J. Phys. Chem. 1990, 94, 3099-3 104
3099
Short and Excitation-Independent Fluorescence Lifetimes of J-Aggregates Adsorbed on AgBr and SiOz Klaus Kemnitz,**t.lKeitaro Yoshihara,*,' and Tadaaki Tani*y* Institute for Molecular Science, Myodaiji, Okazaki 444, Japan, and Ashigara Research Laboratories, Fuji Photo Film Co. Ltd., Minami-Ashigara, Kanagawa 250-01, Japan (Received: July 13, 1989; In Final Form: November 7, 1989)
Excitation intensity independent fluorescence lifetimes of cyanine dye J-aggregates adsorbed on octahedral microcrystals of AgBr and on silica gel have been observed to be as short as 5 and 25 ps, respectively. The fluorescence lifetime of the aggregate/Si02 systems at 4 K is about 120 ps and attributed to the pure radiative lifetimes of the J-aggregate in the adsorbed state; thus the very short fluorescence lifetimes at ambient temperatures seem to be dominated by a fast nonradiative decay channel. The fluorescence lifetimes of the aggregate/microcrystallineAgBr systems are dependent on the size of the J-aggregate, which itself was determined from spectroscopic and kinetic data. The necessity for the development of an inert reference system without electron transfer is discussed for elucidation of the rate of electron transfer of J-aggregates adsorbed on AgBr microcrystals.
1. Introduction Fluorescence decay measurements of large dye aggregates (J-aggregates) in solution have to cope with fast excitation-dependent contributions arising from singletsinglet annihilation at higher excitation intensitie~.I-~ Measurements which were performed with high intensity laser systems generally yielded J-aggregate lifetimes of less than 30 psS-* and have, therefore, recently been suspected of being distorted by high-power effects?JO Single photon counting measurements at low excitation intensities, on the other hand, yielded relatively long fluorescence lifetimes around 100 ps.l-" These measurements, however, were performed with systems of relatively slow instrumental response. We observed fast, excitation-independent J-aggregate decays of 5-30 ps for various dyes adsorbed on octahedral and cubic microcrystals of AgBr and on SO2and for samples prepared at different adsorption temperatures.12 It seems, therefore, that the short fluorescence lifetimes of J-aggregates described in this work represent a general phenomenon. This report describes the results for a 9-ethylthiacarbocyanine dye (Figure 1) adsorbed on octahedral AgBr and on silica gel, embedded in photographic gelatin matrix. The temperature dependences of the fluorescence decays from 295 to 4 K for both systems have been determined and are discussed. We investigated the silica gel system in order to gain a clearer view of the radiative and nonradiative behavior of J-aggregates in the adsorbed state. Since electron transfer in this system is absent, it can be considered as a potential reference system for the electron-transfer system of silver bromide. A comparison of both systems may yield valuable information concerning the mechanism of electron transfer in photographic emulsions.
11. Experimental Section I . Time-Correlated Single Photon Counting System. The photon counting system used and method of data analysis have been described in detail elsewhere." The system used in this work employed a CW mode-locked YAG laser (Coherent Antares 76-s) to synchronously pump a dye laser. It had a system response of 35-50 ps fwhm at 570 nm and a time resolution of 0.64 ps/channel and is able to resolve the 2.3-ps decay of crystal violet in n-hexane s o l ~ t i o n . ' ~This control measurement was performed under identical conditions before acquisition of the sample fluorescence decays. Excitation wavelength was 620 nm at the short-wavelength side of the J-peak and the fluorescence was observed at X 1 640 nm, using several cutoff filters in front of the monochromator slit. The monomer fluorescence lifetime was obtained by excitation
'Institute for Molecular Science. * Ashigara Research Laboratories.
IPresent address: ERATO Micro-Photoconversion Project, JRDC, 1280
Kami-izumi, Sodegaura, Kimitsu, Chiba 299-02, Japan.
at 575 nm. Fluorescence decay and system response function were acquired at identical focusing and collecting optics, using neutral-density filters (ND) in front of the monochromator slit, to equalize fluorescence and scattered light intensity. The system was run at 4 MHz and the excitation beam was slightly focused to 1 mm2. Average power at the sample position was 20 mW, a N D disk was used for reducing the intensity. The decays of all investigated J-aggregate systems could be analyzed by two or three exponentials, where the first component in most cases had a contribution of larger than 90%, when the fluorescence was observed at the emission maximum of the J-aggregate. Second and third components could be attributed to the fluorescence of the photographic emulsion. The low-temperature measurements were performed by using an Oxford cryostat (CF 204). 2. Sample Preparation. The dyes used were synthesized according to the method of Fisher and Hamer.I5 Photographic emulsions containing monodispersed silver bromide microcrystals were prepared by the controlled double jet method,I6 using aqueous gelatine solutions in which the silver ion concentration was kept constant during sample preparation. A methanolic dye solution was added to the emulsion, which was then coated on a triacetate cellulose film base and dried, yielding a film containing 17.7 g of AgBr/m2. The edge lengths of cubic and octahedral silver bromide microcrystals were 0.75 f 0.05 and 0.83 f 0.05 pm, respectively. The silica gel powder used was Syloid 150F (Fuji-Davison Chemical) and had a specific surface area of 3 IO m2/g. The triacetate cellulose film was coated by an aqueous
( I ) Brumbaugh, D. V.; Muenter, A. A,; Knox, W.; Mourou, G.; Wittmershaus, B. J. Lumin. 1984, 31-32, 783. (2) Sundstrom, V.; Gillbro, T.; Gadonas, R. A,; Piskarskas, A. J . Cfiem. Pfiys. 1988, 89, 2754. (3) Stiel, H.; Teuchner, K. Teubner-Texte Pfiys. 1986, 10, 217. (4) Stiel, H.; Daehne, S.; Teuchner, K. J. Lumin. 1988, 39, 351. (5) Yu, Z. X.;Lu, P. Y.; Alfano, R. R. Cfiem. Phys. 1983, 79, 289. (6) Comes, A. S. L.; Taylor, J. R. J . Pfiotocfiem. 1986, 32, 325. (7) Tanaka, M.; Nakazawa, N.; Tanaka, I.; Yamashita, H. Cfiem. Phys. 1985, 97, 457. (8) Rentsch, S. K.; Danielius, R. V.; Gadonas, R. A.; Piskarskas, A. Chem. Pfiys. Left. 1981, 84, 446. (9) Brumbaugh, D. V.; Burberry, M. S. Presented at The International East-West Symposium 11 on the Factors Influencing Photographic Sensitivity, Kona, Hawaii, 1988. (IO) Takahashi, K.; Obi, K.; Tanaka, I.; Tani, T. Cfiem. Pfiys. Lett. 1989, 154, 223. ( 1 1 ) Muenter, A. A. J. Phys. Cfiem. 1976, 80, 2178. (12) Kemnitz, K.; Yoshihara, K.; Tani, T., to be published. (13) Kemnitz, K.; Nakashima, N.; Yoshihara, K. J. Phys. Cfiem. 1988, 92, 3915. (14) Doust, T. Cfiem. Pfiys. Lett. 1983, 96, 522. (15) Fisher, N. 1.; Hamer, F. M. Prof. R. SOC.(London)1936, A145,703. (16) Berry, C. R.: Skillman, D. C. Pfiotogr. Sci. Eng. 1962, 6, 159.
0022-3654/90/2094-3099$02.50/00 1990 American Chemical Society
3100 The Journal of Physical Chemistry, Vol. 94, No. 7 , 1990
Kemnitz et al.
TABLE I: Absorption and Fluorescence Maxima, Bandwidth, Size of J-Aggregate According to Eqs 3, 4, and 5, and Fluorescence Lifetimes of Aeereeate and Monomer in Heteroeeneous and Homoeeneous Systems
M M
M M M M J
MeOH (s) (298 K) MeOH (s) (70 K ) quartz (298 K ) Si0: (298 K ) SO,' (4 K ) AgBr H#
552 560
J
AgBr (70 "C)
J
AgBr (40 "C)
643
SiO:
1
1 1 1
1
I
1
1
I 16
I
I
578 625 620 65 1
J
1
I324
70b 1400-3500"
260-25OO"J 1600-2800" 2400-5000"
653
328 340 286
21
5d
644
586
5
24c
15
230b 26'
538'
"Two-exponentialanalysis. bSingle-exponentialdecay. CSee Figure 3b. dSee Figures 3a and 6. eSee Figure 6. /Equation 5 . BEquation 4. hEquation 3. 'See section IV.2a. /Coverage less than 1/10 monolayer. kAdsorbed on silica gel and embedded in photographic gelatin matrix. 's = solution, M = monomer, J = J-aggregate, L = linear, C = cyclic.
R, = C,H,SO, R', C,H,SO,H
-
R,
-
CI
Figure I . Molecular structure of 5,5'-dichloro-3,3'-disulfopropyl-9ethylthiacarbocyanine.
solution containing silica gel, gelatine, and the dye, and dried, yielding a film containing 1.1 g of silica gel/m2 and 13.8 g of gelatin/m2. Samples of octahedral AgBr microcrystals were prepared at four different temperatures of dye adsorption, TadS = 40, 50,60, and 70 OC. The surface coverage in the J-aggregate/AgBr systems was 0.17. Submonolayers of dye monomer adsorbed on a quartz plate were produced by adsorption from aqueous solution. 3. Measurement of the Relative Sensitization Efficiency. The determination of the sensitization efficiency arof J-aggregates adsorbed on octahedral microcrystals of AgBr was accomplished by applying the standard methodis and performed for Tad$= 40, 50, 60,and 70 O C (see Figure 6b). The above-mentioned sensitization efficiency is the efficiency of the J-aggregate to form a latent image at a certain wavelength, relative to the efficiency of AgBr at direct bandgap excitation of 400
400
3s17
111. Results 1 . Spectroscopy. Figure 2 shows the reflection spectra of the J-aggregates adsorbed on silver bromide. The spectra of the systems obtained at increasing adsorption temperature display increasing red shift of the maximum with simultaneous decrease of the bandwidth. Adsorption and fluorescence maxima and line widths of the adsorbed J-aggregate in the AgBr and Si02systems are listed in Table I , together with the corresponding data of the monomeric adsorption systems. Data of J-aggregate and monomer in aqueous solution are also given. The bandwidth at half-maximum of the J-aggregate systems ranged from 280 to 600 cm-'. The bandwidth was determined from reflection spectra (Figure 2) by using the Kubelka-Munk equationi9and assuming constancy of the scatter coefficient S over the narrow wavelength region of the J-band. The maxima of J-band and, less pronounced, of (17) Kemnitz, K.: Tamai, N.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. J . Pbys. Chem. 1986, 90, 5094.
( 1 8 ) (a) Spence, J.; Carroll, B. H. J . Pbys. Colloid Chem. 1948, 52, 1090. (b) Tani. T.: Urabe, H. J . SOC.Pborogr. Tech. Jpn. 1978, 41, 3 2 5 . (19)ct/S = ( I - R,)2/2R,, with R . the reflectance at infinite depth, S scatter coefficient, c dye concentration, and t molar extinction coefficient.
500
600
700
Wavelength (nm) Figure 2. Reflection spectra of the J-aggregates adsorbed on octahedral
microcrystals of AgBr, produced at various adsorption temperatures, Tab.
adsorbed monomer in the AgBr systems are red-shifted by about 25 nm with respect to the corresponding Si02systems (Table I). The red shift of both monomer and J-aggregate is attributed to the environment, thus, the longer wavelength adsorption peak of the J-aggregate adsorbed on AgBr is not necessarily an indication for larger aggregate size (see eq 4). 2. Kinetics. The fluorescence decays of J-aggregates of AgBr and Si02 systems at 295 K are shown in Figure 3. The fluorescence decay of the AgBr system in Figure 3a was observed at 650 nm and the three-exponential analysis yields 92% contribution of a 5-ps component. This decay has been acquired at maximal resolution of 0.64 ps/channel and shows nearly perfect fitting statistics and residuals.20 The corresponding decay of the J-aggregate adsorbed on silica gel (Figure 3b) is about 5 times slower, in comparison. The fluorescence decays observed in this work were independent (F10%) of excitation intensity over an intensity range of 400. In agreement with this observation was the fact that the intensity of the collected fluorescence was directly proportional to the excitation intensity.42 The lowest employed intensity corresponded to 3 X IO9 photons pulse-'. Annihilation between excited singlet states, as revealed by excitation intensity dependent fluorescence lifetimes and nonlinear behavior of fluorescence intensity, has been observed, so far, only for large J-aggregates ~
~~
~
(20) Van Den Zegel, M : Boens, N , DeSchryver, F C Chem Phys 1986, 101, 31 1
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 3101
Fluorescence Lifetimes of J-Aggregates i L
4t
'
128
256
3
g
a)
11
AgBr
A4K
2
-0
0)
1
1 -1
128
0
256
I
4th
SiO,
I I\\
4 -4
t TK
2 C
2 c 3
3
a
8
,
0)
-0
0
8
b)
1
~~
298
2
2 1 I
0
128
256
384
512
640
Time/ps Figure 3. Fluorescence decays of the J-aggregate adsorbed on AgBr (a)
and S O 2(b). Fit according to I ( t ) = Ed,exp(-fir,); T = 298 K, AgBr: TI = 5.2 PS ( A I = 91.6%), T2 = 19.3 PS (A2 8.2%). 7 3 = 338 PS (A3 = 0.2%). Statistics: CHISQ = 1.01, ZCHISQ = 0.22, ZRUN = -1.69, DW = 1.93. SO,: T~ = 26.3 ps ( A , = 87.9%), 72 = 125 ps (A2 = 12.0%),73 = 2000ps (A3= O,l%). Statistics: CHISQ = 1.12, ZCHISQ = 2.46, ZRUN = -3.64, DW = 1.56; 0.64 ps/channel. The statistical parameters CHISQ, ZCHISQ, ZRUN, DW, from above, as well as the residuals (res) and the autocorrelation (corr) in the figure are defined in ref 20.
1280
1920
Timelps Figure 4. Temperature dependence of fluorescence decays of the J-ag-
gregate in the AgBr (a) and Si02 (b) systems, Tab = 70 OC, 2.56 ps/ channel.
in solution,14 e+, for aggregates with a domain size of N = (20-50) X lo3 (ref 2). Annihilation in these systems, therefore, could be observed down to very low intensities ( 1Olo photons pulse-I). In the case of adsorbed J-aggregates, the domain size is much smaller, N < 50 (section IV.2.), and annihilation was absent in our systems up to the highest intensity employed of 10I2 photons cm-2 pulse-'. The temperature-dependent fluorescence decays of the J-aggregate in the AgBr and SiOzsystems in the range 295 to 4 K are shown in Figure 4. The temperature dependence is rather pronounced for the SiO, system and displays independence from 295 to 100 K, followed by a steep rise of lifetimes and renewed constancy below 25 K. This behavior is described in detail in Figure 5, which displays the analyses of the temperature dependences of Figure 4. The analysis of the temperature behavior of the SiOz system assumes that the radiative rate constant k, is independent of temperature.21 The temperature dependence of the observed fluorescence rate constant, kob, is then given by the sum of radiative rate constant, k,, and nonradiative rate constant (21) Preliminary studies of temperature-dependentquantum yield in combination with fluorescence decay measurements in a related dye/SiO, system indicate, however, that k, increases at lower temperatures (Kemnitz, K., et al., to be published), in agreement with predictions from theory.2"
300
200
100
Temperature /K Figure 5. Analyses of temperature dependences of the AgBr and SiO, systems. The lifetimes of 18 ps from 300 to 100 K of the AgBr system were obtained by nonperfect fits, due to the slightly distorted data of this particular experiment. Repeated measurements at room temperature show that the real lifetime in this temperature range is about 5 ps, as seen in Figure 2a. Analysis of the Si02 system with solid line: kob = k, + k6, + konrexp(-hE*/RT), where k, + k6, = 8.5 X lo9 s-l, konr= 5 X 1Olo s-l, and Al? = 0.01 eV.
k,, where the latter is comprised of temperature-independent and -dependent parts, k,, = k',,, + k',,,(n: kobs = k, + k',,, + k',,,(T) = k, k',, + konrexp(-AE*/kT)
+
(1)
The solid line for SiOz in Figure 5 represents the best fit with k, k'", = 8.5 X lo9 s-I (1/118 ps), k,,: = 5 X 1Olo s-I, and the
+
3102
The Journal of Physical Chemistry, Vol. 94, No. 7 , 1990
.
erization in the excited state around the central double bond. There is evidence that the rate of isomerization increases with decreasing length of the central polymethine chain.23 In the following we are giving a few examples, which are related to the present work. The radiative lifetime of thiadicarbocyanine monomer, T , ( M ) , is 4.2 ns in MeOH at 298 K (4.0 ns in this work, section 111.2) with T,,,(M) = 2.2 ns.24 It is assumed that isomerization in this molecule is slow. A structurally very similar dye to the one in this work is 3,3'-diethylthiacarbocyanine iodide, with monomer and dimer fluorescence lifetimes T ~ ( H * O=) 145 ps (70 ps in this The longer work, Table I ) and r,(H20) = 460 ps, re~pectively.~~ fluorescence lifetime of the dimers is attributed to slower isomerization of the monomer, due to increased rigidity of the monomer in the dimer. The very fast isomerization of the pseudoisocyanine (PIC, l,l'-diethyl-2,2'-cyanine) monomer, qso= 15 ps in water,8*26 is attributed to steric enhancement, due to a twisted ground-state conformation. 2. Aggregate Size. The size of a given J-aggregate can independently be deduced from two spectral properties, band shift and bandwidth. The size N obtained by these spectroscopic parameters is compared with N from kinetic data according to eq 3. We would like to emphasize that no method exists so far to precisely determine the size N of a given J-aggregate and that the values of N determined below are only approximate. We can confidently say, however, that the size of the J-aggregates in the adsorption systems of this work is well below 50 and is thus much smaller than that of corresponding systems in homogeneous solution. (a) For isolated small aggregates the following equation holds27-29
25 -
v)
n
0
20 -
c Q
15 -
E .-c
-1 0)
C 0
8
10 -
VI
g! 0 3
5-
ii 50
40
60
70
Adsorption TemperaturePC b,
0.1
r;es, CB
Kemnitz et al.
-
0.2 0.3 0.4 0.5 0.6
Efficiency of Sensitization,@, Figure 6. Dependence of J-aggregate fluorescence lifetime on adsorption temperature, Tadlrfor octahedral AgBr microcrystals (a) and plot of k , and (k, knr) vs 9, (b), using eq 2 and experimental values of ar and kob, where kob = k , + k , knr.
+
+
small activation energy A ?!, = 0.01 eV, which is discussed in section IV.3.f. The solid line for AgBr is not based upon calculation and is merely a qualitative description of the temperature dependence. Note, however, the qualitatively similar behavior. The influence of adsorption temperature, T,&, on the observed fluorescence lifetime of J-aggregates adsorbed on octahedral AgBr microcrystals is shown in Figure 6a. The observed fluorescence lifetime decreases with increasing adsorption temperature, i.e., with increasing size of the formed aggregate (Table I). Figure 6b shows a plot of the calculated values of k, and ( k , + knr)vs the experimental values of the sensitization efficiency a,, using @r
= ke/(ke
+ kr +
4 1 )
(2)
where k, is the rate constant of electron transfer. In addition to the above studies of the J-aggregate systems in the adsorbed state, J-aggregate and monomer fluorescence lifetimes in aqueous solution were also determined. They are listed in Table I together with the lifetimes at 295 and 4 K of the monomer adsorbed on Si02. The fluorescence decay of the latter system was analyzed in terms of two exponentials and the longer lifetime of the system at 4 K is taken to represent the radiative lifetime of the adsorbed monomer, k , = 1 /(4 ns) (Table I). The nonexponential decay is attributed to molecules adsorbed at different sites and will be discussed in detail elsewhere.22
IV. Discussion I . Kinetic and Spectroscopic Properties of Cyanine Dye Monomers. This paragraph briefly reviews properties of various monomer and dimer systems of cyanine dyes, which form J-aggregates. The monomer fluorescence lifetime at 300 K of cyanine dyes can range from 1.5 ns to 5 ps, depending on dye and environmental condition. The short lifetimes are attributed to isom(22) Kemnitz, K.; Yoshihara, K., to be submitted for publication.
k,(J) = Nk,(M)
COS*
a
(3)
with a being the angle between transition moment of monomer and the long axis of the aggregate, k,(M) the radiative rate constant of the monomer, and k,(J) the radiative rate constant of the J-aggregate. This kinetic relation can be used to estimate the size N of a J-aggregate: N = k , ( J ) / [ k , ( M )cos2 a ] . Using a = 20" (ref 30), k,(M) = 4.0 ns (section 111.2), and k,(J) I 1/(118) ps (Figure 5 and section 111.2) yields N 5 38 for the Si02 system. The value of a in the above calculation was taken from J-aggregate crystal data of a related dye.30 The radiative rate constant of the J-aggregate, k , ( J ) , was obtained as upper limit from the temperature-independent fluorescence lifetime at 25-4 K, under the assumption that contributions by k ; can be neglected below 25 K (I?',,,313-nm light irradiation; the ketone is quantitatively photoreduced to 2-butanol without much H2, and S2-and S 0 3 2 -ions are photooxidized to S2032-ion. The photoreaction is successfully applied to other acyclic and cyclic ketones, and the similar quantitative ions and quantum size photoreactions to the corresponding alcohols are confirmed. The synergistic effect of S2-and effect on the photocatalytic system are discussed in terms of spectral properties, and the selective and highly efficient photoredox reactions of this system are attributed to suppression of the formation of the localized surface states, which may cause competitive water photoreduction to H2 or deactivation of the catalysts.
Recent studies on semiconductor photocatalysis have revealed that semiconductor particulate systems induce efficient photo0022-3654/90/2094-3 104$02.50/0
reactions of organic substrates and have been focused on their synthetic applications, because photoreductions as well as pho-
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