TY - JOUR

T1 - Time-domain shape of electron spin echo signal of spin-correlated radical pairs in polymer/fullerene blends

AU - Popov, Alexander A.

AU - Lukina, Ekaterina A.

AU - Rapatskiy, Leonid

AU - Kulik, Leonid V.

N1 - Publisher Copyright: © 2017 Elsevier Inc.

PY - 2017/3/1

Y1 - 2017/3/1

N2 - Temporal shape of electron spin echo (ESE) signal of photoinduced spin-correlated radical pairs (SCRP) in composite of conductive polymer P3HT and substituted fullerene PCBM is studied in details. ESE signals of radical pairs (RP) P3HT+/PCBM− are calculated in realistic model, taking into account finite microwave pulse length. Inhomogeneous broadening of resonant lines and interradical distance distribution are included. Experimentally observed ESE time-domain shape was found to contradict predictions of conventional SCRP theory, which would be valid in the case of very fast electron transfer. Thus, instantaneous formation of singlet SCRP is not the case for P3HT+/PCBM− pair, and spin system has enough time to evolve coherently during sequential electron transfer. While it is impossible to reproduce experimental data within simple singlet SCRP model, assumption of presence of additional – with respect to what is predicted by singlet SCRP theory – AE (absorption/emission) spin polarization gives convincing accordance with the experiment. Density matrix of RP P3HT+/PCBM− is a superposition of two contributions, namely the parts reflecting (i) antiphase polarization of original singlet-born SCRP and (ii) additional AE-polarization which is generated during initial stage of charge separation. AE-polarization affects experimental ESEEM (electron spin echo envelope modulation) traces, as well as ESE shape, making impossible their interpretation via simple singlet SCRP model. However, this effect can be eliminated by averaging of ESEEM traces over EPR spectral positions. Finally, choosing the optimal gate for ESE time-domain integration and proper microwave detection phase tuning are considered.

AB - Temporal shape of electron spin echo (ESE) signal of photoinduced spin-correlated radical pairs (SCRP) in composite of conductive polymer P3HT and substituted fullerene PCBM is studied in details. ESE signals of radical pairs (RP) P3HT+/PCBM− are calculated in realistic model, taking into account finite microwave pulse length. Inhomogeneous broadening of resonant lines and interradical distance distribution are included. Experimentally observed ESE time-domain shape was found to contradict predictions of conventional SCRP theory, which would be valid in the case of very fast electron transfer. Thus, instantaneous formation of singlet SCRP is not the case for P3HT+/PCBM− pair, and spin system has enough time to evolve coherently during sequential electron transfer. While it is impossible to reproduce experimental data within simple singlet SCRP model, assumption of presence of additional – with respect to what is predicted by singlet SCRP theory – AE (absorption/emission) spin polarization gives convincing accordance with the experiment. Density matrix of RP P3HT+/PCBM− is a superposition of two contributions, namely the parts reflecting (i) antiphase polarization of original singlet-born SCRP and (ii) additional AE-polarization which is generated during initial stage of charge separation. AE-polarization affects experimental ESEEM (electron spin echo envelope modulation) traces, as well as ESE shape, making impossible their interpretation via simple singlet SCRP model. However, this effect can be eliminated by averaging of ESEEM traces over EPR spectral positions. Finally, choosing the optimal gate for ESE time-domain integration and proper microwave detection phase tuning are considered.

KW - Interspin distance measurement

KW - P3HT

KW - PCBM

KW - Pulsed EPR

KW - Spin-correlated radical pairs

KW - PHOTOSYNTHETIC REACTION CENTERS

KW - PULSED EPR

KW - lnterspin distance measurement

KW - ESEEM

KW - BULK HETEROJUNCTIONS

KW - ENVELOPE MODULATION

KW - DOUBLE-QUANTUM COHERENCE

KW - POLYMER

KW - DISTANCE DETERMINATION

KW - CHARGE SEPARATION

KW - EPR SPECTROSCOPY

UR - http://www.scopus.com/inward/record.url?scp=85010976327&partnerID=8YFLogxK

U2 - 10.1016/j.jmr.2017.01.016

DO - 10.1016/j.jmr.2017.01.016

M3 - Article

C2 - 28157560

AN - SCOPUS:85010976327

VL - 276

SP - 86

EP - 94

JO - Journal of Magnetic Resonance

JF - Journal of Magnetic Resonance

SN - 1090-7807

ER -