced excited-state energy transfer (EET) processes play an important role in solar energy conversions. Owing to their excellent photoharvesting and exciton-transport properties, phenylene ethynylene (PE) dendrimers display great potential for improving the efficiency of solar cells. In this work, we investigated the intramolecular EET dynamics in a dendrimer composed of two linear PE units (2-ring and 3-ring) using a fully quantum description based on the tensor network method. We first constructed a diabatic model Hamiltonian based on the electronic structure calculations. Using this diabatic vibronic coupling model, we tried to obtain the main features of the EET dynamics in terms of the several diabatic models with different numbers of vibrational modes (from 4 modes to 129 modes) and to explore the corresponding vibronic coupling interactions. The results show that the EET in this PE dendrimer is ultrafast. Four modes of A' symmetry play dominant roles in the dynamics; the remaining 86 modes of A' symmetry can dampen the electronic coherence; and the modes of A″ symmetry do not exhibit significant influence on the EET process. Overall, the first-order intrastate vibronic coupling terms show the dominant role in the EET dynamics, while the second-order intrastate vibronic coupling terms cause damping of the electronic coherence and slow down the overall EET process. This work provides a microscopic understanding of the EET dynamics in PE dendrimers.

Bending a photonic wire into a ring.

Natural light-harvesting systems absorb sunlight and transfer its energy to the reaction centre, where it is used for photosynthesis. Synthetic chromophore arrays provide useful models for understanding energy migration in these systems. Research has focused on mimicking rings of chlorophyll molecules found in purple bacteria, known as 'light-harvesting system 2'. Linear meso-meso linked porphyrin chains mediate rapid energy migration, but until now it has not been possible to bend them into rings. Here we show that oligo-pyridyl templates can be used to bend these rod-like photonic wires to create covalent nanorings that consist of 24 porphyrin units and a single butadiyne link. Their elliptical conformations have been probed by scanning tunnelling microscopy. This system exhibits two excited state energy transfer processes: one from a bound template to the peripheral porphyrins and one, in the template-free ring, from the exciton-coupled porphyrin array to the π-conjugated butadiyne-linked porphyrin dimer segment.

Quantum Dynamics Simulations of Excited State Energy Transfer in a Zinc–Free-Base Porphyrin Dyad

Rational design of artificial light-harvesting molecular architectures entails building systems that absorb strongly in the visible and near-IR region of the electromagnetic spectrum and also funnel excited state energy to a single site. The ability to model nonadiabatic processes, such as excited-state energy transfer (EET), that occur on a picosecond time scale can aid in the development of novel artificial light-harvesting arrays. A combination of density functional theory (DFT), time-dependent DFT, tight-binding molecular dynamics, and quantum dynamics is employed here to simulate EET in the ZnFbΦ dyad, a model artificial light-harvesting array that undergoes EET with an experimentally measured rate constant of (3.5 ps)-1 upon excitation at 550 nm in toluene [ Yang et al. J. Phys. Chem. B 1998 , 102 , 9426 - 9436 ]. We find that to successfully simulate the EET process, it is important to (1) include coupling between nuclear and electronic degrees of freedom in the QD simulation, (2) account for Coulomb coupling between the electron and hole wavepackets, and (3) parametrize the extended Hückel model Hamiltonian employed in the QD simulations with respect to the DFT.

Photoinduced Ultrafast Intramolecular Excited-State Energy Transfer in the Silylene-Bridged Biphenyl and Stilbene (SBS) System: A Nonadiabatic Dynamics Point of View.

The photoinduced intramolecular excited-state energy-transfer (EET) process in conjugated polymers has received a great deal of research interest because of its important role in the light harvesting and energy transport of organic photovoltaic materials in photoelectric devices. In this work, the silylene-bridged biphenyl and stilbene (SBS) system was chosen as a simplified model system to obtain physical insight into the photoinduced intramolecular energy transfer between the different building units of the SBS copolymer. In the SBS system, the vinylbiphenyl and vinylstilbene moieties serve as the donor (D) unit and the acceptor (A) unit, respectively. The ultrafast excited-state dynamics of the SBS system was investigated from the point of view of nonadiabatic dynamics with the surface-hopping method at the TDDFT level. The first two excited states (S1 and S2) are characterized by local excitations at the acceptor (vinylstilbene) and donor (vinylbiphenyl) units, respectively. Ultrafast S2-S1 decay is responsible for the intramolecular D-A excitonic energy transfer. The geometric distortion of the D moiety play an essential role in this EET process, whereas the A moiety remains unchanged during the nonadiabatic dynamics simulation. The present work provides a direct dynamical approach to understand the ultrafast intramolecular energy-transfer dynamics in SBS copolymers and other similar organic photovoltaic copolymers.

Carotenoids as electron or excited-state energy donors in artificial photosynthesis: an ultrafast investigation of a carotenoporphyrin and a carotenofullerene dyad

Photophysical investigations of molecular donor-acceptor systems have helped elucidate many details of natural photosynthesis and revealed design principles for artificial photosynthetic systems. To obtain insights into the factors that govern the partition between excited-state energy transfer (EET) and electron transfer (ET) processes among carotenoids and tetrapyrroles and fullerenes, we have designed artificial photosynthetic dyads that are thermodynamically poised to favor ET over EET processes. The dyads were studied using transient absorption spectroscopy with ∼100 femtosecond time resolution. For dyad , a carotenoporphyrin, excitation to the carotenoid S2 state induces ultrafast ET, competing with internal conversion (IC) to the carotenoid S1 state. In addition, the carotenoid S1 state gives rise to ET. In contrast with biological photosynthesis and many artificial photosynthetic systems, no EET at all was detected for this dyad upon carotenoid S2 excitation. Recombination of the charge separated state takes place in hundreds of picoseconds and yields a triplet state, which is interpreted as a triplet delocalized between the porphyrin and carotenoid moieties. In dyad , a carotenofullerene, excitation of the carotenoid in the S2 band results in internal conversion to the S1 state, ET and probably EET to fullerene on ultrafast timescales. From the carotenoid S1 state EET to fullerene occurs. Subsequently, the excited-state fullerene gives rise to ET from the carotenoid to the fullerene. Again, the charge separated state recombines in hundreds of picoseconds. The results illustrate that for a given rate of EET, the ratio of ET to EET can be controlled by adjusting the driving force for electron transfer.

Effects of Mo on phase structure and up-conversion emissions of Er:Al2O3 nanocrystals

A simple and efficient approach was presented to enhance up-conversion emissions significantly for the Er:Al2O3 nanocrystals by Mo support (Er-Mo:Al2O3) with a 976 nm laser diode excitation. Mo support had evident effects on the phase structure and up-conversion emissions for the Er:Al2O3 nanocrystals, which promoted the θ-(Al,Er)2O3 transformed to α-(Al,Er,Mo)2O3 phase. Compared with the Er:Al2O3, the maximal green and red up-conversion emissions intensities increased about 3×103 and 1.4×102 times for the Er-Mo:Al2O3 nanocrystals, respectively. It suggests that the enhancement of up-conversion emissions is caused by the high excited state energy transfer process from |4I15/2, 3T2> state of the Er3+-MoO42− dimer to the 4F7/2 level of Er3+.

PH Control of Intramolecular Energy Transfer and Oxygen Quenching in Ru(II) Complexes Having Coupled Electronic Excited States

This work illustrates the control of excited state energy transfer processes via variation of pH in transition metal complexes. In these systems a Ru(II) complex having two carboxylated bipyridyl ligands is covalently linked to pyrene via one of two different pyrene derivitized bipyridyl ligands. The energy of the Ru to carboxy-bipyridine (3)MLCT state is pH dependent while the pyrene triplet energy remains unchanged with solution acidity. At pH 0 the (3)MLCT state is the lowest energy state, and as the pH is raised and the carboxy-bipyridyl ligands are successively deprotonated, the energy of the (3)MLCT state rises above that of the pyrene triplet, resulting in a significant increase in the lifetime of the observed emission. Detailed analysis of ultrafast and microsecond time-resolved excited state decays result in a description of excited state decay that involves initial equilibration of the (3)MLCT and pyrene triplet states followed by relaxation to the ground state. The lifetime of excited state decay is defined by the position of the equilibrium, going from 2 μs at pH 0 to >10 μs at higher pH as the equilibrium favors the pyrene triplet. In addition, quenching of the excited state by dissolved oxygen exhibits a pH dependence that parallels that of the excited state lifetime. The results illustrate the utility of exploiting excited state equilibria of this type in the development of highly effective luminescent oxygen sensors.

Exciplex Formation and Energy Transfer in a Self‐Assembled Metal–Organic Hybrid System

Exciting assemblies: A metal-organic self-assembly of pyrenebutyric acid (PBA), 1,10-phenanthroline (o-phen), and Mg(II) shows solid-state fluorescence originating from a 1:1 PBA-o-phen exciplex. This exciplex fluorescence is sensitized by another residual PBA chromophore through an excited-state energy-transfer process. The solvent polarity modulates the self-assembly and the corresponding exciplex as well as the energy transfer, resulting in tunable emission of the hybrid (see figure).

Highly Selective Recognition on Enatioselective of R(+) and S(−)-1,1-Bi(2-Naphthol) Using of Eu(dap)3 Complex as Photo Probe

The chiral recognition phenomenon was observed in enantioselective and excited-state energy transfer processes. Based on bimolecular luminescence quenching kinetics for a system containing chiral molecules, the quenching efficiency was evaluated by Stern-Volmer equation for a system containing a chiral R(+) and S(-) resolved quencher species. The utility of this methodology is confirmed by examining the enantio-selective excited-state quenching between Eu(dpa)(3) complex (where dpa = pyridine-2,6-dicarboxylate) acting as the energy donor to R(+)-1,1-Bi(2-Naphthol) and S(-)-1,1-Bi(2-naphthol) as the energy acceptor was studied in solution. The results of this study confirm the utility of luminescence measurements as a probe of chiral discriminatory behavior.

Excited-State Energy Transfer Processes in Phenylene- and Biphenylene-Linked and Directly-Linked Zinc(II) and Free-Base Hybrid Diporphyrins

The photoinduced energy transfer processes in 1,4-phenylene-, 1,3-phenylene, 1,2-phenylene, and 4,4‘-biphenylene-linked and directly-linked Zn(II)-free base porphyrin heterodimers in THF were investigated by femtosecond transient absorption spectroscopy. The energy transfer rates were compared between TPP-type and OEP-type heterodimers respectively as A2u-HOMO and A1u-HOMO subunits, for evaluating the relative contribution of the through-bond and through-space interactions. The rate difference becomes smaller with a decrease of spacer, more than 10 for 1,4-bis(phenylethynyl)phenylene and 1,4-diphenylethynylene, 4 for 4,4‘-biphenylene-linked heterodimer, and 3 for 1,3- and 1,4-phenylene-linked spacers. In the meso−meso directly-linked case, the energy transfer rates are the same ((0.55 ps)-1) for 5,5,15,15-tetrakis(3,5-bis(octyloxy)phenyl)-substituted and 5,5,15,15-tetrakis(pentafluorophenyl)-substituted heterodimers, featuring only a minor influence of the frontier orbital characteristics on the energy transfer rate. The energy transfer rates are identical (0.55 ps)-1 for the directly-linked meso−meso heterodimers substituted with 3,5-bis(octyloxy)phenyl and pentafluorophenyl groups regardless of the difference in the HOMO orbital symmetry characteristics, suggesting the predominant Coulombic interaction for the energy transfer in these close proximity porphyrin dimers. In the case of 1,2-phenylene-linked heterodimers, the choice of the peripheral substituents can lead to a state-to-state rapid energy transfer with a rate of (0.55 ps-1) for the TPP-type model or a delocalized excimer-like diporphyrin excited state for the OEP-type model. Collectively, these results indicate that even for the covalently-linked models the relative contribution of the through-space Coulombic interaction becomes increasingly important upon the decrease of the center-to-center separation. Especially, the fast and efficient energy transfer occurring in the directly-linked heterodimer illustrates that this porphyrin unit can be utilized as a good candidate for energy transfer functional arrays in molecular photonic devices.