Preparation and Photophysical Characterization of New Anthracene‐ and Tetracene‐Doped Acenaphthene Luminophores

A series of novel acenaphthene (Acnp) luminophores were prepared by doping varying concentrations of anthracene (AN) and perylene (Pery) by the solid-state reaction method, and studied for their fluorescence characteristics. The prepared Acnp luminophores were excited at the excitation wavelength of the host, Acnp. The fluorescence excitation energy transfer (EET) was observed in these bi- and tri-component luminophores. The complete quenching of weak violet fluorescence of Acnp was noted with the appearance of new AN like emission in bi-component while monomeric emission of Pery in tri-component luminophores. The prepared luminophores were further characterized by X-ray diffraction, TGA, and cyclic voltammetry to study structural parameters, thermal stability, and electrical properties, respectively. The particle size of the prepared luminophores was calculated by scanning electron microscopy (SEM). The present study reveals that the prepared novel Acnp luminophores show two-step EET from the host Acnp to AN and from AN to the second guest Pery in the Acnp matrix. The prepared luminophores could be used in organic light-emitting devices and in optoelectronic devices.

Luminophors of α-Naphthol doped with varying concentrations of anthracene and perylene were prepared by conventional solid state reaction method to explore its fluorescence characteristics. The fluorescence spectra of bicomponent and tricomponent luminophors reveals the fluorescence excitation energy transfer (FRET) process. The prepared bi and tri component luminophors were excited at 290nm which corresponds to host excitation wavelength. The weak violet fluorescence of host, α-Naphthol, gets quenched and give anthracene like emission in bicomponent luminophor systems. Further doping with second host, perylene, in anthracene containing α-Naphthol luminophors, red shifted emission was observed at 605nm. The structural parameters, thermal stability and electrical properties of doped luminophors were studied by using X-ray diffraction, TGA-DSC and Cyclic Voltammetry respectively. The particle size of the prepared luminophors was confirmed by scanning electron microscopy (SEM).

In photosynthetic organisms, light energy is converted into chemical energy through the light absorption and excitation energy transfer (EET) processes. These processes start in light-harvesting complexes, which contain special photosynthetic pigments. The exploration of unique mechanisms in light-harvesting complexes is directly related to studies, such as artificial photosynthesis or biosignatures in astrobiology. We examined, through ab initio calculations, the light absorption and EET processes using cluster models of light-harvesting complexes in purple bacteria (LH2). We evaluated absorption spectra and energy transfer rates using the LH2 monomer and dimer models to reproduce experimental results. After the calibration tests, a LH2 aggregation model, composed of 7 or 19 LH2s aligned in triangle lattice, was examined. We found that the light absorption is red shifted and the energy transfer becomes faster as the system size increases. We also found that EET is accelerated by exchanging the central pigments to lower energy excited pigments. As an astrobiological application, we calculated light absorptions efficiencies of the LH2 in different photoenvironments.

Understanding the fundamentals behind the photophysical response of a fluorescing species in the vicinity of plasmonic nanoparticles is of great interest due to the importance of this event in various applications. The present work has been carried out to throw light on how plasmonic nanoparticles electronically interact with non-plasmonic nanoparticles. Specifically, in this work, the excitation energy transfer (EET) from fluorescence bimetallic silver capped gold (F-AgAu) to gold nanoparticles (AuNPs) and how this process can be modulated by cetyltrimethylammonium bromide (CTAB) have been investigated at both ensemble average and single particle levels. Steady-state and time-resolved fluorescence studies have revealed that the fluorescence intensity and lifetime of F-AgAu in the presence of AuNPs are significantly quenched. Cyclic voltammetry (CV) and polarity-dependent studies have ruled out the possibility of an electron transfer mechanism. The increased non-radiative decay rate has substantiated that the photoluminescence quenching is due to excitation energy transfer from F-AgAu to AuNPs. Interestingly, investigations have revealed that the energy transfer efficiency is reduced from 87% to 28% in the presence of CTAB due to the formation of a CTAB bilayer over AuNPs. Analysis of the data by conventional EET, nano surface energy transfer (NSET), and stretched exponential models have firmly established that the EET process follows a 1/d4 distance dependence (NSET) rather than conventional 1/d6 distance dependence as predicted with the Förster resonance energy transfer model. Additionally, single particle level measurements through fluorescence lifetime imaging microscopy (FLIM) studies have clearly demonstrated that the surfactant (CTAB) can play an important role in controlling the EET process from non-plasmonic to plasmonic nanoparticles. The outcome of the present EET between two different classes of nanoparticles is expected to be useful in developing nanoscale systems for various optoelectronic applications.

The bridging ligands in d-f bimetallic complexes play an important role in the excitation energy transfer (EET) process. To elaborate on the effect of the ligand on the EET process, a series of bridging ligands (μ-L), 1-(1',10'-phenanthrolin-2'-yl)-4,4,4-trifluorobutane-1,3-dione (phen3f), 1-(1',10'-phenanthrolin-2'-yl)-4,4,5,5,5-pentafluoropentane-1,3-dione (phen5f), 1-(2,2'-bipyridine-6-yl)-4,4,4-trifluorobutane-1,3-dione (bpy3f), and 1-(2,2'-bipyridine-6-yl)-4,4,5,5,5-pentafluoropentane-1,3-dione (bpy5f), and their corresponding iridium complexes, [(dfppy)2 Ir(μ-L)] (dfppy=2-(4',6'-difluorophenyl)pyridinato-N,C2'), as well as their corresponding heteroleptic IrIII -EuIII complexes [{(dfppy)2 Ir(μ-L)}3 EuCl]Cl2 were synthesized and characterized. Photophysical and kinetic results revealed that the alternation of the bridge ligand resulted in a systemic difference in the lowest triplet-state energy (T1 ) of the iridium complexes, the EET efficiency from iridium complexes to the EuIII ion, and a significant difference in the total luminescence quantum yields. Based on the nanosecond time-resolved phosphorescence spectra, a model for the energy-transfer mechanism was proposed for d-f bimetallic complexes, which indicated that the nonradiative relaxation of the excited energy of EuIII , especially energy dissipation by means of the T1 state, was the main reason for the discrepancy in the quantum yields of the four IrIII -EuIII complexes.

Effects of hydrogen bonding interaction and solvent polarity on the competition between excitation energy and photo-induced electron transfer processes in hydroxy(1-pyrenebutoxy)phosphorus(V)porphyrin

The dynamics of delocalized excitons in light-harvesting complexes (LHCs) can be investigated using different experimental techniques, and transient absorption (TA) spectroscopy is one of the most valuable methods for this purpose. A careful interpretation of TA spectra is essential for the clarification of excitation energy transfer (EET) processes occurring during light-harvesting. However, even in the simplest LHCs, a physical model is needed to interpret transient spectra as the number of EET processes occurring at the same time is very large to be disentangled from measurements alone. Physical EET models are commonly built by fittings of the microscopic exciton Hamiltonians and exciton-vibrational parameters, an approach that can lead to biases. Here, we present a first-principles strategy to simulate EET and transient absorption spectra in LHCs, combining molecular dynamics and accurate multiscale quantum chemical calculations to obtain an independent estimate of the excitonic structure of the complex. The microscopic parameters thus obtained are then used in EET simulations to obtain the population dynamics and the related spectroscopic signature. We apply this approach to the CP29 minor antenna complex of plants for which we follow the EET dynamics and transient spectra after excitation in the chlorophyll b region. Our calculations reproduce all the main features observed in the transient absorption spectra and provide independent insight on the excited-state dynamics of CP29. The approach presented here lays the groundwork for the accurate simulation of EET and unbiased interpretation of transient spectra in multichromophoric systems.

Neorhodopsin (NeoR) is a newly discovered fungal bistablerhodopsinthat reversibly photoswitches between UV- and near-IR absorbing statesdenoted NeoR367 and NeoR690, respectively. NeoR367 represents a deprotonated retinal Schiff base (RSB), whileNeoR690 represents a protonated RSB. Cryo-EM studies indicatethat NeoR forms homodimers with 29 Å center-to-center distancebetween the retinal chromophores. UV excitation of NeoR367 takes place to an optically allowed S3 state of 1Bu+ symmetry, which rapidly converts to a low-lying opticallyforbidden S1 state of 2Ag– symmetry in39 fs, followed by a multiexponential decay to the ground state onthe 1–100 ps time scale. A theoretically predicted nπ*(S2) state does not get populated in any appreciable transient concentrationduring the excited-state relaxation cascade. We observe an intradimerretinal to retinal excitation energy transfer (EET) process from theNeoR367 S1 state to NeoR690, in competitionwith photoproduct formation. To quantitatively assess the EET mechanismand rate, we experimentally addressed and modeled the EET processunder varying NeoR367-NeoR690 photoequilibriumconditions and determined the EET rate at (200 ps)−1. The NeoR367 S1 state shows a weak stimulated emissionband in the near-IR around 700 nm, which may result from mixing withan intramolecular charge-transfer (ICT) state, enhancing the transitiondipole moment of the S1–S0 transition and possibly facilitatingthe EET process. We suggest that EET may bear general relevance tothe function of bistable multiwavelength rhodopsin oligomers.

The experimental observation of long-lived quantum coherence in the excitation energy transfer (EET) process of the several photosynthetic light-harvesting complexes at low and room temperatures has aroused hot debate. It challenges the common perception in the field of complicated pigment molecular systems and evokes considerable theoretical efforts to seek reasonable explanations. In this work, we investigate the coherent exciton dynamics of the phycoerythrin 545 (PE545) complex. We use the dissipation equation of motion to theoretically investigate the effect of the local pigment vibrations on the population transfer process. The result indicates that the realistic local pigment vibrations do assist the energy transmission. We demonstrate the coherence between different pigment molecules in the PE545 system is an essential ingredient in the EET process among various sites. The coherence makes the excitation energy delocalized, which leads to the redistribution of the excitation among all the chromophores in the steady state. Furthermore, we investigate the effects of the complex high-frequency spectral density function on the exciton dynamics and find that the high-frequency Brownian oscillator model contributes most to the exciton dynamic process. The discussions on the local pigment vibrations of the Brownian oscillator model suggest that the local heterogeneous protein environments and the effects of active vibration modes play a significant role in coherent energy transport.

The number of anthracene (AN) and anthracene-tetracene (AN-TN) doped 4-nitrophenol (4-NP) luminophors (AN/4-NP and AN-TN/4-NP) were prepared at different proportion by conventional technique called solid state reaction. Excitation energy transfer was observed by fluorescence spectroscopy and cyclic voltammetry. 4-NP acts as an outstanding light emitting matrix. The effect of the donor emission was perceived by changing the dopant concentration; this showed that AN/4-NP and TN-AN/4-NP exhibit fluorescence emission ranging from blue to green i.e. in the range from 400 to 525nm and 535-550nm, respectively. Structural properties and thermal stability was studied by XRD, SEM and TGA-DSC techniques. The study reveals that the prepared materials show excellent properties which can meet the demands of optoelectronic devices as well as for light emitting devices.

In this paper, we model the excitation energy transfer (EET) of photosystem I (PSI) of the common pea plant Pisum sativum as a complex interacting network. The magnitude of the link energy transfer between nodes/chromophores is computed by Forster resonant energy transfer (FRET) using the pairwise physical distances between chromophores from the PDB 5L8R (Protein Data Bank). We measure the global PSI network EET efficiency adopting well-known network theory indicators: the network efficiency (Eff) and the largest connected component (LCC). We also account the number of connected nodes/chromophores to P700 (CN), a new ad hoc measure we introduce here to indicate how many nodes in the network can actually transfer energy to the P700 reaction centre. We find that when progressively removing the weak links of lower EET, the Eff decreases, while the EET paths integrity (LCC and CN) is still preserved. This finding would show that the PSI is a resilient system owning a large window of functioning feasibility and it is completely impaired only when removing most of the network links. From the study of different types of chromophore, we propose different primary functions within the PSI system: chlorophyll a (CLA) molecules are the central nodes in the EET process, while other chromophore types have different primary functions. Furthermore, we perform nodes removal simulations to understand how the nodes/chromophores malfunctioning may affect PSI functioning. We discover that the removal of the CLA triggers the fastest decrease in the Eff, confirming that CAL is the main contributors to the high EET efficiency. Our outcomes open new perspectives of research, such comparing the PSI energy transfer efficiency of different natural and agricultural plant species and investigating the light-harvesting mechanisms of artificial photosynthesis both in plant agriculture and in the field of solar energy applications.

The observation of bridge-mediated excitation energy transfer (EET) has raised questions on the physical origin of such an effect. In this work, we studied the effect of bridge fragments in the Coulomb coupling, the major contribution to the electronic coupling in an EET process. For a series of ortho-phenyleneethynylene oligomers spaced donor-acceptors, we found that a large influence of the bridge fragment in EET coupling is through changes in the Coulomb couplings. Both enhancement and screening effects of the bridge were observed as the EET rates were modified by a factor of 0.3-23 with an intervening bridge in our calculations. The dependency of EET couplings on the orientation of transition dipoles of the donor and acceptor from quantum mechanical computations is very similar to that of a simple classical dielectric model. Our work shows that the bridge fragments can modify the Coulomb coupling with their polarizability by providing an optical dielectric medium between the donor and acceptor. In particular, when the transition dipoles of the donor and acceptor were longitudinal to a polarizable bridge, the EET rates were enhanced by one order of magnitude, as compared to the values of through-space models. Our results offer important insights into the design of efficient energy transfer systems.

1,4-bis[β-(2-benzoxazoly1) vinyl] benzene (BBVB) laser dye and sodium salt of meso-tetrakis (4-sulfonatophenyl) porphyrin (TPPS); spectroscopic investigation and DFT, NBO and TD-DFT calculations

Excitation energy transfer (EET) is a key photoinduced process in biological chromophoric assemblies. Here we investigate the factors which can drive EET into efficient ultrafast sub-ps regimes. We demonstrate how a coherent transport of electronic population could facilitate this in water solvated NADH coenzyme and uncover the role of an intermediate dark charge-transfer state. High temporal resolution ultrafast optical spectroscopy gives a 54±11 fs time constant for the EET process. Nonadiabatic quantum dynamical simulations computed through the time-evolution of multidimensional wavepackets suggest that the population transfer is mediated by photoexcited molecular vibrations due to strong coupling between the electronic states. The polar aqueous solvent environment leads to the active participation of a dark charge transfer state, accelerating the vibronically coherent EET process in favorably stacked conformers and solvent cavities. Our work demonstrates how the interplay of structural and environmental factors leads to diverse pathways for the EET process in flexible heterodimers and provides general insights relevant for coherent EET processes in stacked multichromophoric aggregates like DNA strands.

In this project we studied both natural photosynthetic antenna complexes and various artificial systems (e.g. chlorophyll (Chl) trefoils) using high resolution hole-burning (HB) spectroscopy and excitonic calculations. Results obtained provided more insight into the electronic (excitonic) structure, inhomogeneity, electron-phonon coupling strength, vibrational frequencies, and excitation energy (or electron) transfer (EET) processes in several antennas and reaction centers. For example, our recent work provided important constraints and parameters for more advanced excitonic calculations of CP43, CP47, and PSII core complexes. Improved theoretical description of HB spectra for various model systems offers new insight into the excitonic structure and composition of low-energy absorption traps in very several antenna protein complexes and reaction centers. We anticipate that better understanding of HB spectra obtained for various photosynthetic complexes and their simultaneous fits with other optical spectra (i.e. absorption, emission, and circular dichroism spectra) provides more insight into the underlying electronic structures of these important biological systems. Our recent progress provides a necessary framework for probing the electronic structure of these systems via Hole Burning Spectroscopy. For example, we have shown that the theoretical description of non-resonant holes is more restrictive (in terms of possible site energies) than those of absorption and emission spectra. We have demonstrated that simultaneous description of linear optical spectra along with HB spectra provides more realistic site energies. We have also developed new algorithms to describe both nonresonant and resonant hole-burn spectra using more advanced Redfield theory. Simultaneous description of various optical spectra for complex biological system, e.g. artificial antenna systems, FMO protein complexes, water soluble protein complexes, and various mutants of reaction centers continues; this work is supported by the new DOE BES grant.