Effects of temperature and host concentration on the supramolecular enantiodifferentiating [4 + 4] photodimerization of 2-anthracenecarboxylate through triplet-triplet annihilation catalyzed by Pt-modified cyclodextrins

Article abstract of DOI:10.3390/molecules24081502

Visible-light-driven photocatalytic supramolecular enantiodifferentiating dimerization of 2-anthracenecarboxylic acid (AC) through triplet-triplet annihilation (TTA), mediated by the Schiff base Pt(II) complex (Pt-1, Pt-2, and Pt-3) was studied. The host concentration and the temperature effects on the stereoselectivity were comprehensively investigated. Increasing the concentration of sensitizers/hosts significantly enhanced the conversion of the photoreaction but led to reduced enantioselectivities of the chiral photodimers 2 and 3 when the photoreaction was triggered by a 532 nm laser, which was in contrast with the results obtained by direct irradiation of AC with a 365 nm light-emitting diode (LED) lamp, due to the aggregation of the sensitizer/host in water. The cyclization of AC through triplet-triplet annihilation displayed significant temperature dependency when Pt-3 was employed as the sensitizer/host. Increasing the temperature from 0 ^?C to 30 ^?C with 5% equiv. of Pt-3 led to a great increase of the ee of 2 from 2.1% to 31.6%. However, hardly any temperature dependency was observed when the photodimerization was mediated by other sensitizers and/or hosts, or the photoreaction was triggered directly with a 365 nm LED lamp.

Full text of DOI:10.3390/molecules24081502

molecules
Article
Effects of Temperature and Host Concentration on the
Supramolecular Enantiodifferentiating [4 + 4]
Photodimerization of 2-Anthracenecarboxylate
through Triplet-Triplet Annihilation Catalyzed by
Pt-Modified Cyclodextrins
Ming Rao , Wanhua Wu * and Cheng Yang *
Key Laboratory of Green Chemistry & Technology, College of Chemistry, and Healthy Food Evaluation
Research Center, Sichuan University, 29 Wangjiang Road, Chengdu 610064, China; raomingrm@163.com
*
Correspondence: wuwanhua@scu.edu.cn (W.W.); yangchengyc@scu.edu.cn (C.Y.)
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀ
ꢁꢂꢃꢄꢅꢆ
Received: 29 March 2019; Accepted: 16 April 2019; Published: 17 April 2019
Abstract: Visible-light-driven photocatalytic supramolecular enantiodifferentiating dimerization of
-anthracenecarboxylic acid (AC) through triplet-triplet annihilation (TTA), mediated by the Schiff
base Pt(II) complex (Pt-1 Pt-2, and Pt-3) was studied. The host concentration and the temperature
2
,
effects on the stereoselectivity were comprehensively investigated. Increasing the concentration
of sensitizers/hosts significantly enhanced the conversion of the photoreaction but led to reduced
enantioselectivities of the chiral photodimers 2 and 3 when the photoreaction was triggered by a 532 nm
laser, which was in contrast with the results obtained by direct irradiation of AC with a 365 nm
light-emitting diode (LED) lamp, due to the aggregation of the sensitizer/host in water. The cyclization
of AC through triplet-triplet annihilation displayed significant temperature dependency when Pt-3
◦
◦
was employed as the sensitizer/host. Increasing the temperature from 0 C to 30 C with 5% equiv.
of Pt-3 led to a great increase of the ee of from 2.1% to 31.6%. However, hardly any temperature
2
dependency was observed when the photodimerization was mediated by other sensitizers and/or hosts,
or the photoreaction was triggered directly with a 365 nm LED lamp.
Keywords: photochirogenesis; supuramolecular chemistry; chiral photochemistry; catalysis; TTA;
host-guest; 2-anthracenecarboxylate
1
. Introduction
Asymmetric photochemical reaction is particularly intriguing due to its unique ability to gain
chiral polycyclic, highly constrained and/or thermally unstable organic compounds which are difficult
to synthesize by traditional thermochemical or enzymic reactions [ ]. However, the short-lived, highly
1
reactive and weakly interactive properties of the excited photosubstrates make it extremely challenging
to manipulate the enantioselectivity of excited state reactions. Supramolecular photochirogenesis
provides a promising solution to such problems, as “confined space” for the prochiral substrate(s)
was created before photoexcitation through the much longer and more intimate supramolecular
interactions at both ground and excited states. A variety of chiral hosts, such as cyclodextrin
derivatives [
2
–
15], chiral hydrogen-bonding templates [16
–19], chiral macrocyclic molecules [20,21]
and biological macromolecules [22
,
23], have been developed and utilized as chiral sources for efficient
photochirogenic control. However, an excess amount of chiral host is demanded in most supramolecular
photochirogenesis for the purpose of inhibiting the undesired racemic photoproduct resulting from the
photoreaction of the uncomplexed photosubstrates in the bulk solution [24]. Catalytic supramolecular
Molecules 2019, 24, 1502; doi:10.3390/molecules24081502
www.mdpi.com/journal/molecules

Molecules 2019, 24, 1502
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photochirogenesis is more desirable considering the not easy accessibility and atom economy of the
chiral hosts, which is however much more difficult to realize [25 26]. Several endeavors have been
,
made to achieve catalytic enantiodifferentiating photoreaction, and among them photosensitization
is the most frequently applied strategy [27]. Supramolecular photosensitization has been realized
through the much more efficient in situ electron and/or energy transfer from the hydrophobic sensitizer
covalently grafted on the host to the substrates embedded in the host cavity. Moreover, photocatalysis
also has been realized by sophisticated wavelength control to provide the possibility to exclusively
excite the complexed substrate and avoid the background reaction [28].
We have recently reported the first triplet-triplet annihilation (TTA)-based catalytic photochirogenesis
exemplified with the enantiodifferentiating photodimerization of 2-anthracenecarboxylic acid (AC)
sensitized by Schiff base Pt(II) complex-grafted
of the supramolecular photosensitization and wavelength control strategies. TTA upconversion is
a well-developed technology to spectrally blue-shifting the wavelength of the excitation photons [30 31],
which have been comprehensively applied in various fields, such as solar cells [32 33], photocatalysis [34
and bioimaging [35]. TTA-based catalytic photochirogenesis was realized through the following 4 steps:
i) the photosubstrate and the sensitizer-grafted hosts form a host-guest complex; (ii) excitation of the
γ-cyclodextrins (CDs) [29], which combined the advantages
,
,
]
(
photosensitizers by the low-power continuous-wave laser and the triplet-states of the photosensitizers was
populated through intersystem crossing (ISC); (iii) the triplet energy of the sensitizer was in situ transferred
to the complexed acceptor and (iv) two triplet acceptors generate a singlet acceptor through TTA while
the other deactivates to the ground state, and the highly reactive singlet acceptor will dimerize with the
deactivated acceptor to give the photodimers. A nice trick for this strategy is that the triplet sensitizer
was exclusively photoexcited, and the triplet energy transfer from the sensitizer to the substrate in the
cavity of the hosts will be much more effective than to the unbound substrate [36,37], and, therefore, good
enantioselectivity has been achieved with photocatalyst as low as 0.5% equivalent.
Supramolecular enantiodifferentiating[4 +4] photocyclodimerizationofAC hasbeenestablishedasthe
benchmark reaction for photochirogenesis [24
, –41]. This photoreaction will afford classical cyclodimers
38
0
0
0
0
0
0
1
(
–
4
, among which syn-9,10:9 10 -head-to-tail (syn-9,10:9 10 -HT) dimer
2
and anti-9,10:9 10 -head-to-head
are chiral, and the absolute configuration of these photoreaction products
were determined on the basis of theoretical and experimental comparsion of circular dichroism spectra [42].
It is also reported that AC can form 2:2 high order complex with -CD to obtain irregular cyclodimers
0 0
anti-9,10:9 10 -HH) dimer 3
β
0
0
0
0
0
0
0
0
anti-5,8:9 10 -head-to-tail (anti-5,8:9 10 -HT) dimer
5
and syn-5,8,9 10 -head-to-tail (syn-5,8:9 10 -HT) dimer
-CD
6
(Scheme 1) [43 45]. Herein, a new sensitizing chiral host Pt-2 was synthesized by grafting β
–
to the photosensitizer Pt-1 (Scheme S1) to realize photocatalytic supramolecular enantiodifferentiating
dimerization of AC triggered by low-power visible light and better TTA efficiency is expected, moreover,
the temperature effects on the photoreaction mediated by the chiral sensitizers (Pt-2 and Pt-3) were
systematically investigated to gain further insight into the mechanisms of the supramolecular catalytic
photochirogenesis through TTA.

Molecules 2019, 24, 1502
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Scheme 1. Enantiodifferentiating [4 + 4] photocyclodimerization of 2-anthracenecarboxylate (AC)
mediated by photosensitizers Pt-1, Pt-2, and Pt-3.
2
2
2
. Results and Discussion
.1. Enantiodifferentiating Photocyclodimerization of 2-Anthracenecarboxylate (AC) Mediated by Pt-2
.1.1. Characterization of Pt-2
Schiff base Pt(II) complex photosensitizer grafted
β
-CD (Pt-2) was synthesized by reacting Pt-1 with
6A-amino-6 -deoxy-
A
β-CD and was used as a photosensitive chiral host to trigger a visible-light-driven
photocyclodimerization of AC. Pt-1 was also investigated as a reference sensitizer for the comparison
purpose. Pt-2 showed moderate visible absorption and intensive red phosphorescence in deaerated
aqueous solution (Figure 1a), and the maximum absorption and phosphorescence were slightly
red-shifted relative to Pt-1. In addition, Pt-2 shows thermally induced Stokes shifts (∆ES) and
structured emission spectra (Figure S7), indicating a metal-to-ligand charge transfer (MLCT) feature
of the triplet excited state. The triplet state lifetimes slightly longer probably due to the partial
self-inclusion of Schiff-base unit by
β-CD (Table S1) [46]. The triplet energy of Pt-2, estimated
from the phosphorescent emission spectra, was 2.03 eV, which is higher than that of AC (1.82 eV),
guaranteeing an efficient triplet-triplet energy transfer to AC. To investigate the energy transfer
efficiency, phosphorescence quenching titration of photosensitizers Pt-1 and Pt-2 by acceptor AC were
performed (Figure S8). The Stern–Volmer analysis of the emission quenching data showed an apparent
linear relationship (Figure 1b), from which the Stern–Volmer constants K were derived. Pt-2 showed
SV
4
−1
4
−1
a KSV value of 1.57
the host-guest interaction between AC and Pt-2, as well as the slightly longer triplet lifetime of Pt-2
= 2.0 µs, τPt-2 = 3.1 µs) should be responsible [47].
×
10 M , being slightly larger than that of Pt-1 (1.28
×
10 M ), for which
(τ
Pt-1

Molecules 2019, 24, 1502
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0
0
.5
.4
(a)
Pt-1
Pt-2
Pt-1
Pt-2
(b)
1
1
0
0
0
0
0
.2
.0
.8
.6
.4
.2
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Pt-1
Pt-2
0.3
0
0
0
.2
.1
.0
.0
3
0
.000 0.005 0.010 0.015 0.020 0.025
00
400
500
600
700
800
[AC]
Wavelength / nm
Figure 1.
spectra (solid line) of Pt-1 and Pt-2, C = 5
AC] in deaerated water, C[Pt-1, Pt-2] = 5 × 10 M, C[AC] = 0 − 2.31 × 10 M, 20 C.
(a) Ultraviolet-visible (UV-vis) absorption spectra (dotted line) and normalized emission
−5
◦
×
10 M, 20 C. (
b
) Stern-Volmer Plots of (I
−
I)/I versus
0
−
5
−5
◦
[
2
.1.2. Effects of Pt-2/AC Ratio
Photolyses of AC in the presence of Pt-1 and Pt-2 were carried out by using a 532 nm diode
pumped solid state laser or a 365 nm LED lamp in pH 9.0 borate buffer solution. The photoreaction
results were shown in Table 1. Photosensitization of 0.2 mM AC with 0.05 mM Pt-2 for 2 h with 532 nm
laser led to 80% conversion, which is higher than that obtained with Pt-1 (53%), and could be ascribed to
the improved triplet-triplet energy transfer (TTET) efficiency by the host–guest complexation between
β
-CD and AC [48]. This host–guest interaction was further verified by NMR studies, as all of the
1
H-NMR signals for aromatic protons of AC were shifted upfield upon the complexation with Pt-2
Figure S5). In order to determine the inclusion rate between Pt-2 and AC, the ultraviolet-visible
UV-vis) spectral Job plots for the solutions of AC and Pt-2 at a fixed total concentration of 0.2 mM
(
(
showed that the maximum UV vis changes at 0.5 molar fraction, demonstrating a 1:1 complexation
−
stoichiometry (Figure S10a). Unfortunately, due to the poor solubility of Pt-2 in water, we failed to get
the binding constants of Pt-2 and AC, either by UV-vis or isothermal titration calorimeter (ITC) titration.
We have demonstrated that AC forms 2:2 complex with
stacked in the capsule formed by two -CD cavities [49], which led to the slipped photodimers
when the complex was directly photolyzed by using a 365 nm LED. Interestingly, photodimerizing
AC (0.2 mM) in the presence of 0.1 mM Pt-2 only afforded the normal photodimers , completely
no slipped photodimers or were observed, in all photolyses either using the 365 nm LED lamp or
32 nm laser as the light source. This result demonstrates that Pt-2 is more difficult to form 2:2 complex
β-CD, in which two AC molecules slipped
β
5
and
6
1–4
5
6
5
with AC than native β-CD.
On the other hand, photodimerize of AC (0.2 mM) in the presence of catalytic amount of Pt-2
(
0.01 mM) by using the 532 nm laser led to HT dimer
2
in 2.2% ee and HH dimer
3
in 2.0% ee. This result
−
demonstrated a chirality transfer from Pt-2 to AC dimers and the fact that the AC pair included in the
capsule of two Pt-2 are not slip arranged. The circular dichroism spectrum of Pt-2 showed a negative
signal at the MLCT transition (500
sector rule” proposed by Kajitar [50], we conclude that the sensitizer’s square-coordinated plane
reclines on the primary rim of -CD and partially inserting into -CD cavity and this probably prohibit
the formation of cyclodimers 5 and 6.
−
580 nm) which is different from Pt-3 (Figure S8), according to the
“
β
β
The host concentration effect on the photoreaction of AC sensitized by Pt-2 was further investigated.
When irradiating with a 532 nm laser and increase the concentration of Pt-2 from 0.001 mM to 0.1 mM
significantly increased the conversion from 1% to 97%, suggesting the acceleration effect of Pt-2 towards
the cyclization reaction of AC, the enantioselectivity of the photoreaction was slightly changed with
the increasing of the concentration of Pt-2, with the ee value of dimer
and dimer changed from 1.6% to 1.8%. When the light source changed to a LED lamp at 365 nm,
however, the ee values of dimer increased with the increment of the concentration of the sensitizer/host
3
increased from
−0.2% to
−3.5%,
2
3

Molecules 2019, 24, 1502
5 of 15
Pt-2, and the change of dimer
2
was inconsistent with that of the 532 nm laser (Table 1). Considering the
relatively low ee values, every experiment was carried out independently for 3 times. The ee values from
the three times experiments agree very well and the average values are listed in Table 1. The altered chiral
environment due to the aggregation of Pt-2 with increased concentration should be partially responsible
for the changed optical outcomes. The aggregation of Pt-2 in aqueous solution was supported by the
1
following facts: (i) the H nuclear magnetic resonance (NMR) signals of the aromatic protons of Pt-2
showed clear peaks in DMSO-d , when increasing the amount of D O, the signals shifted upfield and
6
2
became broad, and in D O there is no signals observed (Figure S4), (ii) UV-vis absorption maxima
2
showed a bathochromic shift of 11 nm upon increasing the concentration of Pt-2 from 1
and then a modest bathochromic shift of only 2 nm upon further increasing the concentration to 500
Figure S6b), demonstrating that Pt-2 begins to form aggregation at concentration as low as 20
On the other hand, as the optical outcomes of both and demonstrated completely different variation
µM to 20 µM
µM
(
µM.
2
3
trend vs. the concentration of Pt-2 when irradiated by a 532 nm laser and a 365 nm LED, we conclude that
maybe photocyclodimerization of AC sensitized by Pt-2 through TTA underwent different mechanisms
comparing with that by direct irradiation with 365 nm LED.
Table 1. Photocyclodimerization of AC (0.2 mM) mediated by Pt-1 and Pt-2 a.
Relative Yield (%) b
ee% b
Conv% b
Sen.
λ/nm
[Sen.]/µM
1
2
3
4
5
6
2
3
5
6
3
1
β-CD
365
100
37
42
31
15
9
2
1
−1
0
46
[49
]
c
c
c
c
γ-CD
Pt-1
Pt-2
365
532
365
10
50
1
97
53
90
93
98
97
1
75
80
97
40
37
39
42
39
38
40
40
40
41
37
37
36
36
36
36
38
38
39
40
14
16
15
14
15
16
14
14
13
12
9
10
9
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
8
4
-
-
-
-
-
-
-
-
-
-
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
-
-
c
0.0
1.3
−0.1
−5.1
1.6
2.2
2.0
1.8
0.3
0.3
0.7
-
-
-
-
-
-
-
-
c
1
5
0
0
8
c
10
10
9
8
8
c
1
00
1
2.3
c
5
32
−0.2
−2.0
−2.8
−3.5
c
1
0
c
5
0
-
c
1
00
7
-
a
Irradiation with a light-emitting diode (LED) lamp at 365 nm for 2 h or a solid state laser at 532 nm for 2 h, in borate
◦
b
c
buffer solution (pH 9.0), 20 C. Conversion ratio, relative yield and ee determined by chiral HPLC. not detected.
2
.1.3. Effects of Temperature
In order to gain further insight into the mechanisms of the catalytic photodimerization of AC
through triplet-triplet annihilation, we studied the influence of temperature on this photoreaction.
◦
The photocyclodimerization of AC was performed over a temperature range of 0–30 C in the presence
of 0.01 mM or 0.1 mM Pt-2, by irradiation with 532 nm laser or 365 nm LED lamp and the results were
listed in Table 2. Lowering the temperature did not significantly change the product distribution but
enhanced the enantioselectivities of the dimers
TTA or by direct irradiation of AC with a 365 nm LED lamp. For instance, the ee value of dimer
increased from 0.7% to 7.9% and dimer was increased from 1.2% to 5.9% by direct irradiation of
.2 mM AC with 0.1 mM Pt-2 with 365 nm LED lamp when the temperature decreased from 30 C to
2 and 3, regardless by sensitization process through
2
−
−
3
◦
0
0
◦
C, while for the sensitization process, the ee value of dimer
2 was increased from 0.3% to 5.0%. It is
noteworthy that photocyclodimerization of AC through TTA sensitization is less sensitive to the host
concentration change, as photoirradiation of 0.01 mM Pt-2 and 0.2 mM AC by a 532 nm laser afforded
2
in 3.7% ee, while direct irradiation of AC only led to 1.6% ee under the same condition, which indicates
the catalytic characteristics for the cyclization of AC through TTA.

Molecules 2019, 24, 1502
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Table 2. Photocyclodimerization of AC (0.2 mM) with Pt-2 as the sensitizer/host under different temperatures a.
Relative Yield (%) b
ee% b
[
Sen.]
Anti/syn Anti/syn
◦
Conv% b
λ/nm T/ C
HH/HT
/
mM
1
2
3
4
2
3
1/2
3/4
3
65
0
98
96
93
71
80
96
97
96
53
54
75
76
75
89
97
95
0.01
0.01
0.01
0.01
0.1
0.1
0.1
0.1
0.01
0.01
0.01
0.01
0.1
0.1
0.1
0.1
39
41
42
44
37
37
38
37
43
42
40
42
42
41
41
45
38
40
37
36
40
37
36
34
38
39
38
37
37
37
40
40
14
12
13
12
13
15
16
18
12
12
14
14
12
12
12
9
9
7
8
1.6
1.5
1.3
1.2
−7.9
−7.0
−5.1
−0.7
3.7
3.4
2.2
0.5
5.0
3.4
3.2
2.9
2.7
5.9
3.5
2.3
1.2
−3.2
−2.9
−2.0
−2.1
1.6
−1.1
−3.5
−5.3
0.3
0.2
0.3
0.3
0.3
0.3
0.4
0.4
0.2
0.2
0.3
0.3
0.3
0.3
0.2
0.2
1.0
1.0
1.1
1.2
0.9
1.0
1.0
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.0
1.1
1.5
1.6
1.7
1.5
1.4
1.5
1.5
1.6
1.8
1.7
1.8
1.9
1.4
1.4
1.6
1.8
b
1
2
3
0
0
0
8
0
10
10
10
11
7
7
8
8
9
1
2
3
0
0
0
5
32
0
10
20
30
0
10
20
30
9
7
5
2.9
1.8
0.3
a
Irradiation with a LED lamp at 365 nm for 2h or a solid state laser at 532 nm for 2 h, in borate buffer solution (pH 9.0).
Conversion ratio, relative yield and ee determined by chiral high-performance liquid chromatography (HPLC).
To analyze the temperature effect quantitatively, we plotted the natural logarithm of the relative
enantiomer ratios of dimer
2 and 3, as a function of the reciprocal temperature, as illustrated in
Figure 2, all of the data obtained with different chiral host concentrations and excitation wavelengths
◦
in borate buffer solution over a temperature range of 0–30 C, gave excellent straight lines. According
‡
‡
to the Eyring equation, the differential activation enthalpy (∆∆H ) and entropy changes (∆∆S ) were
calculated from the intercept and slope of the plots and were listed in Table 3 [29].
0.2
0.1
0.0
(a)
(b)
0
.2
.0
0
-
-
0.2
0.4
-
0.1
-0.2
3.2 3.3 3.4 3.5 3.6 3.7 3.8
3.2 3.3 3.4 3.5 3.6 3.7 3.8
-
1
-1
1000/T (K )
1
000/T (K
Figure 2. Representative temperature-dependence (differential Eyring) plots ( , (
ee of obtained in the photoreaction of AC irradiated with 365 nm LED and 0.01 mM Pt-2 (ꢀ) or 0.1 mM
)
a
) of the ee of 2 b) of the
3
Pt-2 (ꢀ), with 532 nm laser and 0.01 mM Pt-2 (N) or 0.1 mM Pt-2 (H) in 25 mM borate buffer solution.
Table 3. Differential activation parameters determined from the temperature dependence of the ee of
cyclodimers 2 and 3 in the photocyclodimerization of AC catalyzed by Pt-2.
Dimer 2
Dimer 3
[
Sen.]/mM
.01
.1
λ/nm
‡
−1
‡
−1 −1
‡
−1
‡
−1 −1
∆
∆H (KJ mol
)
∆∆S (J mol
K
)
∆∆H (KJ mol
)
∆∆S (J mol K )
0
365
−0.2
−1.5
3.2
−2.1
−0.4
−4.7
10.3
−6.9
−0.3
−0.6
1.6
−6.8
5
32
365
32
0.6
0
−2.1
−3.2
5
−11.4

Molecules 2019, 24, 1502
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In general, the residence time of AC in the host–guest complex is much longer than the lifetime of
a singlet excited AC molecule (16 ns), and AC pairs is not possible to exchange their relative arrangement
during the short excited lifetime. The product ee is primarily decided by the stability difference
of the two precursory diastereomeric host-guest complexes. For the photoreaction through TTA
upconversion, however, as the lifetime of the reactive singlet excited AC arising from the triplet-triplet
annilation upconverison will fall in the
µs–ms range, which is much longer than that of the singlet
state formed by direct irradiation and is comparable with the dissociation of AC from the complexes
−
3
−6
(
10 ~10 s depending on the association constants). Therefore, the kinetics of AC entering into the
cavity of -cyclodextrin may play a role in influencing the reaction stereoselectivity. However, due to
β
the poor solubility of Pt-2 in water, we are not able to obtain the exact binding constants between Pt-2
and AC to get the accurate dissociation time. Hence, we turned our research focus on the temperature
dependence of the photocyclodimerization of AC sensitized by Pt-3, which give much better water
solubility and the binding constants of AC, to get deep insight into the mechanisms of the catalytic
photoreaction through TTA.
2
.2. Temperature Dependence of the Photocyclodimerization of AC Mediated by Pt-3
Pt-3 sensitized TTA dimerization of AC have been reported by us to afford higher enantioselectivity
for HT dimer
2
in the presence of catalytic amounts of chiral hosts when irradiated by a 532 nm solid
in
laser [29]. Indeed, photocyclodimerization of AC in the presence of 1 mM Pt-3 gave HT dimer
2
only 16.9% ee, while reducing the concentration of Pt-3 to 0.01 mM, which is 5% equiv. of AC, the ee of
◦
2
can be enhanced to 31.6% ee at 30 C. We ascribed this host concentration effect to the aggregation of
Pt-3 at high concentrations, which was further confirmed by the photoreaction of AC mediated by Pt-2
in the present study.
Before we investigated the temperature-dependence behaviors of the enantiodifferentiating
photodimerization of AC catalyzed by Pt-3, we firstly examined the binding behavior between Pt-3 and
AC. The inclusion rate between Pt-3 and AC was determined by the Job’s plot, based on the continuous
variation method by holding AC and Pt-3 at a fix total concentration of 0.2 mM, the maximum UV
−
vis
changes were at 0.67 molar fraction, demonstrated a 1:2 complexation stoichiometry (Figure S10b).
◦
◦
The binding constants between Pt-3 and AC at different temperatures of 0 C and 25 C was determined
with an ITC (Figure 3a). Pt-3 binds with AC stepwisely to give 1:1 (K ) and 1:2 (K ) association constants
1
2
−
1
−1
◦
◦
−1
as 3810 M and 5350 M at 0 C, respectively. While at 25 C, K and K were determined as 10,400 M
and 719 M , respectively [43]. The overall association constant K K was determined as 2.04
10 M at 25 C (Table S2), which were comparable to that of native
10 M at 25 C) [43], indicating that Pt-3 remains strongly binding toward AC.
Intrestingly, K at high temperature is much smaller than that at lower temperature, demonstrating that
1
2
−1
7
−2
×
10 M at
1
2
◦
6
−2
6
◦
7
−2
0
C and 7.48
at 5 C and 6.20
×
γ
-CD (4.14 × 10 M
◦
−2
◦
×
2
the residence time of AC in the 1:2 host-guest complex is much shorter at high temperatures, from which,
we can legitimately infer that, the thermodynamic enantioselectivity derived from the stability difference
of the prochiral complexes formed by enantioface-selective inclusion of the second AC molecule into
a 1:1 Pt-3-AC complex will become less predominant. However, the enantioselectivity come from the
kinetics of AC entering into the cavity of CD become more and more non-negligible by increasing the
temperature; therefore, the selectivity of the enantiodifferentiating photodimerization of AC though TTA
should display different temperature-dependent behaviors comparing with the photoreaction triggered
with a 365 nm LED lamp.
The photocyclodimerization of AC mediated by Pt-3 were carried out over the temperature ranges
◦
from 0–30 C in borate buffer solution (pH 9.0) and the photoreaction results are listed in Table 4.
Interestingly, in the photoreaction mediated by 0.01 mM Pt-3 in 25 mM borate buffer solution (pH 9.0),
◦
◦
increasing the reaction temperature from 0 C to 30 C, led to an increase of the ee value of HT dimer
2
from 2.1% to 31.6%, while the ee of HH dimer
3
decreased from
−
12.4% to
−
3.0%. Particularly,
◦
when employing 1 mM Pt-3 as the sensitizer/host, lowering temperature from 30 to 0 C afforded
an antipodal product for HT dimer 2. However, when the photoreaction was triggered by a 365 nm

Molecules 2019, 24, 1502
8 of 15
LED lamp, the outcomes of both dimers
temperature range of 0–30 C (Table 5).
2
and
3
showed no apparent temperature dependency at the
◦
Figure 3.
(a) Isothermal titration calorimeter (ITC) data for Pt-3 (2 mM) and AC (0.2 mM) in 25 mM
◦
borate buffer (pH 9.0) at 0 C. The top is the raw data for power versus time, and the bottom is the
integrated enthalpy values versus the molar ratio of Pt-3: AC = 1:2, and obtained K as 3810 M 1, K2
as 5350 M . (
−
1
−1
b
) Representative temperature-dependence (differential Eyring) plots obtained in the
) or 1 mM Pt-3 ), with 532 nm
photoreaction of AC irradiated with 365 nm LED and 0.01 mM Pt-3
(
ꢀ
(ꢀ
laser and 0.01 mM Pt-3 (N) or 1 mM Pt-3 (H). The top is the ee of 2, and the bottom is the ee of 3.
Table 4. Photocyclodimerization of AC (0.2 mM) and Pt-3 mediated by temperature a.
Relative Yield (%) b
ee% b
◦
Conv% b
λ/nm T/ C
[sen.] /mM
HH/HT
Anti/syn 1/2 Anti/syn 3/4
1
2
3
4
2
3
3
65
0
74
90
92
95
85
88
81
87
63
81
90
95
93
93
96
99
0.01
0.01
0.01
0.01
1
1
1
1
0.01
0.01
0.01
0.01
1
1
1
1
37
36
37
37
25
28
28
30
38
37
34
35
39
36
33
32
39
38
38
37
24
28
29
30
54
56
56
59
44
47
48
47
13
14
14
15
26
23
23
21
5
5
7
4
11
11
13
14
11
11
11
11
26
22
21
18
3
2
4
2
6
5.5
5.7
6.0
4.9
9.9
10.7
10.2
0.3
0.3
0.3
0.3
1.1
0.8
0.8
0.7
0.1
0.1
0.1
0.1
0.2
0.2
0.2
0.3
0.9
0.9
1.0
1.0
1.1
1.0
1.0
1.0
0.7
0.8
0.6
0.6
0.9
0.8
0.7
0.7
1.2
1.2
1.3
1.4
1.0
1.1
1.1
1.2
1.9
2.3
1.9
1.8
1.7
2.0
2.0
2.0
1
2
3
0
0
0
6.3
0
21.1
21.4
20.8
19.1
2.1
16.7
27.4
31.6
−11.8
−12.7
−12.8
−13.4
−12.4
−10.5
−4.5
10
20
30
5
32
0
10
20
30
−3.0
0
−11.9 −22.3
10
20
30
6
6
7
1.3
−20.3
−12.7
−7.6
13.0
16.9
a
Irradiation with a LED lamp at 365 nm for 2 h or a solid state laser at 532 nm for 2 h. In borate buffer solution
(
pH 9.0). b Conversion ratio, relative yield and ee determined by chiral HPLC.

Molecules 2019, 24, 1502
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Table 5. Differential activation parameters determined from the temperature dependence of the ee of
cyclodimers 2 and 3 in the photocyclodimerization of AC catalyzed by Pt-3.
Dimer 2
Dimer 3
Host
Pt-3
[Sen.]/mM λ/nm
‡
−1
‡
−1 −1
‡
−1
‡
−1 −1
K )
∆
∆H (KJ mol
)
∆∆S (J mol
K
)
∆∆H (KJ mol
)
∆∆S (J mol
0.01
1
365
32
365
32
0.4
2.3
53.0
0.3
2.4
4.7
9.7
5
14.3
−0.9
13.7
15.2
−4.5
22.6
−0.7
5
48.4
7.3
The apparently different temperature-dependent behaviors for Pt-3 when irradiation with 532 nm
laser comparing with the direct irradiation of AC with 365 nm LED lamp, should be ascribed to the
different mechanisms of the photoreaction through TTA. For direct irradiation of AC, the enantioselectivity
of the chiral product
2
and
3 derive from the thermodynamic stability of the prochiral 1:2 complex of
γ-CD and AC, while for TTA-UC triggered photoreaction, the kinetics of AC entering into the cavity
of
γ-CD also played vital roles as TTA-UC comprise several inter-molecular energy transfer processes.
Considering that the variation trend of the ee versus temperature is opposite when changing the exciting
wavelength from 365 nm to 532 nm, we conclude that the kinetic process gave antipodal product to
the thermodynamic control product, and kinetic control products is dominant for visible light-driven
photoreaction mediated by Pt-3. When the temperature increases, molecular diffusion rate become
higher, the kinetic control products become more dominant to give enhanced optical yields of 2 and 3.
To more quantitively analyze the temperature-behavior of the chiral product 2 and 3, the natural
logarithm of the relative enantiomer ratios of the chiral dimers, obtained with the irradiation of 365 nm
LED lamp and 532 nm laser mediated by different concentrations of Pt-3, were plotted against the
reciprocal temperature (Figure 3b), all the fittings showed good linear relationship and from the slope
‡
‡
and intercept of the fittings, differential activation enthalpy (∆∆H ) and entropy changes (∆∆S ) were
calculated, the results were listed in Table 5. Interestingly, the photoreaction triggered by a 532 nm laser
‡
‡
showed obviously large ∆∆H and ∆∆S value than that trigged by 365 nm irradiation. The 365 nm
irradiation will produce a singlet excited AC, which has a short lifetime of 16 ns in aqueous solution.
No rearrangement of AC pairs in the CD cavity are possible during this short time, and the differential
thermodynamic parameters reflect more the difference between diastereomeric complexes at the ground
state. For the 532 nm-induced photodimerization, the triplet excited AC has much longer lifetime, and
it is possible to establish a new equilibrium at the excited state. The differential entropy and enthalpy
values for 532 nm irradiation are mainly the activation differential thermodynamic parameters for the
formation of new equilibrium at the excited state. The much larger differential entropy and enthalpy
values under 532 nm irradiation indicated that activated state for the formation of triplet-excited AC
pairs are greatly different from each other.
‡
‡
2 and 3 in this study are plotted against the relevant ∆∆S
All of the ∆∆H values obtained for
values to give the enthalpy–entropy compensation plot (Figure 4) [29]. The differential parameters
‡
‡
obtained at 365 and 532 nm falled on a single straight line, satisfying the equation ∆∆H = 0.28∆∆
S
−
‡
‡
0
.21 (r = 0.998), ∆∆H = 0.30∆∆S + 0.13 (r = 0.995) for
chirality origin of this enantiodifferentiating photodimerization is from the charity of the CD cavity.
From the slope of the line, the equipodal temperature for the formation of and are determined to be
2 and 3, respectively. This is reasonable, as the
2
3
◦
◦
2
80 K (7 C) and 300 K (27 C), separately.
Energy transfer efficiency from the sensitizer to AC at different temperatures was further
investigated by the quenching of the phosphorescence of the complexes Pt-3 with AC as the triplet
quencher. The emission of the complex was progressively reduced with the increase of the concentration
of AC due to the triplet energy transfer from the complex to AC. It should be pointed out that dynamic
and static quenching by the free and bonded AC are responsible for the quenching of phosphorescence
of the sensitizer, and static quenching process should be much more efficient and highly dependent
on the distance between γ-CD and Schiff-base complexes unit. Stern–Volmer quenching curves were

Molecules 2019, 24, 1502
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constructed with the quenching data which shows a good linear relationship in the temperature range
◦
of 0–30 C (Figure S11). In accordance with association constants, at the lower temperature Pt-3
showed much larger KSV, which demonstrates that static quenching in Pt-3 is much more efficient due
to the intimate contact of AC with Schiff-base Pt complex unit. Whereas, much higher KSV at lower
temperature together with the fact that Pt-3 showed much higher overall association constant K K ,
1
2
jointly confirmed that thermodynamic photoreaction competed with the kinetic ones, so that decreases
the enantioselectivity of the photoreaction.
Figure 4. Enthalpy–entropy compensation plot for the differential parameters of the enantiomers of
2
obtained in the photocyclodimerization of AC catalyzed by Pt-2
(ꢀ) and Pt-3 (ꢀ). (a) of the ee of 2,
(
b) of the ee of 3.
3
. Materials and Methods
3
.1. Chemicals and Instruments
All the chemicals used in synthesis are analytically pure and were used as received. Solvents were
1
13
dried and distilled before use for synthesis. H-NMR and C-NMR spectra were recorded at room
1
13
temperature on Bruker AMX-400 (operating at 400 MHz for H-NMR and 151 MHz for C-NMR)
with TMS as the internal standard (Bruker, Bremen, Germany). High resolution mass spectrum
(
(
HRMS) data were measured in quadrupole time-of-fight liquid chromatography-mass spectrometer
Q-TOF-LCMS) (Shimadzu, Tokyo, Japan) and matrix-assisted laser desorption ionization time-of-fight
mass spectrometer (MALDI-TOF MS, Bruker, Bremen, Germany). UV-vis. spectra were obtained on
JASCO v-650 (Jasco, Tokyo, Japan). CD spectra were acquired using J-1500 CD spectrometer (Jasco,
Tokyo, Japan). Fluorescence spectra and Fluorescence lifetime decay were taken on Fluoromax-4
spectrofluorometer (Horiba, New Jersey, NJ, USA). Photocyclodimerization data was acquired by
high-performance liquid chromatography (HPLC) (UFLC SHIMADZU system equipped with SPD-20A
and RF-20A as a detector, Shimadzu, Tokyo, Japan).
3
.2. Photoreaction and Product Analysis
Photolyses. The mixture aqueous solution of AC (0.2 mM) and sensitizer (Pt-1, Pt-2, and Pt-3 in
different concentration) was irradiated for 2 h under an argon atmosphere using a LED lamp (365 nm)
or a diode pumped solid state (DPSS) laser (532 nm).
HPLC analysis. The resulting solution was analyzed by Chiral HPLC, performed on a tandem
◦
column of Inertsil ODS-2 and Daicel Chiralcel OJ-R, and operated at 35 C using 0.1% trifluoroacetic
acid (TFA) dissolved in H O and acetonitrile (62:38, volume ratio), at a flow rate of 0.5 mL/min.
2
The relative yield and ee value were determined from the peak area of HPLC chromatogram.

Molecules 2019, 24, 1502
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3
.3. Synthesis
Synthesis of Pt-1: After degassing of DMSO (5 mL), compound L1 (180 mg, 0.5 mmol), K PtCl
2
4
(
210 mg, 0.5 mmol), and K CO (210 mg, 1.5 mmol) were added, the flask was placed under vacuum
2 3
◦
and backfilled with argon several times. Then the reaction mixture was heated to 80 C for 18 h.
The solvent was evaporated under reduced pressure at 80 C and the crude product was washed
◦
1
with water (2 × 100 mL) to give the reddish-brown product (230 mg, 83.18%). H-NMR (400 MHz,
DMSO-d6)
δ (ppm): 9.54 (s, 1H), 9.52 (s, 1H), 8.78 (s, 1H), 8.31 (d, J = 8.8 Hz, 1H), 8.04 (d, J = 8.1,
13
Hz, 1H), 7.94–7.82 (m, 2H), 7.67–7.43 (m, 2H), 7.10 (d, J = 8.3 Hz, 2H), 6.86–6.62 (m, 2H). C-NMR
101 MHz, DMSO-d6) (ppm): 164.37, 151.17, 144.79, 143.90, 136.07, 135.69, 135.37, 128.72, 122.13,
21.85, 121.22, 121.03, 120.55, 117.64, 116.29, 116.17, 115.21, 113.60, 112.19. HRMS (ESI-): m/z calcd for
(
1
δ
−
−
C H N O Pt [M − H] , 552.0522, found [M − H] , 552.0519.
21
14
2
4
Synthesis of Pt-2: The solution of compound Pt-1 (55.3 mg, 0.1 mmol) in DMF (5 mL) were
added EDC (1-(3-dimethylaminopropyl)-3-ethylcarbodiimide) (33
µ
L, 0.15 mmol) and HOBT
◦
(
1-hydroxybenzotriazole) (20.3 mg, 0.15 mmol), and the mixture was stirred at
−20 C for 3 h. Then
A
A
compound 6 -amino-6 -deoxy-β-CD (136 mg, 0.12 mmol, 1.2 equiv.) was added, and the mixture was
stirred at room temperature for another 6 h. After completion of the reaction, the mixture was dropwise
added into acetone (200 mL) to obtain the crude product as a red precipitate, which was purified by
reverse phase chromatography, using ethanol aqueous solution in gradient elution from 0–50% to afford
1
the product as red solid (45.6 mg, 27.32%). H-NMR (400 MHz, DMSO-d )
δ
(ppm): 9.51 (d, J = 6.8 Hz,
6
2
H), 8.84 (s, 1H), 8.53–8.39 (m, 2H), 7.86 (d, J = 9.1 Hz, 3H), 7.59 (d, J = 6.1 Hz, 2H), 7.12 (d, J = 8.6 Hz,
H), 6.81 (d, J = 7.0 Hz, 2H), 5.79 (d, J = 18.8 Hz, 14H), 5.14–4.68 (m, 7H), 4.49 (t, J = 36.1 Hz, 7H), 4.08
(ppm):
2
(
d, J = 7.2 Hz, 1H), 3.94–3.80 (m, 1H), 3.63 (d, J = 26.2, 40H). 13C-NMR (151 MHz, DMSO-d6)
δ
1
1
65.53, 165.26, 165.09, 152.34, 152.11, 146.86, 144.74, 136.35, 136.29, 136.12, 133.96, 127.18, 122.43, 122.35,
21.82, 121.71, 116.98, 116.63, 116.23, 102.77, 102.41, 102.03, 84.83, 82.10, 81.93, 81.76, 81.70, 73.51, 73.44,
7
2.89, 72.76, 72.62, 72.56, 72.48, 72.39, 70.35, 60.59, 60.42, 60.34, 60.23, 59.86. MALDI-TOF-HRMS: m/z
+
+
calcd for C H N O Pt M, 1668.4353, [M + H] , 1669.4431; found [M + H] , 1669.4962.
63
83
3
37
Synthesis of Pt-3: The synthesis procedure is the same as Pt-2. 1H-NMR (400 MHz, DMSO-d6)
ppm): 9.50 (s, 2H), 8.81 (s, 1H), 8.44 (d, 1H), 7.86 (d, J = 8.0 Hz, 3H), 7.56 (s, 2H), 7.11 (d, J = 8.7 Hz,
H), 6.77 (t, 2H). 13C-NMR (151 MHz, DMSO-d6)
(ppm): 165.68, 165.20, 165.04, 146.84, 144.64, 136.31,
δ
(
2
δ
1
6
33.84, 122.35, 121.72, 117.01, 116.61, 116.02, 102.86, 102.18, 101.94, 81.48, 81.04, 80.51, 73.05, 72.80, 72.56,
+
0.36, 56.43. MALDI-TOF-HRMS: m/z calcd for C H N O Pt [M], 1830.4881, [M + Na] , 1853.4779,
69
93
+
3
42
+
+
+
[M + K] , 1869.4518, found M , 1830.4833, [M + Na] , 1853.4727, [M + K] , 1869.4523.
4
. Conclusions
In conclusion, a new
driven photocatalytic supramolecular enantiodifferentiating dimerization of 2-anthracenecarboxylic acid
AC) through triplet-triplet annihilation. The photoreaction mediated with Pt-2 failed to form irregular
β
-CD based sensitizer/host Pt-2 was synthesized and was applied for visible-light-
(
0
0
chiral slipped 5, 8: 9 , 10 -cyclodimers due to aggregation of Pt-2 in water, thus hard to form the 2:2 complex
with AC. Decreasing the concentration of sensitizers/hosts significantly enhanced the enantioselectivities of
the optical photodimers 2 and 3 when employing Pt-2, and Pt-3 as the hosts as well as the sensitizers, and
was triggered by 532 nm laser, making them good candidates for organic photocatalyst. The photolysis of
AC through triplet-triplet annihilation sensitized by Pt-3 displayed significant temperature dependency,
the ee value of photodimer
2
was enhanced from 2.1% to 31.6% when increasing the temperature from
◦
◦
0
C to 30 C in the presence of 5% equivalent of the photocatalyst. This is unprecedented as for traditional
enantiodifferentiating photodimerization of AC, the enantioselectivity was deduced with the increasing
temperature. This work presents a new strategy for catalytic supramolecular photochirogenesis, and
provides a more comprehensive understanding for photochirogenesis through triplet-triplet annihilation.

Molecules 2019, 24, 1502
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Supplementary Materials: The following are available online: Scheme S1: synthesis of Pt-1 and Pt-2, Figure S1-3:
1
13
1
H-NMR; C-NMR and HRMS spectra of Pt-2, Figure S4: H-NMR spectra of Pt-2 in different deuterated solvent,
1
Figure S5: H-NMR spectra of the mixture of Pt-2 and AC, Figure S6a: UV-vis spectra of Pt-1
,
Pt-2, and Pt-3
,
,
,
Figure S6b: Normalized UV-vis spectra of Pt-2 at different concentrations, Figure S7: Emission spectra of Pt-2
Figure S8: Phosphorescence quenching spectra of Pt-2 upon increasing AC, Figure S9: CD spectra of Pt-2 and Pt-3
Figure S10: Job plots of Pt-2 and Pt-3, Figure S11: Stern-Volmer plots of Pt-3; Table S1: photophysical properties of
the sensitizers/hosts Pt-1, Pt-2, and Pt-3, and Stern–Volmer quenching constant (KSV) between sensitizers/host and
◦
AC at 20 C. Table S2: association constants of Pt-3 and AC at a different temperature.
Author Contributions: Conceptualization, M.R., W.W. and C.Y.; methodology, C.Y.; software, M.R.; validation,
M.R., W.W. and C.Y.; formal analysis, M.R.; investigation, M.R.; resources, M.R.; data curation, M.R.;
writing—original draft preparation, M.R.; writing—review and editing, W.W. and C.Y.; visualization, M.R.;
supervision, W.W. and C.Y.; project administration, C.Y.; funding acquisition, C.Y.
Funding: This research was funded by the National Natural Science Foundation of China, grant number 21402129,
2
1871194 and 21572142; National Key Research and Development Program of China, grant number 217YFA0505903;
Science & Technology Department of Sichuan Province, grant number 2017SZ0021, 2019YJ0090 and 2019YJ0160.
Acknowledgments: We acknowledge the support of the Comprehensive Training Platform of Specialized Laboratory,
College of Chemistry and Peng Wu of Analytical &Testing Center, Sichuan University for characterization and
lifetime measurement.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds Pt-1, Pt-2, Pt-3 are available from the authors.
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