Photoinduced Energy and Electron Transfer between Ketone Triplets and Organic Dyes
S. Jockusch,† H.-J. Timpe,‡ W. Schnabel,§ and N. J. Turro*,†
Department of Chemistry, Columbia UniVersity, New York, New York 10027, Polychrome GmbH,
An der Bahn 80, 37520 Osterode, Germany, and Hahn-Meitner-Institut Berlin GmbH, Glienicker Str. 100,
14109 Berlin, Germany
ReceiVed: June 13, 1996; In Final Form: October 10, 1996X
Cationic organic dyes (phenosafranine, safranine T (ST+
), thiopyronine, and methylene blue) are efficient
quenchers of the triplet states of aromatic ketones. The triplets of benzophenone (BP), xanthone, thioxanthone,
benzil, and N-methylacridone are quenched by the dyes via energy transfer, generating triplet excited dyes
(typically kq ) 5.5 × 109
M-1
s-1
for 3
BP*
+ ST+
in acetonitrile solution). Regarding Michler’s ketone,
p-(dimethylamino)benzophenone, and 3,6-bis(dimethylamino)thioxanthone, the mechanism of triplet quenching
is solvent-dependent. Electron transfer occurs in polar solvents like acetonitrile, and energy transfer is the
dominant reaction pathway in less polar solvents, such as dichloromethane. Delayed fluorescence of the dye
caused by heterotriplet-triplet annihilation involving ketone and dye was detected upon studying the reaction
of the dyes with triplet excited ketones (BP, xanthone, thioxanthone, and benzil). By applying two successive
laser pulses (λex ) 532 and 308 nm, respectively), the rate constant of the reaction 3BP* + 3(ST+)* was
determined: kht ) 1.1 × 1010 M-1 s-1.
1. Introduction
Cationic organic dyes have received considerable attention
in connection with commercial applications: They are employed,
e.g., as laser dyes, dyes for textiles, and initiators for
photopolymerization.1-3 In many of these applications, the
electronically excited triplet state of the dye is of particular
importance. Mostly, however, the quantum yield of triplet
formation is rather low, and therefore, triplet sensitizers are
employed.
In this work, we report on the reaction of cationic organic
dyes with certain ketones that frequently serve as triplet
sensitizers. In this case, two photophysical pathways are
possible, triplet-triplet energy transfer (eq 1a) or electron
transfer (eqs 1b and 1c). The predominant pathway depends
on the physical and photochemical properties of both dye and
ketone. The structures of the cationic dyes used in this work
are shown in Chart 1.
2. Experimental Section
2.1. Materials. Methylene blue (MB+Cl-) (Aldrich), phenosafranine
(PS+Cl-) (Fluka), and safranine T (ST+Cl-) (Aldrich)
were recrystallized from ethanol-water (4:1, v/v).
Thiopyronine (TP+Cl-) (Fluka) was recrystallized 3 times from
ethanol. Hydrogen chloride bonded to crystalline thiopyronine
(TP+Cl- × 1/2HCl) was removed by heating at 100 °C in Vacuo
at 0.1 Torr.4 Benzophenone (BP) (Aldrich) was purified with
the aid of neutral Al2O3 using n-hexane-ether (2:1, v/v) as
eluent. Xanthone (X), thioxanthone (TX), benzil (BZ), Nmethylacridone
(MAC), Michler’s ketone (MK), and p-(dimethylamino)benzophenone
(ABP) were recrystallized from
ethanol and benzene. 3,6-Bis-(dimethylamino)thioxanthone
(ATX) was synthesized as described in ref 5 and recrystallized
3 times from ethanol.
2.2. Measurements. Quantum yields of dye bleaching Φdye
were determined by continuous irradiation at 365 nm in a
previously described setup 6 containing a high-pressure mercury
lamp (HBO 200, Narva, Berlin) operated in conjunction with a
metal interference filter (UVIF 365, Zeiss, Jena). The absorbed
dose was determined by ferrioxalate actinometry.7,8
The excitation source utilized for flash photolysis studies was
either a ruby laser operated in conjunction with a frequency
doubler (λex ) 347 nm; flash duration ca. 15 ns) or a frequencydoubled
or -tripled Nd-YAG laser (λex ) 532 and 355 nm,
respectively; flash duration ca. 15 ns). Time-resolved emission
spectra were recorded using an optical multichannel analyzer
(Princeton Applied Research) with a 10-ns gate. The computercontrolled
flash photolysis setup (Nd-YAG laser) has been
described elsewhere.9 Actinometry for λex ) 347 and 355 nm
was performed using the benzophenone/naphthalene triplet
system.10 Prior to irradiation, the solutions were bubbled with
argon for 20 min in order to remove oxygen. Second-order
rate constants were determined by laser flash photolysis (error
limit: (10%).
Redox potentials of the dyes were measured using a cyclic
voltammeter (GWP 673 at 100 mV s-1) using platinum
† Columbia University.
‡ Polychrome GmbH.
§ Hahn-Meitner-Institut Berlin GmbH.
X Abstract published in AdVance ACS Abstracts, December 15, 1996.
(1a)
(1b)
(1c)
3K* + D+
K + 3(D+)*
K•+ + D•
K• – + D•2+ke′
kq
ke
CHART 1: Structures of the Dyes
440 J. Phys. Chem. A 1997, 101, 440-445
S1089-5639(96)01744-6 CCC: $14.00 © 1997 American Chemical Society
electrodes and SCE as reference. Tetrabutylammonium hexafluorophosphate
(Fluka) was employed as the electrolyte for 5
× 10-4 M solutions of the dye in acetonitrile. Ferrocene served
as the internal standard (Eox
P ) 0.42 V SCE).11
3. Results and Discussion
3.1. Reaction of Ketone Triplets with PS+. 3.1.1. The
BP-PS+ System. Acetonitrile solutions containing BP and PS+
were irradiated at 347, 355, or 365 nm. At these wavelengths,
the light is almost exclusively absorbed by BP. Continuous
irradiation at λex ) 365 nm resulted in a slight decrease in the
PS+ concentration corresponding to a quantum yield of about
0.01. Interestingly, flash photolysis at λex ) 347 nm revealed
that BP triplets react effectively with PS+. Transient optical
absorption spectra recorded at various times after the laser flash
are shown in Figure 1. The spectrum recorded at the end of
the flash possesses strong absorptions at about 510 and 320 nm
corresponding to BP triplets.12 The ground-state absorption of
PS+ (λmax ) 518 nm) bleaches simultaneously with the decay
of the BP triplet absorption at 320 nm. The bleaching process
is paralleled by a buildup of a new absorption at 800 nm
corresponding to 3(PS+)*.13 Accordingly, 3BP* is quenched by
PS+ via triplet-triplet energy transfer (eq 1a). Furthermore,
absorptions characteristic of redox intermediates of either
reaction partner were not observed. Importantly, after a certain
time, the absorption of ground-state PS+ is regenerated by a
first-order process corresponding to a triplet lifetime τ3 ) 25
µs, quite similar to the previously reported value.13 kq
BP ) 5.5
× 109 M-1 s-1, the rate constant of the reaction 3BP* + PS+,
was obtained by a pseudo-first-order kinetic treatment of the
changes in the absorption of 3BP* at 320 nm (Figure 2).
The emission spectrum recorded with the system BP/PS+
consists of three bands at 420, 450, and 555 nm (Figure 3).
The bands at 420 and 450 nm correspond to the phosphorescence
of 3BP*, which decays simultaneously with the absorption of
3BP* at 320 nm (Figures 1 and 3). The band around 555 nm
strongly resembles the steady-state fluorescence of PS+ in
acetonitrile solution (λex ) 500 nm), formed after the flash (see
inset in Figure 3). Its decay is synchronous with the decay of
3BP* and the bleaching of the dye (Figures 1 and 3). Therefore,
the 555-nm band is attributed to “delayed fluorescence” emitted
from 1(PS+)*, the excited singlet state of PS+. Generally, three
different mechanisms can be discussed regarding the origin of
the delayed fluorescence in this case:
Accordingly, 1(PS+)* can be generated by homotriplet-triplet
annihilation involving 3(PS+)* (eq 2), by triplet-to-singlet energy
transfer from 3(BP)* to PS+ (eq 3), and by heterotriplet-triplet
annihilation involving 3(BP)* and 3(PS+)* (eq 4). Provided the
Figure 1. Transient optical absorption spectra recorded (a) at the end
of the flash, (b) at 2.5 µs, and (c) 10 µs after irradiation of an argonsaturated
acetonitrile solution containing BP (1 × 10-3
M) and PS+
(4
× 10-5 M) with a laser flash (λex ) 347 nm; 23 °C; Dabs ) 5 × 10-6
einstein L-1). Insets: Kinetic traces recorded at different wavelengths
following the laser flash ([BP] ) 1 × 10-3
M; [PS+
] ) 2 × 10-5
M;
Dabs ) 6.2 × 10-6 einstein L-1).
Figure 2. Pseudo-first-order rate constant of the 3BP* decay at 320
nm vs the dye concentration (PS+
, ST+
, TP+
, MB+
; [BP] ) 1 × 10-3
M; λex ) 347 nm).
Figure 3. Transient optical emission spectra recorded 2 µs after
irradiation of an argon-saturated acetonitrile solution containing BP (1
× 10-3 M) and PS+ (2 × 10-5 M) with a laser flash (λex ) 355 nm).
Insets: Kinetic emission traces recorded at different wavelengths
following the laser flash (λex ) 347 nm; 23 °C; Dabs ) 6.2 × 10-6
einstein L-1
).
3
(PS+
)* + 3
(PS+
)* f PS+
+ 1
(PS+
)* (2)
3
(BP)* + PS+
f BP + 1
(PS+
)* (3)
3
(BP)* + 3
(PS+
)* f BP + 1
(PS+
)* (4)
Transfer between Ketone Triplets and Dyes J. Phys. Chem. A, Vol. 101, No. 4, 1997 441
delayed fluorescence originated from 1(PS+)*, formed according
to eq 2, its decay should follow the decay of the optical
absorption of 3(PS+)*. However, the lifetime of the delayed
fluorescence was found to be much shorter than τ ) 25 µs, the
lifetime of 3(PS+). Moreover, delayed fluorescence was not
observed when 3(PS+)* was generated by irradiation of PS+ at
λex ) 532 nm in acetonitrile solution containing no other
compound. Provided the mechanisms according to eq 3 or eq
4 would be operative, the decay rate of the delayed fluorescence
should correspond to that of 3(BP)*, which was actually
observed. To distinguish between these two mechanisms, the
dependence of Iem, the intensity of light emitted at λem ) 555
nm, on Iex, the intensity of the exciting light, was measured. In
the case of triplet-to-singlet energy transfer, Iem should depend
linearly on Iex (Iem ∝ Iex), and in the case of heterotriplet-triplet
annihilation, Iem should depend exponentially on Iex (Iem ∝ Iex
2).
The experimental results revealed an exponential dependence,
and therefore, it was concluded that the delayed emission is
caused by heterotriplet-triplet annihilation.
It is of particular interest to compare the reactions of 3(BP)*
with PS+ in the ground state (eq 1a) and with PS+ in the triplet
state (eq 4). To determine the rate constant of the latter reaction,
the following experiment was carried out (see Scheme 1): A
solution containing both BP and dye was at first irradiated with
a 532-nm light flash to generate dye triplets and subsequently,
delayed by 1 µs, with an additional 308-nm light flash to
generate benzophenone triplets. Such experiments require highintensity
laser pulses to convert sufficient ground-state molecules
into the triplet state. Another prerequisite is a high quantum
yield of triplet formation; i.e., Φisc must be large. ST+, a
derivative of PS+ (see Chart 1), undergoes intersystem crossing
with a larger quantum yield than PS+.14 Since, moreover, ST+
exhibits a quenching behavior similar to that of PS+ (kq
BP(PS+)
) kq
BP(ST+)), it was chosen for these experiments. Groundstate
ST+ was converted into the triplet state with a ca. 50%
yield (Scheme 1). The exact degree of conversion was obtained
from the change in the optical density at 520 nm (Figure 4d).
The decay of 3(BP)* that also reacts with ground-state dye was
monitored at 320 nm (Figure 4b). Typical kinetic traces
illustrating the changes in the optical density both at 320 and
520 nm induced by irradiating a solution with pulses from both
lasers are shown in Figure 4c and 4f. Because of the (weak)
absorption of 3(ST+)* at 320 nm (Figure 4a), it was necessary
to deconvolute the decay curves recorded at 320 nm (Figure
4c). Upon performing experiments at different ST+ concentrations
with and without application of the pump pulse of 532nm
light and applying a pseudo-first-order kinetic treatment
regarding the decay in the absorption at 320 nm, the bimolecular
rate constant of the heterotriplet-triplet annihilation was
determined with the aid of
where kexp is the pseudo-first-order rate constant of the decay
of the absorption at 320 nm obtained after deconvolution with
respect of the absorption of 3(ST+)*, k0 is the deactivation
constant of 3(BP)* in the absence of the dye, kq(BP) is the
bimolecular rate constant of quenching of 3(BP)* by groundstate
dye, and kht is the bimolecular rate constant of the
heterotriplet-triplet annihilation (eq 4). In this way, kht ) 1.1
× 1010 M-1 s-1 was determined. The high value indicates that
the dye triplet is more reactive toward 3(BP)* than the dye
singlet.
3.1.2. The MK-PS+ System. The electronic characteristics
of the triplet state of MK differ from that of BP. Because of
its CT character,15,16 triplet excited MK (3MK*) is prone to
undergo electron-transfer reactions. Actually, flash photolysis
of acetonitrile solutions containing both PS+ and MK at λex )
347 nm revealed that 3(MK)* reacts with PS+ via electron
transfer (eq 1b) and not via energy transfer (eq 1a). Evidently,
3(MK)* reacts with PS+ much faster than 3(BP)*. A rather high
rate constant (kq(MK) ) 1.3 × 1010 M-1 s-1), typical for
electron transfer, was found by pseudo-first-order kinetic
treatment of the ground-state bleaching of PS+ at λobs ) 520
nm, which follows synchronously the decay of the weak T-T
absorption of MK at 650 nm. Typical kinetic traces recorded
during the flash photolysis experiments are presented in Figure
5. Interestingly, a transient absorption at λobs ) 800 nm
indicating the formation of 3(PS+)* was not detected. On the
other hand, an absorption at λobs ) 440 nm attributed to PS•
radicals (the semireduced form of the dye13) grew in after the
flash (see Figure 5b). The rate of the formation of the absorption
at 440 nm (Figure 5b) corresponds to the rate of dye bleaching
(Figure 5a) as expected on the basis of the occurrence of
The PS• radical decayed, in the present case, with a half-life
of about 200 µs. As can be seen from the traces in Figure 5c
and 5d, the absorption of ground-state PS+ recovers simultaneously
with the decay of the absorption of PS•, indicating the
occurrence of back electron transfer
SCHEME 1
kexp
320
) k0 + kq[ST+
] + kht[3
(ST+
)*
] (5)
Figure 4. Double-laser experiment. Kinetic traces recorded at 320 nm
(a-c) and 520 nm (d-f) following laser flash photolysis (λex
1
) 532
nm; λex
2
) 308 nm) of an argon-saturated acetonitrile solution
containing BP (4 × 10-3 M) and ST+ (2 × 10-5 M). λex
1
) 532 nm (a,
d); λex
2
) 308 nm (b, e); double-laser experiment, λex
1
) 532 nm; λex
2
) 308 nm; delay 1.2 ms (c, f).
3
(MK)*
+ PS+
f MK•+
+ PS•
(6)
442 J. Phys. Chem. A, Vol. 101, No. 4, 1997 Jockusch et al.
The remarkable photostability of the acetonitrile solution
against continuous irradiation at 365 nm is in accordance with
this result. The quantum yield of dye conversion is very low
(Φdye ) 0.005).
Notably, the mechanism of the reaction 3(MK)* + PS+ is
solvent-dependent. Triplet-triplet energy transfer is the dominant
reaction pathway when the reaction is performed in the
less polar solvent dichloromethane ( acetonitrile ) 35.9;
dichloromethane ) 8.917). Flash photolysis experiments with
dichloromethane solutions revealed that in this case, the
absorption of 3(PS+)* at 800 nm is formed, indicating the
occurrence of
The lifetime of 3(PS+)* is about 25 µs. It is equal to the
lifetime of the recovery of the ground-state absorption of PS+.
As can be seen from Figure 6a, the recovery is not quantitative.
The trace approaches a saturation value definitely lower than
zero, indicating the occurrence of a dye-consumptive deactivation
reaction. In this connection, it is noticeable that, in
dichloromethane solution, also the formation of PS• was
evidenced. The transient absorption at 430 nm (see kinetic trace
in Figure 6b) contains, apart from the absorption attributed to
3(MK)*, also a portion attributed to the radical PS•. This
suggests that electron transfer occurs in dichloromethane
solution. The simultaneousness of the two deactivation reactions
is also reflected by the magnitude of the rate constant of the
quenching reaction 3(MK)* + PS+. kq(MK) ) 8.7 × 109 M-1
s-1 in dichloromethane is lower than kq(MK) ) 1.3 × 1010 M-1
s-1 in acetonitrile, but it is higher than kq(BP) ) 5.5 × 109
M-1 s-1 measured for the reaction 3(BP)* + PS+ in acetonitrile,
which solely involves energy transfer.
3.1.3. Other Ketones. The results concerning the reaction
of PS+ with various other ketones in the excited triplet state
are summarized in Table 2. Electron transfer was observed only
with MK and ABP in the strongly polar solvent acetonitrile. In
the cases of the other ketones, triplet quenching occurs via
energy transfer with rate constants ranging from 5.5 × 109 to
7.4 × 109 M-1 s-1. The slightly higher rate constant measured
in the case of ATX indicates a contribution of electron transfer.
Notably, 3ATX* reacts with PS+ exclusively via electron transfer
in propylene carbonate (PC), a solvent more polar than
acetonitrile ( PC ) 6518). The rate constant ke
ATX ) 1.4 × 109
M-1 s-1 measured in PC is lower than ke
MK and ke
ABP measured
in acetonitrile because of the higher viscosity of this solvent
(ηPC ) 2.53 cP19; ηacetonitrile ) 0.36 cP17).
Hetero-T-T annihilation, evidenced by delayed fluorescence
of PS+, was only found with solutions containing BP, X, TX,
or BZ. Obviously, only the triplets of these ketones are capable
of reacting with PS+ triplets. So far, it has not been elucidated
why triplets of the other ketones examined in this work do not
undergo hetero-T-T annihilation. A straightforward correlation
of the aptitude to this reaction and the triplet energy of the
ketones could not be established. Actually, such a correlation
seems feasible on the basis of the fact that in the cases of several
ketones (BP, X, TX, BZ), the relative intensity of the delayed
fluorescence decreases in the same order, namely, I(BP) > I(X)
> I(TX) > I(BZ), as the triplet energy: ET(BP) > ET(X) >
ET(TX) > ET(BZ). However, in the cases of other ketones
(MAC, ABP, MK) possessing triplet energies higher than that
of BZ, delayed fluorescence is not observed at all.
3.2. Reaction of Ketone Triplets with MB+. Low quantum
yields of dye bleaching (ΦMB
BP ) 0.004 and ΦMB
MK ) 0.015)
were obtained upon continuous irradiation of ketones at 365
Figure 5. Kinetic traces recorded at different wavelengths following
laser flash photolysis (λex ) 347 nm; 23 °C; Dabs (10-6
einstein L-1
) )
1.5 (a, c), 2.8 (b, d)) of an argon-saturated acetonitrile solution
containing MK (4 × 10-6 M) and PS+ (4 × 10-5 M).
Figure 6. Kinetic traces recorded at different wavelengths following
laser flash photolysis (λex ) 347 nm; 23 °C; Dabs ) 3 × 10-6 einstein
L-1) of an argon-saturated dichloromethane solution containing MK
(4 × 10-6
M) and PS+
(4 × 10-5
M).
TABLE 1: Characterization of Dye Cationsa
PS+ ST+ TP+ MB+
ES
, eV 2.34b
2.36b
2.21b
1.89b
ET
, eV 1.77c
1.78b
1.80d
1.50b
Ered
1/2
, eV -0.66e -0.64e -0.36e
3τ 25f 80e 36g
kq
BP
109
M-1
s-1
5.5 5.5 6.7 8.5
a ES: singlet energy. ET: triplet energy. Ered
1/2
: reduction potential
(SCE). 3
τ: triplet lifetime. k: rate constant of the reaction 3
BP*
+
dye in acetonitrile. b
Reference 14. c
Reference 22. d
Reference 4.
e Measured in this work. f Reference 23. g Reference 20.
TABLE 2: Characterization of Ketones
k, 109
m-1
s-1
ketone ET, eV Eox
1/2
, V ∆RG,b eV CH3CN CH2Cl2 I
BP 3.0c
2.39g
0.05 5.5 q 1.0
X 3.2c 1.8g -0.74 7.4 q 0.6
TX 2.8c
1.69g
-0.45 6.2 q 0.45
BZ 2.4d
2.0h
0.26 5.8 q 0.3
MAC 2.6c 1.47i -0.47 5.4 q 0.0
ABP 2.7e
1.08i
-0.96 12 e 9 q,e 0.0
MK 2.7c
0.95i
-1.09 13 e 8.7 q,e 0.0
ATX 2.7f
1.10i
-0.94 8.7 e,q 0.0
a
ET
: triplet energy. Eox
1/2
: oxidation potential (SCE). ∆RG: free
reaction enthalpy for reduction of PS+ by ketone triplets. k: rate
constant of the reaction of PS+
+ ketone triplets in acetonitrile and
dichloromethane. q: dominant energy transfer. e: electron transfer.
q,e: energy and electron transfer equally important. I: relative intensity
of the delayed fluorescence of PS+
(normalized to BP). b
Calculated
by ∆RG ) Eox
1/2
(ketone) - Ered
1/2
(PS+) - ET(ketone). c Reference 24.
d
Reference 25. e
Reference 26. f
Determined from the phosphorescence
spectrum at 77 K in ether/ethanol/toluene glass. g Reference 5. h Reference
27. i
Measured in this work.
MK•+
+ PS•
f MK + PS+
(7)
3
(MK)*
+ PS+
f 1
(MK) + 3
(PS+
)*
(8)
Transfer between Ketone Triplets and Dyes J. Phys. Chem. A, Vol. 101, No. 4, 1997 443
nm in Ar-saturated acetonitrile solutions containing BP or MK,
respectively.
Laser flash photolysis experiments at λex ) 347 nm revealed
results similar to those found in the case of PS+/ketone systems.
Since MB+ possesses a more positive reduction potential than
PS+ (Table 1), the reaction enthalpy of the reduction of the
dye by ketone triplets, calculated with the aid of the RehmWeller
equation, is more negative. Consequently, electron
transfer was expected to be the favorable reaction mode.
However, in comparing the reaction of MB+ and PS+ with
ketone triplets, only marginal differences regarding the competition
between energy and electron transfer were detected. The
ketones examined in this case were BP, MAC, MK, and ATX.
In acetonitrile solution, energy transfer was observed for the
reaction of MB+ with 3BP* and 3MAC* and electron transfer
for the reaction MB+ + 3MK*. Both reaction pathways seem
to be involved in the reaction MB+ + 3ATX*. In analogy to
the system PS+/MK, the mode of the reaction pathway changed
from electron transfer to energy transfer when the reaction MB+
+ 3MK* was performed in dichloromethane instead of aceto-
nitrile.
Typical results concerning the system MB+/BP are presented
in Figure 7, where transient absorption spectra recorded at
various times after the 15-ns flash are shown. The absorption
bands at 320 and 510 nm, which are formed during the flash,
are attributed to 3BP*. The absorption of 3BP* decreases
simultaneously with the bleaching of the ground-state absorption
of MB+ (around 600 nm) and the growth of the T-T absorption
of 3(MB+)* at 800 nm.20 The kinetic traces, shown as insets in
Figure 7, demonstrate the simultaneousness of these processes.
The formation of the absorption of 3(MB+)* suggests the
occurrence of energy transfer (eq 1a). Obviously, absorptions
due to redox intermediates were not observed. Importantly, the
initial absorption of MB+ is regenerated within about 35 µs
(not shown in Figure 7). In this connection, it is notable that
a triplet lifetime of similar magnitude was found for 3(MB+)*
by Kamat and Lichtin.20 The recovery of the ground-state
absorption of MB+ shows that irreversible processes leading to
permanent dye bleaching do not occur. Pseudo-first-order
kinetic treatment of the changes in the absorption of 3BP* yielded
the rate constant of the reaction 3BP* + MB+:kq
BP ) 8.5 × 109
M-1 s-1. This rate constant is only slightly larger than that of
the reaction 3BP* + PS+ (Table 2).
The rate constant of the reaction 3MAC* + MB+ could not
be determined because the absorptions of 3MAC* and MB+
(ground state) are strongly overlapping. But the appearance of
the absorption of 3(MB+)* at 770 nm confirms the energytransfer
mechanism concerning the reaction 3MAC* + MB+.
The reaction 3MK* + MB+, on the other hand, proceeds via
electron transfer (in acetonitrile solution). The rate constant is
high: ke
MK ) 1.5 × 1010 M-1 s-1. MB•, generated by this
reaction, was evidenced by its characteristic absorption at 430
nm.20 On the other hand, 3(MB+)*, which should be formed by
energy transfer, was not detected in this case.
In analogy to the PS+/MK system dealt with in section 3.1.2.
the mode of reaction pathway (electron or energy transfer) is
expected to be influenced by solvent polarity. It turned out
that in the weakly polar solvent dichloromethane, the reaction
3MK* + MB+ proceeds predominantly via energy transfer as
was evidenced by the formation of 3MK*, detected by its
characteristic absorption at 770 nm. Moreover, a rather strong
absorption at 430 nm due to MB• was observed, indicating that
electron transfer also occurs. Apparently, the partitioning is in
favor of electron transfer in the present case. In this respect,
the MB+/MK system differs from the PS+/MK system where
the energy transfer pathway is favored in dichloromethane. The
difference probably reflects differences in the reduction potential,
which in the case of MB+ is somewhat more positive than that
of PS+ (see Table 1).
Heterotriplet-triplet annihilation, according to eq 4, was also
observed with the system BP/MB+: as is seen from kinetic trace
b in Figure 8, delayed fluorescence originating from reaction
3BP* + 3(MB+)* is emitted from 1(MB+)* at 677 nm. The
emission at 450 nm (trace a in Figure 8) is due to the
phosphorescence of 3BP*.
3.3. Reaction of Ketone Triplets with TP+. TP+ possesses
photophysical properties similar to those of PS+ and MB+ (Table
1), which suggests also that its reactivity toward ketone triplets
should be similar. This was confirmed by laser flash photolysis
experiments at λex ) 347 nm. It turned out, for example, that
TP+ , in analogy to PS+ and MB+, reacts with 3BP* via triplettriplet
energy transfer. Moreover, delayed fluorescence was
emitted from 1(TP+)*, thus demonstrating the occurrence of
heterotriplet-triplet annihilation also in this case.
Different reaction pathways were observed for the reaction
3MK* + TP+ in acetonitrile and dichloromethane solution. In
Figure 7. Transient optical absorption spectra recorded (a) at the end
of the flash, (b) at 0.3 µs, and (c) 10 µs after irradiation of an argonsaturated
acetonitrile solution containing BP (1 × 10-3 M) and MB+
(2 × 10-5
M) with a laser flash (λex ) 347 nm; 23 °C; Dabs ) 4 ×
10-6 einstein L-1). Insets: Kinetic traces recorded at different
wavelengths following the laser flash ([BP] ) 1 × 10-3
M; [MB+
] )
5 × 10-5 M; Dabs ) 5.7 × 10-6 einstein L-1).
Figure 8. Kinetic emission traces recorded at different wavelengths
following laser flash photolysis (λex ) 347 nm; 23 °C; Dabs ) 6 ×
10-6
einstein L-1
) of an argon-saturated dichloromethane solution
containing BP (1 × 10-3
M) and MB+
(5 × 10-5
M).
444 J. Phys. Chem. A, Vol. 101, No. 4, 1997 Jockusch et al.
polar acetonitrile, 3MK* is quenched via electron transfer (ke
MK
) 1.1 × 1010 M-1 s-1) and in less polar dichloromethane by
energy transfer (kq
MK ) 8.7 × 109 M-1 s-1). (Detailed
information concerning these results is contained in the supporting
material.)
Upon continuous irradiation of BP/TP+ in acetonitrile solutions
at λex ) 365 nm, a rather high quantum yield of dye
conversion was found: ΦTP
BP ) 0.1. The fact that this value
is much higher than comparable values (ΦMB
BP ) 0.004 and
ΦPS
BP ) 0.01) very likely reflects the high probability of the
occurrence of reaction 9, i.e., the reaction of 3(TP+)* with
ground-state dye
By this disproportionation reaction (ksq ) 7.7 × 108 M-1 s-1)21
semireduced and semioxidized dye is formed 4.
Notably, the rate constants of the corresponding reactions of
3(PS+)* and 3(MB+)* are lower (ksq
MB ) 3.6 × 108 M-1 s-1).20
Moreover, the lifetimes of 3(PS+)* and 3(MB+)* are shorter than
that of 3(TP+)* (Table 1). Therefore, in these cases, the triplets
deactivate almost quantitatively to the ground state and the
quantum yield of dye conversion at continuous irradiation is
quite low (see above).
4. Conclusions
Heteroanthracene-type dyes, such as PS+, TP+, and MB+,
effectively quench the triplet states of the aromatic ketones BP,
X, TX, BZ, and MAC by energy transfer, and thereby, triplet
excited dyes are formed. The triplet states of other ketones
(MK, ABP, and ATX) can be quenched by the dyes also via an
electron-transfer mechanism. It turned out that the mode of
reaction pathway strongly depends on the solvent polarity.
Whereas electron transfer is favored in polar solvents like
acetonitrile, energy transfer is the dominant reaction pathway
in less polar solvents such as dichloromethane. Generally, the
rate constant of electron transfer is larger than that of energy
transfer.
Heterotriplet-triplet annihilation occurs in the reaction of
triplet excited dyes with triplet excited ketones in several cases
(BP, X, TX, BZ). This process gives rise to delayed fluorescence
of the dye which can be readily detected by time-resolved
luminescence measurements. Moreover, it was found that dyes
in the excited triplet state are more reactive toward triplet excited
ketones than in the excited singlet state.
Acknowledgment. S.J. and N.J.T. at Columbia thank the
National Science Foundation and the Air Force Office of
Scientific Research for their generous support. S.J. thanks the
German Academic Exchange Service for a postdoctoral fellow-
ship.
Supporting Information Available: A detailed description
of laser flash photolysis results for the reaction of ketone triplets
with TP+ along with figures (5 pages). Ordering information
is given on any current masthead page.
References and Notes
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3
(TP+
)*
+ TP+
f TP•
+ TP•2+
(9)
Transfer between Ketone Triplets and Dyes J. Phys. Chem. A, Vol. 101, No. 4, 1997 445