J. Phys. Chem. 1991, 95, 4406-4410
4406
Photochemistry of Sensitlzhtg Dyes. Spectroscopic and Redox Properties of Cresyl Vlolet David I. Kreller and Prasbant V. Kamat* Notre Dame Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: September 25, 1990; In Final Form: December 20, 1990)
The photosensitizing properties of cresyl violet have been investigated by characterizing singlet and triplet excited states and the reduced and oxidized forms of the dye by picosecond and nanosecond laser flash photolysis. The dye readily forms a charge-transfer complex with amines with complexation constants in the range of 49-3350 M-'in methanol. The dye in its singlet and triplet excited states has lifetimes of 2.46 ns and 68 ~ s respectively, , in acetonitrile. The excited triplet is quenched by ground-state dye molecules with a bimolecular rate constant of 1.45 X lo9 M-'6'. The semioxidized radical of the dye has been generated by oxidation of CV+ with pulse radiolytically generated azide radicals (k = 1.6 X 1O1OM-' s-') in a ueous medium. The semireduced dye has been generated by quenching of triplet dye with triphenylamine (k = 1.4 X 108 M-'s-I) in methanol. Photoelectrochemical reduction of cresyl violet in colloidal Ti02 suspension has also been carried out, with a quantum yield of 0.013.
Introduction Cresyl violet is a member of the oxazine class of dyes that absorb strongly in the red region and have high fluorescence quantum yields.' Long-lived excited states and reversible redox behavior make dyes such as cresyl violet potentially useful as photosensitizers in energy- and electron-transfer reactions." Willig et aLs8 have used this dye to probe ultrafast charge injection processes in large-bandgap semiconductors. Applications of oxazines in dye lasers, storage-type photoelectrochemical cells, and luminescent solar collectors have also been demonstrated.'-9 The use of cresyl violet as a primary standard for emission in the red region has been demonstrated by Magde et al.' Because of the wide range of applications of cresyl violet it is essential that its photophysical and photochemical properties be well understood. We have now employed laser flash photolysis and pulse radiolysis techniques to investigate singlet and triplet excited-state properties as well as the behavior of the reduced and oxidized forms of cresyl violet. Photoelectrochemical reduction
H2N
J3:aH* uesyl videt, CV*
of cresyl violet in colloidal T i 0 2 suspension has also been performed.
Experimental Section Materials. Cresyl violet (Exciton, laser grade) and 9, IO-dibromoanthracene (Aldrich) were used as supplied. n-Butylamine, dibutylamine, diethylamine, triethylamine, and diisopropylamine were of the highest available purity from Aldrich. 1-Pyrenecarboxaldehyde and benzophenone were purified by recrystallization. All solvents were of spectrophotometric grade. Colloidal T i 0 2 suspension in acetonitrile was prepared by the method described earlier.* Methods. All experiments were performed at room temperature (-296 K) unless otherwise stated, and solutions were deaerated by bubbling with nitrogen or argon. Absorption spectra were measured with a Perkin Elmer 3840 diode array spectrophotometer. Corrected fluorescence emission spectra were recorded with a SLM photon-counting spectrofluorometer. Laser Flash Photolysis. Nanosecond laser flash photolysis experiments were performed with a Quanta-Ray CDR-I Nd:YAG system to generate 532-nm (second harmonic) laser pulses ( - 6 ns pulse width).I0 The experiments were performed in a rec~
*Address correspondence to this author.
0022-365419 112095-4406302.50/0
TABLE I: Absorption and Emission Characteristics of Cresyl Violet absorption fluorescence excited singlet medium max, nm max, nm lifetime. ns ethanol 600 630 2.78 methanol 590 626 4.54 acetonitrile 588 622 2.46 water 586 63 1 1.27
tangular quartz cell of 6 mm pathlength with a right angle configuration between the directions of laser excitation and analyzing light. A typical experiment consisted of a series of 5-10 replicate shots per measurement, and the average signal was processed with an LSI-11 microprocessor interfaced to a PDP 11/55 computer. Picosecond laser flash photolysis experiments were performed with a mode-locked 532-nm laser pulse from Quantel YG-SOlDP Nd:YAG (output 4 mJ/pulse, pulse width 18 ps). The white continuum picosecond probe pulse was generated by passing the residual fundamental output through a D 2 0 / H 2 0 solution. The excitation and the probe pulse were incident on the sample cell at right angles. The output was fed to a spectrograph (HR-320, ISDA Instruments, Inc.) with fiber optic cables and was analyzed with a dual diode array detector (Princeton Instruments, Inc.) interfaced with an IBM AT computer. The details of the experimental setup and its operation are described in detail elsewhere."J2 The time zero in these experiments corresponds to the time of maximum overlap of the probe and excitation pulses. All the lifetime and rate constants reported in this study are within the experimental error of is%. Pulse Radiolysis. For pulse radiolysis experiments, irradiation was performed with electron pulses (5 ns, -10'' eV g-I pulse-') from the Notre Dame 7-MeV ARCO-LP-7 linear accelerator. A description of the computer-controlled kinetic spectrophotometer and data collection system is available elsewhere."
-
(1) Magde, D.; Brannon, J. H.; Cremers, T.; Olmsted. 111, J. J . Phys. Chem. 1979,83,696. (2) Gerischer. H.; Willig. F. Top. Curr. Chem. 1976, 61, 31. (3) Meier, H. Photochem. Photobiol. 1972, 16, 219. (4) Spitler, M.; Parkinson, B. A. Lmgmuir 1986, 2, 549. (5) Eichberger, R.; Willig, F. Chem. Phys. 1990, 141. 159. (6) Willig. F.; Eichkrger, R.; Sundaresan, N . S.;Parkinson, B. A. J. Am. Chem. Soc. 1990, 112, 2702. (7) Kamat, P. V.; Lichtin. N . N . Isr. J . Chem. 1982, 22, 113. (8) Kamat, P. V. J . Chem. Soc., Faraday Trans. I 1985,81, 509. (9) Batchelder, J. S.; Zewail, A. H.; Cole, T. Appl. Opr. 1981, 3733. (IO) Nagarajan, V.; Fcssenden, R. W. J. Phys. Chem. 1985, 89, 2330. (11) Ebbescn, T. W. Rev. Sci. Inrtrum. 1988, 59, 1307. (12) Kamat. P. V.; Ebbescn, T. W.; DimitrijeviE, N . M.; Nozik, A. J. Chem. Phys. Lett. 1989, 157, 384. (1 3) (a) Patterson, L. K.; Lilie, J. IN.J . Radial. Phys. Chem. 1974,6, 129. (b) Schuler, R. H.; Buzzard, G. K. Ibid 1976, 8, 563.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 11, 1991 4407
Photochemistry of Sensitizing Dyes
TABLE II: Triplet-Triplet Sensitization of Cresyl Violet in Acetonitrile
I \I
0.8
u 0.6
z2
triplet k,, lifetime: ET,'' kJ mol-' IO9 M-'s-l
sensitizer 9,10-DBA 167.4 I-pyrenecarboxaldehyde 186.2 benzophenone 288.8
-
0
U
0.4 -
o*2
0.986
19.0 41.0 68.0
1.13
2.02
c7701b
M-'cm-l 247W 22500" 1622W
"Triplet energy of sensitizer. bCresyl violet triplet. CRelativeto E420
= 48000 for 39,10-DBA*(ref 17). dRelative to = 18700 for '1PCAH* (ref 19). CRelativeto E,,, for 3BP* = 7640 (ref 18).
t 800
Wavelength (nm)
Figure 1. Absorbance and emission spectra of cresyl violet in methanol.
-0.029
t
400
300 450
500
550
Wavelength ( n m )
Figure 2. Transient absorption spectra of singlet excited cresyl violet in
aqueous medium. The spectra were recorded following the 355-nm laser pulse excitation at time intervals: (a) 0 ps; (b) 500 ps; (c) 1.0 ns; (d) 2.0 ns; and (e) 5.0 ns.
Results and Discussion Absorption and Emission Spectra. The absorption and emission spectra of cresyl violet (CV+) in MeOH are shown in Figure 1 and its characteristics in various solvents are summarized in Table I. CV+ has a strong absorption in the red region with a maximum extinction coefficient of 6.9 X 10" M-I s-I at 590 nm in methanol. The fluorescence emission has a maximum at 626 nm in methanol. The singlet excited-state energy (Es) of CV+ determined from the cross-over point between the normalized absorption and emission spectra was 197 kJ mol-' in methanol. (This value should be considered with caution since the cross-over point depends on many variables such as dye concentration, fluorescence quantum yield, and Franck-Condon factors.) The fluorescence quantum yield was determined by the optically dilute relative method with a solution of rhodamine 6G in air-saturated ethanol (dr = 0.95)14*15 and R ~ ( b p y ) , ~in+ water (& = 0.042)16as the standard. These measurements gave an average value of 0.5 as the fluorescence quantum yield of CV+ in methanol and agreed = 0.54).' well with the earlier reported value Excited Singlet State. The excited singlet of CV+ was generated by direct excitation of the dye with a 532-nm laser pulse in a picosecond laser flash photolysis apparatus. The transient absorption spectra recorded a t different time intervals following the laser pulse (1 8 ps) excitation of CV+ in water are shown in Figure 2. The difference absorption spectrum of VV+* exhibited a maximum a t 5 0 0 nm, with an isosbestic point around 545 nm. The dccay of the absorption at 500 nm matched well with the recovery of the bleached dye, which confirmed that the transient formation originated from CV+. In a similar way the transient absorption of ICV+* was recorded in different nonaqueous solvents and the decay was analyzed with first-order kinetics. The lifetimes (14) Demas, J. N.In Opticul Rudiution Meusurements; Mielenz, K. D., Ed.; Academic Press: New York, 1982;Vol. 3. pp 195-248. (15) Kubin, R. F.; Fletcher, A. N. J . Lumin. 1982, 27, 455. (16) Van Houten, J.; Watts, R. J. J . Am. Chem. Soc. 1976,98, 4853.
350
400
450
500
550
600
650
700
750
800
WAVELENGTH, nm
Figure 3. Energy transfer from triplet excited 9,lO-dibromoanthracene to cresyl violet in acetonitrile. Transient absorption spectra were recorded
following 355-nm laser pulse excitation of a solution containing 10-4 M 9,lO-dibromoanthraceneand 5 X IO-, M CV+ at time intervals: (a) (0) 0.2;(b) (+) 1.0;and (c) (A) 1.5 ps. The absorption-time profiles in the insets show the decay of 9,lO-dibromoanthracenetriplet at 424 nm and the formation and decay of 'CV+* at 750 nm. of ICV+* are summarized in Table I. r, of 'CV+* varied from 1.27 to 4.5 ns as the medium was changed from water to methanol. This suggests that solvent polarity may be influencing the nonradiative and radiative pathways of the excited singlet decay. Excited Triplet State. Direct excitation of CV+ in methanol did not exhibit any detectable amount of transients in the nanosecond to microsecond time domain. This shows that the intersystem crossing efficiency for the generation of triplet excited state is very small. Similar low intersystem crossing efficiency has been observed in the case of other oxazine dyes? Hence, it is necessary to employ the triplet-triplet energy-transfer method for generating the triplet excited state of CV+ (reaction 1). 3Sens* + CV+
kel
Sens
+ ,CV+*
Sensitizers, such as 9,10-dibromoanthra~eneene,'~ benzophenone,18 and 1 -pyrenecarboxaldehyde,I9were found to generate ,CV+* in detectable amounts, thus facilitating characterization of the triplet excited dye. The bimolecular rate constant, k,, for the energytransfer process (reaction 1) was determined by recording pseudo-first-order decay of 3Sens* at various CV+ concentrations. The rate constant for the T-T energy transfer and the characteristics of 3CV+* are summarized in Table 11. The rate constants for the energy-transfer process for all three sensitizers employed in the present study were close to the value of a diffusion-controlled process. This indicated that the energy level, E T , of ,CV+*lies well below the ET of these three sensitizers (167.4-288.8W mol-'). Time-resolved transient absorption spectra recorded with two different sensitizers are shown in Figures 3 and 4. The transient absorption spectrum recorded immediately after 355-nm laser (17) Darmanyan, A, P. Chem. Phys. Len. 1984, 110, 89. (18)Carmichacl, I.; Hug,G. L. J . Phys. Chem. Ref Dura 1986,IS, 26. (19)Kumar, C. V.; Chattopadhyay, S.K.; Das, P. K. Phorochem. P h e tobiol.-1983,38, 141.
Kreller and Kamat
4408 The Journal of Physical Chemistry, Vol. 95, No. 1I. I991 0.11
I
350
400
450
500
550
600
650
700
750
800
WAVELENGTH, nm
Figure 4. Energy transfer from triplet excited benzophenone to cresyl violet in acetonitrile. The transient absorption spectra were recorded following 355-nm laser pulse excitation of a solution containing lo-' M benzophenone and 5 X lW5 M CV+ at time intervals: (a) ( 0 )0; (b) (+) 1.0; and (c) (A) 3.75 p s . The absorption-time profiles in the insets indicate the decay of benzophenone triplet at 335 nm and the formation and decay of 'CV+* at 795 nm. pulse excitation corresponds to the sensitizer triplet. The difference absorption spectrum recorded at longer time intervals exhibits a maximum at 770 nm which corresponds to 'CV+*. The growth in the 770-nm absorption was found to match the decay of the sensitizer triplet in all these cases. Similar triplet absorption in the infrared region has also been observed for another member of the oxazine class of dyes7 The extinction coefficient of 'CV+* at the absorption maximum (770 nm) was determined by recording the maximum absorbance values of 'Sens* and 'CV+* and from eq 2.18
Pa is the probability of energy transfer estimated by the expression pet
= ke,[CV+l/(ket[CV+l + ko)
(3)
where ko is the intrinsic decay of the sensitizer triplet which was determined separately in an acetonitrile solution. The values of the extinction coefficient of at 770 nm with various sensitizers are listed in Table 11. The discrepancy between the values of (770 in these three experiments can be attributed to the error in assumed extinction coefficient values of sensitizer triplet. The average value of €770 of 'CV+* obtained from these measurements was 21 140 M-' cm-I. The lifetime of 'CV+* in these experiments varied from 19 to 68 ps depending upon the dye concentration, sensitizer, and the medium. This indicates participation of the sensitizer as well as the ground-state dye molecules in the quenching of 'CV+*. Nevertheless, one can consider 68 ps as the upper limit for the lifetime of 'CV+* in the present experiments. The self-quenching of triplet excited state by the ground-state dye molecules has been the topic of earlier investigation^.^^^^ Up to 50% electron-transfer efficiency has been observed in the quenching of triplet methylene blue by the ground-state dyeZo (reaction 4). The bimolecular rate constant for the quenching
'D* + D(So)
-
Do+ + D'-
(4)
WAVELENGTH, nm M CV+ in methanol with Figure 5. Absorption spectra of 1.3 X successive addition of n-butylamine. Inset shows Benesi-Hildebrand plot of reciprocal of change in absorbance versus reciprocal of n-butylamine concentration.
TABLE III: Apparent Association Constants for the CV+-Amine ComDlex in Methanol at 298 K triphenylamine triethylamine dibutylamine
49 610 3114
diisopropylamine diethylamine n-butylamine
3350 3340 3300
9110%.
was negligible. The rate constant for the self-quenching process was found to be (1 -45 f 0.2) X lo9 M-'s-l in acetonitrile. The transient absorption spectrum recorded at longer time scales did not indicate formation of long-lived products. The lack of formation of electron-transfer products in the self-quenching of ' C V * does not directly support an electron-transfer quenching mechanism as described in reaction 4. However, if the efficiency of net back electron transfer between CV' and CV2+ is very high, the detection of electron-transfer products will be difficult. The efficiency of net electron transfer in a self-quenching process has been shown to be dependent on the solvent polarity and back electron transfera20 Charge-Transfer Interactions. CV+ undergoes charge-transfer complexation with amines such as triphenylamine. This charge-transfer interaction is characterized by appearance of a new absorption band in the region of 460-485 nm. CV+
+ TPA
[CV+-.TPA]
(5)
To study the nature of charge-transfer interactions, the apparent association constants for the association between CV+ and a series of amines in methanol were measured by the Benesi-Hildebrand method.2' The absorption spectra of CV+ recorded with increasing concentration of n-butylamine are shown in Figure 5 . The inset in Figure 5 shows the straight line fit of the BenesiHildebrand plot. Isosbestic points between CV+ absorbance and charge-transfer bands indicated that the complex and free dye molecules were the only absorbing species in the 350-700-nm spectral region and that the complex is unimolecular with respect to the dye. The measured association constants are listed in Table 111.
of 'CV+* by CV+(So) was determined by measuring the pseudo-first-order decay rate constants for 'CV+* at various concentrations of CV+ in the T-T sensitization experiments. The concentration of CV+ was kept relatively higher than 'CV++ so that the concentration change of CV+ during the triplet decay
The emission and excitation spectra of the CV+-n-butylamine complex were recorded in the presence of excess n-butylamine (Figure 6 ) . Interestingly, when solutions of complexed dye were excited in its absorption band, an emission with energy distribution similar to that of uncomplexed CV+ in MeOH was observed. No other emission band which might be attributed to the complex
(20) (a) Kamat, P.V.;Lichtin, N.N. J . Phys. G e m . 1981,85,814. (b) Kamst, P. V.;Lichtin, N. N. J. Photochem. 1982, 18, 197.
(21) (a) Benesi, H. A.; Hildebrand, J. H. J . Am. G e m . Soc. 1949, 71, 2703. (b) Person, W . B. J . Am. Chem. Soc. 1965,87, 167.
The Journal of Physical Chemistry, Vol. 95, No. 11, 1991 4409
Photochemistry of Sensitizing Dyes
a
a 0.00
400
'
' 440
'
'
I
480
1
1
520
560
I
I
600
I
I
640
o b c d
I
680
720
WAVELENGTH, nm
Figure 6. Corrected fluorescence emission and excitation spectra of CV+-n-butylamine complex in MeOH (1.5 X M CV+ with excess n-butylamine). was observed. This suggests that the complex is nonfluorescent and the observed fluorescence arises from the dissociation of the complex in the excited state or from the uncomplexed CV+. To further check the origin of the observed emission band at 630 nm, an excitation spectra was recorded. Both the absorption peaks corresponding to uncomplexed CV+ (absorbance maximum 590 nm) and CV+-butylamine complex (absorbance maximum 475 nm) appeared in the excitation spectrum (Figure 6). The appearance of a charge-transfer band in the excitation spectrum supported the fact that a substantial fraction of emission of CV+ in the presence of n-butylamine originates from the ground-state charge-transfer complex. While most of the excited state is quenched by amine, a fraction of this complex dissociates upon excitation to yield amine and dye. This is supported by the fact that the complexed and free dye exhibit similar fluorescence emission spectra but differ considerably in their excitation spectra. It is possible that the complex has different K, in the ground and excited states. A smaller value of K, for the excited dye and amine complex would weaken the charge-transfer interaction between these two components upon excitation. Picosecond laser flash photolyses are now being carried out to probe the photochemical events associated with the excitation of such charge-transfer complexes. Such a complexation between the dye and the amine was also evident in fluorescence quenching analysis. For example, the Stern-Volmer plot for the quenching of CV+ by triphenylamine exhibited a positive curvature at lower triphenylamine concentrations. Such a deviation arises as a result of charge-transfer interaction between the two reactants. However, at higher triphenylamine concentrations the deviation in the Stern-Volmer plot was minimum. The slope ( k q ~ Sof) the linear portion of this plot was 129. By taking the value of singlet lifetime in methanol as 4.54 ns (Table I), one obtains the value of the bimolecular quenching rate constant as 2.84 X 1Olo M-' s-l. Semioxidized Cresyl Violet. Cresyl violet undergoes irreversible oxidation around +0.5 V vs SCE. The lower oxidation potential of the dye facilitates its oxidation by pulse radiolytically generated radicals such as N3' and (SCN)2'- (reaction 6). The transient N,' CV+ N3- CV2+ (6)
-
+
-
+
spectra recorded during the course of reaction 6 are shown in Figure 7. The bimolecular rate constant for the reaction between N3' and CV+ was determined to be 1.6 X 1O'O M-' s-I. The transient spectra recorded in an aqueous medium, 7 CIS after the pulse, show an absorption maximum at 465 nm and a broad absorption around 700 nm with isosbestic points at 510 and 630 nm. The semioxidized radical of the dye can also be generated by photoionization.
-
+
CV+ + nhu CV2+ e(7) The transient spectrum was recorded following the excitation of CV+ in methanol with a high-intensity 266-nm laser pulse. The transient absorption spectrum was similar to the spectrum of CV2+
-18,000
300
I
I
400
500
-
0 . 5 ~ ~ 1.0,~' 2.0pa 7.0ps I
600
700
Wavelength, nm Figure 7. Transient absorption spectra of pulse radiolytically generated cation radical of cresyl violet in H20. The spectra were recorded following the pulse radiolysis of N20-saturated aqueous solution containing 0.05 M NaN,, 1 X IO" M CV+ at time intervals: (a) 0.5; (b) 1.0; (c) 2.0; (d) 7.0 ps. Absorption-time profile shows the formation of CV2+. 0.051
I
T
1
W 0
z 4
m
a
5: m a
a0
t -0.02
i
400
450
550
500
600
650
700
750
800
1 850
WAVELENGTH, nm
Figure 8. Photochemical reduction of cresyl violet triplet with triphenylamine. The transient spectra were recorded following 355-nm M laser pulse excitation of lo-" M 9,1O-dibromoanthracene, 5 X CV+,and 25 pM triphenylamine at time intervals (a) 2.8; (b) 6.8; and (c) 68 ps. The absorption-time profile at 845 nm (inset) shows the formation of CV'. generated by pulse radiolysis (with absorption band a t 470 nm and bleaching at 650 nm). The linearity of the plot of CV2+yield versus (intensity)2 indicated the photoionization process to be biphotonic. The lifetime of CV2+ was estimated to be 250 and 145 CIS in H 2 0 and MeOH, respectively. Semireduced Cresyl Violet. Both one- and two-electron reductions of CV+ are expected to occur at potentials 1 4 4 6 V vs SCE since the dye exhibits reversible cyclic voltammograms a t -0.46 V vs SCE corresponding to the two-electron-reduction process. The semireduced cresyl violet (CV') was produced by direct electron-transfer quenching of 3CV+* by triphenylamine in methanol solution (reaction 8). The triplet excited state of the
+
-
+
3CV+* TPA CV' TPA+ (8) dye was produced by T-T sensitization with 9,lO-dibromoanthracene. The bimolecular rate constant for reaction 8 was determined to be 1.4 X lo9 M-I s-I. The transient absorption spectra recorded after 355-nm laser pulse excitation of methanol solution containing 9,lO-dibromoanthracene, cresyl violet, and triphenylamine are shown in Figure 8. The transient absorption spectra recorded at an initial stage of the reaction exhibits absorption maxima corresponding to 3DBA* (Amx 425 nm) and 3CV+* (A, 750 nm). However, the
Kreller and Kamat
4410 The Journal of Physical Chemistry, Vol. 95, No. 11, 1991 0.06
m
0
UJ
m 4
O b
of colloidal Ti02. CV' exhibits absorption maxima at 790 and 470 nm similar to the absorption maxima observed in the case of photochemical reduction of 3CV+* by triphenylamine. The extinction coefficient of CV' at 470 nm was estimated to be 20000 M-'cm-I, assuming CV' to have negligible absorptions in the region of ground-state bleaching. This value of the extinction coefficient was used to determine the yield of the photoelectrochemical reduction process. The upper limit for the interfacial charge-transfer efficiency was determined by the methodology described earlier.,, Anthracene in cyclohexane was used as a standard for actinometry. The maximum quantum yield for the photoelectrochemical reduction is estimated to be 0.013. This value is much less than the quantum ields (0.02-0.10) observed for other thiazine and oxazine dyes.8* Compared to these dyes, CV+ has a more negative reduction potential and lies close to the conduction band of Ti02 ( - 4 . 5 V vs SCE). Such a low-energy difference makes the interfacial charge transfer less efficient and facilitates recombination of photogenerated charge carriers. Steady-state irradiation of an argon-saturated colloidal TiOl suspension containing CV+ with 320-nm monochromatic light led to the bleaching of the dye. The colorless product (leuco form of the dye), which is a two-electron-reduction product is formed as the result of disproportionation of CV' (reaction 11). CV-
Y
T I Y E , ps
-0.08
'
450
500
550
I
1
600
650
700
750
800
I
WAVELENGTH, nm
Figure 9. Photochemical reduction of cresyl violet in colloidal TiOz suspension. The spectrum was recorded 2 p s after 337-nm laser pulse excitation of 2 mM colloidal TiOl in acetonitrile containing 7 X M CV+. Inset is an absorption-time profile at 565 nm indicating groundstate bleaching of the dye.
transient absorption spectrum recorded at longer time intervals corresponds to semireduced cresyl violet. The absorption bands of CV' were in the visible (450 nm) and infrared regions (790 nm). The formation of CV' is indicated by the absorption-time profile at 845 nm in the inset of Figure 8. The lifetime of CV' in deaerated CH$N is estimated to be 40 fis. Photoelectrochemical Reduction of Cresyl Violet. In our earlier studies we have shown that thiazine (thionine and methylene blue) and oxazine (oxazine 725 and nile blue) dyes can be reduced in colloidal Ti0, suspension with ultra-bandgap excitation of the semiconductor.E*22 A similar approach was applied to reduce cresyl violet in TiO, suspension (reactions 9 and 10).
+ hv>E,
TiO,
Ti02(e-)
TiO, (h+.-e-)
CV+
TiO,
+ CV'
(9)
(10) The transient absorption spectrum of CV' in acetonitrile solution containing colloidal Ti0, is shown in Figure 9. The dye has negligible absorbance at the excitation wavelength (337 nm). Hence, the dye reduction was initiated by the bandbap excitation (22) Kamat, P. V. J . Phofochem. 1985, 28, 513.
2CV'
- cv- + cv+
(11)
is stable in an inert atmosphere but quickly recovers to the parent dye when exposed to an oxidative atmosphere such as 0,.The storage capability in an inert atmosphere and excellent electroactivity of CV- make the CV+-TiO, system suitable for storage-type photoelectrochemical cells. The salient features of such a method of light energy conversion have been described earlier.8 Conclusion
Transient absorption studies have enabled us to characterize excited and redox states of cresyl violet dye. Elucidation of mechanistic and kinetic details of these studies is essential in understanding the photosensitization behavior of the dye. The high fluorescence yield and long-lived redox states of cresyl violet indicates its usefulness as a sensitizer in developing systems for photoelectrochemical conversion of light energy.
Acknowledgment. The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Contribution No. NDRL-3313 from the Notre Dame Radiation Laboratory.
