Excited state dynamics and photochemistry of nitroaromatic compounds.

Nitrated aromatic molecules have unique photoinduced channels. Due to the presence of oxygen-centered non-bonding orbitals, they can undergo sub-picosecond intersystem crossing showing one of the strongest couplings between the singlet and triplet manifolds among organic molecules. Several nitroaromatic compounds also have a distinctive nitric oxide photodissociation channel which occurs through a complex sequence of atom rearrangements and state changes. These remarkable processes have stimulated the attention of several research groups over the last few years who have applied modern femtosecond spectroscopies and new computational methods to these topics. Nitroaromatic molecules also have demonstrated their value as case-studies, where they can serve to understand the influence of torsional motions between the nitro substituent and the aromatic system in the conversions between states. In this contribution we highlight several of the recent results in this area. Due to the importance of the atmospheric photochemistry of nitrated compounds and their accumulating applications as nitric oxide release agents, continued research about the effects of the different state orderings, substitution patterns, and solvent effects is central to the development of future applications and for a better understanding of their environmental pathways. From this analysis, several pending issues are highlighted, which include the nature of the dominant singlet state involved in intersystem crossing, the role of the formation of charge-transfer states, the yield of the internal conversion channel to the electronic ground state, and a more generalized understanding of the sequence of steps which lead to nitric oxide dissociation.

Unravelling the solvent polarity effect on the excited state intramolecular proton transfer mechanism of the 1- and 2-salicylideneanthrylamine. A TD-DFT case study.

Time dependent density functional theory has been used to investigate the photochemical and photophysical processes involved in the excited states relaxation of 1- and 2-salicylideneanthrylamine in different solvent environments. This investigation reveals that the pathways involved in the relaxation of the first excited state depend on the solvent polarity. The emission spectrum in acetonitrile and methanol is dominated by the cis-keto tautomers, while in cyclohexane, the spectrum is dominated by the fluorescence emission of the locally excited trans-enol form. Our results showed that, for each compound, two nearly isoenergetic trans-enol conformers can coexist in equilibrium, which upon photoexcitation, can relax by two competitive processes: rotation about the azomethine N[double bond, length as m-dash]C bond leading to the twisted-enol conformer, and the excited state intramolecular proton transfer leading to the fluorescent cis-keto tautomer, which can undergo a cis-trans isomerization producing the trans-keto photochromic product. The TD-DFT relaxed potential energy profiles for the ESIPT show that the effect of changing the solvent from polar to nonpolar solvents results on an increment of the energy barrier, and therefore, the ESIPT become kinetically less favoured. In constrast, this change favours the relaxation of the excited trans-enol form towards the twisted conformers, in both the enol and keto regions.

A theoretical study of the photodynamics of salicylidene-2-anthrylamine in acetonitrile solution.

The ultrafast photoinduced processes of salicylidene-2-anthrylamine (2-AntSA) in acetonitrile solution have been investigated using DFT/TD-DFT static electronic structure calculations and excited state ab initio molecular dynamics simulations. Two different isoenergetic enol ground-state structures with suitable geometry for excited state intramolecular proton transfer (ESIPT) where chosen for the excited-state dynamics. The S1 relaxed potential energy profiles for the excited state intramolecular proton transfer and the N[double bond, length as m-dash]C double bond isomerization reactions predict that both reactions occur over an energy barrier and that they are competitive processes in the deactivation of the Franck-Condon state. The photodynamic simulations show that the ESIPT occurs in the femtosecond time scale for both conformers (77 and 213 fs for IA and IIA, respectively) and that the speed is modulated by the ability of the conformers to evolve toward a planar conformation in the S1 state. The trajectories predict two conical intersections which provide nonradiative relaxation pathways to the S0 state. The first one is located in the twisted enol region, where the proton transfer process is unlikely, and only occurs for the conformer IIA in a time scale ≥600 fs. The second conical intersection is located in the cis-keto region, and represents an effective depopulation channel toward the trans-keto form. All our results are in remarkably good agreement with experiments.

Synthesis, characterization, X-ray crystal structure and DFT calculations of 4-([2,2':6',2 '' -terpyridin]-4'-yl)phenol / Síntesis, caracterización, estructura cristalina y cálculos DFT para el 4-([2,2':6',2 '' -terpiridin]-4'-il)fenol / Síntese, caracterização, estrutura de cristal e cálculos DFT para o 4-([2,2':6',2 '' -terpiridina]-4'-il)fenol

Abstract The synthesis of new terpyridine (Tpy) derivatives has been subject of extensive research due to its potential as functional materials for solar energy conversion, among other applications. In this contribution, the 4-([2,2':6',2"-terpyndm]-4'-yl)phenol (TpyOH) was synthesized, characterized and studied through several methods, including X-ray crystallography and computational approaches. Single crystal X-ray structure analysis shows that TpyOH is essentially planar, with dihedral angles of about 5.03° between the central pyridinyl and the phenolic ring, and also 6.05 and 12.2° in the terpyridine moiety. In the crystal, molecules are linked by intermolecular hydrogen bonds and through П- П stacking interactions. Using a time dependent density functional theory approach and taking into account bulk solvent effects, the absorption and fluorescence spectra of TpyOH were investigated and compared. The TD-DFT S0→Sn and S1 →S0 transition energies are in good agreement with experimental results. The frontier molecular orbitals analysis showed that the low-energy absorption band has an intraligand charge transfer character (ICT), while the high-energy band is a common feature of П- П* transitions of the Tpy moiety. The S1→S0 emission transition also has an ICT character, with a 90% contribution from the HOMO→LUMO transitions.

Resumen La síntesis de derivados terpiridinicos (Tpy) se ha investigado ampliamente debido a su potencial para la conversión de energía solar En este artículo se sintetizó y caracterizó el 4-([2,2':6',2"-terpiridin]-4'-il)fenol (TpyOH), a través de varias metodologías como la cristalografía de rayos X y herramientas computacionales. El análisis de rayos X de monocristal mostró que el TpyOH es plano, con ángulos diedros de 5,03° entre el piridinilo central y el anillo fenólico, con presencia de ángulos de 6,05 y 12,2° en la porción terpiridínica. En el cristal, las moléculas están unidas por enlaces de hidrógeno intermoleculares y mediante interacciones de apilamiento n-n. Utilizando cálculos DFT dependientes del tiempo (TD-DFT) y teniendo en cuenta el efecto de los disolventes, se investigaron y compararon los espectros de absorción y fluorescencia de TpyOH. Las energías de transición TD-DFT de S0→Sn y S1→S0 concuerdan con los resultados experimentales. El análisis de orbitales moleculares de frontera mostró que la banda de absorción de baja energía corresponde a transferencia de carga intraligando (ICT); mientras que la banda de alta energía es común en las transiciones П-П* del resto Tpy. La emisión debido a la transición S1→S0 corresponde a ICT, con una contribución del 90% proveniente de transiciones HOMO→LUMO.

Resumo A síntese de derivados de terpiridina (Tpy) tem sido estudada devido ao seu potencial para a conversão de energia solar. Nesta contribuição, o 4-([2,2':6',2"- terpindina]-4'-il) fenol (TpyOH) foi sintetizado, caracterizado e estudado por vários métodos A análise de estrutura de raios X de cristal único mostra que o TpyOH é plano, com Ångulos diedros de 5,03 ° entre o piridinilo central e o anel fenólico, e também 6,05 e 12,2 ° na porção de terpiridina No cristal, as moléculas são ligadas por ligações intermoleculares de hidrogênio e através de interações de empilhamento n-n. Usando uma abordagem da teoria funcional da densidade dependente do tempo e levando em consideração os efeitos do solvente em massa, foram investigados e comparados os espectros de absorção e fluorescência do TpyOH As energias de transição TD-DFT S0→Sn e S1→S0 estão de acordo com os resultados experimentais A análise de orbitários moleculares de fronteira mostrou que a banda de absorção de baixa energia possui um caráter de transferência de carga intraligando (TIC), enquanto a banda de alta energia é uma característica comum das transições П-П* da fração Tpy. A transição de emissão S1→S0 também tem um caráter TIC, com uma contribuição de 90% das transições HOMO→LUMO.

Exciton localization and dissociation dynamics in CdS and CdS-Pt quantum confined nanorods: effect of nonuniform rod diameters.

One-dimensional colloidal multicomponent semiconductor nanorods, such as CdSe-CdS dot-in-rod, have been extensively studied as a promising class of new materials for solar energy conversion because of the possibilities of using the band alignment of component materials and the rod-diameter-dependent quantum confinement effect to control the location of electrons and holes and to incorporate catalysts through the growth of Pt tips. Here we used CdS nanorods as an example to study the effect of nonuniform diameters along the rod on the exciton localization and dissociation dynamics in CdS and (platinum tipped) CdS-Pt nanorods. We showed that, in CdS nanorods prepared by seeded growth, the presence of a bulb with a larger diameter around the CdS seed resulted in an additional absorption band lower in energy than the exciton in the CdS rod. As a result, excitons generated in the CdS rod could undergo ultrafast localization to the bulb region in addition to trapping on the CdS rod. We observed that the Pt tip led to fast exciton dissociation by electron transfer. However, excitons localized on the CdS bulb showed slower average ET rates than those localized in the rod region. Our findings suggested that the effect of rod morphology should be carefully considered in designing multicomponent nanorods for solar energy conversion applications.

An inorganic chromophore based on a molecular oxide supported metal carbonyl cluster: [P2W17O61{Re(CO)3}3{ORb(H2O)}(µ3-OH)]9-.

A polyoxometalate-supported trirhenium carbonyl cluster, mimicking metal oxide supported interfacial dyadic structures, has been synthesized and characterized. Multiple techniques, including computational and transient absorption spectroscopy, have been applied to characterize the charge-transfer dynamics occurring at the interfaces of this "double cluster". The stepwise kinetics of charge separation and recombination has been thoroughly investigated.

Plasmon-induced hot electron transfer from the Au tip to CdS rod in CdS-Au nanoheterostructures.

The plasmon-exciton interaction mechanisms in CdS-Au colloidal quantum-confined plexcitonic nanorod heterostructures have been studied by transient absorption spectroscopy. Optical excitation of plasmons in the Au tip leads to hot electron injection into the CdS rod with a quantum yield of ~2.75%. This finding suggests the possibility of further optimization of plasmon-induced hot electron injection efficiency through controlling the size and shape of the plasmonic and excitonic domains for potential light harvesting applications.

Dynamics of the formation of a charge transfer state in 1,2-bis(9-anthryl)acetylene in polar solvents: symmetry reduction with the participation of an intramolecular torsional coordinate.

We have studied 1,2-bis(9-anthryl)acetylene as a model compound for the characterization of the process of solvent-mediated symmetry reduction in an excited state. Thanks to the acetylenic bridge that joins the two anthracenic moieties, this system maintains minimal steric hindrance between the end chromophores in comparison with the classic 9,9'-bianthryl model compound. The acetylenic bridge also allows for significant electronic coupling across the molecule, which permits a redistribution of electron density after light absorption. Femtosecond resolved fluorescence measurements were used to determine the spectral evolution in acetonitrile and cyclohexane solutions. We observed that, for 1,2-bis(9-anthryl)acetylene, the formation of a charge transfer state occurs in a clear bimodal fashion with well separated time scales. Specifically, the evolution of the emission spectrum involves a first solvent-response mediated subpicosecond stage where the fluorescence changes from that typical of nonpolar solvents (locally excited) to an intermediate, partial charge transfer state. The second stage of the evolution into a full charge transfer state occurs with a much longer time constant of 37.3 ps. Since in this system the steric hindrance is minimized, this molecule can undergo much larger amplitude motions for the torsion between the two anthracenic moieties associated with the charge redistribution in comparison with the typical model compound 9,9'-bianthryl. Clearly, the larger range of motions of 1,2-bis(9-anthryl)acetylene gives the opportunity to study the electron transfer process with a good separation of the time scales for the formation of a partial charge transfer state, determined by the speed of solvent response, and the intramolecular changes associated with the formation of the fully equilibrated charge transfer state.

Beyond band alignment: hole localization driven formation of three spatially separated long-lived exciton states in CdSe/CdS nanorods.

Colloidal one-dimensional semiconductor nanoheterostructures have emerged as an important family of functional materials for solar energy conversion, although the nature of the long-lived exciton state and their formation and dissociation dynamics remain poorly understood. In this paper we study these dynamics in CdSe/CdS dot-in-rod (DIR) NRs, a representative of 1D heterostructures, and DIR-electron-acceptor complexes by transient absorption spectroscopy. Because of a quasi-type II band alignment of CdSe and CdS, it is often assumed that there exists one long-lived exciton state with holes localized in the CdSe seed and electrons delocalized among CdSe and CdS. We show that excitation into the CdS rod forms three distinct types of long-lived excitons that are spatially localized in the CdS rod, in and near the CdSe seed and in the CdS shell surrounding the seed. The branching ratio of forming these exciton states is controlled by the competition between the band offset driven hole localization to the CdSe seed and hole trapping to the CdS surface. Because of dielectric contrast induced strong electron-hole interaction in 1D materials, the competing hole localization pathways lead to spatially separated long-lived excitons. Their distinct spatial locations affect their dissociation rates in the presence of electron acceptors, which has important implications for the application of 1D heterostructures as light-harvesting materials.

Interfacial charge separation and recombination in InP and quasi-type II InP/CdS core/shell quantum dot-molecular acceptor complexes.

Recent studies of group II-VI colloidal semiconductor heterostuctures, such as CdSe/CdS core/shell quantum dots (QDs) or dot-in-rod nanorods, show that type II and quasi-type II band alignment can facilitate electron transfer and slow down charge recombination in QD-molecular electron acceptor complexes. To explore the general applicability of this wave function engineering approach for controlling charge transfer properties, we investigate exciton relaxation and dissociation dynamics in InP (a group III-V semiconductor) and InP/CdS core/shell (a heterostructure beween group III-V and II-VI semiconductors) QDs by transient absorption spectroscopy. We show that InP/CdS QDs exhibit a quasi-type II band alignment with the 1S electron delocalized throughout the core and shell and the 1S hole confined in the InP core. In InP-methylviologen (MV(2+)) complexes, excitons in the QD can be dissociated by ultrafast electron transfer to MV(2+) from the 1S electron level (with an average time constant of 11.4 ps) as well as 1P and higher electron levels (with a time constant of 0.39 ps), which is followed by charge recombination to regenerate the complex in its ground state (with an average time constant of 47.1 ns). In comparison, InP/CdS-MV(2+) complexes show similar ultrafast charge separation and 5-fold slower charge recombination rates, consistent with the quasi-type II band alignment in these heterostructures. This result demonstrates that wave function engineering in nanoheterostructures of group III-V and II-VI semiconductors provides a promising approach for optimizing their light harvesting and charge separation for solar energy conversion applications.

Ultrafast excited state dynamics of allopurinol, a modified DNA base.

The decay of electronically excited allopurinol riboside was studied through the fluorescence up-conversion technique and high level ab initio calculations. For the allopurinol system with a pyrazolic five-membered ring, we observed an ultrafast decay of the fluorescence signal in water (τ < 0.2 ps), similar to what has been observed for hypoxanthine and inosine (with an imidazolic five-membered ring). These results show that the S(1) dynamics in this type of heterocyclic systems are general and dominated by the distortion in the pyrimidinic six-membered ring with a negligible influence of the rest of the heterocycle. The measurements are consistent with the presence of a highly accessible conical intersection between the S(1) (π-π*) excited state and S(0), as calculated by MR-CIS/CASSCF computations. Our calculations show that the loss of planarity of the six-membered ring is responsible for direct access to the S(1)-S(0) degeneracy region without requiring distortions in the rest of the molecule.

Multiple exciton generation and dissociation in PbS quantum dot-electron acceptor complexes.

Multiple exciton generation (MEG) in quantum dots (QDs), a process by which one absorbed photon generates multiple electron-hole pairs, has provided exciting possibilities for improving the energy conversion efficiency of photovoltaic and photocatalytic devices. However, implementing MEG in practical devices requires the extraction of multiple charge carriers before exciton-exciton annihilation and the development of materials with improved MEG efficiency. In this report, using PbS QD/methylene blue complexes as a QD/electron acceptor model system, we demonstrate that the presence of electron acceptors does not affect the MEG efficiency of QDs and all generated excitons can be dissociated by electron transfer to the acceptor, achieving MEG and multiple exciton dissociation efficiencies of 112%. We further demonstrate that these efficiencies are not affected by the charging of QDs.

Ultrafast charge separation and long-lived charge separated state in photocatalytic CdS-Pt nanorod heterostructures.

Colloidal semiconductor-metal nanoheterostructures that combine the light-harvesting ability of semiconductor nanocrystals with the catalytic activity of small metal nanoparticles show promising applications for photocatalysis, including light-driven H(2) production. The exciton in the semiconductor domain can be quenched by electron-, hole-, and energy transfer to the metal particle, and the competition between these processes determines the photocatalytic efficiency of these materials. Using ultrafast transient absorption spectroscopy, we show that, in CdS-Pt heterostructures consisting of a CdS nanorod with a Pt nanoparticle at one end, the excitons in the CdS domain dissociate by ultrafast electron transfer (with a half-life of ∼3.4 ps) to the Pt. The charge separated state is surprisingly long-lived (with a half-life of ∼1.2 ± 0.6 µs) due to the trapping of holes in CdS. The asymmetry in the charge separation and recombination times is believed to be the key feature that enables the accumulation of the transferred electrons in the Pt tip and photocatalysis in the presence of sacrificial hole acceptors.

Photoinduced energy transfer in bichromophoric pyrene-PPV oligomer systems: the role of flexible donor-acceptor bridges.

In this contribution, we report on the electronic energy transfer dynamics of bichromophoric systems incorporating two pyrene chromophores tethered by variable-length flexible alkyloxy chains to p-phenylenevinylene oligomers. These were studied using UV-vis absorption and both steady state and time-resolved fluorescence spectroscopy. Time-resolved emission measurements showed an efficient photoinduced energy transfer process in all the multichromophoric systems, which occurs on the time scale of tens of picoseconds after excitation at 265 nm. The energy transfer process is especially efficient in systems where the linker is formed by eight atoms (up to k(ET) ≈ 2.7 × 10(10) s(-1)), which, despite not being the shortest bridge studied, allows the approach of the donor and acceptor chromophores due to an appropriate number of flexible single bonds. Using Förster theory, we calculated the donor-acceptor distance in each triad from the experimental energy transfer rate, finding them to be in the range 8.8-10 Å.

Wave function engineering for efficient extraction of up to nineteen electrons from one CdSe/CdS quasi-type II quantum dot.

Solar-to-fuel conversion devices require not only efficient catalysts to accelerate the reactions, but also light harvesting and charge separation components to absorb multiple photons and to deliver multiple electrons/holes to the catalytic centers. In this paper, we show that the spatial distribution of electron and hole wave functions in CdSe/CdS quasi-type II quantum dots enables simultaneous ultrafast charge separation (0.18 ps to adsorbed Methylviologen), ultraslow charge recombination (0.4 µs), and slow multiple-exciton Auger annihilation (biexciton lifetime 440 ps). Up to nineteen excitons per QD can be generated by absorbing multiple 400 nm photons and all excitons can be dissociated with unity yield by electron transfer to adsorbed methylviologen molecules. Our finding demonstrates that (quasi-) type II nanoheterostructures can be engineered to efficiently dissociate multiple excitons and deliver multiple electrons to acceptors, suggesting their potential applications as light harvesting and charge separation components in artificial photosynthetic devices.

Strong electronic coupling and ultrafast electron transfer between PbS quantum dots and TiO2 nanocrystalline films.

Hot carrier and multiple exciton extractions from lead salt quantum dots (QDs) to TiO(2) single crystals have been reported. Implementing these ideas on practical solar cells likely requires the use of nanocrystalline TiO(2) thin films to enhance the light harvesting efficiency. Here, we report 6.4 ± 0.4 fs electron transfer time from PbS QDs to TiO(2) nanocrystalline thin films, suggesting the possibility of extracting hot carriers and multiple excitons in solar cells based on these materials.

Synthesis and characterization of a metal-to-polyoxometalate charge transfer molecular chromophore.

[P(4)W(35)O(124){Re(CO)(3)}(2)](16-) (1), a Wells-Dawson [α(2)-P(2)W(17)O(61)](10-) polyoxometalate (POM)-supported [Re(CO)(3)](+) complex containing covalent W(VI)-O-Re(I) bonds has been synthesized and characterized by several methods, including X-ray crystallography. This complex shows a high visible absorptivity (ε(470 nm) = 4000 M(-1) cm(-1) in water) due to the formation of a Re(I)-to-POM charge transfer (MPCT) band. The complex was investigated by computational modeling and transient absorption measurements in the visible and mid-IR regions. Optical excitation of the MPCT transition results in instantaneous (<50 fs) electron transfer from the Re(I) center to the POM ligand.

Role of upper triplet states on the photophysics of nitrated polyaromatic compounds: S(1) lifetimes of singly nitrated pyrenes.

The photophysics of most nitrated polycyclic aromatic compounds is dominated by an ultrafast intersystem crossing channel, which makes their first singlet excited states decay with rates on the order of 10(12) to 10(13) s(-1). Some questions, however, remain about the nature of the receiver triplet states, which have been in principle assigned to specific triplets of a different electronic configuration from T(1). In particular, it could be suggested that even a small degree of n-π* character of the T(1) state may be enough to allow the S(1) state to couple to upper vibronic states of the lowest energy triplet, without the requirement for specific upper triplet states. In this report, we show that there are, in fact, nitroaromatic compounds that do not show the ultrafast intersystem crossing channel but instead have S(1) states that are two to three orders of magnitude longer lived. Our studies focused on the time resolution of the emission from singly nitrated pyrenes, which show a strong photophysical dependence on the position of the NO(2) group: Whereas S(1) in 1-nitropyrene is short-lived (up to 3 ps), in 4-nitropyrene and 2-nitropyrene this state has 0.41 and 1.2 ns lifetimes, respectively, in acetonitrile solution. Computational work at the TD-DFT level of theory indicates that such remarkable increase in the first excited singlet lifetime can indeed be explained by a loss of the energy coincidence between the S(1) state with specific upper triplet states formed from transitions that involve the nonbonding orbitals at the oxygen atoms. These results are in strong support of the previous descriptions about the requirement for intermediacy of specific triplet states in the ultrafast decay of the fluorescent state present in most nitroaromatics. The implications for the photochemistry of this group of toxic atmospheric pollutants, including the channel that redounds in the dissociation of the NO· fragment, are discussed in view of the present results.

Ultrafast charge separation and recombination dynamics in lead sulfide quantum dot-methylene blue complexes probed by electron and hole intraband transitions.

Lead salt quantum dots (QDs) have emerged as attractive materials for solar energy conversion because of their broad spectral response, long exciton lifetime, and efficient multiexciton generation. However, charge separation dynamics from these QDs remain poorly understood. In this study we investigate charge separation and recombination dynamics in PbS-methylene blue (MB(+)) complexes by femtosecond transient absorption spectroscopy. We show that while the 1S electrons and holes in excited PbS QDs lead to overlapping transient absorption features in the visible and near-IR regions, their intraband absorptions in the mid-IR can be monitored independently to directly follow the charge separation and recombination processes. The charge separation and recombination rates in PbS-MB(+) complexes were found to be (2.7 ± 0.2) × 10(12) and (1.1 ± 0.2) × 10(11) s(-1), respectively. The ultrafast charge separation rate suggests the possibility of hot electron injection and multiexciton dissociation from these strongly quantum confined QDs, consistent with recent reports of these phenomena at lead salt QD/TiO(2) interfaces.

Ultrafast photosensitization of phthalocyanines through their axial ligands.

We have studied the energy transfer properties of a novel silicon phthalocyanine that coordinates two anthracene-9-carboxylate groups in the form of trans axial ligands. Our objectives were to generate a system with auxiliary chromophores that enhance the light absorption properties of the macrocycle in a specific region in the UV and to evaluate the efficiency and time scales for energy transfer. The ligand coordination through a carboxylate group directly attached to the anthracenic system allows for close proximity of the donor and acceptor chromophores. The energy transfer process was observed to be nearly 100% efficient and to occur on a time scale of 370 fs. From the energy relations of the donor and acceptor states and the observed dynamics, the initial energy transfer step is likely to involve upper electronic states of the phthalocyanine rather than the states of the lowest-energy vibroelectronic Q bands.