Samir Farid

Radiative and nonradiative electron transfer in contact radical-ion pairs

Abstract The relationship between radiative and nonradiative electron transfer is explored for return electron transfer processes in the contact radical-ion pairs formed by excitation of ground state CT complexes. Using a conventional nonadiabatic theory of electron transfer, absolute rate constants for nonradiative return electron transfer, varying over more than two orders of magnitude, can be predicted from information obtained from analyses of the corresponding radiative processes. The effects of solvent polarity, driving force and molecular dimension on the rates of nonradiative return electron transfer are studied.

Ketocoumarins: A new class of triplet sensitizers

Abstract Several derivatives of 3-ketocoumarins were prepared and are shown to have many of the photophysical criteria required for efficient triplet sensitizers. These compounds include 3-aroylcoumarins ( 1 ) and 3,3′- carbonylbiscoumarins ( 2 ). The aryl groups in 1 are either phenyl and substituted phenyl derivatives or heterocyclic groups such as thienyl and benzofuryl. The substituents on the coumarin moiety in 1 and 2 , if any, are alkoxy or dialkylamino. These compounds, with absorption maxima between 330 and 450 nm, have extinction coefficients in the range of 10 4 to almost 10 5 , which is an important criterion for efficient sensitization of thin films of polymers as those used in photoresists and lithography. The singlet-triplet intersystem crossing (isc) efficiencies of several derivatives approach unity. In others, however, a radiationless decay process competes with the isc. The decay process is particularly dominant in the asymmetrically substituted derivatives of 2 , but seems to be considerably suppressed in polymeric matrices. The triplet energies of these compounds range from ca . 48 to 60 kcal/mol. Some of these ketocoumarins show phosphorescence spectra that suggest the presence of “frozen-in” rotamers.

Accurate Oxidation Potentials of Benzene and Biphenyl Derivatives via Electron-Transfer Equilibria and Transient Kinetics

Nanosecond transient absorption methods were used to determine accurate oxidation potentials (E(ox)) in acetonitrile for benzene and a number of its alkyl-substituted derivatives. E(ox) values were obtained from a combination of equilibrium electron-transfer measurements and electron-transfer kinetics of radical cations produced from pairs of benzene and biphenyl derivatives, with one member of the pair acting as a reference. Using a redox-ladder approach, thermodynamic oxidation potentials were determined for 21 benzene and biphenyl derivatives. Of particular interest, E(ox) values of 2.48 +/- 0.03 and 2.26 +/- 0.02 V vs SCE were obtained for benzene and toluene, respectively. Because of a significant increase in solvent stabilization of the radical cations with decreasing alkyl substitution, the difference between ionization and oxidation potentials of benzene is approximately 0.5 eV larger than that of hexamethylbenzene. Oxidation potentials of the biphenyl derivatives show an excellent correlation with substituent sigma+ values, which allows E(ox) predictions for other biphenyl derivatives. Significant dimer radical cation formation was observed in several cases and equilibrium constants for dimerization were determined. Methodologies are described for determining accurate electron-transfer equilibrium constants even when dimer radical cations are formed. Additional equilibrium measurements in trifluoroacetic acid, methylene chloride, and ethyl acetate demonstrated that solvation differences can substantially alter and even reverse relative E(ox) values.

Exciplexes and Electron Transfer Reactions

Electronically excited molecules, being better electron donors and acceptors than their ground states, form charge-transfer complexes (exciplexes) which can lead to radical ions. Exciplex emission is widely used to probe polymers and organized media such as membranes and micelles. Exciplexes are also intermediates in photoreactions that lead to unique products. Photochemical electron-transfer processes, which are the basis of silver halide photography and electrophotography, are involved in many reactions of wide scope. Recent studies have led to the discovery of several electron-transfer photooxygenations with a diversity that will probably rival that of singlet oxygen. Both exciplex emission and photochemical electron transfer play important roles in organic photochemistry.

The design of photoreactive polymer systems for imaging processes

Abstract The use of polymers in photoreactive imaging systems depends upon the interrelationship between the polymer physical properties and the photosensitive response. Modifications of the structure and physical properties by synthetic means permit control over the physical properties, such as solubility, melting point, glass transition temperature, and crystallinity. Specific polymer imaging systems depend upon these properties in order to function. Modifications of light absorbing chromophores and understanding the factors controlling sensitization permit adjustment of the wavelength response over the range from 250–650 nm. Triplet energy transfer from optical sensitizers to the photoreactive moieties is the most probable mechanism of sensitization. We have found that competing side reactions such as oxidation and photoreactions of the sensitizer can reduce the efficiency of sensitization.

Bimolecular Electron Transfers That Follow a Sandros―Boltzmann Dependence on Free Energy

Rate constants (k) for exergonic and endergonic electron-transfer reactions of equilibrating radical cations (A(•+) + B ⇌ A + B(•+)) in acetonitrile could be fit well by a simple Sandros-Boltzmann (SB) function of the reaction free energy (ΔG) having a plateau with a limiting rate constant k(lim) in the exergonic region, followed, near the thermoneutral point, by a steep drop in log k vs ΔG with a slope of 1/RT. Similar behavior was observed for another charge shift reaction, the electron-transfer quenching of excited pyrylium cations (P(+)*) by neutral donors (P(+)* + D → P(•) + D(•+)). In this case, SB dependence was observed when the logarithm of the quenching constant (log k(q)) was plotted vs ΔG + s, where the shift term, s, equals +0.08 eV and ΔG is the free energy change for the net reaction (E(redox) - E(excit)). The shift term is attributed to partial desolvation of the radical cation in the product encounter pair (P(•)/D(•+)), which raises its free energy relative to the free species. Remarkably, electron-transfer quenching of neutral reactants (A* + D → A(•-) + D(•+)) using excited cyanoaromatic acceptors and aromatic hydrocarbon donors was also found to follow an SB dependence of log k(q) on ΔG, with a positive s, +0.06 eV. This positive shift contrasts with the long-accepted prediction of a negative value, -0.06 eV, for the free energy of an A(•-)/D(•+) encounter pair relative to the free radical ions. That prediction incorporated only a Coulombic stabilization of the A(•-)/D(•+) encounter pair relative to the free radical ions. In contrast, the results presented here show that the positive value of s indicates a decrease in solvent stabilization of the A(•-)/D(•+) encounter pair, which outweighs Coulombic stabilization in acetonitrile. These quenching reactions are proposed to proceed via rapidly interconverting encounter pairs with an exciplex as intermediate, A*/D ⇌ exciplex ⇌ A(•-)/D(•+). Weak exciplex fluorescence was observed in each case. For several reactions in the endergonic region, rate constants for the reversible formation and decay of the exciplexes were determined using time-correlated single-photon counting. The quenching constants derived from the transient kinetics agreed well with those from the conventional Stern-Volmer plots. For excited-state electron-transfer processes, caution is required in correlating quenching constants vs reaction free energies when ΔG exceeds ∼+0.1 eV. Beyond this point, additional exciplex deactivation pathways-fluorescence, intersystem crossing, and nonradiative decay-are likely to dominate, resulting in a change in mechanism.

Relationship between exciplex fluorescence and electron transfer in radical ion pairs

Abstract The relationship between non-radiative (thermal) return electron transfer within radical ion pairs and the fluorescence spectra of exciplexes is discussed. Those exciplexes in which charge transfer from the donor to the acceptor is nearly complete are essentially contact radical ion pairs, and their fluorescence is a return electron transfer process. In particular, the energy and width of the exciplex fluorescence spectrum are directly related to the electron tranfer reorganization parameters. From the emission spectra of the exciplexes formed by several common acceptors with alkylbenzenes as donors it is found that the reorganization energy for electron transfer decreases with increasing molecular size of the acceptor. The spectra also indicate that in the solvents used in this study, the reorganization parameters with dicyanoanthracene, tetracyanoanthracene, dicyanonaphthalene and dicyanobenzene as acceptors are essentially constant for different alkylbenzene donors. With tetracyanobenzene and, most strikingly, pyromellitic dianhydride, however, the reorganization energy is found to decrease with decreasing oxidation potential of the alkylbenzene donor. The relevance of these observations to the interpretation of the rates of return electron transfer in these system as a function of reaction exothermicity is discussed.

Selected aspects of photochemistry in polymer media

Abstract A number of selected aspects of the reciprocal interactions of polymers with excited solutes or polymer-bound chromophores are given. The role of free volume, glass-transition temperature (Tg), microscopic viscosity, and polarity or environments to which excited molecules are exposed is emphasized in order to illustrate possible manners in which the photophysical, photochemical, and subsequent chemistries can be influenced. Many of these factors can affect several monomolecular and bimolecular processes encountered in the sensitization of photocrosslinkable polymers: the photophysics of the sensitizer, energy transfer to the reactive sites on the polymer, and the formation of crosslinks. In fact, the triplet yields of aroylnaphthothiazole derivatives, a widely used class of sensitizers, are considerably higher in the more viscous polymeric matrices. These 0. in the polymer approach, however, a value of only ca. 0.7. As a result a search for more efficient triplet sensitizers led to a new class of compounds: 5- and 7-substituted 3-ketocoumarins. As a model for study of the bimolecular processes in polymers, we chose exciplex and excimer probes. The following conclusions were drawn: 1. Several exciplex and excimer emissions in polymeric media are considerably shifted to shorter wavelengths as compared with the maxima measured in fluid media, indicating that interactions are impaired in polymeric matrices. 2. Emissions from the polymer matrix above the glass-transition temperature are similar in wavelength and temperature dependence thereof to those observed in fluid solutions. 3. Improper orientation of the reactants in polymers is responsible for the shift observed in excimer emission and for a part or all of the shift in exciplex emissions. 4. Little if any difference is observed between exciplex emissions in polymers of low and moderate macroscopic polarities. This may be attributed to two causes: a) Due to improper orientation the dipole moment of the exciplex in the polymer is expected to be smaller and, therefore, less solvation energy can be gained. b) The segmental motion of the polymer required to properly solvate the complex is probably too slow at room temperature compared with the lifetime of the exciplex. 5. Bichromophoric molecules, which form exciplexes in fluid media, fail to reach an exciplex configuration when dissolved in polymers.

Photooxidative Cleavage of Organosilanes

Abstract Avoiding energy wasting return electron transfer reactions remains a central challenge in the design of high efficiency photoinduced electron transfer processes. One strategy for achieving this goal is to utilize compounds which as a result of electron transfer undergo rapid and irreversible chemical reaction in competion with the return electron process. Organosilanes represent a promising class of compounds that may fulfill this requirement. We have found that some organosilanes undergo rapid carbon-silicon bond cleavage when oxidized to their radical cations in photoinduced electron transfer reactions. Interestingly, these radical cation cleavage reactions occur by a rare nucleophile-assisted mechanism. We have investigated the reactivity of different classes of nucleophiles, and have determined the activation parameters for several of these reactions.

Inclusion complexes of pyrenylbutyrate with γ-cyclodextrin

Mixing 4-pyren-1-ylbutyrate ion with γ-cyclodextrin in aqueous solution leads to inclusion complexes that become evident from changed absorption spectra as well as protection of the included pyrene moieties from fluorescence quenchers. The inclusion proceeds in stages, yielding first a 1:1 complex, which then dimerizes to form a 2:2 complex. At 25 °C the respective equilibrium constants of complex formation are 1.3 × 103 and 5.2 × 104 l mol–1. The dimerization equilibrium has an unusual temperature dependence, with ΔH≠–16.7 kcal mol–1 and ΔS≠–35 cal K–1 mol–1.