RIKEN Review No. 44 (February, 2002): Focused on Magnetic Field and Spin Effects in Chemistry and Related Phenomena Quantum beats in confined radical pairs at a strong pulse microwave field Vitaly Gorelik, ∗1 Kiminori Maeda, ∗2 and Hisao Murai ∗1 ∗1 Department of Chemistry, Graduate School of Science, Tohoku University ∗2 Department of Chemistry, University of Tsukuba The effect of short microwave pulse irradiation on the spin correlated radical pairs generated by the hydrogen abstraction reaction of triplet excited xanthone with tert-butyl-substituted phenols in a micellar solution is investigated. The quantum beats with a doubled Rabi frequency are observed under spin-locking conditions. Theoretical analysis of the quantum beats is performed and shows that the spin dephasing process between the triplet states of the radical pair is the main cause of the decay of the double beats. Modulation of dipole-dipole interaction by molecular dynamics is proposed as a source of the triplet-triplet dephasing. Introduction The yield of a radical reaction can be affected by both static andresonantmicrowave(mw)externalmagneticfields. 1) The time-resolved transient absorption detected magnetic reso- nance(ADMR)techniqueusedinourexperimentsbelongsto the RYDMR family and is quite convenient for studying co- herent time evolution and dephasing processes. The resonant mwmagneticfieldinitiatesperiodicalchanginginradicalpair (RP) spin state populations. This phenomenon, referred to asquantumbeats,hasbeenasubjectofmajorinterestduring last several years. 2, 3) As was noted by Salikhov et al., 2) one can expect two kinds of oscillations for the population of the RP spin states depending on the amplitude of the irradiating resonant mw field (ω 1 = γ e B 1 ). At weak B 1 (ω 1 ≪ δ) the populations of triplet (T + ,T - ,T 0 ) and singlet (S) RP spin states oscillate with the Rabi frequency ω 1 (further referred to as single beats). Here δ = ω a - ω b , and ω a , ω b are the resonant frequencies of radicals a and b forming the RP, re- spectively. In the case of strong B 1 (ω 1 ≫ δ), mw irradiation affects both radicals in the RP and spin-locking occurs. In this case the oscillation frequency doubles (2ω 1 ) (called dou- ble beats). Itwasshown 3) thatthedecayofasinglebeatscan be described using singlet-triplet dephasing (STD) caused by random fluctuation of electron exchange interaction between radicals in the RP. Inthisreviewwepresenttheinvestigationofthemw-induced double quantum beats in RP of xanthone (Xn) with tert- butyl-substituted phenols in micellar solutions. We discuss the quantum beats observed employing another dephasing process between spin states of the RP (triplet-triplet dephas- ing, TTD). For calculations we use numerical solution of the modified Liouville equation in a single-site model. 4) Experimental We employed the technique of time-resolved ADMR to ob- serve quantum oscillations. A flow system is used to transfer the sample into the quartz optical cell where the reactions were initiated by a laser pulse (at the wavelength of 355nm). Axenonlampwasusedasaprobelightsource. Thetransient absorption signal was detected by monochromator at 490nm corresponding to the absorption band of xanthone ketyl rad- ical. 5) The quartz cell was positioned into an X-band sap- phire dielectric ring resonator. The setup could change the mw pulse duration with 0.25ns steps. More detailed setup description has appeared. 6) Aqueous micellar solutions of 2 × 10 -3 M Xn, and 2 × 10 -3 M tert-butyl-substituted phenols (2,4,6-tri -tert-butylphe- nol (TTBPh), 2,6-di -tert-butylphenol (DTBPh), para -tert- butylphenol (PTBPh)) were used as samples. SDS deter- gent (Wako) and other micelles (Landcaster) were used as received. Results and discussion ThetripletexcitedxanthonemoleculegeneratestheRPcom- posed of xanthone ketyl and phenoxyl-type radicals in the triplet state formed through hydrogen abstraction reaction from the phenols. The recombination reaction occurs exclu- sively in the singlet RP. 7, 8) The mw irradiation was applied with a delay time of 230ns after laser pulse. The delay time wasdeterminedbythetripletexitedxanthonequenchingpro- cessandthedepletionoftheS-T 0 mixedspinstates(αβ, βα) due to the recombination reaction. Since the mw pulse par- tially transfers the population of T + and T - states into the reactive S-T 0 mixed ones, it facilitates the decay of the RP absorbance signal (see in Fig. 1). Let us designate k rec , k esc , k rlx as the rate constants of geminate recombination, escape of radicals into the bulk, and the longitudinal relaxation pro- cesses from T ± into the S-T 0 mixed spin states, respectively. Denoting the total population of S-T 0 mixed states asM,the sum of the T + and T - state population asT , the concentra- tion of free radicals escaping from the micellar cage as R; assuming that k rec >> 4k rlx and using kinetic equations for electronspinstatepopulations,onecaneasilyderivesthefol- lowing approximated analytical expression for the mw pulse effect: I on - I off ≡ I (B 1 =0) - I (B 1 =0)= I (P 0 +ΔP ) - I (P 0 ) ∼ = - ΔP 2 (1 - X)exp{-(k rlx + k esc )t}- (1 - Y ) 97