Broadband visible light-absorbing [70]fullerene-BODIPY-triphenylamine triad: Synthesis and application as heavy atom-free organic triplet photosensitizer for photooxidation

Article abstract of DOI:10.3390/molecules26051243

A broadband visible light-absorbing [70]fullerene-BODIPY-triphenylamine triad (C₇₀-B-T) has been synthesized and applied as a heavy atom-free organic triplet photosensitizer for photooxidation. By attaching two triphenylmethyl amine units (TPAs) to the -core of BODIPY via ethynyl linkers, the absorption range of the antenna is extended to 700 nm with a peak at 600 nm. Thus, the absorption spectrum of C₇₀-B-T almost covers the entire UV-visible region (270-700 nm). The photophysical processes are investigated by means of steady-state and transient spectroscopies. Upon photoexcitation at 339 nm, an efficient energy transfer (ET) from TPA to BODIPY occurs both in C₇₀-B-T and B-T, resulting in the appearance of the BODIPY emission at 664 nm. Direct or indirect (via ET) excitation of the BODIPY-part of C₇₀-B-T is followed by photoinduced ET from the antenna to C₇₀, thus the singlet excited state of C₇₀ (¹C₇₀ ^*) is populated. Subsequently, the triplet excited state of C₇₀ (³C₇₀ ^*) is produced via the intrinsic intersystem crossing of C₇₀. The photooxidation ability of C₇₀-B-T was studied using 1,5-dihydroxy naphthalene (DHN) as a chemical sensor. The photooxidation efficiency of C₇₀-B-T is higher than that of the individual components of C₇₀-1 and B-T, and even higher than that of methylene blue (MB). The photooxidation rate constant of C₇₀-B-T is 1.47 and 1.51 times as that of C₇₀-1 and MB, respectively. The results indicate that the C₇₀-antenna systems can be used as another structure motif for a heavy atom-free organic triplet photosensitizer.

Full text of DOI:10.3390/molecules26051243

molecules
Article
Broadband Visible Light-Absorbing
[70]Fullerene-BODIPY-Triphenylamine Triad: Synthesis and
Application as Heavy Atom-Free Organic Triplet
Photosensitizer for Photooxidation
San-E Zhu 1,*, Jian-Hui Zhang 1 , Yu Gong 1, Li-Feng Dou 1, Li-Hua Mao 1, Hong-Dian Lu 1, Chun-Xiang Wei 1,
Hong Chen 1, Xue-Fei Wang 2,* and Wei Yang 1,
*
1
School of Energy, Materials and Chemical Engineering, Hefei University, Hefei 230601, China;
zhangjianhuiafter@163.com (J.-H.Z.); Huayu202035@163.com (Y.G.); dou_lifeng2010@126.com (L.-F.D.);
mlh18970341520@163.com (L.-H.M.); luhdo@hfuu.edu.cn (H.-D.L.); weichunxiang@hfuu.edu.cn (C.-X.W.);
chenh113@hfuu.edu.cn (H.C.)
School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences,
Beijing 100049, China
2
*
Correspondence: zhuse@hfuu.edu.cn (S.-E.Z.); wangxf@ucas.ac.cn (X.-F.W.); weyang@ustc.edu.cn (W.Y.);
Tel.: +86-551-62158394 (S.-E.Z. & W.Y.); +86-10-69672551 (X.-F.W.)
Abstract: A broadband visible light-absorbing [70]fullerene-BODIPY-triphenylamine triad (C70-B-T
has been synthesized and applied as a heavy atom-free organic triplet photosensitizer for photooxi-
dation. By attaching two triphenylmethyl amine units (TPAs) to the -core of BODIPY via ethynyl
)
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
π
Citation: Zhu, S.-E; Zhang, J.-H.;
Gong, Y.; Dou, L.-F.; Mao, L.-H.; Lu,
H.-D.; Wei, C.-X.; Chen, H.; Wang,
X.-F.; Yang, W. Broadband Visible
Light-Absorbing [70]Fullerene-
BODIPY-Triphenylamine Triad:
Synthesis and Application as Heavy
Atom-Free Organic Triplet
linkers, the absorption range of the antenna is extended to 700 nm with a peak at 600 nm. Thus,
the absorption spectrum of C70-B-T almost covers the entire UV–visible region (270–700 nm). The
photophysical processes are investigated by means of steady-state and transient spectroscopies.
Upon photoexcitation at 339 nm, an efficient energy transfer (ET) from TPA to BODIPY occurs both
in C70-B-T and B-T, resulting in the appearance of the BODIPY emission at 664 nm. Direct or indirect
(via ET) excitation of the BODIPY-part of C70-B-T is followed by photoinduced ET from the antenna
to C70, thus the singlet excited state of C70 (1C70*) is populated. Subsequently, the triplet excited
state of C70 (3C70*) is produced via the intrinsic intersystem crossing of C70. The photooxidation
ability of C70-B-T was studied using 1,5-dihydroxy naphthalene (DHN) as a chemical sensor. The
photooxidation efficiency of C70-B-T is higher than that of the individual components of C70-1 and
B-T, and even higher than that of methylene blue (MB). The photooxidation rate constant of C70-B-T
is 1.47 and 1.51 times as that of C70-1 and MB, respectively. The results indicate that the C70-antenna
systems can be used as another structure motif for a heavy atom-free organic triplet photosensitizer.
Photosensitizer for Photooxidation.
Molecules 2021, 26, 1243. https://doi.
org/10.3390/molecules26051243
Academic Editor: Jason Peter Mansell
Received: 6 February 2021
Accepted: 22 February 2021
Published: 25 February 2021
Keywords: C70; BODIPY; triphenylamine; triplet photosensitizer; photooxidation
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
1. Introduction
Organic triplet photosensitizers (PSs) have received tremendous attention in recent
years due to their wide application in photocatalysis [
triplet–triplet annihilation upconversion (TTA) [ ]. Broadband visible light-absorbance,
hypotoxicity, high efficiency of intersystem crossing (ISC) and long triplet excited state
1,2], photodynamic therapy [3–5], and
6–8
lifetimes are the desired characteristics for triplet PSs [
to attain the overall properties in one triplet PS.
9,10]. However, it is still a challenge
Copyright:
© 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
PSs with heavy atoms can produce the triplet state, whereas the narrow visible light-
absorbance, toxicity and high cost limit their applications [11 15]. PSs without heavy
atoms usually suffer from the unpredictable ISC ability. In order to address the aforemen-
tioned problems, strategies such as using a twisted -conjugation system [16 17], singlet
fission [18,19], spin–orbit charge transfer [20 21], and radicals [22 23] have been devel-
oped to enhance the ISC of heavy atom-free PSs. However, the synthetic methods of
–
π
,
,
,
Molecules 2021, 26, 1243. https://doi.org/10.3390/molecules26051243
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 1243
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these compounds are highly demanding. Using fullerene as a spin converter is a useful
method to construct PSs with predictable ISC ability, because it possesses an ISC efficiency
of unity even after derivatization [24]. Furthermore, fullerene also shows unique prop-
erties in biological systems. For example, fullerene exhibits excellent antioxidant [25],
antiamyloid [26] and antibacterial [27] properties, has an adhesive and membrane-like
potential [28], possesses excellent abilities in penetrating cell membranes and modulating
ion transport [29,30], etc. Thus, fullerene-based PSs are promising scaffolds for designing
high-technology nanomaterials and drugs in biological field.
C70, a higher molecular weight fullerene in the shape of a rugby ball [31], has a
much more extended
C60 32]. Like C60, C70 also possesses high ISC efficiency (near 1.0) and can produce a high
quantum yield of 1O2 (0.81
0.15) [32]. The photodynamic activity and TTA quantum
yield of C70 are higher than that of the C60 counterparts [33 35]. In consequence, it is highly
π system and higher absorption in the visible region compared to
[
±
–
promising to synthesize heavy atom-free organic triplet sensitizers with better properties
using C70 as a spin converter. Similar to C60, C70 itself is not a good PS due to its low
molar extinction coefficient in the visible region. Grafting suitable antennas onto a C70 cage
seems to be a useful strategy, because significantly greater photosensitization efficiency has
been achieved in C60-based triplet PSs due to improved visible light absorption [36–40].
Until now, most of the reported studies on the photosensitization of fullerenes have mainly
been focused on C60. C70-antenna systems as triplet photosensitizers have rarely been
reported [41].
Herein, we designed and synthesized a broadband visible light-absorbing [70]fullerene-
BODIPY-triphenylamine triad (C70-B-T) as a heavy atom-free organic triplet photosensitizer
for photooxidation. BODIPY, owing to its high chemical stability, simple synthetic route
and adjustable π-conjugation framework [42–49], was selected as the light-harvesting an-
tenna in this triad. As we know, the absorption wavelength of unsubstituted BODIPY is
at ~500 nm and the absorption range is relatively narrow. Therefore, the triphenylmethyl
amine (TPA) units were grafted at the “2” and “6” positions of the BODIPY core to im-
prove the absorption range. Direct carbon–carbon coupling at the “2” and “6” positions of
the BODIPY will weaken the electronic conjugation and bend the molecule due to steric
hindrance [50]. Thus, in this work, alkyne linkers were inserted between the TPA units
and BODIPY core to release the steric congestion. In order to place C70 and antenna at
a fixed distance, TPA-fused BODIPY (B-T) and C70 were connected by a rigid phenyl
acetylene bridge via the Prato reaction. The structures of C70-B-T, B-T and C70-1 are shown
in Figure 1. Both C70-B-T and B-T show broadband visible light-absorbance from 270 to
700 nm, covering almost the entire UV–visible region.
Figure 1. The structures of compounds [70]fullerene-BODIPY-triphenylamine triad (C70-B-T)
, C70-1
and B-T.

Molecules 2021, 26, 1243
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2. Results and Discussion
2.1. Synthesis
The synthetic procedures of C70-B-T and B-T are outlined in Scheme 1, and the details
are given in the Materials and Methods section.
H
TMS
I
TMSA
K2CO3
N
CuI, PPh3, Pd(PPh3)2Cl2
N
N
CH2Cl2/MeOH
rt, 1.5 h, N2
(90%)
Et3N/THF
40 oC, 5 h, N2
(83%)
3
1
2
CHO
CHO
H
I
CHO
Compound 3
CuI, PPh3, Pd(PPh3)2Cl2
CuI, PPh3, Pd(PPh3)2Cl2
NIS
Et3N/THF
40 oC, 3.5 h, N2
(83%)
CHCl3/CH3COOH
60 oC, 12 h
(89%)
Et3N/Tol
60 oC, 48 h, N2
(68%)
N
F
N
F
B
I
I
N
F
N
B
N
F
N
F
B
F
4
6
5
N
CHO
C70
sarcosine
N
N
Tol
N
N
N
F
N
F
N
F
N
F
B
130 oC, 8 h, N2
(45%)
B
B-T
C70-B-T
Scheme 1. Synthetic procedures for C70-B-T and B-T.
By coupling with aromatic compounds at the “2” and “6” positions, the
framework of BODIPY could be extended. In order to construct TPA-fused BODIPY,
the preparation of 4-ethynyl-N,N-diphenylaniline ( ) and 2,6-diiodo-BODIPY ( ) was
required. Compound was synthesized by a standard Sonogashira reaction between
π-conjugation
3
6
3
trimethylsilylacetylene (TMSA) and 4-iodo-N,N-diphenylaniline, followed by deprotection
of the trimethylsilyl group.
BODIPY
coupling reaction of
was prepared in 89% yield by treating
of CH3CO2H. A subsequent double Sonogashira coupling reaction of
4
was synthesized according to the reported procedures [36]. The cross-
with 4-iodobenzaldehyde afforded in 83% yield. BODIPY
with N-iodosuccinimide (NIS) in the presence
with afforded
4
5
6
5
6
3
B-T in 68% yield. Finally, C70-B-T was obtained in 45% yield by treating B-T with C70
and sarcosine under nitrogen atmosphere in toluene. The reference compound C70-1
was prepared according to the Prato procedure [51] as described in the Materials and
Methods section.
The structures of all the intermediates and final compounds were fully confirmed
by NMR, mass and IR techniques. Due to the lower structural symmetry of C70, C70-B-T
and C70-1 are mixture of isomers. The NMR spectra of all the compounds are given in the
Supplementary Materials. For the sake of clarity, the expansion of the 1H-NMR spectrum
of C70-B-T in CDCl3 is shown in Figure 2.

Molecules 2021, 26, 1243
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Figure 2. Expansions of the 1H-NMR spectrum of C70-B-T in CDCl3.
The 1H-NMR spectrum shows that there are more than three isomers in C70-B-T and
gives all the expected proton signals. For instance, the peaks at ~7.90–6.96 ppm assign
to the phenyl ring protons; peaks at ~5.30–2.39 ppm assign to the pyrrolidine protons,
protons of N-CH3 and pyrrole ring CH3 and peaks at ~1.59–1.43 ppm assign to the protons
of pyrrole ring CH3. The 13C-NMR spectrum of C70-B-T also shows the expected signals.
For example, the peaks from 122–151 ppm assign to the sp2-C of C70 and benzene ring,
peaks at ~80–117 ppm are carbons of alkyne, peaks from 58–71 ppm assign to the sp3-C of
C70 and the carbons of pyrrolidine. The mass spectrum of C70-B-T gives a molecular peak
at m/z 1854.4471, which is consistent with the calculated data.
2.2. UV–Vis Absorption and Steady-State Fluorescence
The UV–vis absorption and steady-state fluorescence of C70-B-T, and the reference com-
pounds C70-1 and B-T were recorded in toluene in 1.0
Triad C70-B-T and dyad B-T show broadband absorption in the entire UV–visible region
270–700 nm) with a strong absorption peak at about 600 nm (ε = 58,804 L mol−1 cm−1). Com-
×
10−5 M and are shown in Figure 3
.
(
pared with traditional BODIPY, a bathochromic shift of about 100 nm of the low-energy ab-
sorption peak is found in B-T, indicating that the TPA units extend the conjugation length
effectively. The UV–vis spectrum of C70-B-T is the sum of C70-1 and B-T (the data are given in
Table 1), suggesting no significant electronic communication between C70 and the antenna at
ground state.
The steady-state fluorescence spectra of C70-B-T, C70-1 and B-T in toluene upon
excitation at 339 and 605 nm are presented in Figure 3b and 3c, respectively. The emission
maxima and fluorescence quantum yields of C70-B-T and B-T are listed in Table 1. When
the TPA part was excited at 339 nm, only emissions of BODIPY moiety are observed both in
B-T and C70-B-T. The fluorescence of TPA (447 nm) part is largely quenched, showing that
efficient excitation energy transfer from TPA to BODIPY occurs [52]. The emission peaks
of both the triad and dyad are located at 664 nm, but the emission intensity of C70-B-T
is relatively weak compared with that of B-T (the quenching efficiency is 88%). Direct
excitation of the BODIPY moiety of C70-B-T and B-T at 605 nm results in fluorescence
spectra resemble those ones obtained upon TPA-part excitation. B-T still gives intense

Molecules 2021, 26, 1243
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fluorescence with an emission peak at 664 nm (
Φ
F = 0.22), whereas the emission of C70-B-T
is largely quenched (82%, ΦF = 0.04) due to intramolecular energy or electron transfer.
Figure 3.
C70-1
(
a
) UV–vis absorption spectra of C70-1
,
B-T and C70-B-T in toluene (c = 1.0
×
10−5 mol/L). (
B-T and C70-B-T in
b) Fluorescence of
−5
,
B-T and C70-B-T in toluene (excited at 339 nm, c = 1.0
×
10 mol/L). (c) Fluorescence of C70-1
,
−5
toluene (excited at 605 nm, c = 1.0 × 10 mol/L).
Table 1. Characteristic spectroscopic data of C70-1, B-T and C70-B-T in toluene and THF a.
b
Φ
F
Compound
λabs (nm)
λ
(nm)
Solvent
em
Ex. = 339 nm
Ex. = 605 nm
284, 307, 398,
462, 537, 666
toluene
710
-
-
C70-1
B-T
THF
toluene
THF
-
-
-
0.29
-
-
330, 443, 603
664
668
0.22
0.01
-
284, 310, 344,
443, 600
-
toluene
664
0.03
0.04
C70-B-T
THF
666
-
0.005
a
c = 1.0
×
10−5 M. b Using (4-((trimethylsilyl)ethynyl)phenyl)-BODIPY as reference, ΦF = 0.46 in CHCl3, λexc
484 nm [53].
The emissions of C70-B-T and B-T in THF were also measured to investigate the effect
of solvent polarity on the emission behavior. The results are shown in Figure 4. The
emission intensities of C70-B-T and B-T are quite sensitive to solvent polarity due to the
presence of dipole moments inside the molecules. For instance, the emission intensities
drop largely in THF in comparison to that in toluene. Whereas the emission peak positions
of both C70-B-T and B-T do not show an obvious shift. The low solvent polarity effect
suggests the formation of a neutral excited state in C70-B-T [37,54]. Thus, the emission
quenching of the BODIPY part observed in C70-B-T should mainly be ascribed to the
intramolecular energy transfer from B-T to C70, and the electron transfer from the antenna
to C70 is not significant [41
possible, because the energy of the S1 state of B-T is higher than that of C70
,
55]. The intramolecular energy transfer from B-T to C70 is
32 41]. In the
[
,
magnified spectrum of Figure 3b, weak fluorescence emissions of C70 at 687 nm, 702 nm
and 710 nm are observed. However, these emission bands are not observed in C70-B-T
because of the fluorescence overlap.
Figure 4. Emission of (
(1 × 10 mol/L, exited at 605 nm).
a
)
B-T and (
b
)
C70-B-T in THF (redline) and toluene (black curve)
−5

Molecules 2021, 26, 1243
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2.3. Time-Resolved Fluorescence Spectroscopy
For deeper insight into the photoinduced intramolecular transfer processes, the fluo-
rescence decays of C70-B-T and B-T were investigated using time-resolved fluorescence
spectroscopy techniques. The results are shown in Figure 5.
Figure 5. Fluorescence decay traces obtained with a time-correlated single photon counting (TCSPC)
of (a) B-T and (b) C70-B-T in toluene. Excited at 510 nm and recorded at 664 nm.
When excited at 510 nm and recorded the fluorescence signal at 664 nm, B-T gives
monoexponential decay with fluorescence lifetime of 0.86 ns. Similar results are obtained
when C70-B-T is excited, but the fluorescence lifetime of C70-B-T is reduced to 0.19 ns,
shorter than that of B-T. The much shorter fluorescence lifetime of BODIPY part detected
in C70-B-T indicates an effective excitation energy quenching of the BODIPY part by C70,
which is consistent with the result obtained in the steady-state fluorescence spectra.
The efficiency of energy transfer from BODIPY to C70 in C70-B-T can be calculated by
using Equation (1).
k1
1/τ1 − 1/τ0
Φ1 =
=
(1)
(k0 + k1)
1/τ1
where τ0 and τ1 are the fluorescence lifetimes of B-T and C70-B-T, respectively. Thus, the
energy transfer efficiency Φ1 is determined as 0.78 for C70-B-T, which is in consistence
with the result of the steady-state fluorescence spectra
2.4. Nanosecond Time-Resolved Transient Absorption Spectroscopy
Nanosecond time-resolved transient absorption spectroscopy was used to investigate
the triplet excited states of the triad and C70-1. The nanosecond transient absorption spectra
were recorded by a nanosecond flash photolysis system (LP980 ENDINBURGH) with a
pulse laser (7 ns, 1 Hz) from a Neodymium-doped Yttrium Aluminium Garnet (Nd:YAG)
laser at a wavelength of 532 nm. The results are shown in Figure 6.
Upon excitation at 532 nm, bleaching at about 474 and 538 nm was observed for both
C70-B-T and C70-1 due to ground state absorption of C70. C70-1 shows a sharp transient
absorption band at 420 nm and a broad transient absorption band from 644 to 735 nm
(Figure 6c). In the time-resolved transient absorption spectrum of C70-B-T, no bleaching
of the steady-state absorption of B-T part is observed. C70-B-T also shows a characteristic
3
absorption band of C70* at 418 nm. However, the broad transient absorption band of
C70-B-T is blue-shifted and splits into three bands with peaks at ~585, 622 and 695 nm
due to the derivatization of C70 by the antenna [41]. The dynamic decay behaviors of the
three bands are similar to that of the peak at 418 nm, suggesting that the three bands also
belong to the absorptions of 3C70*, agreed well with the reported studies [41]. Therefore,
the peaks observed in the transient absorption spectrum of C70-B-T should be ascribed
to the absorptions of the 3C70*. When excited, the triplet state of C70-B-T is exclusively
localized on the C70 unit. The lifetime of the triplet state of C70-B-T is 13.9
longer than that of C70-1 (12.5 µs).
µs, slightly

Molecules 2021, 26, 1243
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Figure 6.
(a) Nanosecond time-resolved transient absorption spectrum of C70-B-T upon excitation at
532 nm with a pulse laser (7 ns, 1 Hz) in deaerated toluene at room temperature. (
b) Decay of C70-B-T
at 700 nm. ( ) Nanosecond time-resolved transient absorption spectrum of C70-1 upon excitation at
c
532 nm with a pulse laser (7 ns, 1 Hz) in deaerated toluene at room temperature. (d) Decay of C70-1
at 700 nm. Arrows indicate the spectral trend with time increasing.
2.5. Photooxidation of 1,5-Dihydroxy Naphthalene Mediated by 1O2
The photooxidation ability of C70-B-T was studied by using 1,5-dihydroxy naph-
thalene (DHN) as a 1O2 scavenger. In the presence of 1O2, DHN can be easily oxidized
to juglone [56]. The kinetics of the photooxidation could be measured by following the
decrease in the absorption of DHN at 301 nm or the increase in the absorption of juglone
at 427 nm with time. The spectral responses of DHN using C70-B-T
methylene blue (MB) as the sensitizers upon broadband excitation with a xenon lamp
are presented in Figure 7 and Figure S1, respectively. For C70-B-T C70-1, and MB, the
, B-T, C70-1, and
,
change of the absorption at 301 nm is obvious, indicating the significant consumption
of DHN and the efficient photosensitization ability of the triplet PSs, whereas, nearly no
UV–vis absorption change is observed in the spectral responses of DHN with B-T as the
photosensitizer. The photostability of C70-B-T was also investigated by exposing to light
for 1 h and no decrease is observed in the absorption (Figure S2). This further proves
that the decrease in the absorption at 301 nm is caused by photooxidation instead of the
decomposition of the photosensitizers.
The photooxidation ability of the triplet photosensitizers was quantitatively compared
by plotting the ln[(A
constant and the yield of singlet oxygen (
and the data are listed in Table 2.
−
A0)/A0] against the irradiation time. The photooxidation rate
Φ∆) of the photosensitizers were calculated [37,57],
Table 2. The photooxidation rate constant and yield of singlet oxygen of the photosensitizers a.
c
b
d
−1
Photosensitizers
υi
kobs /min
Φ∆
C70-B-T
C70-1
B-T
67.5
45.9
2.0
6.75
4.59
0.2
0.78
0.81 e
-
MB
44.8
4.48
0.57
a
b
In CH2Cl2/CH3OH = 9:1 (v/v). c = 1.0
×
10−5 mol/L. The rate constant kobs was calculated by the rule:
c
ln[(A
−
A0)/A0] =
−
kobst. In 10−3 min−1
.
Initial consumption rate of DHN, υi = kobs[DHN]. In 10−6 M min−1
.
d
e
Quantum yield of singlet oxygen (1O2), with MB as standard (Φ∆ = 0.57 in CH2Cl2). Literature values [32].

Molecules 2021, 26, 1243
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Figure 7. Absorption spectral evolution for the photooxidation of DHN using (
a
)
C70-1, (
b
)
B-T and
0
(
c
)
C70-B-T as sensitizers. (
d
) Plots of ln[(A
−
A )/A0] vs. irradiation time (t) for the photooxidation
of DHN using dif−fe4rent sensitizers (collected at 301 nm). c[sensitizers] = 1.0
×
10−5 mol L−1
,
c[DHN] = 1.0 × 10 mol L−1. In CH2Cl2/MeOH (9:1, v/v). A, A , and A0 were the absorbances at
0
0
301 nm, where A was the absorption of DHN and sensitizer, A was the absorption of sensitizer, A0
was the initial absorption of DHN.
Among all the investigated photosensitizers, C70-B-T gives the highest photooxida-
tion efficiency. The photooxidation rate constant (kobs) of C70-B-T is 67.5
×
10−3 min−1
,
which is 1.47 times as that of C70-1 (45.9
(
×
10−3 min−1) and 1.51 times as that of MB
44.8 × 10−3 min−1). B-T does not show obvious photooxidation ability. The more efficient
photooxidation of C70-B-T compared with that of C70-1 demonstrates that the broadband
visible light absorption antenna improves the photosensitizing ability. Here, the enhanced
photosensitizing ability of C70-B-T compared with that of C70-1 and B-T should be at-
tributed to the synergetic effect of C70 and B-T. First, B-T harvested a broadband visible
light and be excited to its excited singlet easily, then efficient intramolecular energy transfer
from B-T to C70 occurred and formed the 1C70*, finally the highly efficient ISC of C70 would
eventually lead to the population of 3C70*. These photophysical processes can be supported
by the steady-state and transient data mentioned above.
3. Materials and Methods
3.1. Materials
All reagents were obtained from commercial sources. C70, ethyl 2-(methylamino)acetate
hydrochloride, ethyl glyoxylate, 4-iodo-N,N-diphenylaniline, cuprous iodide (CuI), triph-
enylphosphine (PPh3), trimethylsilylacetylene (TMSA), potassium carbonate (K2CO3), anhy-
drous sodium sulphate (Na2SO4), N-iodosuccinimide (NIS), sodium thiosulfate (Na2S2O3) and
DHN were purchased from Alfa Aesar. Trans-dichlorobis(triphenyl-phosphine)Palladium(II)
(Pd(PPh3)2Cl2) was purchased from Shanxi Kaida Chemical engineering Co., Ltd. Sarcosine,
4-iodobenzaldehyde and 1,2-dichlorobenzene (ODCB) were purchased from J&K Scientific LTD
(Beijing, China).
Silica gel, carbon disulfide (CS2), dichloromethane (DCM), petroleum ether (PE),
methanol (MeOH), tetrahydrofuran (THF), triethylamine (Et3N), acetic acid (CH3COOH),
trichloromethane (CHCl3), and toluene were purchased from Sinopharm Chemical Reagent
Co., Ltd (Shanghai, China). THF was distilled over sodium and benzophenone, other
reagents used for the synthesis were used directly.
All synthesis compounds were characterized by 1H and 13C-NMR spectroscopy on
a BRUKER 400 MHz spectrometer. The mess analyses were performed using a Bruker
ultrafleXtreme MALDI TOF/TOF (Bremen, Germany).

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3.2. Synthesis
3.2.1. Synthesis of C70-1
The solution of C70 (140 mg, 0.16 mmol), ethyl 2-(methylamino)acetate hydrochloride
(49.1 mg, 0.32 mmol) in ODCB (15 mL) was bubbled with N2 for 30 min at room temperature.
Ethyl glyoxylate (158 µL, 0.8 mmol, in a 50% toluene solution) was added and the reaction
mixture was stirred at 130 ◦C for 90 min monitored by thin layer chromatography (TLC).
After the solvent was removed by reduced pressure, the mixture was purified by silica
gel column chromatography using CS2/DCM (2/1, v/v) as the eluent to give C70-1 as
a brown black powder (70.0 mg, 42%). UV-vis (toluence) λ
max/nm 284 (77230 L mol−1
cm−1), 307 (51491 L mol−1cm−1), 398 (32516 L mol−1 cm−1), 462 (26537 L mol−1 cm−1),
1
537 (14119 L mol−1cm−1), 666 (2426 L mol−1cm−1). H-NMR (400 MHz, CDCl3)
δ 5.83
(s, CHCO2), 5.35 (s, CHCO2), 5.30 (s, CHCO2), 4.81(s, CHCO2), 4.63–4.37(m, OCH2CH3),
4.32–4.17(m, OCH2CH3), 4.15-3.99 (m, OCH2CH3), 3.10 (s, NCH3), 2.83 (s, NCH3), 2.68 (s,
NCH3), 2.62 (s, NCH3), 1.54 (t, J = 7.1 Hz, OCH2CH3), 1.42 (t, J = 7.2 Hz, OCH2CH3), 1.17
(t, J = 7.1 Hz, OCH2CH3), 1.04 (t, J = 7.2 Hz, OCH2CH3). 13C-NMR (100 MHz, CDCl3)
δ
170.18, 170.06, 157.73, 155.60, 155.32, 154.87, 151.56, 151.20, 151.14, 150.89, 150.83, 150.50,
150.46, 150.01, 149.78, 149.46, 149.37, 149.23, 149.20, 149.06, 148.87, 148.83, 147.60, 147.54,
147.29, 147.25, 147.18, 147.14, 147.12, 147.06, 146.98, 146.32, 146.10, 145.99, 145.91, 143.52,
143.50, 143.48, 143.40, 143.21, 141.42, 140.74, 140.46, 140.23, 138.52, 137.64, 133.90, 133.70,
133.27, 131.86, 131.73, 131.53, 131.49, 131.36, 75.93, 72.28, 65.54, 64.58, 62.07, 61.65, 35.18,
29.86, 14.84, 14.50, 14.70, 14.35, 14.28. FT-IR υ
/cm−1 (KBr) 2922, 2851, 1746, 1733, 1632,
1427, 1369, 1340, 1190, 1131, 1052, 795, 671, 637, 580, 534. HRMS (MALDI-TOF) m/z calcd
for C79H15NO4[M−•]1041.1007, found 1041.1001.
3.2.2. Synthesis of Compound 2
4-Iodo-N,N-diphenylaniline 1 (500.0 mg, 1.35 mmol), CuI (128.3 mg, 0.67 mmol) and
PPh3 (176.6 mg, 0.67 mmol) were dissolved in 20 mL of THF/Et3N (1/3, v/v) in an oven-
dried, 50 mL one neck flask equipped with a gas inlet adaptor. The mixture was stirred
at room temperature under N2 atmosphere for 30 min. Then Pd(PPh3)2Cl2 (189.0 mg,
0.27 mmol) and TMSA (945
40 C under N2 atmosphere for 5 h until TLC monitoring indicated the disappearance
µ
L, 6.74 mmol) were added. The mixture was stirred at
◦
of compound 1. The solvent was removed in vacuo, and the crude reaction mixture was
purified by silica gel column chromatography using PE/CH2Cl2 (2/1, v/v) as eluent to
1
give
2
[
58] in 83% yield (391.0 mg). H-NMR (400 MHz, CDCl3)
δ
7.34 (d, J = 8.7 Hz, 2H),
7.31-7.27 (m, 4H), 7.12–7.06 (m, 6H), 6.98 (d, J = 8.7 Hz, 2H), 0.27 (s, 9H).
3.2.3. Synthesis of Compound 3
Compound 2 (120.0 mg, 0.35 mmol) was dissolved in 7 mL of CH2Cl2/MeOH (1/1,
v/v) in an oven-dried, 25 mL one neck flask equipped with a gas inlet adaptor. The mixture
was stirred at room temperature under N2 atmosphere for 30 min. Then, K2CO3 (145.5 mg,
1.05 mmol) was added. The mixture was stirred at room temperature under N2 atmosphere
for 1.5 h until TLC monitoring indicated the disappearance of compound 2. The crude
mixture was extracted with CH2Cl2 and H2O three times. The organic layer was dried
by Na2SO4 and the solvent was then removed. The crude product was purified by silica
gel column chromatography using PE/CH2Cl2 (2/1, v/v) as eluent to give
3
[
58] in 90%
1
yield (85.8 mg). H-NMR (400 MHz, CDCl3)
δ
7.33 (d, J = 8.7 Hz, 2H), 7.29–7.25 (m, 4H),
7.11–7.04 (m, 6H), 6.97 (d, J = 8.6 Hz, 2H), 3.02 (s, 1H).
3.2.4. Synthesis of Compound 5
Compound
4 was synthesized by adapting the literature procedure and used di-
rectly [36]. (394.4 mg, 1.13 mmol), 4-iodobenzaldehyde (313.2 mg, 1.35 mmol), CuI
4
(107.6 mg, 0.57 mmol) and PPh3 (148.2 mg, 0.57 mmol) were dissolved in 24 mL of
THF/Et3N (1/2, v/v) in an oven-dried, 50 mL one-neck flask equipped with a gas in-
let adaptor. The mixture was stirred at room temperature under N2 atmosphere for 30 min.

Molecules 2021, 26, 1243
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Then, Pd(PPh3)2Cl2 (158.6 mg, 0.23 mmol) was added. The mixture was stirred at 40 ◦C
under N2 atmosphere for 3.5 h until TLC monitoring indicated the disappearance of
4.
The solvent was removed in vacuo, and the crude reaction mixture was purified by silica
gel column chromatography using PE/CH2Cl2 (4/1, v/v) as eluent to give
5
in 83% yield
1
(425.4 mg). H-NMR (400 MHz, CDCl3)
δ
10.04 (s, 1H), 7.90 (d, J = 8.2 Hz, 2H), 7.72–7.69
(m, 4H), 7.33 (d, J = 8.2 Hz, 2H), 6.00 (s, 2H), 2.56 (s, 6H), 1.43 (s, 6H). 13C-NMR (100 MHz,
CDCl3) 191.52, 156.02, 143.05, 140.63, 135.85, 135.80, 132.64, 132.31, 131.26, 129.80, 129.19,
128.53, 123.47, 121.57, 92.61, 89.91, 14.75. FT-IR
/cm−1 (KBr) 2923, 2852, 1698, 1600, 1542,
δ
υ
1510, 1468, 1436, 1402, 1369, 1304, 1260, 1193, 1156, 1120, 1082, 1050, 1019, 979, 830, 766, 707,
477. HRMS (MALDI-TOF) m/z calcd for C28H23BF2N2O[M+•]452.1871, found 452.1876.
3.2.5. Synthesis of Compound 6
Compound 5 (200.1 mg, 0.44 mmol) and NIS (217.7 mg, 0.97 mmol) were dissolved
in 12 mL of CHCl3/CH3COOH (2/1, v/v) in a 25 mL one-neck flask and the mixture
◦
was stirred at 60 C for 12 h, monitored by TLC. Then the mixture was extracted with
CH2Cl2 and a saturated sodium thiosulfate aqueous solution three times. The combined
organic layer was dried with anhydrous Na2SO4, filtered and concentrated. The crude
reaction mixture was subjected to silica gel column chromatography using PE/CH2Cl2
1
(2/1, v/v) as eluent to give
6
in 89% yield (280.0 mg). H-NMR (400 MHz, CDCl3)
δ
10.05
(s, 1H), 7.91 (d, J = 8.2 Hz, 2H), 7.73 (d, J = 8.2 Hz, 2H), 7.72 (d, J = 8.1 Hz, 2H), 7.30 (d,
J = 8.2 Hz, 2H), 2.66 (s, 6H), 1.45 (s, 6H). 13C-NMR (100 MHz, CDCl3)
191.50, 157.37,
145.26, 140.27, 135.90, 135.45, 132.91, 132.36, 131.15, 129.82, 129.03, 128.37, 124.15, 92.28,
90.37, 86.07, 17.36. FT-IR
/cm−1 (KBr) 2923, 1690, 1599, 1544, 1530, 1489, 1439, 1399, 1345,
δ
υ
1307, 1204, 1181, 1113, 1089, 995,918, 823, 765, 706, 588, 525. HRMS (MALDI-TOF) m/z
calcd for C28H21BF2I2N2O[M+•]703.9804, found 703.9809.
3.2.6. Synthesis of Compound B-T
Compound
6 (100.0 mg, 0.14 mmol), CuI (13.5 mg, 0.07 mmol) and PPh3 (18.6 mg,
0.07 mmol) were dissolved in 12 mL of toluene/Et3N (1/3, v/v) in an oven-dried, 25 mL one
neck flask equipped with a gas inlet adaptor. The mixture was stirred at room temperature
under N2 atmosphere for 30 min. Then Pd(PPh3)2Cl2 (19.9 mg, 0.03 mmol) and compound
◦
3
(152.8 mg, 0.57 mmol) were added. The mixture was stirred at 60 C under N2 atmosphere
for 48 h until TLC monitoring indicated the disappearance of compound 6. The solvent
was removed in vacuo, and the crude reaction mixture was purified by silica gel column
chromatography using PE/CH2Cl2 (4/1, v/v) as eluent to give B-T in 68% yield (95.4 mg).
UV–vis (toluence) λ
max/nm 330 (67,683 L mol−1 cm−1), 443 (13,215 L mol−1 cm−1), 603
−1
1
(59,703 L mol cm−1). H-NMR (400 MHz, CDCl3)
δ 10.04 (s, 1H), 7.90 (d, J = 8.4 Hz, 2H),
7.74–7.71 (m, 4H), 7.34 (d, J = 8.3 Hz, 2H),7.31–7.28 (m, 6H), 7.27–7.24 (m,6H), 7.11–7.08 (m,
8H), 7.07–7.03 (m, 4H), 6.99–6.97 (m, 4H), 2.71 (s, 6H), 1.56 (s, 6H). 13C-NMR (100 MHz,
CDCl3)
131.10, 129.82, 129.53, 129.14, 128.49, 125.06, 123.89, 123.69, 122.51, 116.89, 116.22, 97.05,
92.51, 90.19, 80.69, 13.90, 13.77. FT-IR
/cm−1 (KBr) 2923, 2853, 1635, 1590, 1525, 1492, 1400,
δ 191.53, 158.87, 148.03, 147.28, 143.25, 141.02, 135.91, 135.38, 132.79, 132.43, 132.35,
υ
1318, 1264, 1186, 1173, 1078, 1011, 795, 753, 710, 695, 586, 533, 506. HRMS (MALDI-TOF)
m/z calcd for C68H49BF2N4O[M+•] 986.3973, found 986.3976.
3.2.7. Synthesis of Compound C70-B-T
The mixture of C70 (35.4 mg, 0.04 mmol), compound B-T (25.0 mg, 0.025 mmol) and
◦
sarcosine (18.8 mg, 0.21 mmol) was dissolved in 5 mL of toluene and stirred at 130 C
under N2 atmosphere for 8 h, monitored by TLC. Then the solvent was removed under
reduced pressure. The mixture was subjected to silica gel column chromatography using
CS2 as the eluent to afford unreacted C70, then using CS2/CH2Cl2 (2/1, v/v) as the eluent
to give C70-B-T as a mixture of isomers (21.3mg, 45%). UV–vis (toluene) λmax/nm 284
(
111272 mol−1 cm−1), 310 (98534 mol−1 cm−1), 344 (83490 L mol−1 cm−1), 443 (31724 L
mol−1 cm−1), 600 (58804 L mol−1 cm−1). 1H-NMR (400 MHz, CDCl3)
δ 7.90–7.61 (m,

Molecules 2021, 26, 1243
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phenyl ring H), 7.54–7.43 (m, phenyl ring H), 7.35–7.19 (m, phenyl ring H), 7.11–7.03 (m,
phenyl ring H), 7.00–6.96 (m, phenyl ring H), 5.30 (s, NCHPh), 5.07 (s, NCHPh), 5.01 (s,
NCHPh), 4.97 (s, NCHPh), 4.70 (d, J = 9.4 Hz, NCH2), 4.32–4.21 (NCHPh, NCH2), 4.08 (d,
J = 9.7 Hz, NCH2), 4.04 (s, NCHPh), 3.86 (s, NCHPh), 4.67–4.51 (NCHPh, NCH2), 3.41 (d,
J = 9.4 Hz, NCH2), 3.22 (d, J = 9.5 Hz, NCH2), 2.72 (s, NCH3, pyrrole ring CH3), 2.70 (s,
NCH3, pyrrole ring CH3), 2.69 (s, NCH3, pyrrole ring CH3), 2.56 (s, NCH3, pyrrole ring
CH3), 2.55 (s, NCH3, pyrrole ring CH3), 2.46 (s, NCH3, pyrrole ring CH3), 2.39 (s, NCH3,
pyrrole ring CH3), 1.59 (s, pyrrole ring CH3), 1.56 (s, pyrrole ring CH3), 1.55 (s, pyrrole
ring CH3), 1.52 (s, pyrrole ring CH3), 1.51 (s, pyrrole ring CH3), 1.43 (s, pyrrole ring CH3).
13C-NMR (100 MHz, CDCl3)
δ 167.85, 167.53, 158.95, 158.73, 158.50, 158.03, 156.54, 156.21,
155.52, 155.00, 154.96, 154.81, 154.77, 153.49, 152.54, 151.82, 151.66, 151.58, 151.56, 151.52,
151.47, 151.43, 151.21, 151.16, 151.02, 150.95, 150.83, 150.78, 150.76, 150.73, 150.64, 150.85,
150.55, 150.36, 150.32, 150.17, 150.07, 150.01, 149.96, 149.89, 149.83, 149.80, 149.77, 149.49,
149.44, 149.39, 149.34, 149.27, 149.21, 149.18, 149.10, 149.07, 149.05, 148.95, 148.91, 148.89,
148.79, 148.73, 148.43, 148.39, 148.37, 148.33, 148.25, 148.19, 148.15, 147.95, 147.89, 147.83,
147.60, 147.50, 147.24, 147.16, 147.02, 146.97, 146.95, 146.91, 146.75, 146.65, 146.57, 146.39,
146.32, 146.29, 146.23, 145.95, 145.89, 145.83, 145.78, 145.73, 145.65, 145.53, 145.44, 145.25,
145.17, 145.01, 144.87, 144.77, 144.64, 144.52, 144.48, 144.45, 144.39, 143.99, 143.90, 143.81,
143.73, 143.56, 143.51, 143.43, 143.36, 143.32, 143.28, 143.13, 142.99, 142.92, 142.85, 142.81,
142.40, 142.18, 142.08, 142.06, 142.02, 141.75, 141.28, 141.23, 141.15, 141.06, 140.95, 140.88,
140.81, 140.74, 140.67, 140.64, 140.54, 140.44, 140.29, 140.17, 138.75, 138.02, 137.97, 137.75,
137.58, 137.53, 137.42, 134.67, 134.61, 134.58, 133.92, 133.87, 133.75, 133.67, 133.60, 133.07,
132.67, 132.44, 132.11, 131.88, 131.82, 131.71, 131.68, 131.66, 131.60, 131.42, 131.35, 131.30,
131.19, 131.09, 131.05, 129.52, 128.97, 128.38, 128.28, 125.02, 124.57, 124.40, 123.66, 123.13,
122.86, 122.50, 116.87, 116.25, 97.12, 91.25, 91.14, 90.97, 89.61, 89.52, 89.44, 89.35, 88.09, 86.09,
83.15, 82.83, 82.18, 80.84, 79.93, 70.85, 70.27, 69.98, 68.86, 68.27, 66.68, 66.44, 66.28, 65.70,
62.18, 60.46, 58.87, 58.47, 39.69, 39.62, 32.06, 30.69, 30.44, 30.30, 30.18, 29.84, 29.50, 14.28,
13.89, 13.81. FT-IR υ
/cm−1 (KBr) 2959, 2923, 2852, 1708, 1673, 1592, 1502, 1402, 1339, 1317,
1283, 1174, 1090, 796, 546, 419. HRMS (MALDI-TOF) m/z calcd for C21H19BF2N2 [M+•
]
1857.4479, found 1854.4471.
3.3. Photooxidation Experiment
Compounds C70-B-T
,
C70-1
,
B-T and MB in a concentration of 2.0
×
10−5 mol L−1 and
DHN in a concentration of 2.0
×
10−4 mol L−1 were dissolved in CH2Cl2/MeOH (9:1, v/v),
respectively. Then, the above solutions of sensitizers and DHN were mixed in a volume
ratio of 1:1, and O2 was bubbled through the mixture for 10 min. The mixture was then
placed in a quartz cell and irradiated with a broadband light source-xenon lamp using 0.72
M NaNO2 aqueous solution as a cutoff filter (0.17 mW/cm2). The consumption of DHN
was monitored by a decrease in the absorption at 301 nm using a UV–vis spectrophotometer
(UV-1800, Mapada, Shanghai, China) at intervals of 5 min.
The singlet oxygen quantum yield (Φ∆) was determined by using Equation (2).
k
k
obs(x)•I(std)
obs(std)•I(x)
Φ∆ = Φ∆ (std)
(2)
In the equation,
Φ∆(std) is the singlet oxygen generation quantum yield of MB (0.57
in CH2Cl2), kobs(x) and kobs(std) were the absolute value of the slopes of ln[(A-A0)/A0]
versus irradiation time for the photooxidation of DHN by sensitizers and MB, respectively.
I(x) and I(std) were the total light intensities absorbed by sensitizers and MB, respectively.
3.4. Photostability Experiment
C70-B-T in a concentration of 1.0
×
10−5 mol L−1 in CH2Cl2/MeOH (9:1, v/v) was
placed in a quartz cell and irradiated with a xenon lamp (0.17 mW/cm2) continuously for
1 h. The spectral response of C70-B-T was recorded using a UV–vis spectrophotometer at
0 h and 1 h, respectively.

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3.5. Measurement of Photophysical Properties
UV–vis absorption and fluorescence spectra were recorded via an absorption spec-
trometer (UV-1800, Mapada) and a fluorescence spectrophotometer (FP8500, JASCO, Tokyo,
Japan) at room-temperature, respectively. The fluorescence lifetime measurements were
conducted using a time-correlated single photon counting (TCSPC) apparatus at room
temperature and a pulsed laser at a wavelength of 510 nm was used as the excitation
source. The nanosecond transient absorption spectra were recorded by a nanosecond flash
photolysis system (LP980, Edinburgh instruments, UK) with a pulse laser (7 ns, 1 Hz) from
a Nd:YAG laser at a wavelength of 532 nm. The samples in 10 mm path length quartz
cuvettes were freshly prepared and deoxygenated by bubbling nitrogen for over 20 min be-
fore measurement. The analyzing light was a 450 W pulsed xenon lamp. A monochromator
equipped with a photomultiplier for collecting the spectral range from 350 to 850 nm was
used to analyze transient absorption spectra. The decay curves were fitted by least-squares
regression using a custom-written algorithm in the Matlab.
The corresponding fluorescence quantum yields were calculated by using Equation (3) [59]
with (4-((trimethylsilyl)ethynyl)phenyl)-BODIPY as the standard, ΦF = 0.46 in CHCl3, λexc
484 nm [53].
ꢀ
ꢁ
2
Astd•Fx
nx
nstd
ΦF
= ΦF
(3)
(x)
(std) Ax•F
std
where
ΦF(x) and ΦF(std) are the fluorescence quantum yields of the sensitizers and standard,
respectively. Ax and Astd are the absorbance of the sensitizers and standard, Fx and Fstd are
the area under the emission curve of the sensitizers and standard, and n is the refractive
index of the solvents used in measurement.
4. Conclusions
In conclusion, a broadband visible light-absorbing [70]fullerene-BODIPY-triphenylamine
triad (C70-B-T) has been synthesized and used as a heavy atom-free organic triplet photo-
sensitizer for photooxidation. Two TPA units were introduced to the π-core of BODIPY and
the absorption spectrum of C70-B-T covered virtually the entire UV–visible region. Upon
the direct or indirect excitation of the BODIPY-part of C70-B-T, the intramolecular singlet
excited state energy transfer from BODIPY to C70 unit occurs and produces 1C70*. Then,
the ISC of C70 produces 3C70*. The photophysical processes were confirmed by steady-state
and transient spectroscopies. The photooxidation ability of the photosensitizers was inves-
tigated using DHN as a chemical sensor. Among all the investigated compounds, C70-B-T
gives the best photooxidation efficiency. The photooxidation rate constant of C70-B-T is 1.47
and 1.51 times as that of C70-1 and MB, respectively. The results indicate that C70-antenna
could be used as another heavy atom-free organic triplet photosensitizer structure motif,
with potential applications in photodynamic therapy, photocatalysis, photovoltaics and
TTA upconversion.
Supplementary Materials: The following are available online: the spectral response of DHN with
MB as the sensitizer (Supplementary Figure S1), the photostability of C70-B-T (Supplementary
Figure S2), high resolution mass spectra (Supplementary Figures S3–S7), 1H-NMR and 13C-NMR
spectra (Supplementary Figures S8–S23) are also provided.
Author Contributions: J.-H.Z. and Y.G. performed the experiments; J.-H.Z., Y.G., L.-F.D., L.-H.M.,
H.-D.L., C.-X.W., and H.C. analyzed the data; S.-E.Z., W.Y. and X.-F.W. conceived and designed the
experiments; J.-H.Z. and S.-E.Z. wrote the paper. All authors have read and agreed to the published
version of the manuscript.
Funding: This work was sponsored by the National Natural Science Foundation of China (21702042,
81961138011), the Anhui Provincial Natural Science Foundation for Distinguished Young Scholar
(2008085J26), the China Scholarship Council in 2019, and the University of Chinese Academy of Sci-
ences.

Molecules 2021, 26, 1243
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Data Availability Statement: The data presented in this study are available in Supplementary Mate-
rials.
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: Samples of the compounds are available from the authors.
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