Photophysical properties of hydroxy-substituted flavothiones

Article abstract of DOI:10.1021/jp000084y

Flavothione and a number of synthesized hydroxy- (mono- and di-) substituted flavothiones have been thoroughly examined, particularly regarding their absorption, emission, photophysical (triplet yields and lifetimes), and oxygen-photosensitizing characteristics. These were all studied as a function of the nature of the solvent (four), which was particularly critical in terms of aiding in determining the energy and configurational nature of the lowest triplet state as well as the mechanism of intersystem crossing. Theoretical calculations were also performed. Both the location and number of hydroxyl groups have a substantial impact on the nature of the lowest excited triplet state as well as on the relative location of the two lowest excited singlet and triplet states. These in turn affect the magnitude and even the existence of triplet-state occupation as well as the ability to sensitize oxygen (to singlet oxygen). Three groups of compounds exist as characterized by the configurational nature of the triplet and the mechanism of intersystem crossing, or the essential absence of intersystem crossing altogether. The quantum yield of singlet oxygen formation is high for one group where the T(π, π*) state is lowest and generally high in another group where the T(n, π*) state is lowest, except in ethanol where competitive H-atom abstraction occurs. The potential of all hydroxy compounds as photosensitizers is evaluated.

Full text of DOI:10.1021/jp000084y

J. Phys. Chem. A 2000, 104, 6095-6102
6095
Photophysical Properties of Hydroxy-Substituted Flavothiones
Fausto Elisei,*,† Joa˜o C. Lima,‡ Fausto Ortica,† Gian G. Aloisi,† Manuela Costa,§
Em´ılia Leita˜o,‡ Isabel Abreu,‡ Anto´nio Dias,‡ Vasco Bonifa´cio,‡ Jorge Medeiros,§
Anto´nio L. Mac¸anita,‡, and Ralph S. Becker‡,|,
*
Dipartimento di Chimica, UniVersita` di Perugia, Perugia, Italy, Instituto de Tecnologia Qu´ımica e Biolo´gica,
R. da Quinta Grande 6, 2800 Oeiras, Portugal, INOVA, R. de S. Gonc¸alo, Ponta Delgada, Portugal, Instituto
Superior Te´cnico, AV. RoVisco Pais, 1000 Lisboa, Portugal, and Department of Chemistry, UniVersity of
Arkansas, FayetteVille, Arkansas 72701
ReceiVed: January 6, 2000; In Final Form: March 26, 2000
Flavothione and a number of synthesized hydroxy- (mono- and di-) substituted flavothiones have been
thoroughly examined, particularly regarding their absorption, emission, photophysical (triplet yields and
lifetimes), and oxygen-photosensitizing characteristics. These were all studied as a function of the nature of
the solvent (four), which was particularly critical in terms of aiding in determining the energy and
configurational nature of the lowest triplet state as well as the mechanism of intersystem crossing. Theoretical
calculations were also performed. Both the location and number of hydroxyl groups have a substantial impact
on the nature of the lowest excited triplet state as well as on the relative location of the two lowest excited
singlet and triplet states. These in turn affect the magnitude and even the existence of triplet-state occupation
as well as the ability to sensitize oxygen (to singlet oxygen). Three groups of compounds exist as characterized
by the configurational nature of the triplet and the mechanism of intersystem crossing, or the essential absence
of intersystem crossing altogether. The quantum yield of singlet oxygen formation is high for one group
where the T(π, π*) state is lowest and generally high in another group where the T(n, π*) state is lowest,
except in ethanol where competitive H-atom abstraction occurs. The potential of all hydroxy compounds as
photosensitizers is evaluated.
Introduction
In addition, it was important that the 5-methoxy-substituted
compound was studied regarding the nature of its lowest singlet
and triplet excited states and triplet quantum yield to help
elucidate the cause of some important changes in photophysical
properties of two of the hydroxy-substituted derivatives com-
pared with all others.
We have a continuing interest in the development of
photosensitizers that will be effective in a wide variety of
systems and circumstances; for example, see refs 1 and 2. Here
we report on hydroxy-substituted flavothiones:
Detailed studies of the solution spectroscopy of the parent
moiety chromothione2 as well as FT itself3 have been reported.
Experimental Section
Substituted flavothiones were synthesized from reaction of
the parent hydroxyflavones with either Lawesson’s Reagent2
or diphosphorus pentasulfide2,4 (Table 1). Flavones were
purchased from Merck and Extrasynthesis. Flavothiones were
purified by column chromatography or PTLC5 and then
recrystallized from methanol-hexane (1:1). Mass spectroscopy
(KRATOS MS 25 RE), 1H NMR (BRUCKER AMX300), FTIR
(MATTSON 7000), and UV-vis absorption spectroscopy (OLIS
15) were used to identify the synthesized flavothiones. The
crystals were stored in the dark to avoid photodegradation.
Melting points were measured with a BUCHI 530 apparatus
(Table 1).
Fluorescence and phosphorescence spectra were run in Spex
Fluorolog 2021 and were corrected for the instrumental response.
Fluorescence decays were measured with a picosecond-time-
resolved fluorimeter earlier described: frequency-doubled emis-
sion, λexc ) 400 nm, from a mode-locked Ti-saphire laser
(Spectra-Physics), pumped by an argon Ion (Spectra-Physics)
laser, repetition rate 800 kHz, pulse width after electronic
distortion 28 ps.6
This paper presents a comprehensive study of the conse-
quences of mono- (n-OHFT with n ) 3, 5, 6, and 7) and
bihydroxyl (n,n′-diOHFT with n,n′ ) 3,6 and 5,7) substitution
of flavothione (FT) on the photophysical properties of the
resulting flavothiones. The primary aims are to rationalize1 the
effect of the location and number of hydroxyl groups on the
energy and orbital origin of the first and the second excited
singlet and triplet states,2 the consequence of the latter on the
photophysical properties including the presence of fluorescence,
the triplet yields and lifetimes, and the singlet oxygen yields,
and3 how the foregoing depend on the nature of the solvent.
Our final goal is to evaluate which compounds and conditions
are best suited to produce good photosensitizers.
† Universita` di Perugia.
‡ Instituto de Technologia Qu´ımica e Biolo´gica.
§ INOVA.
Instituto Superior Te´cnico.
| University of Arkansas.
10.1021/jp000084y CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/06/2000

6096 J. Phys. Chem. A, Vol. 104, No. 25, 2000
Elisei et al.
TABLE 1: Flavothiones: Synthesis Method, Reaction Yield,
and Melting Point
temp irr. time yield
mp
compound reagent
solvent
(°C) (min) (%)
(°C)
FT
R.L. DME
25
25
80
10
10
15
76
80
62
83-85
64-68
P2S5 MeCN
P2S5 MeCN/Dx
(5:2)
3-OHFT
5-OHFT
6-OHFT
R.L. benzene
R.L. benzene
P2S5 MeCN/THF
(1:4)
40
80
25
30
60
10
6
130-132
30 206-208
21
7-OHFT
R.L. THF
65
80
40
70
10
40
20
20
15
10
30
25
14 191-194
23 185-187
17 216-218
35
17 199-202
16
3,6-diOHFT R.L. THF
5,7-diOHFT R.L. benzene
P2S5 MeCN
3,7-diOHFT R.L. benzene/Tol(1:1) 80
P2S5 MeCN
40
40
25
7,8-diOHFT R.L
Tol
77 243-246
6
P2S5 MeCN
Triplet-formation quantum yields and lifetimes were measured
with a flash photolysis setup previously described (Nd:YAG
Continuum, Surelite II, third harmonics, λexc ) 355 nm, pulse
width ca. 7 ns and energy e 1 mJ pulse-1).7,8 First-order kinetics
was observed for the decay of the lowest triplet state (T-T
annihilation was prevented by the low excitation energy). The
triplet lifetimes were measured at absorbance ca. 0.2; the
concentration effect on triplet lifetime was not investigated. The
transient spectra were obtained by monitoring the optical density
change at intervals of 5-10 nm over the 300-800 nm range
and averaging at least 10 decays at each wavelength.
The product ꢀTφT (where ꢀT and φT are the triplet extinction
coefficient and quantum yield, respectively) was measured at
the triplet-triplet absorption maxima. The calibration of the
experimental setup was made with an optically matched solution
of benzophenone in acetonitrile (ꢀTφT ) 6500 M-1cm-1).9
The φT values in benzene were generally measured by
comparing the change of absorbance of â-carotene triplet
sensitized by energy transfer from a substrate, ∆Aâ, and from
benzophenone, ∆Aâ(B), used as standard [φT(B) ) 1,10 under
the same experimental conditions (absorbance at 355 nm and
concentration of â-carotene)].11 The efficiency of energy transfer
from the substrate, Pet, and from benzophenone, Pet(B), was
also taken into account by measuring the triplet lifetime of the
donor in the absence (τT) and in the presence (τT′) of â-carotene:
Figure 1. Absorption spectra of flavothiones in benzene (solid line)
and in water at pH ) 14 (dashed line). Also included are n f π*
absorption obtained in a 10 cm path cell for FT, 6-OHFT, and 7-OHFT.
equilibrated solutions)
(φ∆)EtOH
(S∆)benz
τT
τT - τT′
(φT)EtOH
)
(3)
(
)
(
)
τT - τT′
τT
EtOH
benz
where (S∆)benz ) (φ∆/φT)benz ≈ 0.6 was measured in benzene
and τT and τT′ correspond to the triplet lifetimes measured,
respectively, in Ar-saturated and air-equilibrated solutions.
The ꢀT values in benzene were obtained from the ꢀTφT/φT
ratio (the first two terms from the OD change and the
denominator from â-carotene sensitization). Furthermore, ꢀT of
FT in acetonitrile was measured by energy transfer from
benzophenone at λexc ) 355 nm; ꢀT of 6500 M-1 cm-1 (at 520
nm)9 was used for the donor (benzophenone).
All measurements were carried out at 22 ( 2 °C; the solutions
were saturated by bubbling with argon. The experimental errors
on τT were estimated to be about (10% while those on ꢀT and
φT were (15%.
The singlet oxygen yields (φ∆) were determined by measuring
the phosphorescence intensity of O2(1∆g) with a germanium
diode detector in air-equilibrated solutions.12 The amplified
signal extrapolated at zero time was plotted as a function of
laser dose. Linear relationships were obtained for all substrates
and for phenalenone (in each solvent) used as standard (φ∆ )
0.97 ( 0.03).13 The φ∆ values were then obtained from the ratio
of the slopes of each compound to that of phenalenone
∆Aâ Pet(B)
φT ) φT(B)
Pet )
(1)
(2)
Pet
∆Aâ(B)
τT - τT′
τT
Only for 5-OHFT and 5,7-diOHFT (and 5-OCH3FT) were we
unable to measure the φT by energy transfer (see later discus-
sion).
The smaller solubility of â-carotene in other solvents, the
reactivity of benzophenone in ethanol (EtOH), and the generally
short-lived triplet of flavothiones in EtOH prevented the use of
the same method to obtain φT in EtOH, acetonitrile (MeCN),
and the hydrogen-bonding trifluoroethanol (TFE), which is able
to destabilize the n, π* states. For these reasons, φT in the more
polar solvents was obtained from the singlet-oxygen quantum
yields in air-equilibrated solutions by assuming that the fraction
of triplet quenched by molecular oxygen leading to singlet
oxygen was independent of the solvent (and taking into account
the fraction of triplet quenched by the molecular oxygen in air-

Properties of Hydroxy-Substituted Flavothiones
J. Phys. Chem. A, Vol. 104, No. 25, 2000 6097
TABLE 2: Experimental Absorption Maxima (λexp) and Extinction Coefficients (E) of Flavothiones in Benzene and Electronic
Transitions Calculated by the INDO/1-CI Semiempirical Method after Geometrical Optimization with INDO/1
CI
compound
FT
transition
λ
exp (nm)
ꢀ (104 M-1cm-1
)
λ
cal (nm)
f
%
orbitals
S0 f S1
S0 f S2
S0 f S3
S0 f S4
S0 f S5
569
392
319
0.005
1.85
582
381
320
305
298
0.000 04
0.52
0.089
0.000 90
0.45
78
91
66
81
53
22
86
93
56
35
57
77
88
74
83
42
79
90
43
34
35
49
77
90
60
29
77
26
56
86
92
67
59
77
88
52
44
n
H-1 f πL*
πH f πL*
πH f πL+1
*
n
π
H-1 fπL+1*
316
1.89
H-2 f πL*
πH f πL+1
*
3-OHFT
5-OHFT
6-OHFT
S0 f S1
S0 f S2
S0 f S3
574
404
338
0.000 01
0.62
n
H-1 f πL*
435
354
2.38
1.52
πH f πL*
0.20
π
H-2 f πL*
πH f πL+1
πH f πL+1
nH f πL*
*
*
S0 f S4
S0 f S1
S0 f S2
S0 f S3
S0 f S4
S0 f S5
S0 f S1
S0 f S2
S0 f S3
302
625
372
340
329
316
577
391
337
0.42
608a
398
326
0.000 002
0.53
1.36
1.54
π
π
H-1 f πL*
H-2 f πL*
nH f πL+1
0.13
0.000 38
0.018
0.000 01
0.48
*
π
n
H-1 f πL+1
H-1 f πL*
πH f πL*
πH-2 f πL*
*
562
409
345
0.005
1.80
1.42
0.16
πH f πL+1
*
S0 f S4
311
1.18
313
0.24
π
H-2 f πL*
πH f πL+1
*
7-OHFT
S0 f S1
S0 f S2
S0 f S3
548
393
338
0.006
1.51
1.93
569
383
328
0.000 04
0.42
nH-1 f πL*
πH f πL*
πH-2 f πL*
0.43
πH f πL+1
*
S0 f S4
S0 f S5
303
302
0.001 3
0.39
n
π
H-1 f πL+1*
300(s)
1.00
H-2 f πL*
πH f πL+1
*
3,6-diOHFT
5,7-diOHFT
S0 f S1
S0 f S2
S0 f S3
S0 f S4
S0 f S1
S0 f S2
S0 f S3
S0 f S4
569
412
356
310
651
378
332
330
0.000 08
0.55
n
H-1 f πL*
446
371
1.84
1.49
πH f πL*
0.27
0.24
πH-2 f πL*
πH f πL+1
nH f πL*
*
0.000 08
0.44
381
332
1.90
1.77
π
H-1 f πL*
0.13
0.26
nH f πL+1
*
πH-2 f πL*
a Value obtained with 5-methoxyflavothione.
multiplied by the known φ∆ of the standard. At least 50 kinetic
measurements were averaged for each solution.
Quantum mechanical calculations were performed using the
Cerius2 software. Transition energies and oscillator strengths
were obtained by INDO/1-CI calculations, after geometrical
optimization with the INDO/1 model.14 In all cases, electron
excitations (singly excited) from the 20 highest occupied to the
20 lowest virtual molecular orbitals were used.
Results
Singlet-Singlet Absorption. Figure 1 shows the absorption
spectra of a series of hydroxy-substituted flavothiones in benzene
and water (pH ) 14). The spectra in benzene are strongly red-
shifted with respect to those of the parent flavones, the largest
bathochromic shift being observed with 3,6-dihydroxyflavo-
thione. In polar solvents such as methanol, the spectra (not
shown) are blue-shifted by 2-4 nm with respect to benzene
and an additional absorption band was observed at longer
wavelengths. Addition of a drop of concentrated HCl or the
use of carefully dried methanol removes this band, while
addition of concentrated NaOH increases its intensity at the
expenses of the remaining absorption bands. The spectra
obtained in water at pH ) 14 show this same band, which is
assigned to the anionic forms of the compounds. Acidification
of the aqueous solutions with HCl induces precipitation.
Figure 1 also shows the red edge of the absorption spectra in
benzene, measured in 10 cm optical length cells, for some of
Figure 2. Fluorescence and phosphorescence spectra in 3-methylpen-
tane at 298 K: (a) FT; (b) 6-OHFT; (c) 7-OHFT.
the compounds. The S0 f S1(n, π*) transition is clearly observed
for the following compounds: FT, 6-OHFT and 7-OHFT, and
5-OCH3FT. In Table 2, experimental (in benzene) and calculated
transition wavelengths are given for the hydroxy compounds.
Fluorescence and Phosphorescence. The same three com-
pounds in the series (FT, 6-OHFT, and 7-OHFT) show
fluorescence emission from S2 (Figure 2).
The fluorescence quantum yields (φf) are low, and the
fluorescence decays are single exponential with lifetimes (τf)

6098 J. Phys. Chem. A, Vol. 104, No. 25, 2000
Elisei et al.
TABLE 3: Fluorescence Maxima (λf), Quantum Yields (Of),
and Lifetimes (τf) in 3-Methylpentane (3MeP), Benzene, and
Methanol (MeOH) at 293 K, Triplet Energies (ET) from
Phosphorescence Maxima, and Phosphorescence Lifetimes
(τP) in 3MeP and MeOH at 77 K
λf
φf
τf
ET
τp (µs),
compound
FT
solvent
(nm) (10-4
)
(ps)
(cm-1
)
77 K
3-MeP
benzene
MeOH
3-MeP
benzene
MeOH
3-MeP
benzene
MeOH
490
499
479
484
486
470
496
490
480
5.4
3.8
1.8
4.7
3.7
0.8
4.4
2.5
0.6
11
8.0
7.3 16 200
16 000
15 900
59
49
82
6-OHFT
7-OHFT
<3
3.9 16 400
15 700
3.6
90
58
Figure 4. Triplet-triplet spectra of FT (O), 3-OHFT (4), and 6-OHFT
(]) in benzene (λexc ) 355 nm). Inset: Decay and recovery kinetics
of FT recorded at 360 (a) and 400 (b) nm.
3.4 16 750
62
TABLE 4: Triplet-triplet Absorption Wavelengths (λmax),
Triplet Lifetimes (τT), and Quantum Yields of Triplet
Formation (OT) of Hydroxyflavothiones, in Benzene, Ethanol
(EtOH), Acetonitrile (MeCN), and Trifluoroethanol (TFE) at
295 K
a
ꢀT
τT
(µs) cm-1
(M-1
b
compound solvent
λ
max (nm)
benzene 360, 480
EtOH 355, 480
MeCN 350, 490
TFE 355, 480
benzene 320, 380, 520
EtOH 370, 480
MeCN 370, 480
TFE 370, 510
benzene 370, 480, 750
EtOH 350, 465, 670
MeCN 365, 470
TFE 470
benzene 380, 505, 710
EtOH
MeCN 370, 505
TFE 515
)
φT φT
FT
0.93 7600 1.0
0.04 7450c 0.90 1.0
0.31 7300 0.93 1.0
0.70 7450c 0.95 0.86
7.1 1700 1.0
3-OHFT
6-OHFT
7-OHFT
5.9 2350d
3.6 3500d
6.3 3400d
1.6 4400 0.96
0.10 4800d
0.63 4700d
0.91 1050d
1.1 3800 0.98
0.08
0.95
0.89
1.0
Figure 3. Phosphorecence spectra in 3-methylpentane at 77 K: (a)
FT; (b) 6-OHFT; (c) 7-OHFT.
0.85
0.87
0.83
in the picosecond region (Table 3). Both φf and τf are markedly
decreased in polar solvents. In methanol, the lifetimes of
6-OHFT and 7-OHFT are shorter than the time resolution of
our equipment (∼ 3 ps).
e
0.91
0.73
0.70
0.38 3900d
0.45 1400d
3,6-diOHFT benzene 325, 395, 480, 570 12.0 1250 0.96
The emission spectra of FT, 6-OHFT, and 7-OHFT show a
second emission band, phosphorescence, at longer wavelengths
at room temperature (Figure 2). The phosphorescence spectra
in 3-methylpentane (3MeP) at 77 K, Figure 3, show maxima at
about the same wavelengths (∼25 nm blue-shifted at 77 K).
Data on the phosphorescence maxima (also considered to be
the 0-0 band) and the lifetimes (τP) in 3-methylpentane (3MeP)
and methanol (MeOH), at 77 K, are also shown in Table 3.
Triplet Properties. Upon laser excitation, T1fTn transitions
of FT and its hydroxy derivatives were detected, mainly in the
300-550 nm region, just after the laser pulse. The lowest triplet
state decayed by first-order kinetics and was quenched efficiently
by molecular oxygen (kox ) 2.5-5 × 109 M-1 s-1) and by
â-carotene (low-energy triplet acceptor) to produce singlet
oxygen and the T1 of â-carotene, respectively. Furthermore, the
lowest triplet states of FT and hydroxyflavothiones were
sensitized by benzophenone, a well-known high-energy triplet
donor.
EtOH
MeCN 470, 570
TFE 490, 560
300, 480, 580
9.1
750d
0.93
1.0
0.95
7.1 1540d
8.1 2500d
a Extinction coefficient of the underlined maximum. b Obtained from
the singlet-oxygen quantum yields by assuming the S∆ measured in
benzene (see text). c Average of the measured values in benzene and
MeCN. d Values obtained from the ratio (ꢀTφT/φT). e The triplet-triplet
absorption spectrum is strongly hidden by the absorption of a
photoproduct and by the recovering of the ground state.
the triplet decay only for FT and for 5-OHFT in benzene; see
further analysis in the Discussion section. Triplet-triplet
absorption maxima, triplet lifetimes (τT), extinction coefficients
for T-T absorptions (ꢀT), and quantum yields of triplet
formation (φT) in benzene, MeCN, EtOH, and TFE are given
in Table 4.
Quenching experiments of the lowest triplet state of FT and
3-OHFT by i-ButOH were performed by recording the triplet
lifetime in TFE. The linear plots reported in Figure 5 show the
different behavior of the two triplets. In fact, T1 of FT is
quenched by i-ButOH with a quenching rate constant of 2.4 ×
105 M-1 s-1, while τT of 3-OHFT is practically uneffected by
i- ButOH in a wide range of concentrations.
The spectra of three representative hydroxy cases (FT,
3-OHFT, and 6-OHFT) are shown in Figure 4. Since the
absorptions of the triplet and of the ground state are in the same
spectral region, the triplet transient spectra are strongly affected
by the overlap between the T1 f Tn and S0 f S2 absorption
bands.
Generally, the lowest triplet state was detected by direct
irradiation. Small absorptions due to nontriplet transients
(practically unaffected by molecular oxygen) were detected after
The φT values reported in the sixth column of Table 4 are
generally obtained from energy-transfer experiments to â-car-
otene, while those of the seventh column are obtained via the
singlet oxygen quantum yields measured in four solvents. With
few exceptions, φT is close to unity.

Properties of Hydroxy-Substituted Flavothiones
J. Phys. Chem. A, Vol. 104, No. 25, 2000 6099
not greatly changed by hydroxyl substitution. The calculated
values of energy of these molecular orbitals (before CI) are
plotted in Figure 7.
Discussion
Singlet Excited States. As can be seen from Table 2, for
FT, the energy of S1(n, π*) with a maximum at 569 nm in
benzene (ꢀmax ) 52 M-1 cm-1), and observed by others3 with
a maximum in the vicinity of 588 nm in 3-methylpentane, is
well predicted by INDO/1 at 582 nm. The parent, chromothione,
exhibits2 a lowest S1(n, π*) with a maximum at 561 nm in
toluene (ꢀmax ∼20 M-1 cm-1). In any case, for FT the lowest
singlet state is (n, π*). The increase of solvent polarity,
polarizability, or hydrogen-donating ability strongly blue-shifts
the transition, from 588 nm in 3-MP to 569 nm in benzene and
to 508 nm in methanol.
Hydroxyl substitution has a marked influence on the energy
of the first three singlet excited states of flavothiones. Generally,
it is expected that conjugative-type substitution as OH will blue-
shift S(n, π*) transitions and red-shift S(π, π*) transitions,15
and this trend is in fact theoretically predicted for all but the
5-hydroxy- and the 5,7-dihydroxy-substituted flavothiones
(Table 2). For both compounds, the S2(π, π*) state is theoreti-
cally predicted to be slightly blue-shifted and the S1(n, π*) state
is theoretically predicted to be strongly red-shifted with respect
to the unsubstituted flavothione (Table 2). Note the quite
acceptable agreement of theory with experiment with respect
to these differences (Table 2), namely, the S2(π, π*) state is
experimentally red-shifted with respect to FT, for all compounds,
except for 5,7-diOHFT where it is blue-shifted. The reason for
the exceptional behavior of 5-OHFT and 5,7-diOHFT clearly
emerges from observation of Figure 7, where the theoretical
energies of the molecular orbitals are plotted: the n molecular
orbital (basically a linear combination of the nonbonding orbitals
of sulfur) is raised in energy (becomes the HOMO, nH), and
the π is lowered (becomes HOMO - 1, πH-1) in the cases of
the 5-hydroxy-substituted compounds. Since the S2 state is 88%
the πH-1f πL* configuration and S1 is 77% the nH f πL*, the
S1(n, π*) is lowered in energy (red-shifted) and the S2(π, π*)
is raised (blue-shifted).
Figure 5. Linear plots for the quenching of triplet lifetimes of (O) FT
and (b) 3-OHFT by iso-butanol in TFE.
TABLE 5: Quantum Yield of Singlet-Oxygen
Photosensitization (O∆) by Flavothiones in Benzene,
Acetonitrile (MeCN), Trifluoroethanol (TFE), and Ethanol
(EtOH) in Air-Equilibrated Solutions (λexc ) 355 nm)
φ∆
compound
FT
3-OHFT
5-OHFT
6-OHFT
7-OHFT
3,6-diOHFT
5,7-diOHFT
benzene
MeCN
TFE
EtOH
0.64
0.59
0.045
0.61
0.59
0.61
0.05
0.44
0.62
<0.01
0.48
0.30
0.57
<0.01
0.44
0.62
<0.01
0.48
0.30
0.57
<0.01
0.14
0.56
<0.01
0.15
0.11
0.56
<0.01
The φT values of FT in EtOH, MeCN, and TFE (in the 0.90-
0.95 range) were obtained from the product ꢀTφT and from the
ꢀT’s measured by energy transfer from benzophenone (in the
case of MeCN) or estimated as the mean values of the ꢀT’s in
benzene and MeCN (in the cases of EtOH and TFE).
The triplet yields of the hydroxy derivatives are generally
high and close to unity in benzene, except for 5-OHFT and 5,7-
diOHFT. There appears to be a very small sensitization of
â-carotene and oxygen by the latter compounds, but no oxygen
quenchable signal could be clearly seen. Therefore, we estimate
the amount of any triplet produced was e5%. The 5-OCH3FT
gave a parallel result.
Singlet-Oxygen Quantum Yields. The singlet oxygen yields
are given in Table 5 for four different solvents. Note that for
all of the hydroxy derivatives, except 5-OHFT and 5,7-diOHFT,
the φ∆’s in Bz are similar, ∼0.6, and less than φT. As noted
just above, essentially no singlet oxygen was produced either
from 5-OHFT and 5,7-diOHFT or from 5-OCH3FT. Also these
values are about the same in acetonitrile and TFE as well, but
all except 3-OHFT and 3,6-diOHFT decrease markedly in
ETOH. We will consider the reasons for this in the Discussion
section.
1H NMR Spectra. Proton NMR spectra of 5-OHFT and 5,7-
diOHFT in dimethyl sulfoxide show a sharp signal at 13.6 ppm.
This shift at very low field is typical of a hydrogen-bonded
hydroxyl proton. The hydroxyl proton for 3-OHFT and 3,6-
diOHFT shows a signal at higher field (9.0 ppm), and for
6-OHFT and 7-OHFT no signal is observed.
The influence of the position of substitution on the extent of
energy lowering of the S2(π, π*) and S3(π, π*) states for the
remaining compounds is also understood after observation of
the contour orbitals in Figure 6. The S2(π, π*) state in FT is a
slightly perturbed πH f πL* configuration (Table 2), and the
πH molecular orbital has its largest electron density on carbon
3 (Figure 6). Consequently, the largest bathochromic shifts of
the S2 state are induced by 3-hydroxy substitution.
The n f π* transitions are observable as the lowest energy
transition in three thiones (FT, 6-OHFT, and 7-OHFT) as well
as in 5-OCH3FT, as noted earlier. In the cases of 3-OHFT and
3,6-diOHFT, even though their oscillator strengths are as high
as any of the other four, the S0 f S2 transition is more red-
shifted than any of the others and it is very likely that the weak
n f π* transition is lost in the tail of the much stronger S0 f
S2 transition.
Theoretical Calculations. The theoretical values of wave-
lengths and oscillator strengths of the relevant electronic
transitions (the lowest five singlet states) for FT and the
hydroxyflavothiones are given in Table 3. The contours of the
molecular orbitals involved in the electronic transitions of FT
are represented in Figure 6; the subscript H stands for the
HOMO, H - 1 for next occupied orbital, etc., and L means
LUMO, L + 1, the next unoccupied, etc. These contours are
Also recall that fluorescence was observed only for the same
three thiones (FT, 6-OHFT, and 7-OHFT). The S2 - S1 energy
gap for these is in the 7000-8200 cm-1 region, while for
3-hydroxy and 3,6-dihydroxy it is in the 4500-5100 cm-1 range.
It is likely in the latter two cases that vibrational relaxation is
totally or largely competitive with fluorescence from S2 (knr
.
1 × 1011 s-1). In the cases of the 5-hydroxy-substituted
flavothiones, the S2 - S1 energy gap is as great as for the

6100 J. Phys. Chem. A, Vol. 104, No. 25, 2000
Elisei et al.
Figure 6. Frontier molecular orbitals of FT calculated by INDO/1.
compounds that fluoresce, but we believe that these may show
intramolecular proton transfer (see later discussion), which is
known to be fast and dominant over any fluorescence emission
process from the original molecule.
in EtOH is assigned as essentially the reciprocal rate constant
for hydrogen-atom abstraction from EtOH by the FT triplet to
produce the FT-H• radical. All the foregoing is further verified
by the fact that the decay time of FT in TFE is strongly
quenched by i-ButOH (see Figure 5). The 0.04 µs decay time
is unaffected by the presence of oxygen, while the other
component at 350 nm (decay time of 40 µs) is shortened to 6
µs. Additional information to be published elsewhere utilizing
steady-state photochemical data obtained in several solvents16
clearly indicate that the foregoing result is a consequence of
the fact that the FT-H• radical reacts with oxygen to form the
flavone. The longer 200 µs decay time, seen as a component in
the recover (at 400 nm), could then be the result of reforming
the ground state via radical-radical reactions outside the solvent
cage. On the basis of all of the foregoing, the lowest triplet
state is assigned as n, π*.
In the case of 6-OHFT and 7-OHFT, the decay and recovery
decay times in benzene are the same (∼1.6 µs) and essentially
the same as FT. In EtOH, the foregoing are shortened to ∼0.1
µs, quite similar to that of FT in EtOH. On the basis of the
parallel behavior and lifetimes of 6-OHFT and 7-OHFT
compared to FT in the same solvents, we assign the lowest triplet
state of these two compounds to be n, π*. An appropriate
Triplet States. On the basis of the triplet lifetimes, the
hydroxy compounds can be divided into three groups: group 1
(FT, 6-OHFT, and 7-OHFT), having a τT shorter than 2 µs in
benzene, which is further markedly reduced to less than 100 ns
in EtOH, group 2 (3-OHFT and 3,6-diOHFT), with τT longer
than 6 µs even in EtOH, and group 3 (5-OHFT and 5,7-
diOHFT), showing essentially no triplet-state occupation.
Group 1. First, in the case of FT the spectra in benzene, EtOH,
MeCN, and TFE all look similar (Figure 4). In benzene, there
is one first-order decay of 1 µs, which is similar to those in
MeCN and TFE. However, in EtOH, while the decay at 480
nm is single exponential with a decay time equal to 0.04 µs,
the 350 nm (decay) and 400 nm (recovery) regions are double
exponential: a common decay time of 0.04 µs and a longer
one (40 µs at 350 nm and 200 µs at 400 nm). On the basis of
the well-known H-atom abstraction capability of thiones in
alcohols, but not in benzene, MeCN, or TFE, the single lifetime
in the latter three solvents is assigned as that of the nonreacting
triplet (not including ground-state quenching). The decay time

Properties of Hydroxy-Substituted Flavothiones
J. Phys. Chem. A, Vol. 104, No. 25, 2000 6101
However, as is the case for 5-OHFT, essentially no triplet was
observed and no phosphorescence was observed. Thus, the two
situations look similar but, of course, no proton transfer can
occur in the 5-OCH3FT case. Therefore, there must be another
reason for the results concerning the φT of the 5-OCH3FT (very
small or absent), and that must originate from the relative
relationship of the energy locations of the lowest two excited
singlet and triplet states; see below.
An appropriate energy level diagram based on parallel
information obtained for group 3 is given in Figure 8. Note
that intersystem crossing from either S1(n, π*) to T1(n, π*) or
S2(π, π*) to T2(π, π*) is configurationally forbidden.17 More-
over, the energy separation of S1 and S2 is large (∼10 000 cm-1),
and any second-order contribution via vibronic coupling would
be negligible. A parallel energy level diagram would also be
appropriate for the 5-hydroxy- and 5,7-dihydroxy compounds.
Finally, recall that fluorescence is observed from 5-OCH3-
FT but not from either of the two hydroxy derivatives. We
believe that this could originate from a fast proton transfer from
-OH to the S atom in the hydroxy-substituted cases, which
could quench any potential fluorescence.
Figure 7. Energies before configuration interaction of the frontier
molecular orbitals in the series of hydroxy-substituted flavothione.
Mechanisms of Intersystem Crossing. For group 1 com-
pounds (FT, 6-OHFT, and 7-OHFT), the lowest excited singlet
and triplet states are n, π*. The mechanism of intersystem
crossing is then proposed to be
S1(n, π*) SO8 T2(π, π*) vib8 T1(n, π*)
where SO signifies occurrence via the spin-orbital coupling
operator and vib signifies occurrence via the vibronic operator.
For group 2 compounds (3-OHFT and 3,6-diOHFT), clear
observation of an n f π* transition was not possible. Nonethe-
less, theory (Table 2) predicts the lowest singlet as n, π*. We,
perhaps, can have more faith than usual in the calculation since
the predicted and experimental values for FT and 6-OHFT (and
possibly 7-OHFT) are in very good agreement. Thus, we assign
the lowest energy singlet as n, π*, and the lowest energy triplet
is π, π*, vide infra. Thus, the mechanism proposed for
intersystem crossing is
Figure 8. Schematic energy level diagram for the ordering of the two
lowest excited singlet and triplet states for group 1, 2, and 3 type
compounds (see text for components of each group).
schematic energy level diagram for group 1 compounds, based
on calculations, absorption, flash, and phosphorescence data,
as well as the S(n, π*) - T(n, π*) [∼1000 cm-1] and S(π, π*)
- T(π, π*) [∼8200 cm-1] energy separations determined from
FT and 3-methoxyflavothione respectively, appears in Figure
8.
S1(n, π*) SO8 T1(π, π*)
Group 2. In these cases (the 3-OH substituted cases), the
lifetimes of decay and recovery in benzene are 7 and 12 µs,
very much longer (7-12-fold) than for the compounds in group
1. Furthermore, in EtOH, the lifetimes are essentially the same
(6 and 9 µs). Thus, no quenching process is present; that is, it
is clear that H-abstraction does not occur. Therefore, we assign
the transient as a triplet and the lowest triplet state as π, π*
based on the longer lifetimes and lack of H-atom abstraction in
EtOH. Since recovery is not quite to the baseline, some small
amount of net photochemistry does occur. An appropriate
schematic energy level diagram based on parallel information
as obtained for group 1 is given in Figure 8.
Group 3. For the compounds in this group, essentially no
triplet transients were detected upon laser excitation in any
solvent. At first we believed that the reason for this result for
the two hydroxy compounds was an intramolecular proton
transfer in the singlet manifold competing with triplet formation.
With this occurring, which is typically fast (k g 1010 s-1), this
process would compete with kISC and very significantly lower
φT, as is observed. Recall that ground-state H-bonding occurs,
as determined by NMR, and fluorescence is absent. Also no
phosphorescence is observed for the 5-OH-substituted cases.
To attempt to verify this idea, we synthesized the 5-OCH3FT.
For group 3 recall that essentially no intersystem crossing
occurs.
Singlet-Oxygen Yields. All of the flavothiones in benzene,
acetonitrile, and trifluoroethanol show about the same φ∆ (0.6)
except for 5-OHFT and 5,7-diOHFT. In ETOH, however, the
group 1 compounds undergo a marked decrease in φ∆. We have
assigned the lowest triplet state in these compounds as n, π*.
In the 3-substituted compounds (group 2) it is π, π*. Therefore,
the reason for the foregoing changes in φ∆ in ETOH for
compounds in group 1 is that a competitive H-atom abstraction
reaction occurs from T(n, π*), simultaneously with energy
transfer to oxygen, thus lowering the φ∆ (Table 5) and shortening
the τT (Table 4). For 3-OHFT and 3,6-diOHFT, since the lowest
triplet state is π, π*, relatively insignificant H-atom abstraction
is expected in general, and therefore, the φ∆ remains high even
in ETOH, comparable to the three other solvents (Table 5), and
τT is not shortened (Table 4).
Conclusions
The addition of OH groups to FT generally causes varying
degrees of red-shift of the first strong absorption band, the S0

6102 J. Phys. Chem. A, Vol. 104, No. 25, 2000
Elisei et al.
f S2 transition, with 3,6-diOH substitution causing the largest
red-shift. The exception is 5,7-diOHFT.
The position of OH substitution and the number of OH groups
added can cause some dramatic differences in photophysical
behavior. These are caused by the relative state order changes
of the two lowest excited singlet and triplet states.
photosensitizers via a H-atom extraction mechanism or act via
formation of singlet oxygen in an environment where H-atom
abstraction was relatively noncompetitive (as in a benzene-like,
acetonitrile-like, or perhaps a water environment).
Acknowledgment. This work was supported by the Fun-
dac¸a˜o para o Desenvolvimento da Cieˆncia e Tecnologia (FCT),
Portugal, Grant PRAXIS/2/2.1/QUI/324/94. R.S.B. acknowl-
edges the G.G.P. XXI/BCC/3638/96 grant, J.C.L. and I.A. are
grateful for the PRAXIS 4/4.1/BPD/3410 and PRAXIS 4/4.1/
BD/3589 grants, respectively. F.E., F.O., and G.G.A. are
thankful for the grant from the Italian Consiglio Nazionale delle
Ricerche and for the support of the National Project by the
Ministero dell’Universita` e della Ricerca Scientifica e Tecno-
logica and the University of Perugia.
The 6-OH or 7-OH substitution (group 1) on FT does not
change the nature of the lowest triplet state (n, π*) or its lifetime
compared with FT. In all solvents these have a high φT (0.8-1,
including FT). The φ∆ is generally in the region of ∼0.6 in
benzene and decreases somewhat to 0.5 for 6-OH and still
lowers for 7-OH (0.3) in MeCN and TFE. There is a large
decrease in φ∆ in ETOH, as there is for τT, and these are caused
by an H-atom abstraction reaction from the solvent (triplet n,
π* state lowest) competing with energy transfer to oxygen.
The 3-OH or 3,6-diOH substitution (group 2) results in a
change in the configurational nature of the lowest triplet state
from n, π* (FT, 6-OHFT, and 7-OHFT, group 1) to π, π*. Also,
φT does not change from ∼1. Moreover, φ∆ is ∼0.6 (as for FT,
6-OHFT, and 7-OHFT) in all solvents including ETOH, which
is in marked contrast to the large decrease found for FT,
6-OHFT, and 7-OHFT. The latter result is explained by the fact
that there is essentially no H-atom abstraction reaction compet-
ing with energy transfer to oxygen for the 3-substituted cases
because of the change in the configurational origin of the lowest
triplet state.
The 5-OH or 5,7-diOH substitution results in a very marked
change in the photophysical (and φ∆) properties compared to
all other hydroxy substitutions. Essentially no triplet occupation
is observed by direct excitation, and φ∆ is essentially zero in
all solvents (four considered). Also, no fluorescence or phos-
phorescence are observed. The 5-methoxy compound also
showed parallel behavior but did exhibit a weak fluorescence.
The results are understandable on the basis of the energy
relationship among the two lowest singlet and triplet states of
n, π* and π, π* character, as well some proton transfer from
S2 for the hydroxy derivatives.
References and Notes
(1) Becker, R. S.; Mac¸anita, A. L. ReV. Port. Quim. 1995, 2, 30-44.
(2) Becker, R. S.; Chakrovorti, S.; Gartner, C. A.; Miguel, M. da G. J.
Chem. Soc., Faraday Trans. 1993, 89, 1007-1019.
(3) Maciejewski, A.; Szymanski, M.; Steer, R. P., J. Photochem.
Photobiol. A, 1996, 100, 43-52 and references therein.
(4) Scheeren, J. W.; Ooms, P. H. J.: Nivard, R. J. F. Synthesis 1973,
149-151.
(5) Still, W. C.; Kakin, M.; Mitra, A. In Rapid Chromatographic
Techniques for PreparatiVe Separations with Moderate Resolution; Co-
lumbia University: New York, 1978.
(6) Lima, J. C.; Abreu, I.; Santos, M. L.; Brouillard, R.; Mac¸anita, A.
L. Chem. Phys. Lett. 1998, 298, 189-195.
(7) Go¨rner, H.; Elisei, F.; Aloisi, G. G. J. Chem. Soc., Faraday Trans.
1992, 88, 29-34.
(8) Romani, A.; Elisei, F.; Masetti, F.; Favaro, G. J. Chem. Soc.,
Faraday Trans. 1992, 88, 2147-2154.
(9) Carmichael, I.; Hug, G. L. J. Chem. Phys. Ref. Data 1986, 15,
1-204.
(10) Murov, S. L.; Charmichael, I.; Hug, G. L. In Handbook of
Photochemistry; Marcel Dekker Inc., New York, 1993.
(11) Kumar, C. V.; Qin, L.; Das, P. K. J. Chem. Soc., Faraday Trans.
2 1984, 80, 783-793.
(12) Elisei, F.; Aloisi, G. G., Lattarini, C.; Latterini, L.; Dall’Acqua,
F.; Guiotto, A. Photochem. Photobiol. 1996, 64, 67-74.
(13) Schmidt, R.; Tanielian, C.; Dunsbach, R.; Wolff, C. J. Photochem.
Photobiol., A 1994, 79, 11-17.
(14) Zerner, M. C. Semiempirical Molecular Orbital Methods. In ReViews
in Computational Chemistry, 2; Lipkowitz, K. B., Boyd, D. B., Eds.; VCH
Publishers Inc.: New York, 1991; pp 313-365 and references therein.
(15) Becker, R. S. In Theory and Interpretation of Fluorescence and
Phosphorescence; Wiley-Interscience: New York, 1969; pp 163-167.
(16) Lima, J. C.; et al. To be published.
It is highly probable that the 5-OH and 5,7-diOH compounds
would not be good photosensitizers since they show no triplet-
state occupation and a quenched excited singlet-state behavior.
On the other hand, the best compounds as potential photosen-
sitizers via a singlet-oxygen mechanism (or possibly an electron-
transfer process as well) would be those of group 2, the 3-OH
and 3,6-diOH. The group 1 compounds could also be potential
(17) El Sayed, M. A. J. Chem. Phys. 1964, 41, 2462-2467 and
references therein.