Double-channel contact recombination of radical pairs subjected to spin conversion via the Delta g mechanism

The contact recombination from both singlet and triplet states of a radical pair is studied assuming that the spin conversion is carried out by the fast transversal relaxation and Delta g mechanism. The alternative HFI mechanism is neglected as being much weaker in rather large magnetic fields. The magnetic-field-dependent quantum yields of the singlet and triplet recombination products, as well as of the free radical production, are calculated for any initial spin state and arbitrary separation of radicals in a pair. The magnetic field effect is traced and its diffusional (viscosity) dependence is specified.     

Magnetic isotope effect on kinetic parameters and quantum beats of radical pairs in micellar solution studied by optically detected esr using pulsed microwave

We investigated the quantum beats, the oscillation between singlet and triplet states of radical pairs induced by the microwave field resonant to one of the component radicals. They were observed as the alternation of the yields of the component radicals by a nanosecond time-resolved optical absorption with the X-band (9.15 GHz) resonant microwave pulse. This technique was applied to the photochemical reaction of benzophenone, benzophenone-d(10), and benzophenone-carbonyl-(13)C in a sodium dodecylsulfate micellar solution with a step-by-step increase of the resonant microwave pulse width. The yields of the component radicals showed alternation with an increase of the microwave pulse width. This indicates that the radical pair retains spin coherence in the micellar solution. The magnetic isotope effect on the amplitude of the quantum beat was observed. The MW effect on the quantum beat of BP-(13)C decreases from 80% to 60% of that of BP by irradiation of the pi-pulse MW due to spin-locking. The kinetic parameters were also determined using the X- or Ku-band (17.44 GHz) region. They are almost similar to each other except for the intersystem recombination rate in the system of BP-(13)C, which may be slightly higher than those in other systems.     

Free energy gap laws for the pulse-induced and stationary fluorescence quenching by reversible charge transfer in polar solutions

The Stern-Volmer constants for either pulse-induced or stationary fluorescence being quenched by a contact charge transfer are calculated and their free energy dependencies (the free energy gap laws) are specified. The reversibility of charge transfer is taken into account as well as spin conversion in radical ion pairs, followed by their recombination in either singlet or triplet neutral products. The natural decay of triplets as well as their impurity quenching by ionization are accounted for when estimating the fluorescence quantum yield and its free energy dependence.     

The Origin of Magnetic Field Dependent Recombination in Alkylcobalamin Radical Pairs

Magnetic field effect studies of alkylcobalamin photolysis provide evidence for the formation of a reactive radical pair that is born in the singlet spin state. The radical pair recombination process that is responsible for the magnetic field dependence of the continuous-wave (CW) quantum yield is limited to the diffusive radical pair. Although the geminate radical pair of adenosylcob(III)alamin also undergoes magnetic field dependent recombination (A. M. Chagovetz and C. B. Grissom, J. Am. Chem. Soc. 115, 12152–12157, 1993), this process does not account for the magnetic field dependence of the CW quantum yield that is only observed in viscous solvents. Glycerol and ethylene glycol increase the microviscosity of the solution and thereby increase the lifetime of the spin-correlated diffusive radical pair. This enables magnetic field dependent recombination among spin-correlated diffusive radical pairs in the solvent cage. Magnetic field dependent recombination is not observed in the presence of nonviscosigenic alcohols such as isopropanol, thereby indicating the importance of the increased microviscosity of the medium. Paramagnetic radical scavengers that trap alkyl radicals that escape the solvent cage do not diminish the magnetic field effect on the CW quantum yield, thereby ruling out radical pair recombination among randomly diffusing radical pairs, as well as excluding the involvement of solvent-derived radicals. Magnetic field dependent recombination among alkylcobalamin radical pairs has been simulated by a semiclassical model of radical pair dynamics and recombination. These calculations support the existence of a singlet radical pair precursor.     

Controlling electron transfer dynamics in donor-bridge-acceptor molecules by increasing unpaired spin density on the bridge

A t-butylphenylnitroxide (BPNO*) stable radical is attached to an electron donor-bridge-acceptor (D-B-A) system having well-defined distances between the components: MeOAn-6ANI-Ph(BPNO*)-NI, where MeOAn=p-methoxyaniline, 6ANI=4-(N-piperidinyl)naphthalene-1,8-dicarboximide, Ph=phenyl, and NI=naphthalene-1,8:4,5-bis(dicarboximide). MeOAn-6ANI, BPNO*, and NI are attached to the 1, 3, and 5 positions of the Ph bridge, respectively. Time-resolved optical and EPR spectroscopy show that BPNO* influences the spin dynamics of the photogenerated triradical states 2,4(MeOAn+*-6ANI-Ph(BPNO*)-NI-*), resulting in slower charge recombination within the triradical, as compared to the corresponding biradical lacking BPNO*. The observed spin-spin exchange interaction between the photogenerated radicals MeOAn+* and NI-* is not altered by the presence of BPNO*. However, the increased spin density on the bridge greatly increases radical pair (RP) intersystem crossing from the photogenerated singlet RP to the triplet RP. Rapid formation of the triplet RP makes it possible to observe a biexponential decay of the total RP population with components of tau=740 ps (0.75) and 104 ns (0.25). Kinetic modeling shows that the faster decay rate is due to rapid establishment of an equilibrium between the triplet RP and the neutral triplet state resulting from charge recombination, whereas the slower rate monitors recombination of the singlet RP to ground state.     

Synthesis and photochemical properties of a photochromic iron(II) complex of hexaarylbiimidazole

The photochromic ligand bis(terpyridyl)hexaarylbiimidazole (bistpy-HABI) and the Fe(II) complex of bistpy-HABI with formula [{Fe(tpy)}2.bistpy-HABI](PF6)4.4H2O were synthesized and characterized. Bistpy-HABI is readily cleaved into a pair of terpyridyltriphenylimidazolyl radicals (tpy-TPI*) on irradiation with UV light. This photochemical reaction is completely reversible, and the light-induced radicals can thermally recombine to form bistpy-HABI in the dark. [{Fe(tpy)}2.bistpy-HABI]4+ is the first example of a transition-metal complex of an HABI derivative and was found to show photochromic reaction in solution. The spin state of the light-induced radical pair in a frozen matrix was investigated by ESR spectroscopy. The triplet state of the light-induced radical pair from [{Fe(tpy)}2.bistpy-HABI]4+, as well as that from bistpy-HABI, was confirmed to be a ground state or nearly degenerated with a singlet state. Kinetic studies on the radical recombination reaction in solution elucidated the decrease in the activation energy by forming the Fe(II) complex. This is the first observation of a decrease in the activation energy of the radical recombination reaction by the formation of a metal-coordinated radical complex. The syntheses, photochemical properties, and spin states of bistpy-HABI and [{Fe(tpy)}2.bistpy-HABI](PF6)4 are discussed.     

ANTHRALIN-DERIVED TRANSIENTS—II. FORMATION OF THE RADICAL BY SPONTANEOUS FRAGMENTATION OF BOTH SINGLET AND TRIPLET STATES OF THE 10,10''-DEHYDRODIMER: RADICAL PAIR MULTIPLICITY EFFECTS

The singlet and triplet states of the anthralin (1,8-dihydroxy-9-anthrone) dehydrodimer have been produced selectively in benzene via pulsed laser excitation and pulse radiolysis respectively. The lifetime of S1 is less than or equal to 30 ps, that of T1 short but unspecified. Both states fragment spontaneously to yield a pair of anthralin radicals. The singlet radical pair predominantly undergoes geminate recombination within the solvent cage. In contrast, the corresponding triplet radical pair undergoes essentially exclusive cage escape to give the anthralin free radical (lambda max 370, 490 and 720 nm) which recombines under normal diffusive conditions. Both recombination processes lead, at least in part, to one or more species which have been assigned as tautomeric forms of the original dimer. The anthralin free radical in benzene is insensitive to the vitamin E model 6-hydroxy-2,2,5,7,8-pentamethylchroman and reacts only slowly with oxygen.     

Production of free radicals and triplets from contact radical pairs and from photochemically generated radical ions

The quantum yields of triplets and free radicals (or radical ions) that escaped recombination in photochemically created primary radical pairs (or radical ion pairs) are calculated. As the products of monomolecular photodissociation, the neutral radicals appear at contact, while the ions are initially distributed over the space due to distant photoionization (bimolecular electron transfer) in the liquid solution. The diffusional dependence of the quantum yields is shown to be different when recombination starts from contact or from separated reactants. The experimental data for recombination of ionized perylene with aromatic amine counterions is well fitted with the noncontact initial distribution provided the recombination is also noncontact and even more distant than ionization.     

Ultrafast Decay of the Excited Singlet States of Thioxanthone by Internal Conversion and Intersystem Crossing

The experimental ultrafast photophysics of thioxanthone in several aprotic organic solvents at room temperature is presented, measured using femtosecond transient absorption together with high‐level ab initio CASPT2 calculations of the singlet‐ and triplet‐state manifolds in the gas phase, including computed state minima and conical intersections, transition energies, oscillator strengths, and spin–orbit coupling terms. The initially populated singlet ππ* state is shown to decay through internal conversion and intersystem crossing processes via intermediate nπ* singlet and triplet states, respectively. Two easily accessible conical intersections explain the favorable internal conversion rates and low fluorescence quantum yields in nonpolar media. The presence of a singlet–triplet crossing near the singlet ππ* minimum and the large spin–orbit coupling terms also rationalize the high intersystem crossing rates. A phenomenological kinetic scheme is proposed that accounts for the decrease in internal conversion and intersystem crossing (i.e. the very large experimental crescendo of the fluorescence quantum yield) with the increase of solvent polarity.     

Yields of excited states from geminate recombination of hydrocarbon radical ions

A theory similar to the radical pair model for CIDNP, etc., is used to describe the rate of loss of spin correlation in geminate pairs of radical ions produced by radiolysis. The ratio of the yields of triplet and singlet excited states and the effects of magnetic fields are discussed.     

Nanosecond time-resolved magnetic field effect on radical recombination in solution

The time dependence of the magnetic field effect on radical recombination in solution has been analyzed experimentally and theoretically. For the geminate recombination of anthracene anions and dimethylaniline cations in a polar solvent, the effect originates from a magnetic field dependent production of triplet states in an initially singlet phased radical pair, induced by hyperfine interaction of the unpaired electrons with the nuclei. The magnetic field dependence of the triplet yield shows a lifetime broadening of the energy levels of the radical pair if a short delay-time between radical production and triplet observation is chosen. The agreement of this delay-time dependent broadening effect with the theoretical results proves directly the coherence of the spin motion in the radical pairs.     

Radical pair substitution in chemically induced dynamic nuclear polarization. Co-operative effects

The problem of radical pair substitution in Chemically Induced Dynamic Nuclear Polarization is reconsidered. The singlet—triplet evolution in the radical pairs is described in a continuous fashion, assuming non-disturbance of the electron spin state during the scavenging reaction. The CIDNP effect of the recombination product of the secondary pair is demonstrated to result from the “co-operative effect” of singlet—triplet evolution in both the primary and the secondary pair. The hypothetical one proton case, in which the primary pair has different g-factors and a zero hyperfine interaction, and the secondary pair equal g-factors and a non-zero hyperfine interaction is treated qualitatively. Some examples are discussed in which older models lead to a faulty interpretation of experimental results.     

Mapping the influence of molecular structure on rates of electron transfer using direct measurements of the electron spin-spin exchange interaction

The spin-spin exchange interaction, 2J, in a radical ion pair produced by a photoinduced electron transfer reaction can provide a direct measure of the electronic coupling matrix element, V, for the subsequent charge recombination reaction. We have developed a series of dyad and triad donor-acceptor molecules in which 2J is measured directly as a function of incremental changes in their structures. In the dyads the chromophoric electron donors 4-(N-pyrrolidinyl)- and 4-(N-piperidinyl)naphthalene-1,8-dicarboximide, 5ANI and 6ANI, respectively, and a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor are linked to the meta positions of a phenyl spacer to yield 5ANI-Ph-NI and 6ANI-Ph-NI. In the triads the same structure is used, except that the piperidine in 6ANI is replaced by a piperazine in which a para-X-phenyl, where X = H, F, Cl, MeO, and Me(2)N, is attached to the N' nitrogen to form a para-X-aniline (XAn) donor to give XAn-6ANI-Ph-NI. Photoexcitation yields the respective 5ANI(+)-Ph-NI(-), 6ANI(+)-Ph-NI(-), and XAn(+)-6ANI-Ph-NI(-) singlet radical ion pair states, which undergo subsequent radical pair intersystem crossing followed by charge recombination to yield (3)NI. The radical ion pair distances within the dyads are about 11-12 A, whereas those in the triads are about approximately 16-19 A. The degree of delocalization of charge (and spin) density onto the aniline, and therefore the average distance between the radical ion pairs, is modulated by the para substituent. The (3)NI yields monitored spectroscopically exhibit resonances as a function of magnetic field, which directly yield 2J for the radical ion pairs. A plot of ln 2J versus r(DA), the distance between the centroids of the spin distributions of the two radicals that comprise the pair, yields a slope of -0.5 +/- 0.1. Since both 2J and k(CR), the rate of radical ion pair recombination, are directly proportional to V(2), the observed distance dependence of 2J shows directly that the recombination rates in these molecules obey an exponential distance dependence with beta = 0.5 +/- 0.1 A(-)(1). This technique is very sensitive to small changes in the electronic interaction between the two radicals and can be used to probe subtle structural differences between radical ion pairs produced from photoinduced electron transfer reactions.     

Photophysics and photochemistry of naphthoylnaphthvalene, and photoinduced valence isomerization of highly strained aromatic compounds (Or Quinones) yielding the valene-type isomers as well as related photochromism and photo-electro dualchromism

The lowest excited singlet state of naphthoylnaphthvalene (NNV) undergoes valence isomerization yielding ground-state naphthoylnaphthalene (NN) finally. Neither the lowest excited singlet nor triplet state of NN is formed upon excitation of NNV, and of particular interest in photoinduced NNV→NNV valence isomerization is the existence of an intermediate which is probably either a bond-cloven species or a valence isomer of NNV. The lowest excited triplet state of NNV populated in benzene deactivates to its ground state, but that populated in ethanol abstracts a hydrogen atom from the solvent molecule generating the NNV ketyl radical. Interestingly, this radical also undergoes rapid valence isomerization and recombination of two NN ketyl radicals thus formed yields naphthopinacol. Synthesis of poly-tert-butylpolyacenes, tri-tert-butylisobenzofuran and tri-tert-butylpolyacenequinones, and furthermore, their photoinduced valence isomerization yielding the valene-type isomers as well as related photochromism and photo-electro dualchromism are presented.     

Photoinduced omega-bond dissociation in the higher excited singlet (S2) and lowest triplet (T1) states of a benzophenone derivative in solution

Photochemical properties of photoinduced omega-bond dissociation in p-benzoylbenzyl phenyl sulfide (BBPS) in solution were investigated by time-resolved EPR and laser flash photolysis techniques. BBPS was shown to undergo photoinduced omega-bond cleavage to yield the p-benzoylbenzyl radical (BBR) and phenyl thiyl radical (PTR) at room temperature. The quantum yield (phi(rad)) for the radical formation was found to depend on the excitation wavelength, i.e., on the excitation to the excited singlet states, S2 and S1 of BBPS; phi(rad)(S2) = 0.65 and phi(rad)(S1) = 1.0. Based on the CIDEP data, these radicals were found to be produced via the triplet state independent of excitation wavelength. By using triplet sensitization of xanthone, the efficiency (alpha(rad)) of the C-S bond fission in the lowest triplet state (T1) of BBPS was determined to be unity. The agreement between phi(rad)(S1) and alpha(rad) values indicates that the C-S bond dissociation occurs in the T1 state via the S1 state due to a fast intersystem crossing from the S1 to the T1 state. In contrast, the wavelength dependence of the radical yields was interpreted in terms of the C-S bond cleavage in the S2 state competing with internal conversion from the S2 to the S1 state. The smaller value of phi(rad)(S2) than that of phi(rad)(S1) was proposed to originate from the geminate recombination of singlet radical pairs produced by the bond dissociation via the S2 state. Considering the electronic character of the excited and dissociative states in BBPS showed a schematic energy diagram for the omega-bond dissociation of BBPS.     

Electron donor-bridge-acceptor molecules with bridging nitronyl nitroxide radicals: influence of a third spin on charge- and spin-transfer dynamics

Appending a stable radical to the bridge molecule in a donor-bridge-acceptor system (D-B-A) is potentially an important way to control charge- and spin-transfer dynamics through D-B-A. We have attached a nitronyl nitroxide (NN*) stable radical to a D-B-A system having well-defined distances between the components: MeOAn-6ANI-Ph(NN*)-NI, where MeOAn = p-methoxyaniline, 6ANI = 4-(N-piperidinyl)naphthalene-1,8-dicarboximide, Ph = phenyl, and NI = naphthalene-1,8:4,5-bis(dicarboximide). MeOAn-6ANI, NN*, and NI are attached to the 1, 3, and 5 positions of the Ph bridge. Using both time-resolved optical and EPR spectroscopy, we show that NN* influences the spin dynamics of the photogenerated triradical states (2,4)(MeOAn(+)*-6ANI-Ph(NN*)-NI(-)*), resulting in slower charge recombination within the triradical compared to the corresponding biradical lacking NN*. The observed spin-spin exchange interaction between the photogenerated radicals MeOAn(+)(*) and NI(-)(*) is not altered by the presence of NN*, which only accelerates radical pair intersystem crossing. Charge recombination within the triradical results in the formation of (2,4)(MeOAn-6ANI-Ph(NN*)-(3)NI), in which NN* is strongly spin-polarized. Normally, the spin dynamics of correlated radical pairs do not produce a net spin polarization; however, net spin polarization appears on NN* with the same time constant as describes the photogenerated radical ion pair decay. This effect is attributed to antiferromagnetic coupling between NN* and the local triplet state (3)NI, which is populated following charge recombination. This requires an effective switch in the spin basis set between the triradical and the three-spin charge recombination product having both NN* and (3)NI present.     

Modulation of radical ion pair lifetimes by the presence of a third spin in rodlike donor-acceptor triads

It is well known that the molecular structure of an electron donor-acceptor system can be changed to optimize the electronic coupling between photogenerated radical ion pairs (PRPs), resulting in favorable charge separation (CS) and charge recombination (CR) rates. It would be far more convenient to avoid extensive synthetic modifications to the structure to achieve the same ends by perturbing the electronic properties of the PRP. We present here results on PRPs within rodlike donor-acceptor molecules having a covalently attached stable 2,2,6,6-tetramethylpiperidinoxyl radical (T*). The distances and orientations between all three radicals are highly restricted by the intervening molecular structure, making it possible to directly measure both the CR dynamics and the spin-spin exchange interaction, 2JPRP, between the radicals within the PRPs. The molecular triads studied are MeOAn-6ANI-PI-T* and MeOAn-6ANI-NI-T*, where MeOAn = p-methoxyaniline, 6ANI = 4-(N-piperidinyl)naphthalene-1,8-dicarboximide, NI = naphthalene-1,8:4,5-bis(dicarboximide), and PI = pyromellitimide. These molecules have been characterized using femtosecond and nanosecond transient absorption spectroscopy as well as measurements of 2JPRP using magnetic field effects on the triplet state yield resulting from CR. We find that T* enhances radical pair intersystem crossing (EISC), resulting in an increase or decrease in the PRP lifetime depending on the relative ordering of the energy levels of the PRP and the local neutral triplet states. This is especially pronounced when the PRP is nearly isoenergetic with the neutral triplet state, as is the case for MeOAn-6ANI-NI-T*. The dependence of the 3*NI and 3*6ANI yield on an applied external magnetic field shows a distinct resonance at 2JPRP, the magnitude of which is not perturbed by the presence of the third spin. The sensitivity of this system to changes in spin state may offer ways to externally control the radical ion pair dynamics using pulsed microwaves.     

Formation and decay of localized contact radical ion pairs in DNA hairpins

The dynamics of charge separation and charge recombination in synthetic DNA hairpins possessing diphenylacetylene-4,4'-dicarboxamide linkers have been investigated by means of femtosecond time-resolved transient absorption spectroscopy. The lowest excited singlet state of the linker is capable of oxidizing nearest neighbor adenine as well as guanine. A large wavelength shift in the transient absorption spectrum accompanies the conversion of the singlet linker to its anion radical, facilitating the investigation of electron-transfer dynamics. The rate constants for charge separation are dependent upon the oxidation potentials of the neighboring nucleobase donors but not upon the identity of nonnearest neighbors. Thus, the charge separation processes yield a contact radical ion pair in which the positive charge is localized on the neighboring nucleobase. Rate constants for charge recombination are dependent upon the identity of the first and second nearest-neighbor nucleobases but not more remote bases. This dependence is attributed to stabilization of the contact radical ion pair by interaction with its nearest neighbor. The absence of charge migration to form a base-pair separated radical ion pair is a consequence of Coulombic attraction in the contact radical ion pair and the low effective dielectric constant (epsilon < 7) experienced by the contact radical ion pair. Photoinduced charge injection to form a base-pair separated radical ion pair is necessary in order to observe charge migration.