Laser flash photolysis study of the photochemistry of thioxanthone in organic solvents

Article abstract of DOI:10.1590/S0103-50532010000600003

The photoreactivity of the triplet excited state of thioxanthone (TX) was investigated employing the laser fash photolysis technique. The wavelength for the absorption maximum and the lifetime of the triplet excited state are solvent dependent. When hydrogen donor solvents were employed, a new band at 410 nm was observed in the triplet absorption spectrum, which was attributed to the ketyl radical derived from thioxantone. Quenching rate constants, k_q, ranged from (1.7 0.1) × 10⁶ L mol^-1 s^-1 for toluene to ca. 10⁹ L mol^-1 s^-1 for phenol and its derivatives containing polar substituents, as well as for indole, triethylamine and DABCO.

Full text of DOI:10.1590/S0103-50532010000600003

J. Braz. Chem. Soc., Vol. 21, No. 6, 960-965, 2010.
Printed in Brazil - ©2010 Sociedade Brasileira de Química
0103 - 5053 $6.00+0.00
Laser Flash Photolysis Study of the Photochemistry of Thioxanthone
in Organic Solvents
Janaina F. Rodrigues,a Francisco de Assis da Silvaa and José Carlos Netto-Ferreira*,a,b
aDepartamento de Química, Universidade Federal Rural do Rio de Janeiro, BR 465 km 7,
23970-000 Seropédica-RJ, Brazil
bInstituto de Química, Universidade Federal da Bahia, Campus de Ondina,
40170-290 Salvador-BA, Brazil
A fotorreatividade do estado excitado triplete de tioxantona (TX) foi investigada empregando
a técnica de fotólise por pulso de laser. O máximo do comprimento de onda de absorção e o tempo
de vida para o seu estado excitado triplete são dependetes do solvente. Quando foram utilizados
solventes doadores de hidrogênio, foi observado o surgimento de uma nova banda no espectro de
absorção para os transientes, com máximo a 410 nm, a qual foi atribuída ao radical cetila derivado
da tioxantona. As constantes de velocidade de supressão para o triplete de tioxantona, kq, variaram
de (1,7 0,1) × 106 L mol-1 s-1 para tolueno a ca. 109 L mol-1 s-1 para fenol e seus derivados contendo
substituintes poleres, bem como para indol, trietilamina e DABCO.
The photoreactivity of the triplet excited state of thioxanthone (TX) was investigated
employing the laser flash photolysis technique. The wavelength for the absorption maximum
and the lifetime of the triplet excited state are solvent dependent. When hydrogen donor solvents
were employed, a new band at 410 nm was observed in the triplet absorption spectrum, which was
attributed to the ketyl radical derived from thioxantone. Quenching rate constants, kq, ranged from
(1.7 0.1) × 106 L mol-1 s-1 for toluene to ca. 109 L mol-1 s-1 for phenol and its derivatives containing
polar substituents, as well as for indole, triethylamine and DABCO.
Keywords: laser flash photolysis, tioxanthone, triplet excited state, solvent effect
Introduction
more efficient than similar processes without change of
configuration.7,8 Moreover, the proximity between its
T1 and T2 triplet electronic states leads to photophysical
properties that are strongly influenced by solvent effects
and substitution on the aromatic ring.9,10 TX can easily
form free radicals from the triplet excited state which
permits its use as photoinitiator for polymerization11-13 and
as a photosensitizer.14 In the presence of tertiary amines,
the triplet excited state of TX produces an intermediate
exciplex, through the involvement of an initial excited
charge transfer complex. The exciplex decay leads to the
formation of an ion-radical pair which ultimately can form
the radical pair ketyl/alkylamino, as result of the hydrogen
transfer from the amine radical cation to the thioxanthone
anion radical (Scheme 1). The alkylamino radical is the
effective initiator of polymerization.15
The photophysical and photochemical properties of
several aromatic ketones are strongly influenced by the
energy and configuration of their electronically excited
state.1-4
The nature of the excited state of thioxathone
(9H-thioxanthen-9-one , TX) and its derivatives has
been amply studied. The singlet lowest excited state
of TX has pp* configuration, being very fluorescent
in hydroxylic solvents, with a triplet energy located at
65.5 kcal mol-1.5,6 This efficiency for triplet formation is
attributed to the transition involving S1 (pp*) → T2 (np*)
states, followed by internal conversion T2 → T1 (pp*).6 It is
well known that intersystem crossing processes involving
inversion of configuration are 3 orders of magnitude
In this work the triplet reactivity of TX towards
hydrogen and electron donors was investigated employing
the laser flash photolysis technique (LFP).
*e-mail: jcnetto@ufrrj.br

Vol. 21, No. 6, 2010
Rodrigues et al.
961
Scheme 1.
Materials and Methods
640 nm (CCl4) to 583 nm (CH3OH), depending on solvent
polarity, as recently demonstrated by Ferreira et al.9 Similar
dependency with the solvent polarity was found for the
transient lifetime, which varies from 250 ns (toluene) to
10.5 ms (2-propanol). This species decays by first-order
kinetics with some contribution from a second-order
decay, which can be attributed to triplet-triplet annihilation,
common in laser studies.
The transient absorption spectrum for thioxanthone in
hydrogen-donor solvents, such as n-hexane, cyclohexane,
toluene, methanol, ethanol or 2-propanol, shows a new
species absorbing at 410 nm, independently of the solvent
nature (hydroxylic or non-hydroxylic) (Figure 1). This new
species has been previously assigned to the ketyl radical
derived from TX for the hydroxylic solvents methanol,
ethanol and 2-propanol,9,16 with its formation occurring
through a hydrogen abstraction reaction involving the triplet
excited state of TX. These data are fully in accordance with
previous results from the literature.17
Materials
The solvents acetonitrile, carbon tetrachloride,
dichloromethane, n-hexane, cyclohexane, toluene,
methanol, ethanol and 2-propanol were from Sigma-
Aldrich (spectroscopic grade). The reagents thioxanthone,
trans-stilbene, 1-methylnaphthalene, 1,3-cyclohexadiene,
1,4-diazabicyclo [2.2.2]octane (DABCO), triethylamine
(TEA), 1-hexene, 1-cyclohexene, 1-methyl-cyclopentene,
1,4-cyclohexadiene, indole, phenol, p-cyanophenol and
p-methoxyphenol were from Sigma-Aldrich and used as
received (purity > 99%).
Laser flash photolysis
Nanosecond laser flash photolysis experiments were
carried out at room temperature on deaerated acetonitrile
solutions. The concentrations were adjusted to yield an
absorbance of ca. 0.3 at the excitation wavelength (355 nm).
Samples were contained in custom-built cells fabricated
from 10 mm × 10 mm rectangular Suprasil quartz tubes
and were deaerated for at least 30 min with dry oxygen-free
nitrogen. Stock solutions of quenchers, 10-2-10-3 mol L-1,
were prepared so that it was only necessary to add microliter
volumes to the sample cell to reach appropriated quencher
concentration. Samples were excited by the 3rd harmonic
(lEXC = 355 nm, single pulses of 10 ns duration and
40 mJ/pulse) of a pulsed Nd/YAG Surelite II. The LFP
apparatus used was a LUZCHEM mini-system, model
mLPF112.A 175W Xenon lamp was employed as detecting
lightsource;themonochromatorwasaCUIlaserCorporation
Digikrön monochromator (model CM110), together with a
Hamamatsu model R955 photomultiplier and a Tektronix
TDS 2012 oscilloscope. The Tektronix oscilloscope signal
was transferred to a personal computer for data storing and
processing. Transient absorption spectra were recorded by
time interval selection after the laser pulse (seven pulses at
each wavelength, with an interval of ten nanometers).
Figure 1. Transient absorption spectra obtained upon excitation on 355 nm
of TX in cyclohexane, recorded at 1.66 () and 3.33 ms () after the
laser pulse. Inset: Decay for the transient generated upon excitation and
monitored at 630 nm.
Quenchingexperimentsemploying1-methylnaphthalene
(ET = 60.6 kcal mol-1), 1,3-cyclohexadiene
(ET = 52. 4 kcal mol-1) and trans-stilbene
(ET = 49.3 kcal mol-1),18 in acetonitrile, revealed that the
quenching rate constant in all cases is diffusion-controlled,
which indicates the triplet nature of this transient (Table 1).
Furthermore, the diffusion-control values for these
quenching rate constants confirm that the triplet energy of
TX is above 61 kcal mol-1.10,19
Results and Discussion
Laser excitation (355 nm) of degassed thioxanthone
solutions in several solvents leads to the formation of a
transient which shows absorption maximum varying from

962
Laser Flash Photolysis Study of the Photochemistry of Thioxanthone in Organic Solvents
J. Braz. Chem. Soc.
Table 1. Second order quenching rate constants for thioxanthone triplet
in acetonitrile
Quencher
kq (L mol-1 s-1)
trans-stilbene
1-methylnaphthalene
1,3-cyclohexadiene
DABCO
triethylamine
methanol
ethanol
cyclohexane
2-propanol
(3.9 0.2) × 1010
(1.2 0.1) × 1010
(2.0 0.1) × 1010
(5.4 0.4) × 109
(8.1 0.4) × 109
(2.0 0.1) × 105
(3.1 0.1) × 105
(5.9 0.2) × 105
(1.2 0.1) × 106
(1.7 0.1) × 106
(2.8 0.1) × 106
(1.9 0.1) × 107
(4.5 0.1) × 107
(1.8 0.2) × 108
(1.8 0.1) × 109
(4.3 0.3) × 109
(5.5 0.3) × 109
(8.6 0.7) × 109
toluene
1-hexene
1-cyclohexene
1-methylcyclopentene
1,4-cyclohexadiene
phenol
p-cyanophenol
p-methoxyphenol
indole
The experimentally observed kinetic rate constant, kdecay
,
is related to the quenching rate constant, kq, according to
the equation below:20
kdecay = ko + kq[Q]
where ko is the triplet decay rate constant in the absence of
quencher, kq is the triplet decay rate constant in the presence
of the quencher and [Q] is the quencher concentration.
Linear Stern-Volmer plots for the quenching of TX triplet
were obtained for all quenchers used and showed good
correlation coefficients (Table 1 and Figure 2).
Rate constants for the quenching of thioxanthone
triplet by simple olefins were also measured by laser
flash photolysis, with the values obtained ranging from
(2.8
0.1) × 106 L mol-1 s-1 (1-hexene) to
(1.8 0.2) × 108 L mol-1 s-1 (1,4-cyclohexadiene). The
transient absorption spectrum recorded after laser
photolysis of TX, at an olefin concentration where the entire
triplet signal disappeared, does not show the formation
of any new transient. These hydrogen abstraction rate
constants clearly follow the stability of the intermediate
carbon-centered radical formed from the hydrogen donor,
with the olefin containing a doubly allylic hydrogen, i.e.,
1,4-cyclohexadiene being the most reactive (Table 1).21
Transient absorption spectra for thioxanthone in
methanol, ethanol or 2-propanol are similar to that
observed when 1,4-cyclohexadiene was employed as
quencher. Thus, a weak absorption with maximum at
410 nm is readily observed, which can be assigned to the
ketyl radical derived from thioxanthone.16 Quenching rate
constants measured for these alcohols, in acetonitrile, are
Figure 2. Stern-Volmer plots for the quenching of triplet TX in acetonitrile
by (a) 1-methyl-cyclopentene, 1; cycloexene, 2; 1-hexene, 3; (b) indole,
4; phenol, 5; triethylamine, 6; (c) 2-propanol, 7; ethanol, 8; methanol, 9.
related to the stability of the ketyl radical derived from the
alcohol and follow the order kq(2-propanol) > kq(ethanol) > kq(methanol)
(Table 1).
The high values for the quenching rate constants
obtained for olefins and alcohols in acetonitrile, when
compared to those obtained for other triplet ketones having
np* configuration,21 strongly suggest that the lowest
triplet excited state of TX has np* configuration in this

Vol. 21, No. 6, 2010
Rodrigues et al.
963
solvent. This is fully in accord with a recent paper from
Ferreira et al.9
containing benzylic hydrogen, which is fully in accord with
previous results from the literature.27 This is expected due to
the lower bond dissociation energy of the O-H bonding and
the lower oxidation potential of phenols. In particular, the
reaction with phenols is faster for lowest triplet excited state
of ketones with charge transfer pp* character (pp*[CT])
than for those with lowest np* triplet state.26,28 It has been
suggested that for phenols the hydrogen transfer occurs
through a mechanism involving the initial formation of
an exciplex, resulting from a hydrogen bond interaction
between the phenolic hydroxyl and the carbonyl group. It
is proposed that the hydrogen transfer would occur in two
steps: an initial electron-transfer, followed by an ultra-fast
proton-transfer.10,27 This process results in the experimental
observation of the ketyl/phenoxyl radical pair, as observed
in the present work (Scheme 2 and Figure 3). The ketyl
radical derived from TX can be observed at 410 nm after
quenching by phenol, para-methoxyphenol and para-
cyanophenol (Figure 4). The weak absorption observed at
430 nm in this figure can be associated to the formation
of the aryloxyl radical derived from para-cyanophenol.29
An important difference between np* and pp* states
is that the oxygen atom in the carbonyl group is more
electron deficient in the former than in the later. Thus, in
the photoreduction process by different hydrogen donors,
the T1 (np*) state is more reactive than the T1 (pp*).22-24
Taking as an example the quenching rate constant,
(1.8 0.2) × 108 L mol-1 s-1), for the hydrogen abstraction
from 1,4-cyclohexadiene (a very efficient hydrogen donor),
one can observe that this value is remarkably fast and
comparable to those obtained for typical np* mono or
diketones, such as benzophenone21 or 1,1,4,4-tetramethyl-
1,4-dihydro-2,3-naphthalenedione.25 For comparison,
ketones such as dinaphthylketones, having lowest energy
triplet with pp* character, show quenching rate constant
by 1,4-cyclohexadiene of ca. 106 L mol-1 s-1.26
Table 1 also shows quenching results employing
hydrogen and electron donors in acetonitrile. The reactivity
of triplet TX towards phenols is higher than for alcohols,
olefins containing allylic hydrogen or aromatic compounds
Scheme 2.

964
Laser Flash Photolysis Study of the Photochemistry of Thioxanthone in Organic Solvents
J. Braz. Chem. Soc.
Figure 5. Transient absorption spectrum obtained upon excitation on
355 nm of TX with 0.0025 mol L-1 of indole in acetonitrile recorded at
1.80 ms after the laser pulse.
Figure 3. Transient absorption spectra obtained upon excitation at 355 nm
of TX with 0.00031 mol L-1 of para-methoxyphenol in acetonitrile
recorded at 0.27 (), 0.66 (), 1.16 () and 4.65 ms () after the
laser pulse.
process of triplet TX by this species must also involve an
initial electron transfer followed by a fast proton transfer,
as proposed for phenols.32
The transient absorption spectra obtained upon laser
excitation of thioxanthone in the presence of the well
known efficient electron donorsTEA (2.04 × 10-4 mol L-1) or
DABCO (7.20 × 10-4 mol L-1), in acetonitrile, and recorded
several microseconds after the laser pulse, show bands
at 410 nm and in the 540-680 nm region. The transient
observed at 410 nm can be assigned to the anion radical
derived from thioxanthone (Figure 6).23 The quenching
rate constants for the reaction of TX triplet with DABCO
(5.4 0.4) × 109 L mol-1 s-1 or TEA (8.1 0.3) ×
109 L mol-1 s-1, are in good accordance with a mechanism
involving an electron transfer process.
Figure 4. Transient absorption spectra obtained upon excitation on 355 nm
of TX with 0.00025 mol L-1 of para-cyanophenol in acetonitrile recorded
at 0.73 (), 1.69 (), 2.64 () and 5.50 ms () after the laser pulse.
Analysis of the substituent effect on the quenching ofTX
triplet by phenol and derivatives showing polar substituents,
such as para-methoxy and para-cyano, revealed that the
presence of these substituents did not have a strong influence
in the reaction from the TX triplet. This lack of dependence
on polar substituents in the quenching reaction is an
indication that the primary process involves a charge transfer
complex, which results in an electron transfer, followed by
a fast proton transfer, ultimately leading to the formation of
the corresponding radical pair (Scheme 2).The configuration
of the ketone lowest triplet excited state is considered to be
a factor determining the smaller influence in the electron
transfer-mediated reactions.10,22,30
The transient absorption spectrum observed upon
laser irradiation of TX in the presence of indole show
the disappearance of the absorption band at 610 nm,
corresponding to the triplet excited state of TX, and the
formation of new bands at 410 nm (ketyl radical derived
from TX) and 500 nm (indolyl radical)31 (Figure 5), with
the quenching rate constant (8.6 0.7) × 109 L mol-1 s-1
being compared to those obtained for phenols. This result
indicates that the mechanism involved in the quenching
Figure 6. Transient absorption spectra obtained upon excitation on 355 nm
of TX with 0.00017 mol L-1 of TEA in acetonitrile recorded at 0.60 (),
1.38 () and 2.32 µs () after the laser pulse.
Conclusion
In conclusion, employing the laser flash photolysis
technique it was observed that the triplet excited state of

Vol. 21, No. 6, 2010
Rodrigues et al.
965
TX is easily obtained and dependent of the polarity and
hydrogen bonding properties of the solvent. The triplet
lifetime for TX increases with the change from non-polar to
polar solvents, whereas its lmax for triplet-triplet absorption
shows a blue shift as a function of solvent polarity. The
confirmation of the triplet character of this transient was
made by quenching experiments with very well known
triplet quenchers such trans-stilbene, 1-methylnaphthalene
and 1,3-cyclohexadiene, for which diffusion-controlled rate
constants were obtained. The transient absorption spectrum
in the presence of hydrogen donor solvents shows a weak
band at 410 nm, when compared to the strong triplet-triplet
absorption.Thissameabsorptionat410nmisobservedwhen
quenching experiments were performed with hydrogen-
donor quenchers, which was attributed to the ketyl radical
derived from thioxanthone. Similar values for the quenching
rate constants were observed for DABCO, TEA, indole and
phenols. For the later, there was no dependence of kq with
para-substitution on the phenols, indicating a mechanism
involving electron-transfer followed by a fast proton transfer.
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Acknowledgments
Financial support from the Coordenação de
Aperfeiçoamento do Pessoal de Nível Superior-Programa
Nacional de Cooperação Acadêmica (CAPES-PROCAD,
Brazil), Financiadora de Estudos e Projetos (FINEP) and
Fundação de Amparo à Pesquisa do Estado da Bahia
(FAPESB) are gratefully ackowledged. J. F. R. thanks
the Conselho Nacional de Desenvolvimento Científico
e Tecnológico (CNPq-Brazil) for a graduate fellowship.
J. C. N.-F. thanks CNPq for a research fellowship. F. A. S.
thanks CAPES, Programa de Pós-graduação em Química
of the Instituto de Química, Universidade Federal da Bahia
(UFBA) and Prof. Dr Cristina M. Quintella (LABLASER-
UFBA) for a Pos-Doctoral traineeship. We thank M.Sc.
Eduardo Benes da Silva for assistance.
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Received: March 15, 2009
Web Release Date: January 28, 2010