Photochemical probes of intramolecular electron and energy transfer

Stereochemical Effects on Platinum Acetylide Two-Photon Chromophores

A series of cis-platinum(II) acetylide complexes containing two-photon-absorbing chromophores have been synthesized and characterized to explore the effects of stereochemistry on the nonlinear absorption properties. The molecules feature 4-(phenylethynyl)phenylethynylene (PE2), diphenylaminofluorene (DPAF), and benzothiazolylfluorene (BTF) ligands. The photophysical properties were investigated under one- and two-photon conditions and compared to the known trans analogues via UV-visible absorption, photoluminescence, femtosecond and nanosecond transient absorption (TA), nanosecond z-scan, and femtosecond two-photon absorption (2PA). The bent cis complexes exhibit blue shifts in the absorption, emission, femtosecond, and nanosecond TA spectra along with lower molar extinction coefficients and lower phosphorescence yields relative to the trans complexes suggesting less efficient Pt-induced spin-orbit coupling and intersystem crossing in the cis configuration. The cis chromophores are noncentrosymmetric and therefore show dipolar behavior with a pronounced 2PA in the 0-0 transition of the S₀ → S₁ band, while the trans complexes show quadrupolar behavior with a forbidden 0-0 transition. In the S₀ → S_n region, both cis and trans complexes show intense two-photon-absorption bands (up to 3700 GM by the peak cross section for cis-BTF) which contain a significant contribution from the excited state absorption (S₁ → S_n). All six complexes exhibit comparable nonlinear absorption response with a significant contribution from triplet-triplet absorption that slightly favors trans complexes but is more strongly dependent upon the structure of the π-conjugated chromophore.

Red electroluminescence of ruthenium sensitizer functionalized by sulfonate anchoring groups

We have synthesized five novel Ru(ii) phenanthroline complexes with an additional aryl sulfonate ligating substituent at the 5-position, as both photosensitizers for oxide semiconductor solar cells (DSSCs) and light emitting diodes (LEDs).

Triplet sensitization in an anionic poly(phenyleneethynylene) conjugated polyelectrolyte by cationic iridium complexes

We describe a systematic study of triplet sensitization in a poly(phenyleneethynylene) conjugated polyelectrolyte (CPE) in methanol solution by using a series of three cationic iridium complexes with varying triplet energy. The cationic iridium complexes bind to the anionic CPE by ion-pairing, leading to singlet state quenching of the polymer, and allowing for efficient back-transfer of triplet excitation energy to the polymer. Efficient (amplified quenching) of the polymer's fluorescence is observed for each iridium complex, with Stern-Volmer quenching constants in excess of 10(5) M(-1). Triplet sensitization is confirmed for two of the iridium complexes by monitoring the relative yield of the CPE triplet state by transient absorption spectroscopy. One of the iridium complexes does not sensitize the CPE triplet, and consideration of the energies of the three complexes allows us to bracket the triplet energy of the CPE within the range 1.95-2.26 eV.

Photophysics and light-activated biocidal activity of visible-light-absorbing conjugated oligomers

The photophysical properties of three cationic π-conjugated oligomers were correlated with their visible light activated biocidal activity vs S. aureus. The oligomers contain three arylene units (terthiophene, 4a; thiophene-benzotriazole-thiophene, 4b; thiophene-benzothiadiazole-thiophene, 4c) capped on each end by cationic -(CH2)3NMe3(+) groups. The oligomers absorb in the visible region due to their donor-acceptor-donor electronic structure. Oligomers 4a and 4b have high intersystem crossing and singlet oxygen sensitization efficiency, but 4c has a very low intersystem crossing efficiency and it does not sensitize singlet oxygen. The biocidal activity of the oligomers under visible light varies in the order 4a > 4b ≈ 4c.

Light-induced antibacterial activity of symmetrical and asymmetrical oligophenylene ethynylenes

The light-induced antibacterial activity of symmetric and asymmetric oligophenylene ethynylenes (OPEs) was investigated against Gram-positive (Staphylococcus aureus and Staphylococcus epidermidis) and Gram-negative (Escherichia coli) bacteria. To understand the light-induced biocidal effect better, the transient absorption and triplet lifetime of OPEs were studied in methanol and water. A higher triplet lifetime was observed for OPE samples in water than in methanol. The magnitudes of the changes in optical density (ΔOD) of the S-OPE-n(H) series of symmetric oligomers are much higher than that of the asymmetric OPE-n series in water and are generally correlated with the singlet oxygen yield. It was found that the antibacterial activity against both Gram-positive and Gram-negative bacteria is size-, concentration-, and time-dependent. The light-induced antibacterial activity may result from the coordinated interactions of membrane disruption and interfacial or intracellular singlet oxygen generation, and the dominant factor is most likely the latter. The results obtained in this study will aid in the design of more efficient biocides in the future.

Light and dark biocidal activity of cationic poly(arylene ethynylene) conjugated polyelectrolytes

In this paper we report a study of cationic poly(arylene ethynylene) conjugated polyelectrolytes. The objective of the study was to compare the behavior of a polymer where a thiophene has replaced a phenyl ring in poly(phenylene ethynylene) polycations (PPE) previously investigated. Properties of solution phase and physisorbed suspensions of the polymer on microspheres were investigated. The photophysical properties of the polymer are evaluated and used to understand the striking differences in biocidal activity compared to the PPE polymers previously examined. The principal findings are that the thiophene polymer has remarkable dark biocidal activity against Pseudomonas aeruginosa strain PAO1 but very little light-activated activity. The low light-activated biocidal activity of the thiophene polymer is attributed to a highly aggregated state of the polymer in aqueous solutions and on microspheres as a physisorbed coating. This results in low triplet yields and a very poor sensitization of singlet oxygen and other reactive oxygen intermediates. The highly effective dark biocidal activity of the thiophene-containing polymers is attributed to its high lipophilicity and the presence of accessible quaternary ammonium groups. The difference in behavior among the polymers compared provides insights into the mechanism of the dark process and indicates that aggregation of polymer can reduce light activated biocidal activity by suppressing singlet oxygen generation.

Efficient near-infrared polymer and organic light-emitting diodes based on electrophosphorescence from (tetraphenyltetranaphtho[2,3]porphyrin)platinum(II)

The new metalloporphyrin Pt(tptnp), where tptnp = tetraphenyltetranaphtho[2,3]porphyrin, has been prepared and subjected to photophysical and electrooptical device studies. In degassed toluene solution at room temperature Pt(tptnp) features efficient phosphorescence emission with lambda(max) 883 nm with a quantum efficiency of 0.22. The complex has been used as the active phosphor in polymer and organic light-emitting diodes. Polymer light-emitting diodes based on a spin-coated emissive layer consisting of a blend of Pt(tptnp) doped in poly(9-vinylcarbazole) and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole exhibit near-IR emission with lambda(max) 896 nm, with a maximum external quantum efficiency (EQE) of 0.4% and a maximum radiant emittance of 100 muW/cm(2). Organic light-emitting diodes prepared via vapor deposition of all layers and that feature an optimized multilayer hole injection and electron blocking layer heterostructure with an emissive layer consisting of 4,4'-bis(carbazol-9-yl)biphenyl (CBP) doped with Pt(tptnp) exhibit a maximum EQE of 3.8% and a maximum radiant emittance of 1.8 mW/cm(2). The polymer and organic light-emitting diodes characterized in this study exhibit record high efficiency for devices that emit in the near-IR at lambda >800 nm.

Variable-band-gap poly(arylene ethynylene) conjugated polyelectrolytes adsorbed on nanocrystalline TiO(2): photocurrent efficiency as a function of the band gap

A series of poly(arylene ethynylene) conjugated polyelectrolytes (CPEs) substituted with carboxylic acid side groups have been synthesized and characterized. The polymers feature a backbone consisting of a carboxylated dialkoxyphenylene-1,4-ethynylene unit alternating with a second arylene ethynylene moiety of variable electron demand. The HOMO-LUMO gap is varied across the series, giving rise to a set of four polymers that have absorption maxima ranging from 404 to 495 nm. The CPEs adsorb effectively from solution onto nanostructured TiO(2) films, giving rise to TiO(2)/CPE films that absorb approximately 90% of the incident light at the absorption band maximum. The photocurrent generation efficiency of the TiO(2)/CPE films was examined in a solar cell configuration using an I(3)(-)/I(-) propylene carbonate electrolyte and a Pt/fluorine-doped tin oxide counter electrode. Most of the films exhibit good photocurrent generation efficiency with a peak quantum efficiency of approximately 50% at wavelengths corresponding to the polymers' absorption band maximum. Interestingly, the photocurrent generation efficiency for the lowest-band-gap polymer is substantially lower compared to the other three systems. This effect is attributed to efficient nonradiative decay of excitons at trap sites arising from interchain contacts distal from the TiO(2)/CPE interface.

Light-induced biocidal action of conjugated polyelectrolytes supported on colloids

A series of water soluble, cationic conjugated polyelectrolytes (CPEs) with backbones based on a poly(phenylene ethynylene) repeat unit structure and tetraakylammonium side groups exhibit a profound light-induced biocidal effect. The present study examines the biocidal activity of the CPEs, correlating this activity with the photophysical properties of the polymers. The photophysical properties of the CPEs are studied in solution, and the results demonstrate that direct excitation produces a triplet excited-state in moderate yield, and the triplet is shown to be effective at sensitizing the production of singlet oxygen. Using the polymers in a format where they are physisorbed or covalently grafted to the surface of colloidal silica particles (5 and 30 microm diameter), we demonstrate that they exhibit light-activated biocidal activity, effectively killing Cobetia marina and Pseudomonas aeruginosa. The light-induced biocidal activity is also correlated with a requirement for oxygen suggesting that interfacial generation of singlet oxygen is the crucial step in the light-induced biocidal activity.

Photophysics of Diplatinum Polyynediyl Oligomers: Chain Length Dependence of the Triplet State in sp Carbon Chains

The series of polyynes with the structure trans, trans-[Ar-Pt(P 2)-(C[triple bond]C) n -Pt(P 2)-Ar], where P = tri( p-tolyl)phosphine, Ar = p-tolyl, and n = 3, 4, 5, 6 (6, 8, 10, 12 sp carbon atoms), has been subjected to a comprehensive photophysical investigation. At low temperature ( T < 140 K) in a 2-methyltetrahydrofuran (MTHF) glass, the complexes exhibit moderately efficient phosphorescence appearing as a series of narrow (fwhm < 200 cm (-1)) vibronic bands separated by ca. 2100 cm (-1). The emission is assigned to a (3)pi,pi* triplet state that is concentrated on the sp carbon chain, and the vibronic progression arises from coupling of the excitation to the -C[triple bond]C- stretch. The 0-0 energy of the phosphorescence decreases with increasing sp carbon chain length, spanning a range of over 6000 cm (-1) across the series. Transient absorption spectroscopy carried out at ambient temperature confirms that the (3)pi,pi* triplet is produced efficiently, and it displays a strongly allowed triplet-triplet absorption. In the MTHF solvent glass ( T < 140 K), the emission lifetimes increase with emission energy. Analysis of the triplet nonradiative decay rates reveals a quantitative energy gap law correlation. The nonradiative decay rates can be calculated by using parameters recovered from a single-mode Franck-Condon fit of the emission spectra.

Platinum Acetylide Two-Photon Chromophores

To explore the photophysics of platinum acetylide chromophores with strong two-photon absorption cross-sections, we have investigated the synthesis and spectroscopic characterization of a series of platinum acetylide complexes that feature highly pi-conjugated ligands substituted with pi-donor or -acceptor moieties. The molecules (numbered 1-4) considered in the present work are analogs of bis(phenylethynyl)bis(tributylphosphine)platinum(II) complexes. Molecule 1 carries two alkynyl-benzothiazolylfluorene ligands, and molecule 2 has two alkynyl-diphenylaminofluorene ligands bound to the central platinum atom. Compounds 3 and 4 possess two dihexylaminophenyl substituents at their ends and differ by the number of platinum atoms in the oligomer "core" (one vs two in 3 and 4, respectively). The ligands have strong effective two-photon absorption cross-sections, while the heavy metal platinum centers give rise to efficient intersystem crossing to long-lived triplet states. Ultrafast transient absorption and emission spectra demonstrate that one-photon excitation of the chromophores produces an S1 state delocalized across the two conjugated ligands, with weak (excitonic) coupling through the platinum centers. Intersystem crossing occurs rapidly (Kisc approximately 1011 s-1) to produce the T1 state, which is possibly localized on a single conjugated fluorenyl ligand. The triplet state is strongly absorbing (epsilonTT > 5 x 104 M-1 cm-1), and it is very long-lived (tau > 100 micro s). Femtosecond pulses were used to characterize the two-photon absorption properties of the complexes, and all of the chromophores are relatively efficient two-photon absorbers in the visible and near-infrared region of the spectrum (600-800 nm). The complexes exhibit maximum two-photon absorption at a shorter wavelength than 2lambda for the one-photon band, consistent with the dominant two-photon transition arising from a two-photon-allowed gerade-gerade transition. Nanosecond transient absorption experiments carried out on several of the complexes with excitation at 803 nm confirm that the long-lived triplet state can be produced efficiently via a sequence involving two-photon excitation to produce S1, followed by intersystem crossing to produce T1.

A fulleropyrrolidine end-capped platinum-acetylide triad: the mechanism of photoinduced charge transfer in organometallic photovoltaic cells

The fullerene end-capped platinum acetylide donor-acceptor triad Pt(2)ThC(60) was synthesized and characterized by using photophysical methods and photovoltaic device testing. The triad consists of the platinum acetylide oligomer Ph-[triple bond, length as m-dash]-Pt(PBu3)2-[triple bond, length as m-dash]-Th-[triple bond, length as m-dash]-Pt(PBu3)2-[triple bond, length as m-dash]-Ph (Ph=phenyl and Th=2,5-thienyl, stereochemistry at both Pt centers is trans) that contains fulleropyrrolidine moieties on each of the terminal phenylene units. Electrochemistry of the triad reveals relatively low potential oxidation and reduction waves corresponding, respectively, to oxidation of the platinum acetylide and reduction of the fulleropyrrolidine units. Photoluminescence spectroscopy shows that the singlet and triplet states of the platinum acetylide chromophore are strongly quenched in the triad assembly, both in solution at ambient temperature as well as in a low-temperature solvent glass. The excited state quenching arises due to intramolecular photoinduced electron transfer to produce a charge separated state based on charge transfer from the platinum acetylide (donor) to the fulleropyrrolidine (acceptor). Picosecond time resolved absorption spectroscopy confirms that the charge transfer state is produced within 1 ps of photoexcitation, and it decays by charge recombination within 400 ps. Organic photovoltaic devices fabricated using spin-coated films of Pt2ThC60 as the active material operate with modest efficiency, exhibiting a short circuit photocurrent of 0.51 mA cm(-2) and an open circuit voltage of 0.41 V under 100 mW cm(-2)/AM1.5 illumination. The results are discussed in terms of the relationship between the mechanism of photoinduced electron transfer in the triad and the comparatively efficient photovoltaic response exhibited by the material.

Intramolecular Triplet Energy Transfer in Donor–Acceptor Molecules Linked by a Crown Ether Bridge

Bichromophoric compounds BP-C-NP and BP-C-NBD were synthesized with benzophenone chromophore (BP) as the donor, and 2-naphthyl (NP) and norbornadiene group (NBD) as the acceptor, respectively. Their intramolecular triplet energy transfer was examined. The bridges linking the donor and acceptors in these molecules involve a crown ether moiety complexing a sodium ion. Phosphorescence quenching, flash photolysis and photosensitized isomerization experiments indicate that intramolecular triplet energy transfer occurs with rate constants of about 3.3 x 10(5) and 4.8 x 10(5) s(-1) and efficiencies of about 33 and 42 % for BP-C-NP and BP-C-NBD, respectively. Theoretical calculations indicate that these molecules adopt conformations below room temperature which allow their two-end chromophores conducive to through-space energy transfer.

Synthesis, Photophysics, and Optical Limiting of Platinum(II) 4‘-Tolylterpyridyl Arylacetylide Complexes

A series of 4'-tolylterpyridyl platinum(II) complexes with different arylacetylide ligands, namely, phenylacetylide, 4-bromophenylacetylide, 4-nitrophenylacetylide, 4-methoxyphenylacetylide, 4-dimethylaminophenylacetylide, 1-naphthylacetylide, and 3-quinolinylacetylide, were synthesized. Their photophysical properties, such as electronic absorption spectra, emission characteristics at room temperature and 77 K, and transient difference absorption spectra, have been investigated. All of these complexes exhibit a metal-to-ligand charge-transfer (1MLCT) transition at ca. 420-430 nm in their electronic absorption spectra. For ttpy-Ph, ttpy-C6H4Br-4, ttpy-C6H4OCH3-4, ttpy-C6H4N(CH3)2-4, and ttpy-Np, an additional solvatochromic charge-transfer band appears at ca. 460-540 nm. This band is sensitive to the para substituents on the phenylacetylide ligand and is tentatively assigned to a metal- or/and acetylide-to-terpyridyl charge-transfer transition (i.e., a 1MLCT or/and 1LLCT transition). All of the complexes exhibit room-temperature phosphorescence. The emission can be attributed to a 3MLCT state except for ttpy-C6H4NO2-4, for which the emission likely originates from an intraligand 3pi,pi* state involving the nitrophenylacetylide ligand. For ttpy-C6H4OCH3-4, ttpy-C6H4N(CH3)2-4, and ttpy-Np, there probably is more than one low-energy state in close energy proximity, resulting in multiple exponential decays. In addition, the triplet transient absorption difference spectra of ttpy-Ph, ttpy-C6H4Br-4, ttpy-C6H4NO2-4, and ttpy-Quin exhibit moderately intense, broad absorption bands in the visible region and extending into the near-IR region, which likely originate from the same excited state that emits or from a state that is in equilibrium with the emitting state. It appears that the electron-rich arylacetylide ligands, especially 4-methoxyphenylacetylide and 4-dimethylaminophenylacetylide, cause a decrease of the emission efficiency and disappearance of the transient absorption. In contrast, the complexes that exhibit positive absorption bands in the visible spectral region of the triplet transient difference absorption spectra show substantial optical limiting for nanosecond laser pulses at 532 nm.

A Platinum Acetylide Polymer with Sterically Demanding Substituents: Effect of Aggregation on the Triplet Excited State

The effect of interchain interaction on the triplet excited state is explored in two Pt-acetylide polymers of the type [-trans-Pt(PBu(3))(2)-C triple bond C-Ar-C triple bond C-](n), where Ar is either 1,4-phenylene or is based on the pentiptycene unit (polymers 2 and 3, respectively). To explore the effect of interchain interaction in Pt-acetylide materials, the optical properties of parent polymer 2 are compared with those of polymer 3 in which interchain interaction is precluded by the sterically bulky pentiptycene moiety. Insight into the effect of the pentiptycene unit on packing in the solid state comes from the X-ray structure of monomer 1b, Ph-C triple bond C-[trans-Pt(PBu(3))(2)]-C triple bond C-Ar-C triple bond C-[trans-Pt(PBu(3))(2)]-C triple bond C-Ph. Spectroscopic studies indicate that weak phosphorescence emission from an interchain aggregate is observed from parent polymer 2, both in solution and in the solid state. By contrast, the photophysics of 3 is dominated by the intrachain triplet exciton. Interestingly, the phosphorescence emission of polymer 3 in the solid state is nearly superimposable with that of a single crystal of monomer 1b, suggesting that the solid polymer experiences an environment that is similar to that of the monomer in the crystal.

Facile preparation and photophysics of near-infrared luminescent lanthanide(III) monoporphyrinate complexes

The synthesis, characterization, and single crystal X-ray diffraction structures of a series of monoporphyrinate, trivalent lanthanide complexes with the monoanionic ligands hydridotris(1-pyrazolyl)borate (Tp) and (cyclopentadienyl)tris(diethylphosphinito)cobaltate (L(OEt)) having the general formulas M(TPP)(L) (M = Yb, Tm, Er, Ho, Nd, Pr; TPP = 5,10,15,20-tetraphenylporphyrinate; L = Tp, L(OEt)) are described. The photophysical properties of these complexes are also presented including their absorption, emission, and transient absorption properties.

Photophysics and electron transfer in poly(3-octylthiophene) alternating with Ru(II)- and Os(II)-bipyridine complexes

A series of soluble metal-organic polymers that contain Ru(II)- and Os(II)-polypyridine complexes interspersed within a pi-conjugated poly(3-octylthiophene) backbone are prepared. Detailed electrochemical and photophysical studies are carried out on the polymers and two model complexes to determine the extent that the metal-polypyridine units interact with the pi-conjugated system. The results indicate that there is a strong electronic interaction between the metal-based chromophores and the pi-conjugated organic segments, and consequently the photophysical properties are not simply based on the sum of the properties of the individual components. In the Ru(II) polymers, the metal-to-ligand charge-transfer (MLCT) excited state is slightly higher in energy than the 3 pi,pi* state of the poly(3-octylthiophene) backbone. This state ordering results in a material that displays only a weak MLCT luminescence and a long-lived transient absorption spectrum that is dominated by the 3 pi,pi* state. In the Os(II) polymer the MLCT state is lower in energy than the polythiophene-based 3 pi,pi* state and the "unperturbed" MLCT emission is observed. Finally, all of the metal-organic polymers undergo photoinduced bimolecular electron-transfer (ET) reactions with the oxidative quencher dimethyl viologen. Transient absorption spectroscopy reveals that photoinduced ET to dimethyl viologen produces the oxidized polymers, and in most cases, the transient spectra are dominated by features characteristic of a poly(3-octylthiophene) polaron.

Photooxidation of Diimine Dithiolate Platinium(II) Complexes Induced by Charge Transfer to Diimine Excitation

A photochemical and photophysical investigation was carried out on (tbubpy)Pt(II)(dpdt) and (tbubpy)Pt(II)(edt) (1 and 2, respectively, where tbubpy = 4,4'-di-tert-butyl-2,2'-bipyridine, dpdt = meso-1,2-diphenyl-1,2-ethanedithiolate and edt = 1,2-ethanedithiolate). Luminescence and transient absorption studies reveal that these complexes feature a lowest excited state with Pt(S)(2) --> tbubpy charge transfer to diimine character. Both complexes are photostable in deoxygenated solution; however, photolysis into the visible charge transfer band in air-saturated solution induces moderately efficient photooxidation. Photooxidation of 1 produces the dehydrogenation product (tbubpy)Pt(II)(1,2-diphenyl-1,2-ethenedithiolate) (4). By contrast, photooxidation of 2 produces S-oxygenated complexes in which one or both thiolate ligands are converted to sulfinate (-SO(2)R) ligands. Mechanistic photochemical studies and transient absorption spectroscopy reveal that photooxidation occurs by (1) energy transfer from the charge transfer to diimine excited state of 1 to (3)O(2) to produce (1)O(2) and (2) reaction between (1)O(2) and the ground state 1. Kinetic data indicates that excited state 1 produces (1)O(2) efficiently and that reaction between ground state 1 and (1)O(2) occurs with k approximately 3 x 10(8) M(-)(1) s(-)(1).

Ion-Pair Charge Transfer Photochemistry in Rhenium(I) Borate Salts

The photophysics and photochemistry of the salt [(bpy)Re(CO)(3)(py)(+)][BzBPh(3)(-)] (ReBo, where bpy = 2,2'-bipyridine, py = pyridine, Bz = C(6)H(5)CH(2) and Ph = C(6)H(5)) has been investigated in THF and CH(3)CN solutions. UV-visible absorption and steady-state emission spectroscopy indicates that in THF ReBo exists primairly as an ion-pair. A weak absorption band is observed for the salt in THF solution that is assigned to an optical ion-pair charge transfer transition. Stern-Volmer emission quenching studies indicate that BzBPh(3)(-) quenches the luminescent dpi (Re) --> pi (bpy) metal-to-ligand charge transfer excited state of the (bpy)Re(CO)(3)(py)(+) chromophore. The quenching is attributed to electron transfer from the benzylborate anion to the photoexcited Re(I) complex, (bpy(-)(*))Re(II)(CO)(3)(py)(+) + BzBPh(3)(-) --> (bpy(-)(*))Re(I)(CO)(3)(py) + BzBPh(3)(*). Laser flash photolysis studies reveal that electron transfer quenching leads to irreversible reduction of the Re(I) cation to (bpy(-)(*))Re(I)(CO)(3)(py). Photoinduced electron transfer is irreversible owing to rapid C-B bond fragmentation in the benzylboranyl radical, PhCH(2)BPh(3)(*) --> PhCH(2)(*) + BPh(3)(*). Quantitative laser flash photolysis experiments show that the quantum efficiency for production of the reduced complex (bpy(-)(*))Re(I)(CO)(3)(py) is unity, suggesting that C-B bond fragmentation in the benzylboranyl radical occurs more rapidly than return electron transfer within the geminate radical pair that is formed by photoinduced electron transfer.