Excited-state Photodynamics Research Articles

Investigating the Photophysical Aspects of a Naphthalene-Based Excited-State Proton Transfer Dye 1-(1H-Benzo[d]imidazol-2-yl)naphthalen-2-ol: pH-Dependent Modulation of Photodynamics.

The steady state and time-resolved photophysical behavior of a proton transfer dye 1-(1H-benzo[d]imidazol-2-yl)naphthalen-2-ol (H-BINO) was investigated. The excited state intramolecular proton transfer (ESIPT) reaction in H-BINO is predominant in nonpolar solutions (toluene and DCM) with a lifetime of ∼1.0 ns. However, in polar media (DMF and MeOH), the excited state photodynamics is characterized by a complex equilibrium of emission from the locally excited state (0.1-2.3 ns), the phototautomer (0.5-1.2 ns), and the anionic emission (2.1-5.4 ns). In the solid state, emission from the various aggregated states dictates the photobehavior. Interestingly, the photodynamics in aqueous solution changes starkly as a function of pH with the anionic (2.1 ns) and phototautomeric (0.5-1.0 ns) emissions guiding the photodynamics as the pH of the medium increases. Optimized structural parameters at the proton donor and acceptor sites for the enol and keto forms and the calculated potential energy curve along the proton transfer coordinate at the density functional theory (DFT) level with the B3LYP/6-311G++(d,p) theory support a favorable and barrierless ESIPT process. The current results will surely boost the ongoing research on small molecule emissive materials.

μ-Oxo Dimerization Effects on Ground- and Excited-State Properties of a Water-Soluble Iron Porphyrin CO2 Reduction Catalyst

Iron 5,10,15,20-tetra(para-N,N,N-trimethylanilinium)porphyrin (Fe-p-TMA) is a water-soluble catalyst capable of electrochemical and photochemical CO2 reduction. Although its catalytic ability has been thoroughly investigated, the mechanism and associated intermediates are largely unknown. Previous studies proposed that Fe-p-TMA enters catalytic cycles as a monomeric species. However, we demonstrate herein that, in aqueous solutions, Fe-p-TMA undergoes formation of a μ-oxo porphyrin dimer that exists in equilibrium with its monomeric form. The propensity for μ-oxo formation is highly dependent on the solution pH and ionic strength. Indeed, the μ-oxo form is stabilized in the presence of electrolytes that are key components of catalytically relevant conditions. By leveraging the ability to chemically control and spectrally address both species, we characterize their ground-state electronic structures and excited-state photodynamics. Global fitting of ultrafast transient absorption data reveals two distinct excited-state relaxation pathways: a three-component sequential model consistent with monomeric relaxation and a two-component sequential model for the μ-oxo species. Relaxation of the monomeric species is best described as a ligand-to-metal charge transfer (τ1 = ∼500 fs), an ionic strength-dependent metal-to-ligand charge transfer (τ2 = 2-4 ps), and finally relaxation of a ligand field excited state to the ground state (τ3 = 5 ps). Conversely, excited-state relaxation of the μ-oxo species proceeds via cleavage of an FeIII-O bond to generate transient FeIV═O and FeII porphyrin species (τ1 = 2 ps) that recombine to the ground-state μ-oxo species (τ2 = ∼1 ns). This latter lifetime extends to timescales relevant for chemical reactivity. It is therefore emphasized that further consideration of catalyst speciation and chemical microenvironments is necessary for elucidating the mechanisms of catalytic CO2 reduction reactions.

Excited-State Optically Detected Magnetic Resonance of Spin Defects in Hexagonal Boron Nitride.

Negatively charged boron vacancy (V_{B}^{-}) centers in hexagonal boron nitride (h-BN) are promising spin defects in a van der Waals crystal. Understanding the spin properties of the excited state (ES) is critical for realizing dynamic nuclear polarization. Here, we report zero-field splitting in the ES of D_{ES}=2160 MHz and its associated optically detected magnetic resonance (ODMR) contrast of 12% at cryogenic temperature. In contrast to nitrogen vacancy (NV^{-}) centers in diamond, the ODMR contrast of V_{B}^{-} centers is more prominent at cryotemperature than at room temperature. The ES has a g factor similar to the ground state. The ES photodynamics is further elucidated by measuring the level anticrossing of the V_{B}^{-} defects under varying external magnetic fields. Our results provide important information for utilizing the spin defects of h-BN in quantum technology.

Excited-state photodynamics of pyrene-containing boronated dyes

Three pyrene-containing boronated dyes have been examined in order to evaluate the effect of pyrene moiety localization on the photophysical properties. (3-Fluorophenyl)-1-pyreneborinic 8-oxyquinolinate was obtained by reacting 1-lithiopyrene with 3-fluorophenylboronic diethyl ester followed by complexation with 8-hydroxyquinoline. This compound consists of a pyrene unit (Py) directly bonded to a boron-quinolinate moiety (BQ). Two diarylborinic complexes (boranils) with salicylaldimine ligands (SA) bearing a pyrene moiety were obtained by reacting bis(2,6-difluorophenyl)borinic ethyl ester with salicylaldehyde and 1-aminopyrene or N1-(pyren-1-ylmethyl)benzene-1,4-diamine. The first compound consists of pyrene unit directly bonded to the boron-containing boranil moiety (BA), while the other consists of a pyrene unit connected to the BA moiety via a 1-amino-4-methyleneaminophenyl linker. The obtained compounds were characterized by 1H and 13C NMR spectroscopy and X-ray crystallography. UV–Vis fluorescence measurements showed the strong influence of solvent on the observed emission maximum (λmax) and quantum yield of emission. For (3-fluorophenyl)-1-pyreneborinic 8-oxyquinolinate the excited pyrene unit lead to dual fluorescence (from Py and BQ) which was modulated by solvent polarity. The possible formation of the open form of this complex via B–N bond breakage influences strongly the photoinduced processes which was supported by the fluorescence lifetimes measurements and quantum mechanical calculations. For two boranils the BA emission was observed upon excitation of pyrene regardless on solvent polarity. In the case of boranil complex containing 1-amino-4-methyleneaminophenyl linker a significant increase in quantum yield in the solid state was observed which may be attributed to aggregation-induced emission.

Spatiodynamics, Photodynamics, and Their Correlation in Hybrid Perovskites

The motion of organic cations and its possible correlation with excited state photodynamics are important to the fundamental understanding of the superior performances of 2D and 3D hybrid lead-halide perovskites (LHPs) in optoelectronic applications. Herein, variable-temperature (VT) solid-state 1H and 13C CP (Cross Polarization) NMR spectroscopy without isotope enrichments, combined with VT powder XRD, differential scanning calorimetry (DSC) and time-resolved photoluminescence (TRPL), was employed to study the phase transition, local structure, motion dynamics and photodynamic behaviors of organic cations in hybrid 2D Ruddlesden-Popper perovskites (BA)2(MA)n-1PbnI3n+1 (BA denotes butylammonium and MA denotes methylammonium). Two types of BA conformations in various crystalline phases of 2D RPPs and conformational transition associated with phase transition were clearly revealed. Quantitative NMR analyses suggest that the rotation dynamics of MA in 2D RPPs is comparable to that of MA in 3D MAPbI3. A surprising linear relationship was discovered between the estimated activation energy of the MA rotation and logarithm of photoluminescence (PL) lifetimes (τ) of 2D RPPs and MAPbI3, strongly supporting the presence of coupling between the cation spatial dynamics and the excited state photodynamic behavious.

Modulation of excited-state photodynamics of ESIPT probe 1′-hydroxy-2′-acetonaphthone (HAN) on interaction with bovine serum albumin

The 1′-hydroxy-2′-acetonaphthone (HAN) is a well-known excited-state intramolecular proton-transfer (ESIPT) dye that has been used as a useful fluorescence probe to understand various homogenous and microheterogenous media. However, interaction of HAN with proteins is still largely less explored. The present study is aimed to investigate the interaction of HAN with the most abundant animal circulatory protein, bovine serum albumin (BSA), using multi-spectroscopic techniques and molecular docking studies. The photophysical properties of HAN undergo significant modulation upon its interaction with BSA protein. The confinement and reduced polarity experienced by bound HAN into BSA binding pocket causes a large fluorescence enhancement. The binding constant for HAN-BSA complex is evaluated to be ∼4.3 × 104 M−1. Time-resolved fluorescence measurements reveal the exclusive presence of only keto (K*) form and twisted keto rotamer (KR*) form of the dye in the excited state inside the BSA binding pocket, though in the ground state the dye exists entirely in its enol (E) form. Results suggest a quantitative ESIPT reaction for HAN even inside the BSA pocket. Time resolved fluorescence anisotropy studies indicate very tight binding of HAN into the BSA pocket such that independent rotational diffusion of the dye is completely ceased. Molecular docking studies provide best docking conformation and involved interaction forces, suggesting the preferred binding of HAN to the site IIA (Sudlow's site I) of the BSA protein. Observed photophysical results provide valuable insights of HAN–BSA binding interaction, revealing for the first time that the intriguing excited-state equilibrium between K* and KR* forms of HAN is largely shifted towards KR* form, as the dye binds to the BSA pocket. To the best of our knowledge such an intriguing modulation in the excited state prototautomeric conversion process has not been reported before for the studied dye in the presence of any other host. Results of the present study can be useful in understanding the pharmacology of structurally similar dyes/drugs, having relevance to drug delivery, pharmaceutical sciences, sensors, and so on.

The longer timescale excited state dynamics of isolated benzene.

The excited state photodynamics of isolated benzene have been studied in the nanosecond range by two-step photoionization through various vibrations of the lowest singlet state, with imaging photoelectron spectroscopy detection. Photoelectron spectra are measured as a function of pump-probe delay time, and their time evolution is successfully compared to a biexponential decay function without regard to a particular kinetic model. The only reasonable kinetic model with only two exponentials is the one that involves an intersystem crossing from S1 to T1, although that model has previously been called into question by high-resolution studies that failed to find any singlet-triplet perturbations in Zeeman studies of the S1 spectrum. That contradiction remains unresolved.

Influence of the N atom position on the excited state photodynamics of protonated azaindole.

We present a study of the photofragmentation of three protonated azaindole molecules - 7-azaindole, 6-azaindole, and 5-azaindole - consisting of fused pyrrole-pyridine bicyclic aromatic systems, in which the pyridinic (protonated) nitrogen heteroatom is located at the 7, 6, and 5 positions, respectively. Photofragmentation electronic spectra of the isolated aforementioned azaindolinium cations reveal that their photodynamics extends over timescales covering nine orders of magnitude and provide evidence about the resultant fragmentation pathways. Moreover, we show how the position of the heteroatom in the aromatic skeleton influences the excited state energetics, fragmentation pathways, and fragmentation timescales. Computed ab initio adiabatic transition energies are used to assist the assignation of the spectra, while geometry optimisation in the excited electronic states as well as ab initio calculations along the potential surfaces demonstrate the role of ππ*/πσ* coupling and/or large geometry changes in the dynamics of these species. Evidence supporting the formation of Dewar valence isomers as intermediates involved in sub-picosecond relaxation processes is discussed.

Long-Lived Charge Separation at Heterojunctions between Semiconducting Single-Walled Carbon Nanotubes and Perylene Diimide Electron Acceptors

Nonfullerene electron acceptors have facilitated a recent surge in the efficiencies of organic solar cells, although fundamental studies of the nature of exciton dissociation at interfaces with nonfullerene electron acceptors are still relatively sparse. Semiconducting single-walled carbon nanotubes (s-SWCNTs), unique one-dimensional electron donors with molecule-like absorption and highly mobile charges, provide a model system for studying interfacial exciton dissociation. Here, we investigate excited-state photodynamics at the heterojunction between (6,5) s-SWCNTs and two perylene diimide (PDI)-based electron acceptors. Each of the PDI-based acceptors, hPDI2-pyr-hPDI2 and Trip-hPDI2, is deposited onto (6,5) s-SWCNT films to form a heterojunction bilayer. Transient absorption measurements demonstrate that photoinduced hole/electron transfer occurs at the photoexcited bilayer interfaces, producing long-lived separated charges with lifetimes exceeding 1.0 μs. Both exciton dissociation and charge recombinat...

Erratum to: Investigation of the S1/ICT equilibrium in fucoxanthin by ultrafast pump–dump–probe and femtosecond stimulated Raman scattering spectroscopy

Time-resolved multi-pulse spectroscopic methods-pump-dump-probe (PDP) and femtosecond stimulated Raman spectroscopy-were used to investigate the excited state photodynamics of the carbonyl group containing carotenoid fucoxanthin (FX). PDP experiments show that S1 and ICT states in FX are strongly coupled and that the interstate equilibrium is rapidly (<5 ps) reestablished after one of the interacting states is deliberately depopulated. Femtosecond stimulated Raman scattering experiments indicate that S1 and ICT are vibrationally distinct species. Identification of the FSRS modes on the S1 and ICT potential energy surfaces allows us to predict a possible coupling channel for the state interaction.

Investigation of the S1/ICT equilibrium in fucoxanthin by ultrafast pump-dump-probe and femtosecondstimulated Raman scattering spectroscopy.

Time-resolved multi-pulse spectroscopic methods-pump-dump-probe (PDP) and femtosecondstimulated Raman spectroscopy-were used to investigate the excited state photodynamics of the carbonyl group containing carotenoid fucoxanthin (FX). PDP experiments show that S1 and ICT states in FX are strongly coupled and that the interstate equilibrium is rapidly (<5ps) reestablished after one of the interacting states is deliberately depopulated. Femtosecondstimulated Raman scattering experiments indicate that S1 and ICT are vibrationally distinct species. Identification of the FSRS modes on the S1 and ICT potential energy surfaces allows us to predict a possible coupling channel for the state interaction.

Luminescent Quadrupolar Borazine Oligomers: Synthesis, Photophysics, and Two‐Photon Absorption Properties

A set of monodisperse bent donor-acceptor-donor-type conjugated borazine oligomers, BnNn+1 (n=1-4), incorporating electron-rich triarylamine donor and electron-deficient triarylborane acceptor units has been prepared through an iterative synthetic approach that takes advantage of highly selective silicon-boron and tin-boron exchange reactions. The effect of chain elongation on the electrochemical, one- and two-photon properties and excited-state photodynamics has been investigated. Strong intramolecular charge transfer (ICT) from the arylamine donors to boryl-centered acceptor sites results in emissions with high quantum yields (Φfl >0.5) in the range of 400-500 nm. Solvatochromic effects lead to solvent shifts as large as ∼70 nm for the shortest member (n=1) and gradually decrease with chain elongation. The oligomers exhibit strong two-photon absorption (2PA) in the visible spectral region with 2PA cross sections as large as 1410 GM (n=4), and broadband excited-state absorption (ESA) attributed to long-lived singlet-singlet and radical cation/anion absorption. The excited-state dynamics also show sensitivity to the solvent environment. Electrochemical observations and DFT calculations (B3LYP/6-31G*) reveal spatially separated HOMO and LUMO levels resulting in highly fluorescent oligomers with strong ICT character. The BnNn+1 oligomers have been used to demonstrate the detection of cyanide anions with association constants of log K>7.

Steric and electrostatic effects on photoisomerization dynamics using QM/MM ab initio multiple spawning

Photoisomerization of conjugated systems is a common pathway for photomechanical energy conversion in biological chromophores. There are many examples where the local environment of the chromophore plays an important role in determining the outcome of photoisomerization. We have investigated the effect of simple steric and electrostatic environments on the excited-state photodynamics of ethylene, a simple model for larger conjugated systems. Ab initio electronic structure methods were combined with molecular mechanical force fields to describe the ground and excited-state potential energy surfaces of ethylene embedded in electrostatic and steric environments. The time evolution of the system following photoabsorption was modeled using the ab initio multiple spawning (AIMS) method for quantum dynamics. We introduce a new method for integration of the equations of motion in AIMS, which detects conical intersections automatically and then decreases the timestep adaptively around them. Neither steric hindrance nor electrostatics have a large effect on the excited-state lifetime, even at effective pressures as large as 2 GPa. However, a nearby point charge creates an electric field that stabilizes one of two symmetry-related conical intersections, biasing the reaction toward a particular photoisomerization pathway. For the larger tetramethylethylene, where steric hindrance is expected to be more pronounced, we also see no effect on the excited-state lifetime. Our results suggest that electrostatic interactions are more effective than steric hindrance in modifying the course of excited-state reactions.

Excited-State Interaction of Red and Green Perylene Diimides with Luminescent Ru(II) Polypyridine Complex

Three new perylene diimide (PDI)-based ligands have been synthesized by the covalent attachment of dipyrido[a,c]phenazine moiety to one of the bay-positions of PDI, while the second position has been substituted with either a 4-tert-butylphenoxy or a pyrrolidinyl group to obtain two types of chromophores, Ph-PDI and Py-PDI, respectively, with distinct properties. In the case of Py-PDI, the resultant 1,7- and 1,6-regioisomers have been successfully separated by column chromatography and characterized by (1)H NMR spectroscopy. The ligands have been employed to prepare donor-acceptor-based ensembles incorporating the covalently linked PDI and Ru(II) polypyridine complex as the acting chromophores. A comprehensive study of the excited-state photodynamics of the ensembles has been performed by means of electrochemical and steady state and time-resolved spectroscopic methods. Although, in all the three ensembles, the photoexcitation of either chromophore resulted in a long-lived triplet excited state of PDI ((3)PDI) as the final excited state, the photochemical reactions leading to the triplet states were found to be essentially different for the two types of the ensembles. In the case of the Ph-PDI-based ensemble, the excitation of either chromophore leads to the electron transfer from the Ru(II) complex to Ph-PDI, whereas for the Py-PDI-based ensembles, the electron transfer is observed in the opposite direction and only when the Ru(II) complex is excited. The difference in the behavior was rationalized based on electrochemical study of the compounds, which has shown that the Ph-PDI chromophore is a better electron acceptor and the Py-PDI chromophores are relatively better electron donors. This study shows a chemical approach to control the photoreactions in PDI-based dichromophoric ensembles including the possibility to switch the direction of the photoinduced electron transfer.

Structure and Photoinduced Electron Transfer Dynamics of a Series of Hydrogen-Bonded Supramolecular Complexes Composed of Electron Donors and a Saddle-Distorted Diprotonated Porphyrin

The excited-state photodynamics of intrasupramolecular photoinduced electron transfer was investigated in a series of hydrogen-bonded supramolecular complexes composed of diprotonated 2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrin (H(4)DPP(2+)) and electron donors bearing a carboxylate group. The formation of supramolecular complexes was examined by spectroscopic measurements. The binding constants obtained by spectroscopic titration indicate the strong binding (10(8)-10(10) M(-2)) even in a polar and coordinating solvent, benzonitrile (PhCN). The crystal structure of the supramolecular assembly using ferrocenecarboxylate (FcCOO(-)) was determined to reveal a new structural motif involving two-point and single-point hydrogen bonding among saddle-distorted H(4)DPP(2+) dication and two FcCOO(-) anions. Femtosecond laser flash photolysis was applied to investigate the photodynamics in the hydrogen-bonded supramolecular complexes. Rate constants obtained were evaluated in light of the Marcus theory of electron transfer, allowing us to determine the reorganization energy and the electronic coupling matrix constant of photoinduced electron transfer and back electron transfer to be 0.68 eV and 43 cm(-1), respectively. The distance dependence of electron transfer was also examined by using a series of ferrocenecarboxylate derivatives connected by linear phenylene linkers, and the distance dependence of the rate constant of electron transfer (k(ET)) was determined to be k(ET) = k(0) exp(-beta r), in which beta = 0.64 A(-1).

Excited-State Photodynamics of Perylene−Porphyrin Dyads. 5. Tuning Light-Harvesting Characteristics via Perylene Substituents, Connection Motif, and Three-Dimensional Architecture

Seven perylene-porphyrin dyads were examined with the goal of identifying those most suitable for components of light-harvesting systems. The ideal dyad should exhibit strong absorption by the perylene in the green, undergo rapid and efficient excited-state energy transfer from perylene to porphyrin, and avoid electron-transfer quenching of the porphyrin excited state by the perylene in the medium of interest. Four dyads have different perylenes at the p-position of the meso-aryl group on the zinc porphyrin. The most suitable perylene identified in that set was then incorporated at the m- or o-position of the zinc porphyrin, affording two other dyads. An analogue of the o-substituted architecture was prepared in which the zinc porphyrin was replaced with the free base porphyrin. The perylene in each dyad is a monoimide derivative; the perylenes differ in attachment of the linker (either via a diphenylethyne linker at the N-imide or an ethynylphenyl linker at the C9 position) and the number (0-3) of 4-tert-butylphenoxy groups (which increase solubility and slightly alter the electrochemical potentials). In the p-linked dyad, the monophenoxy perylene with an N-imide diphenylethyne linker is superior in providing rapid and essentially quantitative energy transfer from excited perylene to zinc porphyrin with minimal electron-transfer quenching in both toluene and benzonitrile. The dyads with the same perylene at the m- or o-position exhibited similar results except for one case, the o-linked dyad bearing the zinc porphyrin in benzonitrile, where significant excited-state quenching is observed; this phenomenon is facilitated by close spatial approach of the perylene and porphyrin and the associated thermodynamic/kinetic enhancement of the electron-transfer process. Such quenching does not occur with the free base porphyrin because electron transfer is thermodynamically unfavorable even in the polar medium. The p-linked dyad containing a zinc porphyrin attached to a bis(4-tert-butylphenoxy)perylene via an ethynylphenyl linker at the C9 position exhibits ultrafast and quantitative energy transfer in toluene; the same dyad in benzonitrile exhibits ultrafast (<0.5 ps) perylene-to-porphyrin energy transfer, rapid (∼5 ps) porphyrin-to-perylene electron transfer, and fast (∼25 ps) charge recombination to the ground state. Collectively, this study has identified suitable perylene-porphyrin constructs for use in light-harvesting applications.

Linker Dependence of Energy and Hole Transfer in Neutral and Oxidized Multiporphyrin Arrays

The excited-state photodynamics of the neutral and one-electron-oxidized forms of five porphyrin dyads were studied in benzonitrile containing tetrabutylammonium hexafluorophosphate as the supporting electrolyte. Each dyad contains a zinc porphyrin (Zn) and a free base porphyrin (Fb) joined by a linear biphenylene (Phi(2)), terphenylene (Phi(3)), quaterphenylene (Phi(4)), diphenylbutadiyne (L), or phenylethyne (E) linker (ZnFbPhi(2), ZnFbPhi(3), ZnFbPhi(4), ZnFbL, ZnFbE). The findings along with recent results on the neutral and oxidized forms of ZnFb dyads containing a diphenylethyne or phenylene linker (ZnFbU, ZnFbPhi) and steric hindrance to porphyrin-linker internal rotation at one or both ends of a diarylethyne linker (ZnFbD, ZnFbP, ZnFbB) give insights into the effects of linker characteristics (length, orbital energies, orbital overlap with the porphyrins) on the rate constants for excited-state energy transfer, excited-state hole transfer, and ground-state hole transfer. Analysis of the results is aided by density functional theory molecular orbital calculations and Forster energy-transfer calculations. Although the rate constants for linker-mediated through-bond excited-state energy transfer can be modulated significantly using a number of molecular design criteria (e.g., linker characteristics, interplay between porphyrin orbital characteristics, and linker attachment site), ground-state hole transfer, which also occurs via a linker-mediated through-bond electron-exchange mechanism, is primarily affected by the free-energy driving force for the process as dictated by the redox characteristics of the interacting porphyrins. The insights gained from this study should aid in the design of next-generation multichromophore arrays for solar energy applications.

Optimization of Semiempirical Quantum Chemistry Methods via Multiobjective Genetic Algorithms: Accurate Photodynamics for Larger Molecules and Longer Time Scales

Excited-state photodynamics is important in numerous varieties of important materials applications (e.g., liquid crystal display, light emitting diode), pharmaceuticals, and chemical manufacturing processing. We study the effectiveness of multiobjective genetic and evolutionary algorithms in multiscaling excited-state direct photodynamics via rapid reparameterization of semiempirical methods. Using a very limited set of ab initio and experimental data, semiempirical parameters are reoptimized to provide globally accurate potential energy surfaces, thereby eliminating the need for expensive ab initio dynamics simulations. Through reoptimization, excited-state energetics are predicted accurately via semiempirical methods, while retaining accurate ground-state predictions. In our initial study of small photo-excited molecules, our results show that the multiobjective evolutionary algorithm consistently yields solutions that are significantly better—up to 384% lower error in the energy and 87% lower error in the energy-gradient—than those reported previously. As verified with direct quantum dynamical calculations, multiple high-quality parameter sets obtained via genetic algorithms show near-ideal behavior on critical and untested excited-state geometries. The results demonstrate that the reparameterization via evolutionary algorithms is a promising way to extend direct dynamics simulations of photochemistry to multi-picosecond time scales and to larger molecules, with promise in more application beyond simple molecular chemistry.

Synthesis and Excited-State Photodynamics of Perylene-Bis(Imide)-Oxochlorin Dyads. A Charge-Separation Motif

Three perylene-oxochlorin dyads have been prepared and characterized with the goal of identifying charge-injection or molecular-switching motifs for use in molecular photonics. Each dyad consists of a perylene-bis(imide) dye (PDI) joined at the 10-position of a magnesium, zinc, or free base (Fb) oxochlorin via a diphenylethyne linker. Each dyad has been studied in both polar and nonpolar media using static and time-resolved optical spectroscopy and electrochemical techniques. Dyad PDI−MgO is an excellent charge-separation unit in which the excited perylene (∼3.5 ps lifetime) or the excited oxochlorin (lifetimes of 0.5 ns in benzonitrile and 1.0 ns in toluene) give rise to state PDI- MgO+ in high overall yield (>90%); the charge-separated state has a lifetime of ≥1 ns in both toluene and benzonitrile. The pathway for generating PDI- MgO+ from the excited perylene (PDI*) involves both hole transfer and energy transfer to the oxochlorin followed by electron transfer from the resulting MgO* to PDI. Similar decay of PDI* by energy transfer and hole transfer is found for dyads PDI−ZnO and PDI−FbO. However, electron-transfer quenching of the excited oxochlorin in these two dyads either does not occur or occurs to a much lesser degree than for PDI−MgO in both polar and nonpolar solvents. For PDI−FbO the decay of the charge-separated state occurs significantly by charge recombination to give the excited oxochlorin, making this a good light-harvesting system even though the early stages of the dynamics include charge separation/recombination. The observed differences in the extent of the possible excited-state processes (energy, hole, and electron transfer) among the dyads and in polar versus nonpolar media are consistent with the estimated energy ordering of the excited- and charge-separated states. This study has provided a new class of arrays containing perylene accessory pigments and oxochlorin chromophores that can be utilized for applications in light harvesting and molecular optoelectronics.

Synthesis and Excited-State Photodynamics of A Perylene-Monoimide-Oxochlorin Dyad. A Light-Harvesting Array

A perylene-oxochlorin dyad has been prepared and characterized for potential use as a new light-harvesting motif. The dyad (PMI−ZnO) consists of a perylene-monoimide dye (PMI) joined at the 5-position of a zinc oxochlorin (ZnO) via a diphenylethyne linker. The dyad and its subunits have been studied in both polar and nonpolar media using static and time-resolved optical spectroscopy and electrochemical techniques. Energy flows very rapidly from both unrelaxed (vibrationally, conformationally, or electronically) and relaxed forms of the excited perylene (PMI*) to the ground-state oxochlorin with an effective time constant of ∼4 ps and an efficiency of 99% in both toluene and benzonitrile. Subsequently, there is little or no quenching of the excited oxochlorin (ZnO*) in either solvent. These findings are consistent with the expectation that all charge-separated states such as PMI- ZnO+ lie energetically above both PMI* and ZnO* in both polar and nonpolar media. Furthermore, light absorption by the perylene and oxochlorin occurs in complementary regions, giving broad spectral coverage. Thus, the new perylene-oxochlorin dyad is an excellent light-harvesting unit that can be incorporated into more elaborate architectures for use in solar-energy and molecular-photonics applications.