J . Phys. Chem. 1991, 95, 9310-9314
9310
play an important role in these processes as one of the film-forming molecules.
Conclusion In this work it has been shown that silylene (SiH2) plays an important role in the 1R laser-induced CVD of a-Si:H. It reacts nearly gas kinetically with silane or disilane, forming higher silanes. Because of the high reaction rates it will in most cases not reach the surface to form a layer. The measured behavior of the deposition rates of the studied laser CVD processes can easily be explained by a reaction mechanism involving HzSiSiHz as a film-forming molecule. With this kinetic model it is possible to describe quantitatively all our results from LIF and deposition
experiments. Since many groups suggest that SiH2 is a primary decomposition product in most a-Si:H deposition processes like, e.g., plasma CVD and HOMO CVD, the results of this work have a certain relevance also for these methods. This is even true for the Hg-sensibilized deposition starting with SiH, molecules.
Acknowledgment. This work was funded by the German "Bundesminister for Forschung und Technologie" (BMFT). We thank Miss F. Bolecke for extensive technical help during performance of the experiments. Finally, we thank Mr. Pi0 Gondlez, Universidade de Vigo, Spain, for helpful discussions. Registry No. Si, 7440-21-3; H1, 1333-74-0; SiH2, 13825-90-6; SiH,, 7803-62-5; Si2H6,1590-87-0.
Ultrafast Conformational Relaxation of Triphenylmethane Dyes: Spectral Characterlzatlon M. M. Martin,* P. Plaza, and Y. H. Meyer Laboratoire de Photophysique MolZculaire du CNRS, Brit. 21 3, Universite Paris-Sud. 91 405 Orsay, France (Received: February I , 1991; In Final Form: May 23, 1991)
Absorption cross sections of the fast transient state formed during the excited-statedeactivation process of the triphenylmethane dye ethyl violet are extracted from picosecond time resolved pumpprobe experiments in the 350-750-nm spectral range. Absorption and stimulated emission cross sections of the dye first excited state are also obtained from these experiments and from stationary fluorescence measurements. The transient state absorption spectrum is found to be similar to that of SI but spectrally shifted to higher energies. The transient state is described as an excited-state conformer.
Introduction Solvent viscosity is well-known as the determining factor in the nonradiative deactivation rate of the first excited singlet state of triphenylmethane dyes' but a unified relaxation mechanism has not yet been found for these compounds. Particularly, divergences remain as to the existence and the nature of a highly unstable transient state involved in the excited dye relaxation pathway.'-17 Detailed studies were devoted to experimental and theoretical analysis of the excited-state decay kinetic^'-^*^^"^^ and particular attention was given to the description of the SIdeactivation process (1) (2) (3) 611. (4)
Fiirster, T.; Hoffman, G. Z . Phys. Chem. (Munich) 1971, 75, 63. Magde. D.;Windsor, M. W. Chem. Phys. Len. 1974,24, 144. Ippen, E. P.; Shank, C. V.; Bergman, A. Chem. Phys. Lerf. 1976,38,
Cremers, D. A.; Windsor, M. W. Chem. Phys. krr. 1980,71, 27. (5) SundstrBm, V.; Gillbro, T.; Bergstrijm, H. Chem. Phys. 1982,73,439. (6) Bagchi, B.; Fleming, G. R.; W. Oxtoby, D. J . Chem. Phys. 1983,78,
7375. (7) Sundstriim, V.; Gillbro. T. J . Chem. Phys. 1984, 81, 3463. (8) Menzel. R.; Hoganson, C. W.; Windsor, M. W. Chem. Phys. Lerr. 1985,120, 29. (9) Ben-Amotz, D.;Harris, C. B. Chem. Phys. k r r . 1985, 119, 305. (IO) Ben-Amotz, D.;Harris, C. B. J . Chem. Phys. 1987,86,4856, 5433. ( I I ) Mokhtari, A.; Fini, L.; Chesnoy, J. J . Chem. Phys. 1987,87, 3429. (I 2) Vogel, M.; Rettig, W. Ber. Bunsen-Ges. Phys. Chem. 1985,89,962. (I 3) Voge. M.; Rettig, W. Ber. Bunsen-Ges.Phys. Chem. 1987, 91, 1241. (14) Robl, T.; Seilmeier, A. Chem. Phys. Leu. 1988, 147, 544. (IS) Martin, M. M.; Nesa, F.; Br€h€ret, E.; Meyer, Y. H. In Ulrrafusr Phenomeno rk Yajima, T., Ycshihara. K., Harris, C. B., Shionoya, S.,Eds.; Springer Series in Chemical Physics Vol. 48; Springer-Verlag: New York,
__.
19RR: _ _ nr 4 7 1 (16) Martin, M. M.; Brlhlret, E.; Nesa, F.; Meyer, Y . H. Chem. Phys. 1989, 130, 279. (17) Mokhtari, A.; Chebira, A.; Chesnoy, J. J . Opr. Soc. Am. B 1990, 7,
1551.
0022-3654/91/2095-93 10$02.50/0
as a diffusive internal molecular motion on a barrierless potential in order to explain the nonexponential character of the SIdecay. On the other hand, spectroscopic studies are still very fe~.4**J*.'~ The first excited state and transient state(s) have never been characterized by their absorption spectrum. Generally speaking, very few data on excited-state light interaction cross sections are available a t present in the literature for laser dyes and saturable dyes. In the present paper we report such determination for the triphenylmethane dye ethyl violet in ethanol in the 350-750-nm range from picosecond time-resolved pumpprobe experiments. The cross section determination is based on a simple kinetic model which describes well our observations is low-viscosity solvents ( 9 I 1 cP)16 where under our experimental conditions the SI decay of ethyl violet can be reasonably approximated to an exponential decay. The results support a relaxation mechanism involving a spectroscopically well-defined transient state.
Experimental Section 1. Excitation Laser Source. The subpicosecond dye laser system in these experiments does not use mode locked cavities. The short pulses are generated by original methods previously described in detail in refs 20-22 and reviewed in ref 23. High-power, 600-610 (18) Martin, M. M.; Plaza, P.; Hung, N. D.; Meyer, Y. H. UIrrafosr Phenomeno VII;Harris, C. B., Ippen, E. P., Mourou, G.A., Zewail. A. H., Eds.; Springer Series in Chemical Physics, Vol. 53; Springer-Verlag: New .~ York, 1990; p 504. (19) Martin, M. M.; Plaza, P.; Meyer, Y. H. Chem. Phys. 1991,153,297. ( 2 0 ) Martin, M. M.; BrChCret, E.; Meyer, Y. H. Opr. Commun. 1985.56,
-..
61
(21) Meyer, Y. H.; Martin, M. M.; Brlhlret, E.; Benoist d'Azy, 0. UIrrafusr Phenomenu V; Fleming, G.R., Siegman, A. E., Eds.; Springer Series in Chemical Physics, Vol. 46; Springer-Verlag: New York. 1986; p 89. (22) N a a , F.; Martin, M. M.; Meyer, Y. H. Opr. Commun. 1990,75,294.
0 1991 American Chemical Society
Conformational Relaxation of TPM Dyes nm, 0.6-ps pulses are produced from a single, standard, frequency-doubled IO-Hz, 8 4 s Nd:YAG laser (Quantel) in a twostage dye laser system. In the first stage, 100-ps pulses are produced with a spectro-temporal selection Rhodamine 6G dye laser.20,21In the second stage, the 100-ps pulses are used to pump a Rhodamine 640 microcavity and pulse shortening down to less than 1 ps is obtained by nonlinear extracavity propagation of the microcavity output in amplifying and absorbing saturable dye solutions.22 Amplification up to more than 400 pJ is achieved in dye amplifiers pumped with 100 mJ of the YAG laser. 2. PumpProbe Experiments and Fluorescence Measu-b. Time-resolved absorption spectra were measured by the p u m p probe technique using the subpicosecond laser described above as the excitation source at 603 nm and a supercontinuum of white light as the probe. The supercontinuum is generated by focusing, with a beam splitter and a 10 cm focal length lens, 80% of the input laser energy into a 5 mm flowing water cell. The remaining 20% is attenuated and sent through a variable delay line on to the sample. The supercontinuum beam is spatially and spectrally filtered in order to eliminate the remaining laser light in the water cell output and flatten the supercontinuum spectrum in the probing wavelength range. The continuum beam is split into two beams, respectively sent on to the sample and on to a reference cell containing either the solvent or the unexcited sample. Sample and reference cell paths are 1 mm long. Pump and probe beams have a diameter of less than I .5 mm on the sample and cross at an angle of 1 So. The probe beam spectra are simultaneously analyzed through a polychromator (Jarrell-Ash, entrance slit 200 pm) by a computer-controlled double diode array detector (Princeton Instrument Inc.). A 270-nm spectral range is analyzed in a single experiment. The pumpprobe delay time is adjusted by means of a stepper motor translation (Micro-Controle). A water solution of NdC13 is used for wavelength calibration and a solution of the dye HIDCI in ethanol is used for zero delay time calibration as described in ref 19. The reported experiments were performed at room temperature. Both pump and probe beams had linear polarization, the directions of which were set at the magic angle ( - 5 5 ' ) . The sample excitation energy, measured with a joulemeter (RK 3232 Laser Precision), was 9 pJ. Data were accumulated over 500 laser shots without pulse intensity discrimination. The fluorescence spectra were recorded with a Hitachi-Perkin-Elmer MPF3L fluorimeter equipped with a R928 Hamamatsu photomultiplier. The samples were contained in a 1 X 1 cm cell. The excitation and emission spectral slits were fixed at 6 nm. Samples containing only the solvent were tested under the same experimental conditions. 3. Compounds. The chloride salt of ethyl violet was purchased from Aldrich and used without further purification. Spectroscopy grade ethanol (Merck) was used as the solvent. In pumpprobe experiments the dye concentration was 0.82. X lo-" mol/L. Solutions were diluted down to mol/L for fluorescence measurements.
-
Experimental Results 1. Time-Resoh.edphotoinduced Absorbance. The time-resolved change in optical density spectra (AD) that we measured for an ethyl violet solution in ethanol at the 350-750-nm wavelength range is given in Figure 1A, in the range of 1-1 3 ps pump-probe delays. For comparison, the unexcited sample absorption spectrum is also given in Figure 1 B. The decay of the photoinduced absorbance is observed to be strongly wavelength dependent. Apart from the bleaching observed in the ground-state absorption wavelength range, net transient absorption and net gain are respectively observed below 500 nm and above 650 nm. Transient absorption, bleaching, and/or gain decay kinetics of several triphenylmethane dyes solutions were the object of a number of studies using probe beams a t fixed wavelength under different (23) Hung, N. D.;Meyer, Y. H.; Martin, M. M.; Nesa, F. Ulrrufusr Phenomep in s~crroscopy;Klose, E., Wilhemi. B., Eds.;Springer Proceedings in Physics 49; Springer-Verlag: New York, 1990, p 33.
The Journal of Physical Chemistry, Vol. 95, No. 23, 1991 9311
. .
0.04
-0.04
1 L
0.8
Dl 0.4
II
0
, 350
III
I
I
I
, I I / , I I /
.
450
U I I I I I
550
650
750
Wavelength (nm) Figure 1. (A) Time-resolved photoinduced change in optical density
(AD) of an ethyl violet solution in ethanol (0.82 X lo4 mol/L) after subpicosecond laser excitation at 603 nm. Pumpprobe delays: curve a, 1 ps; b, 3 ps; c, 9 ps; d, 13 ps. (B) Optical density of the unexcited sample (Do)and fluorescence spectrum measured with a spectrofluorimeter, drawn in arbitrary units (FI). 0.04
AD
0
-0.04
-0.12 -0.08
lo/
*
450m
Q
645m
A
625 nm
o
550nm 595m
-0.16 0
5
10
15 20 25 t (picoseconds)
30
35
Figure 2. Typical absorbance kinetics (AD(r)) observed at selected wavelengths for ethyl violet in ethanol and fitted from eqs 3 and 4 using the kinetic scheme shown in Figure 3. The time zero was chosen at the end of the pumping pulse according to the boundary conditions of eq 4 (see text). The fit with the set of rate constants given in Figure 3 leads to the cross sections spectra given in Figures 5 and 6.
pumpprobe polarization conditions. We previously reported the main features of the time-resolved photoinduced absorbance of such solutions on a large wavelength scale when using arallel polarization directions for probe and pump b e a m ~ . ' ~ J ~These ~~*J features are described here for pump and probe beam polarization directions set at the magic angle, which is equivalent to probing an isotropic distribution of the molecule orientations. At short delay times, from 1 to 3 ps (Figure 1, curves a and b), a decrease in the bleaching and/or gain signal (AD < 0) above 600 nm occurs simultaneously with an increase of the transient absorption (AD > 0) in its short wavelength edge around 410 nm. In the same range of delay time, the bleaching observed between 510 and 600 nm remains constant or increases slightly. The initial red gain and blue transient absorption (A? = 1 ps) are attributed to the SIexcited state directly populated by laser excitation? The apparent 'delayed" bleaching in the 5 10-600-nm range (the bleaching keeps on increasing after pumping, A? = 1-3 ps) is attributed to the fast decay of the SIstate toward an intermediate state which absorbs less than SI in this wavelength range,15J6J8J9 At long probe delay, 13 ps (Figure 1, curve d) the gain signal around 650 nm attributed to the SIstate emission has vanished and a slight, but reproducible, red absorption band remains together with the blue absorption. On the other hand, the blue absorption measured at long probe delay has a different spectral distribution from that observed at the end of the exciting pulse. The transient red absorption observed at long probe delays has
B
9312 The Journal of Physical Chemistry, Vol. 95, No. 23, 1991
Martin et al.
Do(X) = O.43ua(X)NL
+
Therefore, with N o ( t ) N , ( t ) + N,(t) = N , the measured change in optical density due to pumping, AD(X,t) = D(X,r) Do(X),is AD(X,t) = O * ~ ~ ( [ U I ( XU~(A)INI(~) ) + [.AX) - ua(X)INx(t)IL (3) We previously showedt6that in our experimental conditions, for ethyl violet in low viscosity solvents ( q I1 cP), the population dynamics can be described by Figure 3. Three-electronic-state molecular model involved to explain the fast deactivation of ethyl violet in ethanol. The transient state S, is formed from S,in k-’ = 3.6 ps and decays nonradiatively in about k;’ = 7 ps. The S, radiative lifetime is taken as k,-I = 4 ns.
already been noted by several author^.^-^^'^*^^ The change in shape of the blue absorption bands in solvents of different viscosity was reported elsewhere.I8J9 Typical kinetics at selected wavelengths are given in Figure 2. We previously interpreted these observations by describing the ethyl violet SIstate relaxation mechanism as a process involving the formation of a transient state to which we attributed the blue and red absorption bands observed at long delay times.I6J9 We can thus explain the absorbance decays measured at any wavelength by taking into account the absorption and/or gain spectra of the ground and excited states So and SI as well as that of the intermediate state S, involved in the excitation pathway. The light interaction c r m sections of these states for an isotropic orientation distribution of the molecules can be extracted from the present time-resolved measurements. 2. Fluorescence Spectrum. The fluorescence spectrum measured when exciting the solution at 550 nm is drawn (in arbitrary units) in Figure 1 (Fl) for comparison with the absorption spectrum and with the transient gain signal. The spectrum extends in the 570-8Wnm spectral range with a maximum around 630 nm. The shape is independent of the excitation wavelength. When the photomultiplier spectral response is taken into account the fluorescence intensity on the red side of the spectrum slightly increases with respect to that at the maximum. In previous fluorescence polarization experiments, this fluorescence emission was attributed to a single species: the SI state.I6
Determination of the SI and S, State Absorption and Stimulated Emission Cross Sections To explain the time-resolved absorption spectra observed for ethyl violet, we consider a three-level molecular model (Figure 3), where So and SIare respectively the ground state and the first excited state formed by excitation at 603 nm, and S, is the intermediate state involved in the deactivation pathway. The corresponding populations are No,N , and N, and their respective absorption cross sections are ua, uuI,and uw. We consider the stimulated emission cross sections uel and uu for S, and S,, respectively, and define the resulting light interaction isotropic cross sections u1 and u, as follows: QI
=
‘JUI
- “e1 - ucx
= flu, (1) We assume that the cross sections are time-independent and that the upper excited states relax nonradiatively, to SI or S,, respectively, so rapidly that they do not contribute to the observed transient phenomena. The generalized ‘transmission” T ( T < 1 for net attenuation, and T > 1 for net gain) for the probe beam through the excited sample of length L, at wavlength X and delay time t between the pump and probe, can be written as T(X-1) exPkua(X) Ndr) - OI(X) N I ( ~-)u,(X) Nx(t)}L (2) for small input signal and weak enough gain. The corresponding excited sample optical density (negative for a net gain) is D(X,t) = 0.43(~,(X)N d t ) + ul(X) N1W+ u,(x) N,(t)JL and the unexcited sample optical density is 0,
NOW = N -
( h / W l exp(-Kt)
+
k / ( k , - K ) [ k , exp(-Kt) - K exp(-k,t)ll N , ( t ) = no exp(-Kt) N,(t) = nolk/(k,
(4)
- K)I(exp(-Kt) - exp(-k,t)l
where we assume that N,(O) = 0, N I ( 0 )= no,and No(-) = N . It must be noted that in this set of equations the time t = 0 corresponds to the end of the exciting pulse and thus to the experimental delay time (At) at which the absorption and bleaching rise curves of the reference HIDCI solution reach their plateau value as explained in ref 19. The rate constants k , k l , and k, correspond respectively to the SI S,, SI So, and S, So transitions with K = k + k l . The spectra of uI and u, can be determined from eq 3, if the respective N , ( t ) and N,(t) populations given by eq 4 are known. They can be deduced from the measure of the initial SI excited-state population no and from the set of rate constants k , k l , and k, properly adjusted to lit the experimental transmission decay curves at all wavelengths. 1. Initial SIExcited-State Population. The solution used had a concentration N of 4.94 X 10l6 molecules/cm3 and an optical density Do of 0.77 at the excitation wavelength 603 nm. For the incident 9 J pump pulse energy (that is, -2.73 X IO” photons) the transmitted pump energy was found to be 4 pJ, showing that 1.5 X loi3photons were absorbed in a volume of solution estimated to be 1.2 f 0.4 mm3. The percentage of initially excited molecules no/N along the pumping beam path is thus found to be 0.28 f 0.10. 2. ul and u, Cross Sections. The uI and u, spectra were obtained from the lit of the absorbance decays with the eqs 3 and 4 on a large wavelength range but cross sections a t particular wavelengths can be deduced from the ratio AD/Do(X). At short probe delays, if one assunies that only SI and So are populated, the ratio AD/Do(X) given by eq 5 reduces to eq 6 and thus is proportional to the ratio h / N . For the particular value of h / N = 0.28, the latter equation allows one to know the wavelength at which u1 = 0.
-
- -
AD/Do(XJ) = l(ui/ua) - l l N I ( t ) / N + ((u.x/ua) - IlNx(t)/N (5) AD/D,(X,shortAt) = {(ul/ua) - l } n o / N (6) AD/Do(LlongAt) = l(Ux/ua) -
lJN.x(t~ong)/N
(7)
From the curve AD/Do(X)obtained at the end of the exciting pulse (Figure 4, curve a) one expects that u1= 0 around 630 nm. On the other hand, this curve shows an isosbestic point for the So SI and SI S, transitions (al = a,) around 515 nm where AD/Do = 0. Assuming that only So and S, states remain populated at long probe delays, q 5 reduces to eq 7 and an isosbestic point for the So SI and S, S, transitions (a, = ua)can be foreseen around 625 nm in the same way from curve c in Figure 4. On the other hand, an isosbestic point for the SI S, and S, S, transitions (al = u,) is expected around 595 nm from curves a and b. As a matter of fact, the change in optical density AD(A,t) remains constant in that wavelength range between 1 and 3 ps. These curves correspond to delays short enough so that the total population NI + N , can be assumed to remain nearly constant. The best fit of the experimental results AD(A,t) reported in Figures 1 and 2 was sought for a large number of wavelengths
-
-
-
-
-
Conformational Relaxation of
-
The Journal of Physical Chemistry, Vol. 95, No. 23, 1991 9313
TPM Dyes
0.2 n
I
b
-0.4
\ aI
" " " ' ~ " ~ ~ " " " " " ' ~ " '
500
540 580 Wavelength (nm)
620
Wavelength (nm)
Figure 4. Time-resolved change in the ratio AD/&, given by eq 5. Pumpprobe delays: curve a, 1 ps; b, 3 ps; c, 11 ps.
t
1
1
4 "XGe
x
x
x
~
1
b' ' ' '
'
a ' I ' ' I ' ' ' 550 650 Wavelength (nm)
'' ''I'' ''I' ''' '
350
450
I
'' '
J
750
0
Figure 5. Spectral distribution of the S,and S, state light interaction cross sections cIand u, extracted from pumpprobe experiments. The stimulated emission ue was obtained from the fluorescence spectrum and from eq 8 using 4 ns for the SIstate radiative lifetime. in the 350-750-nm range using eqs 3 and 4. The initial population of the first excited state no/N was taken as 0.28 as given above. The fit of all decays was obtained for the same set of rate constants, k = 2.8 X 10" s-], k, = 2.5 X 108 s-l, and k, = 1.4 X 10" s-I, by adjusting the values of uI and u,. The spectra obtained for uI and u, are given in Figure 5. It must be noted that all spectra ul(A) and .,(A) given by the following equations are also solutions of eqs 3 and 4:
~ I ( A )= ~ I ( A ) ~ o / + ~ ' ua(X)(1 o - no/nb) where nb is any other possible value of NI(0) within the experimental error estimated above. The uncertainty on the value of no affects the u1and u, spectra mostly in the range of large ua. The spectra obtained for ul and u, show that the first excited singlet state SI and the transient state S, of ethyl violet both interact with light from 350 nm to the near-IR region. a, is found to be positive in the whole wavelength range whereas uI < 0 above 630 nm. S, stimulated emission is expected to become dominant in eq 1 since fluorescence is observed above 570 nm (Figure 1B). Since no fluorescence nor gain signal could be attributed to S,, positive values are indeed expected for a, and this fact can be used to give the lower limit of the calculated ratio n,,/N within the ranges of values 0.28 & 0.10 given above. This limit was found to be 0.22. The S, spectrum obtained shows two absorption peaks around 410 and 610 nm. To determine the SI state absorption cross sections uul in the same wavelength range one must know the stimulated emission cross section ueI. 3. Estimation of the SI State Stimulated Emission Cross Sections. We calculated the stimulated emission cross sections uel from the fluorescence spectrum E(X)24using eq 8, where 9 a,l(A)
Figure 6. S , excited-stateabsorption spectrum u,, and S, transient state absorption spectrum a, compared to the ground state absorption spectrum u, in the visible wavelength range.
=
[email protected](A)/8~c7n2
(8)
is the fluorescence yield of ethyl violet, 7 the first-excited-state lifetime and n the solvent refraction index. E(A) is taken so that .fE(X) dX = 1. We tentatively determined the ratio @ / 7 from (24) Peterson, 0.G.;Webb, J. P.;McColgin, W. C.; Ekrly, J. H.J. Appl. Phys. 1971,42, 1917.
1'
220
"' I
' ' ' I
' '
' ' '
'''
260 300 Wavelength (nm)
'
I
340
Figure 7. Ground-state absorption bands in the UV range. the Strickler and Berg equation.25 The difficulty comes from the fact that the ethyl violet ground-state absorption spectrum given in Figures 1B corresponds to the superposition of the quasi degenerated So SI and So S2 transitions. In the Strickler and Berg equation, the integral of the absorption cross sections over the whole 480-650-nm absorption band leads to a radiative lifetime of 2.2 ns which is close to the value of 1.8 ns reported for the similar dye crystal violet.12 A value of 3.2 ns12 was found for the So SItransition of another triphenylmethane dye malachite green, the two electronic transitions of which are well separated. A preliminary estimation of the respective So SI and So S2 transitions cross sections from fluorescence polarization studies26indicates that a 4 4 s radiative lifetime seems to be reasonable. The values obtained for u, from eq 8 by using a value of 2.5 X lo* s-I for the ratio @ / T are reported in Figure 5, where the stimulated emission cross section spectrum can be compared to that obtained for the total SIlight interaction cross section ul deduced from pumpprobe measurements. 4. SI and S, Absorption Spectra. The SI absorption cross sections uulcalculated from eq 1 are given in Figure 6 together with the S, and S, absorption spectra. Except for the strong nearly degenerate So SI and So S, visible band which is seen around 592 nm, the ground-state spectrum exhibits UV absorption bands a t 253 and 307 nm (Figure 7). From the energy separation between the S1 state and these upper states, one can expect absorption bands from SI to these upper electronic levels to occur around 440 and 640 nm. These bands are indeed observed in the SI absorption spectrum (Figure 6). In comparison, the S, spectrum is blue-shifted with the longer wavelength band now appearing around 610 nm and much less intense, while the 440-nm band shifts to 410 nm with slightly increased intensity.
-
-
-
-
-
-
-
Discussion In previous studies, the excited-state deactivation of triphenylmethane dyes was interpreted and theoretically analyzed in terms of photoinduced large-amplitude internal molecular I biradicaloid charge motion involving phenyl ring transfer state formation,12J3or fast internal conversion followed by heat transfer to the surrounding solvent m o l e c ~ l e s . ~ ~ ~ ~ ~ ~ . ~ ~ ( 2 5 ) Strickler, S. J.; Berg, R. A. J . Chem. Phys. 1962. 37, 814. (26) Plaza, P.; Martin, M.M.;Meyer, Y.H. To be published.
9314
J. Phys. Chem. 1991, 95, 9314-9320
Theoretical descriptions of triphenylmethane dye excited-state deactivation as a diffusive barrierless phenyl ring torsional process leading to a nonradiative distorted form of the excited state foresee nonexponential deactivation kinetics as well as fractional viscosity dependence of the fluorescence yield,1,6*9*'0 but various kinetics are expected depending on the intramolecular potential shape in which the motion occurs and on the type of sink which gives rise to the excited-state population decay. In particular, Gaussian or Lorentzian sink with maximum probability of decay at the potential minimum leads to exponential decay at low viscosity and multiexponential decay at high viscosity.6 The nonexponential character is accompanied by a broadening of the excited-state distribution over a range of phenyl ring torsion angles. As we previously reported,16 exponential SI decay can be reasonably assumed to fit the transient absorption kinetics we observed at selected wavelengths for ethyl violet in low-viscosity solvents such as ethanol, dioxane, or tetrahydrofuran, whereas we measured nonexponential fluorescence decay in viscous solvents such as decanol. In the present paper, SI single-exponential deactivation of ethyl violet in ethanol leading to a well-defined unique transient state S, is shown to describe well the spectral change observed on a large-wavelength scale. In decanol we observed19similar spectral evolution leading to the same blue and red transient absorption bands at long pumpprobe delays. This would indicate that in spite of a nonexponential initial excited-state decay, the deactivation process occurs also in this viscous solvent via a single transient species. On the basis of experiments showing that the S , state formation and deactivation rates were not influenced by the solvent dielectric constant at constant viscosity,I6 we discarded the description of S, as a TlCT state of the type defined by Grabowski et aL2'
Detecting a decay longer than 70 ps for the S, state in viscous solvent such as decanol, we also excluded the observation of a transient vibrationally hot ground state in the case. of ethyl vi01et.I~ In this paper, the absorption spectrum of the transient S, is found to be similar to that of SI but spectrally shifted to higher energies, the red band becoming much less intense and the blue band becoming slightly more intense. Furthermore, S, stimulated emission cross sections are too small to be detected. These results support the description of S, as a well-defined excited-state conformer. As a matter of fact, molecular conformational change leading to a reorganization of the A electron delocalization chain is expected to affect the transition energies and distortion of the excited state leading to a smaller *-orbital overlap is expected to decrease the radiative transition probability. Blue shift of the ground-state absorption spectra of molecules undergoing trans-cis isomerization is well-known. Potential energy diagrams describing polymethine or stilbene derivatives isomerization process in their ground and excited states show that the SI S, absorption wavelength range is also expected to depend much on the internal twisting coordinate.28 Torsional isomerization in propeller-like compounds through one, two- or three-ring flip mechanisms have been reported for triphenylmethanes and related compounds.29 The excited-state conformer S , may be reached through a similar mechanism from the nonequilibrium SIexcited state that still carries the ground-state conformation. Registry No. Ethyl violet, 2390-59-2.
-
(27) Grabowski, Z. R.; Rottkiewicz, K.; Siemiarczuk, A,; Cowley, D. J.; Baumann, W. N o w . J. Chim. 1979, 3, 443. (28) Momicchioli, F.; Baraldi, I.; Berthier, G. Chem. Phys. 1988,123. 103. (29) Mislow, K.Acc. Chem. Res. 1976, 9, 26.
Bromous Acid Perturbations in the Belousov-Zhabotinsky Reaction. Experiments and Model Calculations of Phase Response Curves Peter Ruoff,* Horst-Dieter Forsterling,f Liszlb Gyorgyi,' and Richard M. Noyesf Department of Chemistry, Rogaland University Center, N-4004 Stavanger, Norway, Fachbereich Physikalische Chemie, Philipps- Universitiit Marburg, 0-3550 Marburg, Germany, Department of Chemistry, University of Montana, Missoula, Montana 5981 2, and Department of Chemistry, University of Oregon, Eugene, Oregon 97403 (Received: January 28, 1991; In Final Form: May 21, 1991)
The oscillatory Belousov-Zhabotinsky reaction has been perturbed with sodium bromite solutions of various strengths. Experimental phase response curves have been compared with simulation calculations by using the original Oregonator, a seven-variableOregonator model including reactions of Br2, the Showalter, Noyes, Bar-Eli (SNB) model, and a large reaction scheme containing 80 reactions with 26 kinetic active components. For HBr02-induced transitions to the excitable branch qualitative and almost quantitative agreements between experimental and calculated phase response curves are obtained with the seven-variable Oregonator and the largest model. The original Oregonator and the SNB model are not able to describe experimentally observed positive phase shifts. This work also shows that Br2must be included in models that attempt to provide quantitative description of the Belousov-Zhabotinsky reaction.
Introduction Phase response curves have recently been used to investigate dynamics and mechanisms of chemical oscilIators.l4 In earlier publications3*' we have experimentally and theoretically studied the phase response behaviors of different perturbants in the Belousov5-Zhabotinsky6 (BZ)oscillator.' Experimental behaviors have been found to be in good agreement with calcu'Author to whom correspondence should be addressed at Rogaland University Center. Philipps-UniversitBt Marburg. 'University of Montana. 8 University of Oregon.
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lations when Oregonator-type of models are ~ s e d . ~ . * 9 Typically, ~ for oxidation spikes,I0 the removal of bromide ion causes an ad(1) Sevcikova, H.; Suchanova, D.; Marek, M. Sci. Pap. Prague Inst. Chem. Technol. 1982, KI 7 , 137. (2) D u b , E.; De Kepper, P. Eiophys. Chem. 1983, 18, 211. (3) Ruoff, P. J. Phys. Chem. 1984,88, 2851. (4) Ruoff, P.; Noyes, R. M. J. Chem. Phys. 1988,89,6247. ( 5 ) Belousov, B. P. In Oscillations and Traueling Waues in Chemical Systems; Field, R. J., Burger, M.. Eds.; Wiley-Interscience: New York, 1985. (6) Zhabotinsky, A. M. In Oscillurions and Traueling Waues in Chemical Systems; Field, R.J., Burger, M., Eds.;Wiley-Interscience: New York, 1985. (7) Field, R. J.; KBrbs, E.; Noyes, R. M. J. Am. Chem. SOC.1972, 94, 8649.
0022-3654191 /2095-9314%02.50/0 0 1991 American Chemical Society
