TY - JOUR

T1 - Positive electronic exchange interaction and predominance of minor triplet channel in CIDNP formation in short lived charge separated states of D-X-A dyads

AU - Zhukov, Ivan

AU - Fishman, Natalya

AU - Kiryutin, Alexey

AU - Lukzen, Nikita

AU - Panov, Mikhail

AU - Steiner, Ulrich

AU - Vieth, Hans Martin

AU - Schäfer, Julian

AU - Lambert, Christoph

AU - Yurkovskaya, Alexandra

PY - 2020/1/7

Y1 - 2020/1/7

N2 - Previous transient absorption measurements using the magnetically affected reaction yield (MARY) technique for a series of rigidly linked electron donor/electron acceptor dyads (D-X-A) consisting of a triarylamine donor, a naphthalene diimide acceptor, and a meta-conjugated diethynylbenzene unit as a bridge had revealed the presence of electronic exchange interaction, J, in the photoexcited charge separated (CS) state. Here, we present results obtained by photochemically induced dynamic nuclear polarization (photo-CIDNP) that allows for determining the sign of J. By variation of the magnetic field from 1 mT to 9.4 T, pronounced absorptive maxima of CIDNP were detected for more than 20 1H nuclei disregarding the sign of their hyperfine coupling constants in the transient charge separated state, with positions of maxima close to those found by the MARY technique. Quantitative comparison of the observed CIDNP signals for various D-X-A dyads reveals an increase in the CIDNP enhancement factor with increasing population of the triplet state determined by MARY spectroscopy at zero magnetic field. For CIDNP of the methyl groups of the TAA donor dyads, we found in all studies a good linear dependence between the CIDNP signal amplitude and the initial population of the CS triplet state. The linear relationship together with the absorptive CIDNP allows us to conclude that (i) the sign of the electronic exchange interaction Jex is positive, (ii) CIDNP is formed predominantly in the vicinity of level anticrossing between the T+ and S electronic levels, and (iii) coherent triplet-singlet transitions are induced by hyperfine interaction and accompanied by simultaneous electron and nuclear spin flip, T+β→Sα.

AB - Previous transient absorption measurements using the magnetically affected reaction yield (MARY) technique for a series of rigidly linked electron donor/electron acceptor dyads (D-X-A) consisting of a triarylamine donor, a naphthalene diimide acceptor, and a meta-conjugated diethynylbenzene unit as a bridge had revealed the presence of electronic exchange interaction, J, in the photoexcited charge separated (CS) state. Here, we present results obtained by photochemically induced dynamic nuclear polarization (photo-CIDNP) that allows for determining the sign of J. By variation of the magnetic field from 1 mT to 9.4 T, pronounced absorptive maxima of CIDNP were detected for more than 20 1H nuclei disregarding the sign of their hyperfine coupling constants in the transient charge separated state, with positions of maxima close to those found by the MARY technique. Quantitative comparison of the observed CIDNP signals for various D-X-A dyads reveals an increase in the CIDNP enhancement factor with increasing population of the triplet state determined by MARY spectroscopy at zero magnetic field. For CIDNP of the methyl groups of the TAA donor dyads, we found in all studies a good linear dependence between the CIDNP signal amplitude and the initial population of the CS triplet state. The linear relationship together with the absorptive CIDNP allows us to conclude that (i) the sign of the electronic exchange interaction Jex is positive, (ii) CIDNP is formed predominantly in the vicinity of level anticrossing between the T+ and S electronic levels, and (iii) coherent triplet-singlet transitions are induced by hyperfine interaction and accompanied by simultaneous electron and nuclear spin flip, T+β→Sα.

KW - RADICAL-ION PAIRS

KW - DYNAMIC NUCLEAR-POLARIZATION

KW - WEAK MAGNETIC-FIELDS

KW - CONSECUTIVE BIRADICALS

KW - QUANTUM COHERENCE

KW - SPIN

KW - RECOMBINATION

KW - PHOTOLYSIS

KW - MANIFESTATION

KW - RELAXATION

UR - http://www.scopus.com/inward/record.url?scp=85077680393&partnerID=8YFLogxK

U2 - 10.1063/1.5131817

DO - 10.1063/1.5131817

M3 - Article

C2 - 31914738

AN - SCOPUS:85077680393

VL - 152

JO - Journal of Chemical Physics

JF - Journal of Chemical Physics

SN - 0021-9606

IS - 1

M1 - 014203

ER -