Evaluating brominated thioxanthones as organo-photocatalysts

Article abstract of DOI:10.1002/poc.3738

Bromination of the widely used triplet sensitizer thioxanthone extends the absorption spectrum into the visible range with only minor loss of lowest triplet state energy (3?kcal/mol for di-bromination). Because of bromine substitution, a slight increase in triplet quantum yield was observed. The di-brominated derivative was effective as organo-photocatalyst in performing [2?+?2] cross-photocycloaddition of acrylimide-based compounds under visible light irradiation.

Full text of DOI:10.1002/poc.3738

Received: 2 May 2017
DOI: 10.1002/poc.3738
Revised: 9 June 2017
Accepted: 11 June 2017
S P E C I A L I S S U E A R T I C L E
Evaluating brominated thioxanthones as organo‐photocatalysts
Akila Iyer1 | Anthony Clay1 | Steffen Jockusch2
| J. Sivaguru1
1 Department of Chemistry and
Abstract
Biochemistry, North Dakota State University,
Bromination of the widely used triplet sensitizer thioxanthone extends the absorp-
tion spectrum into the visible range with only minor loss of lowest triplet state
energy (3 kcal/mol for di‐bromination). Because of bromine substitution, a slight
increase in triplet quantum yield was observed. The di‐brominated derivative was
effective as organo‐photocatalyst in performing [2 + 2] cross‐photocycloaddition
of acrylimide‐based compounds under visible light irradiation.
Fargo, ND, 58108, USA
2 Department of Chemistry, Columbia
University, New York, NY, 10027, USA
Correspondence
Steffen Jockusch, Department of Chemistry,
Columbia University, New York, NY 10027,
USA.
Email: sj67@columbia.edu
J. Sivaguru, Department of Chemistry and
Biochemistry, North Dakota State University,
Fargo, North Dakota, 58108 USA.
Email: jayaraman.sivaguru@ndsu.edu
Funding information
NSF, Grant/Award Number: CHE‐1465075
1 | INTRODUCTION
properties, as we expected the heavy atom to facilitate a
more‐efficient intersystem crossing.[12–17] Unlike other
Thioxanthone 1 is a popular triplet sensitizer in photoinitiated
reactions because of its high molar absorptivity in the near‐
UV spectral region (300‐400 nm), high triplet quantum yield
and relatively high triplet energy.[1,2] Thioxanthone and its
derivatives are frequently used in organo‐photocatalytic
reactions[3,4] and as initiator in photopolymerization formula-
tions.[5] Although 1 absorbs strongly in the near‐UV spectral
region (λmax = 381 nm), its absorption shows only a minor
tailing in the visible region above 400 nm, which makes it
only poorly suited as a sensitizer/catalyst for visible light
illumination. We envisioned a simple structural modification
of the thioxanthone scaffold that would result in a
bathochromic shift without altering the triplet energy signifi-
cantly.[6] A wide variety of thioxanthone derivatives have
been reported where substituents not only shift the absorption
bathochromically but also reduce the triplet energy signifi-
cantly.[7–11]
substitutions (eg, amino substitution) that alters the triplet
excited‐state characteristics of thioxanthone, we believed
that substituting bromine will be advantageous because
of its dual nature of both electron donation (by reso-
nance) and electron withdrawal (by induction) that will
minimally alter the triplet state energy, while providing
us an avenue to excite the molecule effectively with visi-
ble light.
2 | RESULTS AND DISCUSSION
Brominated thioxanthones were synthesized by electrophilic
bromination of parent thioxanthone 1. We found that the
use of bromine in glacial acetic acid with catalytic amounts
of iodine (Scheme 1) resulted in a mixture of mono‐bromi-
nated thioxanthone 2 and di‐brominated thioxanthone 3.
The brominated derivatives were chromatographically
separated and characterized by nuclear magnetic resonance
(NMR) spectroscopy and mass spectrometry. The regioselec-
tivity of bromination was ascertained by single crystal X‐ray
powder diffraction (XRD) analysis.[18]
This prompted us to investigate the effect of brominat-
ing thioxanthone on its photochemical and photophysical
The authors dedicate this manuscript to Prof Waldemar Adam, an esteem col-
league and friend on the occasion of his 80th birthday.
J Phys Org Chem. 2017;e3738.
wileyonlinelibrary.com/journal/poc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/poc.3738

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IYER ET AL.
SCHEME 1 Electrophilic bromination of
thioxanthone 1
The absorption spectra of thioxanthone 1 and its bromi-
bathochromic shift of the long‐wavelength absorption band
with increasing bromination. The absorbance of both bromi-
nated thioxanthone derivatives extend well into the visible
spectral region while maintaining high molar absorptivities,
which should make them attractive to be used as triplet
sensitizer.
nated derivatives (2 and 3) are shown in Figure 1. The
absorption maxima and their molar absorptivities are listed in
Table 1 (entries 1 and 2). As anticipated, the spectra display a
Low‐temperature phosphorescence spectra (77 K in
solvent glass) were recorded to determine the triplet state
energies (Figure 2; Table 1, entries 3‐6). For the parent
thioxanthone 1, the triplet energy (ET) was 64 kcal/mol (in
EtOH) and 65 kcal/mol (MCH) with a corresponding
phosphorescence lifetime (τp) of 155 milliseconds (in EtOH)
and 111 milliseconds (in MCH). This is in alignment with
literature reported values, which indicated the lowest triplet
state is of ππ* character.[2] For the mono‐brominated
thioxanthone 2, the triplet energy (ET) was 62 kcal/mol (in
EtOH) and 64 kcal/mol (MCH) with a corresponding
FIGURE
dimethylformamide (DMF)
1
Absorption
spectra
of
1‐3
recorded
in
TABLE 1 Photophysical properties of thioxanthone derivatives 1‐3
Entry
Parameters
λmax (nm)/DMF a
1
2
3
1)
381
388
396
b
2)
3)
4)
5
ε
DMF (M−1 cm−1
)
6328
4941
5234
ET (kcal/mol)/(EtOH)
ET (kcal/mol)/(MCH)
64
62
61
c
c
65
64
62
d
τp (ms)/(EtOH)
155
26
17
d
6
τp (ms)/(MCH)
111
22
0.79
15
0.81
e
7)
8
ΦT
0.76
625
TT
f
λmax (nm)
645
650
f
9
τT (μs)
45
44
40
g
10)
11)
kq4 (M−1 s−1
)
4.0 0.1 × 109 i
1.2 0.1 × 108 i
3.2 0.1 × 109
4.3 0.2 × 107
2.8 0.1 × 109
7.1 0.2 × 106
h
kq6 (M‐1 s−1
)
aAbsorption maximum of the longest wavelength peak.
bMolar absorptivity.
cTriplet state energy determined from the lowest‐energy peak of the phosphorescence spectra at 77 K.
dPhosphorescence lifetimes at 77 K.
eTriplet quantum yields determined by laser flash photolysis (Figure 3B).
fAbsorption maximum of the triplet‐triplet transition (Figure 3A).
gBimolecular triplet quenching rate constants by 4 in acetonitrile.
hBimolecular triplet quenching rate constants by 6 in acetonitrile.
iFrom Iyer et al[4] and Iyer et al.[20]

IYER ET AL.
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FIGURE 2 Normalized phosphorescence spectra of 1‐3 in methylcyclohexane (left) and ethanol (right) glass at 77 K recorded 10 to
30 milliseconds after pulsed excitation (λex = 377 nm: 1 in EtOH and MCH, 2 in MCH; λex = 388 nm: 2 in EtOH, 3 in EtOH and MCH)
FIGURE 3 A), Normalized triplet absorption spectra of 1‐3 in argon saturated acetonitrile solution recorded after pulsed laser excitation
(λex = 355 nm, 5‐nanosecond pulse width). B), Determination of the triplet quantum yield. 1‐Phenylnaphthalene triplet absorption traces
monitored at 480 nm after pulsed laser excitation (λex = 355 nm, 5‐nanosecond pulse width) of argon saturated acetonitrile solutions of 1‐
phenylnaphthalene (1 mM) in the absence (gray) and presence of sensitizers 1‐3 and benzophenone
phosphorescence lifetime (τp) of 26 milliseconds (in EtOH)
and 22 milliseconds (in MCH). The fivefold decrease in the
phosphorescence lifetimes for brominated thioxanthones
(when compared to 1) is a direct manifestation of internal
heavy atom effect because of bromine substitution that (as
expected) enhances the intersystem crossing efficiency
(phosphorescence rate). For the di‐brominated thioxanthone
derivative 3, the triplet energy (ET) was 61 kcal/mol (in
EtOH) and 62 kcal/mol (MCH) with a corresponding
phosphorescence lifetime (τp) of 17 milliseconds (in EtOH)
and 15 milliseconds (in MCH).
An important parameter for the use of the brominated
thioxanthones as sensitizer is the quantum yield of triplet
formation. The triplet quantum yields were determined
by laser flash photolysis in triplet energy transfer experi-
ments to 1‐phenylnaphthalene. The acceptor compound
(1‐phenylnaphthalene) is ideally suited because it is trans-
parent at the laser excitation wavelength (355 nm) and its
triplet energy (59 kcal/mol)[19] is lower than the triplet
To characterize the triplet excited states of the
thioxanthone derivatives at room temperature, we turned to
transient absorption measurements using laser flash photoly-
sis (Figure 3A and Table 1, entries 8 and 9). Laser excitation
(λex = 355 nm, pulse width = 5 nanoseconds) of argon‐
saturated solutions of 1‐3 in acetonitrile generated transient
absorption spectra (Figure 3A). The long wavelength
absorption band for 1, 2, and 3 were 625, 645, and 650 nm,
respectively, with a corresponding triplet lifetime at room
temperature of 45 microseconds (for 1), 44 microseconds
(for 2), and 40 microseconds (for 3).
SCHEME 2 [2 + 2] Photocycloaddition of acrylimides (4 and 6)
facilitated by 3 under purple LED illumination (395 – 405 nm)

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IYER ET AL.
FIGURE 4 Determination of the
bimolecular quenching rate constants kq of
quenching of sensitizer triplet states by 4
(Figure 4A) and 6 (Figure 4B) using laser
flash photolysis (λex = 355 nm, 5‐
nanosecond pulse width). Triplet absorption
decay rate constant monitored at 640 nm (2)
or 650 nm (3) vs the concentration of 4 or 6
in argon saturated acetonitrile solutions
energy of 1‐3. In addition, 1‐penylnaphthalene shows
intense triplet‐triplet absorption at 480 nm, a spectral
region that does not overlap with the triplet‐triplet absorp-
tion of 1‐3. Figure 3B shows kinetic traces of formation
of 1‐phenylnaphtalene triplet states. In the absence of
sensitizer (1‐3) no transient absorbance was observed.
However, in the presence of 1, 2, or 3, strong triplet
absorption of 1‐phenylnaphthalene is generated because
of triplet energy transfer. Using benzophenone as
standard, which has a triplet quantum yield of 1.0, the
triplet quantum yields for 1, 2, and 3 were determined
to be 0.76, 0.79, and 0.81, respectively[19] (Table 1, entry
7). The slight increase in triplet quantum yield is in line
with the expected effect of bromine substitution of
thioxanthone.
Having ascertained the photophysical properties of the
brominated thioxanthones 2 and 3, we shifted our focus
towards using them for photochemical reactions. On the basis
of the substantial bathochromic shift observed for 3
(Figure 1), we evaluated this derivative towards visible light
mediated photocatalysis. We chose to study [2 + 2]
photocycloaddition acrylimides (Scheme 2), as it happens
close to diffusion control (kq > 109 M−1 s−1) while the
quenching by 4 was slower (kq ≈ 106 − 108 M−1 s−1). A
closer look at the quenching rate constants reveals that the
triplet energy of 4 is closer to that of parent thioxanthone 1
while the triplet energy of 6 is lower than that of all the 3
thioxanthones. This is in line with our previously reported
triplet energies of 4 and 6.[4,20]
3 | CONCLUSION
Bromination of thioxanthone resulted in a significant
bathochromic shift into the visible spectral region while the
triplet state energy showed only a minor decrease (≤3 kcal/
mol).[21] In addition, bromination increased the triplet
quantum yields slightly. This enabled the use of brominated
thioxanthone derivatives as a photocatalyst for [2 + 2]
photocycloaddition of under visible light irradiations using
commercially available purple LEDs. Our studies have
shown that brominated thioxanthone derivatives with their
high molar absorptivities, high triplet energies and long
triplet lifetimes should make these compounds attractive to
be used as sensitizers in a variety of photoreactions.
to be
a
well‐investigated photoreaction from our
laboratory.[4,20] The photoreaction when performed with
hydrazide‐based acrylimide 4 with 20 mol% loading of the
photocatalyst 3 under purple LED illumination (395‐
405 nm) afforded the cross [2 + 2] photoproduct 5 in good
yield (75% yield). The same reaction when performed with
acrylimide 6 with 10 mol% loading of the photocatalyst 3
under purple LED illumination afforded the cyclobutane
derivative 7 in good yield (>95% yield). As the reaction
was efficient, we performed quenching studies by laser flash
photolysis to determine the quenching efficiencies of the trip-
let states of the thioxanthone derivatives by substrates 4 and 6
(Figure 4 and Table 1; entries 10 and 11). The bimolecular
quenching rate constant of thioxanthone triplets by 6 was
4 | EXPERIMENTAL SECTION
4.1 | Materials
See Data S1 for synthesis procedures.
4.1.1 | Photophysical measurements
UV‐Vis spectra were recorded on a Shimadzu 2501PC UV‐
Vis spectrometer or a Cary 300 UV‐Vis spectrophotometer.
Phosphorescence spectra were recorded on a Fluorolog‐3P
fluorometer (HORIBA Jobin Yvon). Samples solutions in

IYER ET AL.
5 of 5
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3‐mm quartz tubes were frozen in a quartz liquid nitrogen
Dewar and excited with a pulsed xenon lamp. Luminescence
signal detection was delayed 10 milliseconds after the light
pulse and collected for 20 milliseconds. Phosphorescence
lifetimes were measured by multichannel scaling on an
OB920 spectrometer (Edinburgh Analytical Instruments) in
conjunction with a pulsed xenon lamp. Laser flash photolysis
experiments employed the pulses from a Spectra Physics
GCR‐150‐30 Nd:YAG laser (355 nm, ca 5 mJ/pulse, 5‐nano-
second pulse length) and a computer controlled system that
has been described previously.[22] Solution of the
thioxanthone derivatives were prepared at concentrations
such that the absorbance was 0.3 at the excitation wavelength
(355 nm) and deoxygenated by argon purging.
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purged with N2 for 5 minutes. The sample was then irradiated
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[18] Refer to supporting information.
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ACKNOWLEDGEMENTS
[20] A. Iyer, S. Jockusch, J. Sivaguru, Chem. Commun. 2017, 53, 1692.
The authors thank Dr Angel Ugrinov for solving the single
crystal XRD structure. AI thanks Padmanabha Kavasseri
(Pad) from West Fargo High School for his assistance during
the photochemical investigations. The authors at NDSU
thank the NSF (CHE‐1465075) for generous support of their
research.
[21] S. Z. Vatsadze, Y. D. Loginova, G. dos Passos Gomes, I. V.
Alabugin, Chem. – Eur. J. 2017, 23, 3225.
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SUPPORTING INFORMATION
Additional Supporting Information may be found online in
the supporting information tab for this article.
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How to cite this article: Iyer A, Clay A, Jockusch S,
Sivaguru J. Evaluating brominated thioxanthones as
organo‐photocatalysts. J Phys Org Chem. 2017;e3738.
https://doi.org/10.1002/poc.3738
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