Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
Study of the Spin−Orbit Charge Transfer Intersystem Crossing of Perylenemonoimide−Phenothiazine Compact Electron Donor/ Acceptor Dyads with Steady-State and Time-Resolved Optical and Magnetic Spectroscopies
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Yingjie Zhao,†,# Andrey A. Sukhanov,§,# Ruomeng Duan,∥ Ayhan Elmali,⊥ Yuqi Hou,† Jianzhang Zhao,*,† Gagik G. Gurzadyan,*,‡ Ahmet Karatay,*,⊥ Violeta K. Voronkova,*,§ and Chen Li*,∥ †
State Key Laboratory of Fine Chemicals, School of Chemical Engineering and ‡Institute of Artificial Photosynthesis, State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, P. R. China § Zavoisky Physical-Technical Institute, FRC Kazan Scientific Center of RAS, Kazan 420029, Russia ∥ School of Environment and Civil Engineering, Dongguan University of Technology, Dongguan 523808, Guangdong Province, P. R. China ⊥ Department of Engineering Physics, Faculty of Engineering, Ankara University, 06100 Beşevler, Ankara, Turkey S Supporting Information *
ABSTRACT: Compact electron donor/acceptor dyads were prepared, with perylenemonoimide (PMI) as the electron acceptor and phenothiazine (PTZ) as the electron donor, to study the relationship between the molecular geometry and the spin−orbit charge transfer (CT) intersystem crossing (SOCT-ISC) efficiency. The photophysical properties of the dyads were studied with steadystate and time-resolved optical and magnetic resonance spectroscopies. We found that PTZ is an ideal chromophore for molecular conformation restriction in the compact dyads, which exerts a significant effect on the electronic coupling between the donor and acceptor and thus on the photophysical properties such as UV−vis absorption, fluorescence, and ISC efficiency. Anomalous intensified fluorescence at elevated temperatures was observed, which can be fully rationalized by the bright twisted charge transfer (CT) state. The singlet oxygen quantum yields (ΦΔ) of the dyads are up to 57%, which are highly solvent-polarity-dependent. Femtosecond transient absorption and fluorescence spectroscopies indicate that the charge separation proceeds within 1 ps and the charge-recombination-induced ISC takes 2.8 ns. With time-resolved electron paramagnetic resonance spectroscopy, we confirmed the SOCT-ISC mechanism of the dyad, for which the electron spin polarization and the population rates of the sublevels of the triplet state (zero-field states TX, TY and TZ) are drastically different from those of the spin−orbit-couplinginduced ISC (for monobrominated PMI). Photodriven intermolecular energy-transfer and electron-transfer processes in the presence of a triplet energy acceptor or a sacrificial electron donor were also studied. The radical anion of PMI was reversibly produced with photoreduction for which near-IR absorption band in the range of 750−850 nm was observed. the aromatic molecules showing large planar π-conjugation frameworks. One reason is that, for the aromatic molecules with a planar π-conjugation framework, the electron exchange energy is large (J); thus, the energy gap between the S1 and T1 states of the molecule is large (2J; two states are with the same electronic configuration, except the electron spins are different); as a result, the coupling between S1 and T1 states is weak and the direction ISC S1 → T1 is forbidden to a large extent.18 There are a few strategies to achieve efficient ISC in heavyatom-free organic molecules,15,16 for instance, the approaches based on exciton-coupling effect,19,20 electron spin convert-
1. INTRODUCTION Triplet photosensitizers (PSs), i.e., compounds with the triplet excited state populated upon photoexcitation, are important in areas such as photoredox catalytic synthetic organic reactions, 1−4 photovoltaics, 5,6 photodynamic therapy (PDT),7−9 solar fuels (hydrogen production by water splitting, for example),10 and triplet−triplet-annihilation (TTA) upconversion.11−14 Concerning these applications, strong absorption of visible light, efficient intersystem crossing (ISC), and a long-lived triplet state are the desired photophysical properties for the triplet PSs.15−17 One crucial issue for designing of new triplet PSs is to attain efficient ISC with heavy-atom-free, simple molecular structures. This is challenging in photochemistry because the ISC is almost unpredictable in the absence of heavy atoms (such as Ir, Pt, Ru, I, Br, etc.) for © XXXX American Chemical Society
Received: May 23, 2019 Revised: June 25, 2019 Published: July 8, 2019 A
DOI: 10.1021/acs.jpcc.9b04896 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C er,21,22 radical enhanced ISC,23−27 the population of the double-excited state,28 and singlet fission.29 However, in most cases, the triplet PSs based on these mechanisms are synthetically demanding, and the chromophores used for these mechanisms are still limited. Thus, new strategies to attain efficient ISC based on simple molecular structures are highly desired. Concerning this aspect, charge recombination (CR)-induced ISC is a promising method to attain ISC in heavy-atom-free small organic molecules. CR-induced ISC has been known for decades.30−34 These molecules contain electron donors and acceptors, which are usually connected with a long, rigid linker. This molecular structural profile makes the synthesis difficult. Moreover, it is tricky to achieve triplet production via CR because the charge separation (CS) and CR are all spinselective processes. The first excited state is the singlet state; thus, the 1LE → 1CT → S0 is bound to prevail than 1LE → 1 CT → 3LE (LE: locally excited state).1,35,36 Moreover, the triplet-state quantum yields of these molecular systems,31,37 as well as the application of such electron donor/acceptor hydride triplet PSs, were rarely studied. The reason to use a long, rigid, and nonconjugated linker between the electron donor and acceptors in the conventional electron donor/acceptor dyads showing ISC is to reduce the electron coupling between the donor and acceptor and thus to reduce the electron exchange energy related to the charge transfer (CT) state.31 With extremely small J values (
