Spin-orbit charge transfer intersystem crossing in perylenemonoimide-phenothiazine compact electron donor-acceptor dyads

Article abstract of DOI:10.1039/c8cc07012a

The spin-orbit charge transfer intersystem crossing (SOCT-ISC) of perylenemonoimide-phenothiazine compact dyads was shown to depend on the molecular geometry and the vector dipole orientations of the two chromophores, and the dyads show high triplet state

Full text of DOI:10.1039/c8cc07012a

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Spin–orbit charge transfer intersystem crossing
in perylenemonoimide–phenothiazine compact
electron donor–acceptor dyads†
b
Cite this: Chem. Commun., 2018,
54, 12329
Received 29th August 2018,
Accepted 4th October 2018
Yingjie Zhao, ‡a Ruomeng Duan,‡b Jianzhang Zhao *a and Chen Li
*
DOI: 10.1039/c8cc07012a
rsc.li/chemcomm
The spin–orbit charge transfer intersystem crossing (SOCT-ISC) of One of the critical requirements for these dyads to attain efficient
perylenemonoimide–phenothiazine compact dyads was shown to ISC is the orthogonal geometry between the electron donor and
depend on the molecular geometry and the vector dipole orienta- the acceptor. Under this circumstance, the charge recombination
tions of the two chromophores, and the dyads show high triplet (CR) will generate a molecular orbital angular momentum change,
state quantum yields (57%).
which compensates the electron spin angular momentum change
of the ISC, thus the ISC is facilitated. This is the so-called spin–
Triplet photosensitizers are important and have been used in orbit charge transfer ISC (SOCT-ISC).26,31,32 Naphthal–acridine,32
many areas, for instance, photo-redox catalytic organic reactions,1–5 julolidine–anthracene,26 bodipy–anthracene,27,29,33 and bodipy–
photovoltaics,6,7 hydrogen production with water splitting,8–10 and phenothiazine dyads showing SOCT-ISC were reported.30
photon upconversion.11–16 Strong absorption of the photoexcitation Previously, we studied the orthogonal perylenebisimide (PBI)–
light (UV, visible, or near IR), efficient intersystem crossing (ISC), phenothiazine (PTZ) dyads.34 However, no SOCT-ISC was observed
and a long-lived triplet excited state are the pivotal properties for a for the PBI–PTZ dyads, although photo-induced charge separation
triplet photosensitizer.17–20
(CS) and charge recombination (CR) were detected.34 As such we
A few approaches are available to attain efficient ISC.18,21 The concluded that the orthogonal geometry and CS/CR are not
most typical one is the heavy atom effect, for instance in the exclusively sufficient to achieve the efficient SOCT-ISC process.34
presence of atoms of Pt, Ru, Ir, I, Br, etc. However, the heavy atom It was proposed that the vector dipole orientation may also exert a
effect is not always efficient, especially for large chromophores. significant effect to the ISC efficiency.29 However, some challenges
Another approach is to establish a p–p* 2 n–p* transition. still remain in this area, for instance, the chromophore selection
However, the triplet photosensitizers based on this strategy usually for SOCT-ISC is still limited, the relationship between the mole-
show absorption in the UV or blue spectral range (o450 nm). cular structure and the SOCT-ISC efficiency is unclear, and no
Other approaches for efficient ISC are exciton coupling22–24 and application of the SOCT-ISC in any photophysical processes has
symmetry breaking charge transfer.25 However, the molecules been demonstrated.
showing such processes are usually synthetically demanding. As
In order to address the above challenges, herein we prepared
such, molecules with simple structures showing efficient ISC are perylenemonoimide (PMI)–phenothiazine (PTZ) compact dyads,
highly desirable. Unfortunately, it is still a challenge to design with PTZ as the electron donor and PMI as the electron acceptor
metal-free, simple organic molecules with efficient ISC.
(Scheme 1). The PTZ and PMI units are directly connected
Recently, ISC was observed in compact electron donor–acceptor together with C–N or C–C bonds, so the molecular conformation
dyads, i.e. directly connected electron donors and acceptors.26–30 is constraint and the geometry (co-planarity) of the dyads is
different. Due to the significant steric hindrance between the
two moieties, we assume that the two moieties in PMI-N-PTZ
adopt an energy minimized perpendicular geometry. Thus,
a State Key Laboratory of Fine Chemicals, School of Chemical Engineering,
Dalian University of Technology, E-208 West Campus, 2 Ling-Gong Road,
PMI-N-PTZ is assumed to show efficient SOCT-ISC.
Dalian 116024, P. R. China. E-mail: zhaojzh@dlut.edu.cn
b School of Environment and Civil Engineering, Dongguan University of Technology,
No. 1, Daxue Rd., Songshan Lake, Dongguan, Guangdong Province, P. R. China.
E-mail: lichen@mpip-mainz.mpg.de
On the other hand, an analogue with different conformation
restrictions was prepared (PMI-C-PTZ). The linkage between the two
moieties, i.e. the C–C bond, at the 3-position of PTZ is assumed to
experience minor rotational steric hindrance. With the Buchwald–
Hartwig reaction, PMI-DPA was also prepared as a reference com-
pound with the least restriction between the two moieties.
† Electronic supplementary information (ESI) available: Experimental details,
1H NMR, 13C NMR and HRMS spectra, absorption and fluorescence spectra,
electrochemistry data and DFT calculations. See DOI: 10.1039/c8cc07012a
‡ These authors contributed equally to this work.
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Fig. 2 Nanosecond transient absorption spectra of (a) PMI-N-PTZ.
(b) Decay trace at 480 nm. lex = 500 nm. c = 5.0 Â 10À6 M in deaerated
n-hexane, 20 1C.
Scheme 1 Molecular structures of PMI-N-PTZ, PMI-C-PTZ, PMI-DPA and
the reference compound PMI-Br.
The steady-state UV-Vis absorption spectra of the compounds
were studied (Fig. 1a). The absorption of PMI-N-PTZ is highly
identical to that of PMI-Br (Fig. 1a). This result indicates that
there is no significant electronic coupling between the PMI and
PTZ moieties, otherwise the absorption profile will be different
from that of the PMI moiety.26,35 We assume that the PTZ and
PMI moieties take the orthogonal geometry, and DFT optimiza-
tion indicated a molecular geometry with a dihedral angle of 951
between the two units (see the ESI,† Fig. S24), owing to the
geometry restriction.
For PMI-C-PTZ and PMI-DPA, with less steric hindrance, the
dihedral angle between the two moieties is found to be close to
501 (see the ESI,† Fig. S24), and as such a larger electronic
coupling between the two moieties than that in PMI-N-PTZ is
observed. As a result, the absorption of PMI-C-PTZ and PMI-DPA
becomes broad, structureless, and red-shifted as compared to that
of PMI-N-PTZ.
The fluorescence emission spectra of PMI derivatives were
studied (Fig. 1b). For the three PMI derivatives, compared with
PMI-Br, the structured fluorescence of the PMI moiety was
quenched even in non-polar solvents such as n-hexane (Fig. 1b).
The emission band at ca. 600 nm is broad and solvent polarity-
dependent, i.e., the emission became red-shifted and the inten-
sity decreased with increasing solvent polarity (see the ESI,†
Fig. S18). Thus, this broad emission band is attributed to the
CT emission (1CT - S0). PMI-N-PTZ gives the weakest fluores-
cence, due to complete CS. The fluorescence excitation spectra
of the compounds agree well with the UV-Vis absorption spectra
(see the ESI,† Fig. S25), which confirms the purity of the
compounds as well as the compliance of the Kasha’s rule of
the photophysics of the compounds.
To study the triplet state production of the PMI derivatives,
the nanosecond transient absorption spectra (ns-TA) of the com-
pounds were studied (Fig. 2 and ESI,† Fig. S20). For PMI-N-PTZ,
upon 500 nm pulsed laser excitation, a weak ground state
bleaching (GSB) band at 500 nm was observed. Moreover, two
strong excited state absorption (ESA) bands in the ranges of
330–440 nm and 440–600 nm were observed (in deaerated
n-hexane), which can be attributed to the T1 - Tn absorption
3
of PMI* and they are highly identical to the ns-TA spectra of
PMI-Br (see the ESI,† Fig. S20). The decay trace recorded at
3
480 nm gives a PMI* state lifetime of 200 ms for PMI-N-PTZ.
In aerated solution, the lifetime was reduced to 120 ns (see the
ESI,† Fig. S21), confirming the triplet state feature of the transient
species. To the best of our knowledge, this is the first report of the
triplet state of PMI produced by SOCT-ISC. It should be pointed
out that the triplet state of the dyads is non-emissive, probably
due to the small T1/S0 coupling matrix elements, but it is unlikely
due to any fast non-radiative decay process because the triplet
state is long-lived (123 ms and 200 ms, respectively).
1
Based on the results of ns TA spectra and the O2 quantum
yield (Table 1 and ESI,† Table S1) of the PMI–PTZ dyads in
different solvents, the ISC efficiency is dependent on the mole-
cular geometry as well as the solvent polarity. PMI-N-PTZ takes
the orthogonal geometry and shows the highest 1O2 quantum
yield. However, for PMI-C-PTZ and PMI-DPA, the geometry devi-
ates from orthogonal, the yields become much lower. Moreover,
in a non-polar solvent, the dyads give a strong triplet state
signal and the highest 1O2 quantum yields (FD = 57% in
1
n-hexane). In a polar solvent, however, the O2 quantum yield
decreased dramatically (FD o 1% in DCM). These results
indicate that the charge separated state (CSS) is involved in the
ISC.27,28,33,36,37 Due to the large electronic coupling between the
electron donor and acceptor in the dyads, the ISC pathway
should be the SOCT-ISC.38
The photophysical process of this SOCT-ISC mechanism is
shown in the energy diagram (Scheme 2). There are two kinds
of CR pathways from the CSS state. One is the ISC process to
produce the triplet state 3PMI*, and another is the CR pathway
that directly produces the ground state. Thus, the probable
Fig. 1 (a) UV-Vis absorption spectra of the compounds, c = 1.0 Â 10À5 M
in n-hexane; (b) fluorescence emission spectra (lex = 440 nm and A =
0.162); optically matched solutions were used (the concentration is varied
slightly). In n-hexane, 20 1C.
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Table 1 Photophysical properties of compounds (in n-hexane)
b
c
e
f
g
labs
ea
lem
FF
tF
krd
0.1
9.8
8.0
knr
FD
57 Æ 2
tT
PMI-N-PTZ
PMI-C-PTZ
PMI-DPA
PMI-Br
496
510
545
499
3.55
4.03
3.41
4.43
507/659
502/589
504/597
509
(0.1 Æ 0.03)/(0.5 Æ 0.03)
(1.4 Æ 0.1)/(44.0 Æ 0.6)
(2.4 Æ 0.2)/(42.6 Æ 0.5)
84.5 Æ 1
(4.4/5.2) Æ 0.1
(4.5/4.5) Æ 0.1
(4.6/5.3) Æ 0.1
(4.5) Æ 0.1
1.9
1.2
1.1
0.3
200 Æ 5
7 Æ 0.3
(123 Æ 5)i
44 Æ 5
h
—
18.8
15 Æ 1
(37 Æ 5)j
a
b
Molar absorption coefficient, in 104 MÀ1 cmÀ1
.
Fluorescence quantum yield at room temperature, in %, BDP-1 (FF = 4.4% in toluene, molecular
structure is shown in the ESI) was used as the standard for PMI-N-PTZ. BDP (FF = 72% in THF, molecular structure is shown in the ESI) was used as
c
d
the standard for PMI-C-PTZ, PMI-DPA and PMI-Br. Fluorescence lifetime at room temperature, in ns (lex = 445 nm). Radiative rate constant,
derived from the room temperature fluorescence data, kr = F/t, in 107 sÀ1
Singlet oxygen quantum yield, in %, with diiodoBodipy used as the standard (FD = 85% in toluene).
The measurements were with 1,3-diphenylisobenzofuran (DPBF) as a singlet oxygen scavenger (for details, see the ESI). Lifetime of the triplet
. Non-radiative rate constant, derived from the room temperature
e
f
fluorescence data, knr = (1 À F)/t, in 108 sÀ1
.
g
h
i
j
state at room temperature. Not observed. In toluene. In MeCN. The errors for some items were based on three independent measurements.
Fig. 3 (a) Delayed fluorescence spectra of the mixture of PMI-N-PTZ with
BPEA and (b) decay trace of the fluorescence at 470 nm. [PMI-N-PTZ] =
2.0 Â 10À5 M, [BPEA] = 4.0 Â 10À5 M in n-hexane. lex = 500 nm. The spike
in the delayed fluorescence traces is the scattered laser 20 1C.
Scheme 2 Simplified Jablonski diagram illustrating the photophysical
1
processes involved in PMI-N-PTZ. The energy levels of the LE, 1CT and T1
excited states are derived from the spectroscopic data and TDDFT calcula-
tions based on the optimized geometry of the ground state. The calculations
were performed at the B3LYP/6-31G(d) level using Gaussian 09W.
spectrum is the same as the prompt fluorescence of BPEA
(see the ESI,† Fig. S22). Interestingly, the decay trace exhibits
reason for this solvent-dependent ISC process is that the CR a bi-exponential feature, which contains a rise component with
process giving the ground state becomes faster in polar solvents a lifetime of 3.7 ms and a decay component with a lifetime of
(DCM and MeCN) and competes with the ISC process, therefore 19.7 ms. The rise time is attributed to the triplet–triplet energy-
the decay of the CT state back to the ground state is dominant. transfer (TTET) and triplet–triplet annihilation (TTA) processes
Thus the ISC becomes less efficient in highly polar solvents. in which the singlet excited state of BPEA was populated, and
The non-radiative decay channels of the emissive singlet charge the decay part indicates that the delayed fluorescence lifetime
separated state of the dyads include at least the charge recom- is 19.7 ms. However, in aerated solution, no delayed fluorescence
bination process to the ground state and the formation of the of BPEA was observed, proving that triplet excited states are
triplet state, thus no simple exponential relationship was involved in the generation of the delayed fluorescence, so that
observed between the knr and the energy gap of the emissive the triplet state can be quenched by the O2 and no delayed
CT states/ground state (Table 1). The energy gap of the CSS and fluorescence will be generated. Similar results were observed for
S0 state can be approximated as the Gibbs free energy changes of the PBI and SG5 acceptors (see the ESI,† Fig. S23), with the rise
the electron transfer (see the ESI† for more discussions based times being 8.7 ms and 4.7 ms and the decay time being 44.1 ms
on the Marcus equation).39 In our previous study of PBI–PTZ and 21.2 ms for PBI and SG5, respectively.
dyads, the PTZ was directly linked to PBI at the bay position and
In summary, perylenemonoimide (PMI)–phenothiazine (PTZ)
the two moieties take the orthogonal geometry,34 but no effective compact dyads were prepared to investigate spin–orbit charge
SOCT-ISC was observed, which is quite different from the present transfer intersystem crossing (SOCT-ISC). To study the relationship
PMI–PTZ dyads.
between the molecular structure and the SOCT-ISC efficiency, the
In view of the strong ISC ability and the long lifetime of the mutual orientation of the PMI and PTZ moieties was constrained
3PMI* state, the intermolecular energy transfer with the dyads as either orthogonal or more coplanar by using different linkage
as photosensitizers was exemplified with the generation of the profiles, resulting in different rotational steric hindrances.
delayed fluorescence. 9,10-Bis(phenylethynyl)anthracene (BPEA), PMI-N-PTZ, with the orthogonal geometry, shows efficient
perylenebisimide (PBI) and Solvent Green 5 (SG5) were used as triplet state production (FD = 57%) and a long-lived triplet state
triplet acceptors/emitters (Fig. 3 and ESI,† Fig. S22). The mole- (200 ms). It is found that the vector dipole orientations and
cular structures of the acceptors are presented in the ESI.†
the electronic coupling between the two moieties in the dyads
For PMI-N-PTZ/BPEA in deaerated n-hexane, the delayed as well as the solvent polarity affect the ISC efficiency. With
fluorescence of BPEA was observed (Fig. 3). The emission the dyads as triplet photosensitizers, delayed fluorescence of
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We thank the NSFC (21473020, 21673031, 21761142005,
21603021 and 21421005) for financial support.
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