Photoinduced Charge-Transfer Reaction between Pyrene and N,N‘-Dimethylaniline on Silica Gel Surfaces

Thousand-Fold Enhancement of Charge-Separated State Lifetimes Caused by an Adamantane Bridge in Dimethylaniline-Anthracene and Dimethylaniline-Pyrene Dyads

A long-standing challenge in photoinduced electron transfer research is the design of compact donor-acceptor dyads that can generate long-lived charge-separated (CS) states for use as sensitizers in solar energy harvesting. Reports of dyads exhibiting CS state lifetimes in the microsecond time domain are very rare. Herein, we report two compact donor-bridge-acceptor dyads exhibiting lifetimes in the microsecond domain. We employed an adamantane moiety as a bridge, and the lifetimes obtained are nearly 1000-fold larger when compared to those of the same donor-acceptor dyads bridged through C₃-alkyl chains. In addition to long-lived CS state decays, slow formation of acceptor triplets was also observed via nanosecond flash photolysis. The long lifetime of the CS state is attributed to the extremely small value of the electronic coupling matrix element for the charge recombination compared to charge separation.

Sensitization-initiated electron transfer via upconversion: mechanism and photocatalytic applications

Sensitization-initiated electron transfer (SenI-ET) describes a recently discovered photoredox strategy that relies on two consecutive light absorption events, triggering a sequence of energy and electron transfer steps. The cumulative energy input from two visible photons gives access to thermodynamically demanding reactions, which would be unattainable by single excitation with visible light. For this reason, SenI-ET has become a very useful strategy in synthetic photochemistry, but the mechanism has been difficult to clarify due to its complexity. We demonstrate that SenI-ET can operate via sensitized triplet-triplet annihilation upconversion, and we provide the first direct spectroscopic evidence for the catalytically active species. In our system comprised of fac-[Ir(ppy)₃] as a light absorber, 2,7-di-tert-butylpyrene as an annihilator, and N,N-dimethylaniline as a sacrificial reductant, all photochemical reaction steps proceed with remarkable rates and efficiencies, and this system is furthermore suitable for photocatalytic aryl dehalogenations, pinacol couplings and detosylation reactions. The insights presented here are relevant for the further rational development of photoredox processes based on multi-photon excitation, and they could have important implications in the greater contexts of synthetic photochemistry and solar energy conversion.

Probing the interplay between factors determining reaction rates on silica gel using termolecular systems

In this study we have compared energy and electron transfer reactions in termolecular systems using a nanosecond diffuse reflectance laser flash photolysis technique. We have previously investigated these processes on silica gel surfaces for bimolecular systems and electron transfer in termolecular systems. The latter systems involved electron transfer between three arene molecules with azulene acting as a molecular shuttle. In this study we present an alternative electron transfer system using trans β-carotene as an electron donor in order to effectively immobilise all species except the shuttle, providing the first unambiguous evidence for radical ion mobility. In the energy transfer system we use naphthalene, a structural isomer of azulene, as the shuttle, facilitating energy transfer from a selectively excited benzophenone sensitiser to 9-cyanoanthracene. Bimolecular rate constants for all of these processes have been measured and new insights into the factors determining the rates of these reactions on silica gel have been obtained.

Luminescent charge-transfer complexes: tuning emission in binary fluorophore mixtures

Charge-transfer (CT) complexes composed of a π-electron-poor naphthalene diimide (NDI) derivative combined with a series of π-electron-rich donors were investigated. Solutions of the CT complexes are nonemissive; however, solid-state complexes and aqueous suspensions display emission that is dependent on the energy of the HOMO of the electron donor. Crystallographic analysis of a pyrene-NDI complex reveals columnar packing and a high degree of frontier molecular orbital (FMO) overlap that likely contributes to the observed optical properties. The fluorescent CT particles are utilized as imaging agents; additional luminescent CT complexes may be realized by considering FMO energies and topologies.

Methionine-pyrene hybrid based fluorescent probe for trace level detection and estimation of Hg(II) in aqueous environmental samples: experimental and computational studies

A new fluorescent, Hg(2+) selective chemosensor, 4-methylsulfanyl-2-[(pyren-4-ylmethylene)-amino] butyric acid methyl ester (L, MP) was synthesized by blending methionine with pyrene. It was well characterized by different analytical techniques, viz. (1)H NMR, (13)C NMR, QTOF mass spectra, elemental analysis, FTIR and UV-vis spectroscopy. The reaction of this ligand with Hg(2+) was studied by steady state and time-resolved fluorescence spectroscopy. The Hg(2+) complexation process was confirmed by comparing FTIR, UV-vis, thermal, QTOF mass spectra and (1)H NMR data of the product with that of the free ligand values. The composition (Hg(2+):L=1:1) of the Hg(2+) complex in solution was evaluated by fluorescence titration method. Based on the chelation assisted fluorescence quenching, a highly sensitive spectrofluorometric method was developed for the determination of trace amounts of Hg(2+) in water. The ligand had an excitation and emission maxima at 360 nm and 455 nm, respectively. The fluorescence life times of the ligand and its Hg(2+) complex were 1.54 ns and 0.72 ns respectively. The binding constant of the ligand, L with Hg(2+) was calculated using Benesi-Hildebrand equation and was found to be 7.5630×10(4). The linear range of the method was from 0 to 16 μg L(-1) with a detection limit of 0.056 μg L(-1) for Hg(2+). The quantum yields of the ligand and its Hg(2+) complex were found to be 0.1206 and 0.0757 respectively. Both the ligand and its Hg(2+) complex have been studied computationally (Ab-initio, Hartree Fock method) to get their optimized structure and other related physical parameters, including bond lengths, bond angles, dipole moments, orbital interactions etc. The binding sites of the ligand to the Hg(2+) ion as obtained from the theoretical calculations were well supported by (1)H NMR titration. The interference of foreign ions was negligible. This method has been successfully applied to the determination of mercury(II) in industrial waste water.

Activation energies of photoinduced unimolecular, bimolecular and termolecular processes on silica gel surfaces

Activation energies for energy and electron transfer have been measured in various systems on silica gel. In the case of ion-electron recombination, a facile technique involving fluorescence recovery is described which complements diffuse reflectance spectroscopy in the study of these systems. In bimolecular anthracene/azulene systems, activation energies have been shown to be independent of pre-treatment temperature in the range 25-210 °C, demonstrating that physisorbed water plays little role in determining diffusion rates on silica gel. In a ternary anthracene/azulene/perylene system, we have for the first time presented comparative activation energies for the diffusion of azulene and its radical cation, and have shown a greater activation energy for diffusion of the latter species.

Metal Ion Enhanced Charge Transfer in a Terpyridine-bis-Pyrene System

The synthesis, electrochemical and photophysical properties of a branched molecule 3,5-bis(pyrene-1-yl)-4'-phenyl-2,2':6',2″-terpyridine are reported. Spectroscopy in different solvents reveals that an optical electron transfer from the pyrene donor to the terpyridyl electron acceptor can occur in polar media, as the system displays both charge transfer (CT) absorption and CT emission. Furthermore, the study of the zinc complex as well as the bis-protonated form shows an enhancement of the electron transfer character of the system, by an increase of the acceptor strength. This is accompanied by a large increase of the non-radiative processes. With sub-nanosecond transient absorption spectroscopy, the CT state, consisting of the pyrene radical cation and the terpyridine radical anion, has been detected. At room temperature, the study of the nanosecond transient absorption spectra reveals the formation of a low-lying triplet excited state that we attribute to the pyrene moiety through which the CT state decays. At 77K, the absence of the terpyridine triplet emission also suggests the population of a low-lying triplet state of the pyrene unit.

Pyrene–p-tert-butylcalixarenes inclusion complexes formation: a surface photochemistry study

Diffuse reflectance and luminescence techniques were used to study the photophysics and photochemistry of pyrene within p-tert-butylcalix[n]arenes with n = 4, 6, and 8, and to study their ability to form inclusion complexes in heterogeneous media. Evidences for inclusion complex formation were found for the three hosts under study. Ground state diffuse reflectance results have shown the formation of ground state dimers of pyrene inside the cavity of calix[6]arene and calix[8]arene, with this feature much more evident for calix[6]arene. For calix[4]arene, only a monomer fits inside the cavity and the presence of pyrene microcrystals outside the cavity was detected. A luminescence lifetime distribution analysis was performed, revealing the presence of prompt emissions from the pyrene microcrystals outside the cavity in the case of calix[4]arene and from the constrained dimers inside the cavities of calix[6]arene and calix[8]arene. Transient absorption results have shown the presence of pyrene radical cation and also of trapped electrons for the three hosts under study. The formation of the phenoxyl radical of the calixarene following the laser pulsed excitation of pyrene at 355 nm is increased for calix[6]arene and calix[8]arene. This feature is particularly relevant for calix[6]arene, suggesting a very favourable situation for the hydrogen atom abstraction to occur. The analysis of the degradation products revealed the presence of hydroxypyrene as a major photodegradation product for the three hosts. Dihydro-hydroxypyrene was also formed in the case of calix[6]arene and calix[8]arene. The formation of the calixarene's phenoxyl radical and subsequent hydrogen abstraction is consistent with the formation of dihydro-dihydroxypyrene.

Ion–electron recombination on silica gel surfaces: experiment and modelling

Kinetics on silica gel and other solid, porous surfaces are often complex. In this paper we have studied the decay kinetics of radical cations produced following multiphoton ionisation on silica gel, and have characterised these using an empirical model. Trends in kinetics have been observed both as a function of concentration and of temperature. Concentration dependent studies suggest heterogeneity of surface adsorption, both in terms of the nature of adsorption sites and aggregation effects. Temperature dependent studies show that the activation energies for surface diffusion correlate with the size of the radical cation, suggesting that its movement rather than that of the electron dominates the observed kinetics. Monte Carlo simulations have been shown to give useful qualitative insights into the interpretation of the extracted parameters, in particular into how apparent distributions of rate constants can arise as a result of low surface dimensionality.

Kinetics of diffusion-assisted reactions in microheterogeneous systems

This review is focused on the basic theory of diffusion-assisted reactions in microheterogeneous systems, from porous solids to self-organized colloids and biomolecules. Rich kinetic behaviors observed experimentally are explained in a unified fashion using simple concepts of competing distance and time scales of the reaction and the embedding structure. We mainly consider pseudo-first-order reactions, such as luminescence quenching, described by the Smoluchowski type of equation for the reactant pair distribution function with a sink term defined by the reaction mechanism. Microheterogeneity can affect the microscopic rate constant. It also enters the evolution equation through various spatial constraints leading to complicated boundary conditions and, possibly, to the reduction of dimensionality of the diffusion space. The reaction coordinate and diffusive motion along this coordinate are understood in a general way, depending on the problem at hand. Thus, the evolution operator can describe translational and rotational diffusion of molecules in a usual sense, it can be a discrete random walk operator when dealing with hopping of adsorbates in solids, or it can correspond to conformational fluctuations in proteins. Mathematical formulation is universal but physical consequences can be different. Understanding the principal features of reaction kinetics in microheterogeneous systems enables one to extract important structural and dynamical information about the host environments by analyzing suitably designed experiments, it helps building effective strategies for computer simulations, and ultimately opens possibilities for designing systems with controllable reactivity properties.

Various aspects of the constraints imposed on the photochemistry of systems in porous silica

This manuscript briefly reviews the photochemistry of organic molecules on porous silica (or SiO2). To gain an understanding of the chemistry on silica, data are displayed and discussed with respect to studies in homogeneous solution. In particular, the exact dimensionality of kinetic processes on porous SiO2 is a matter for debate. Hence, units of concentration of an adsorbate on the surface are expressed as moles per nanometer squared and as moles per liter, in order to compare with solution. Many studies show that organic molecules adsorb to SiO2 via the surface silanol (or surface hydroxyl OH) groups. The adsorption is heterogeneous, due to various clusters of silanol groups and to charge transfer (CT) sites. Photophysical studies clearly show these effects. The photo-induced reactions on SiO2 may be described by 'fractal' approaches, but a 'Gaussian' approach is often more useful to the photochemist. Photo-induced reactions occur via movement of the reactants on the surface, as in the case of the Langmuir-Hinshelwood (LH) mechanism or, as in the case of the Eley-Rideal (ER) mechanism, by bombardment of a surface bound excited state by a gaseous reactant, such as O2. Quenching of excited singlet states by O2 produces excited triplet states, which in turn are quenched to give singlet molecular oxygen. At room temperature the O2 quenching process on silica occurs by both mechanisms to approximately the same extent. However, the LH mechanism is dominant at lower temperatures and the ER mechanism is dominant at higher temperatures. Some quenchers, including carbon tetrachloride and tetranitromethane only quench by the LH mechanism giving rise to static quenching and chloro or nitro derivatives of the excited state. Photo-induced electron transfer between excited arenes and amines occurs readily, but the ionic products are short-lived compared to solution. This is due to the limited diffusion of the products on the surface, which in turn promotes back-electron transfer. Photoionization of arenes occurs on SiO2 via a two-photon process and gives very long-lived ions compared to solution. This is due to trapping of the photo-produced electrons by the SiO2 itself. Finally, the effects of co-adsorbants, including solvents, surfactants, and polymers, in photoreactions at the SiO2 surface are considered. The review ends with suggestions for future studies.