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Sub-picosecond fluorescence study of the LDS 751 dye molecule in ethanol
Volume 2 13, number 3.4
CHEMICAL PHYSICS LETTERS
8 October 1993
Sub-picosecond fluorescence study of the LDS 75 1 dye molecule in ethanol P. Hkbert ‘,
G.Baldacchino, T. Gustavsson and J.-C. Mialocq
CEA, Centre d’Etudes de Saclay, D.%WDRECAM/scM/URA33I
CNRS, 91 I91
Gifwr- YvetteCedex, France
Received 28 April 1993;in final form 28 July 1993
A timedependent fluorescence Stokes shift study of the LDS 751 (styvl8) dye molecule in ethanol solution is presented. Fluorescence intensity curves were recorded with sub-picosecond time resolution at various wavelengths using the fluorescence upconversion method. From the experimental curves, time-resolved fluorescence spectra have been reconstructed, allowing the calculationofthespectmlshiftco~elationfunctionc(t)=[Y(t)-~(m)]/[Y(O)-Y(o3)].Wefindc(f)ofLDS751inethanolat room temperature to be well described by a monoexponential function with a I/e time of 5 f I ps.
1. Introduction A great deal of attention is currently given to the role of solvation in chemical reactions involving electron- or proton-transfer and trans-cis photoisomerization [ l-5 1. The experimental data are in most cases obtained by time-resolved emission- or absorption-spectroscopy. However, in non-viscous solvents and at ambient temperature the above-mentioned processes are extremely rapid, typically on the order of some picoseconds or even shorter, i.e. hundreds or tens of femtoseconds. For this reason, detailed studies of these phenomena became technically possible only with the development of ultrashort (pica- and femto-second) laser pulses. Today, such lasers are readily available in photochemistry laboratories, presenting a wide choice of wavelength, pulse energy and repetition rate. Unfortunately, the potentially high temporal resolution offered by such systems is often marred by the electronics used in the detection system (time-correlated single-photon counting, streak camera etc. ) . One way to get around this problem is to use the method of sum-frequency generation (or “fluorescence upconversion” when applied to a laser-induced emission spectrum ). The first “photochemical” application of this method was made in 1975 by Mahr and Hirsch
[6], who attempted to study the time-evolution of the luminescence from a rhodamine 6G dye jet pumped by a mode-locked argon-ion laser. Rapidly, several research groups adopted this new technique for ultrafast emission spectroscopy [ 7- 131. Detailed technical descriptions of the sum-frequency generation technique can be found in refs. [ 14,151. In this Letter we present a time-resolved study of the fluorescence Stokes shift of the unsymmetrical cyanine dye molecule LDS 75 1 (scheme 1) in ethanol solution, based on data obtained using the fluorescence upconversion technique. Part of this material has been presented recently [ 161. It should be mentioned here that a nice study of the time-dependent fluorescence Stokes shift (TDFSS) of a related molecule, LDS 750 (styryl7) has already been reported by Fleming and co-workers [ 12,17,181.
’ DRECAM/SPAM 0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
Scheme 1.
345
Volume 2 13, number 3,4
CHEMICAL PHYSICS LETTERS
2. Experimental A1.7x10-4MsolutionofLDS751 (styryl8; 2-(4(4-dimethylaminophenyl)-1,3-butadienyl)-3-ethylbenzothiazolium perchlorate) (Lambda Physik) was prepared in absolute ethanol (Merck, pro analysi). Steady-state absorption and emission spectra recorded with a CARY 1E spectrophotometer and a SPEX Fluorolog 2Fl llA1 spectrofluorometer respectively are shown in fig. 1. Time-resolved fluorescence intensity curves were obtained by the fluorescence upconversion technique. The laser source was a hybridly mode-locked dye laser running with rhodamine 6G and pinacyanol chloride as gain/saturable absorber couple and pumped by a cw mode-locked and frequency-doubled Nd: YAG laser. This cavity-dumped laser system has been described in detail elsewhere [ 19 ] and has been shown to provide pulses as short as 650 fs, but, due to the weak fluorescence and low peak power of the laser pulses we were forced to increase the pulse duration in order to maximize the pulse energy. The typical pulsewidth was x 1.6 ps (fwhm supposing a sech* pulse form). We tuned the laser to 610 nm, 35 nm to the red of the LDS 75 1 absorption maximum, (A,= 575 nm), in order to excite mainly the v=O vibrational level of the first excited singlet state S,. The laser pulse was split in two halfs by a bearn-splitter, one half serving as a gating pulse and one half exciting the sample. The gating pulse passed through a @ plate, turning the polarization by 90” to horizontal, and a delay line consisting of a comer cube (Precision Lapping) mounted on a motorized translation stage (Microcontrdle). The exciting pulse passed also through a f4 plate, which could be ad-
0
400
500
600
Wavelength
700
800
(nm)
Fig 1. Steady-state absorption and emission spectra of LDS 75 1 in ethanol.
346
8 October 1993
justed for either vertical or horizontal excitation before being focalized into the sample. The fluorescence from the sample cell was collected by a 1: 1 imaging system consisting of two f=S cm achromatic lenses. The solid angle obtained with this system was 0.12 sr. The fluorescence was focalized at small angle with the gating pulse in a 0.5 mm typeII urea crystal (INRAD) cut at 72”. By choosing the polarization of the excitation pulse, we could record the fluorescence polarized either parallel I,,= Zvv) or perpendicular (1, =Zuv) to the excitation (the suffixes denote excitation/detection polarizations in the laboratory) pulse, thus allowing for the construction of the rotation-free fluorescence intensity curves as described below. The sum-frequency generated in the UV around 320 nm, according to the fluorescence wavelength examined, was passed through a 0.25 m focal-length monochromator (Instruments SA HR25) and a Schott UG5 UV bandpass filter. The resolution for the sum-frequency at 320 nm was set to about 0.5 nm corresponding to 2 nm for the red fluorescence, limited by the laser bandwidth. The filtered UV light was detected by a UV-sensitive photomultiplier-tube (RTC 20204) coupled to a lockin photon counter (Stanford SR400). The combined use of a type II crystal and a spectral filtering was found necessary in order to diminish the contribution at 305 nm from the SHG of the gating pulse. During optimization of the signal the excitation pulse was modulated by a mechanical chopper and phase-sensitive detection used in order to suppress the residual SHG-signal. However, once optimized, the chopper was eliminated, and the full excitation intensity was used. The experimental setup is shown in fig. 2. Fluorescence intensity curves for both vertical and horizontal excitation were recorded at ten different wavelengths between 650 and 780 nm, thus covering the main part of the steady-state fluorescence spectrum (see fig. 1) . For each wavelength, both a “short” and a “long” time interval were recorded with steps of 667 fs and 3.333 ps, respectively (covering 50 and 250 ps, respectively), in order to get information regarding the fast rise time on the one hand, and the relaxed fluorescence decay on the other. In order to compensate for fluctuations in the laser intensity, several tens of recordings were averaged before further treatment. The rotation-free fluorescence decay
Volume 213, number 3,4
CHEMICAL PHYSICS LETTERS Delay-line
I
I
8 October 1993
curves, normalized to unity, have been given an arbitrary time-origin in order to coincide at half-maximum (for display purpose only).
3. Observations
Fig 2. The experimental setup: M: monochromator; PM: photomultiplier tube.
,z 1.0
z 0.8 al
-2 ;
0.6
It] : 0.4 i 2
750
nm
700 660 665 650
nm nm nm nm
0.2 0.0
Fig 3. Examples of rotation-free fluorescence intensity curves of LDS 751 in ethanol (610 nm excitation). The decays are arbitrarily normalized to unity at maximum intensity.
G(t) can be constructed from Z,( t) and I, (t ) using the relation
G(O=J,(Q+2Z,(O,
(1)
but this implies that I,,(t) and I, (t) have been recorded under exactly the same experimental conditions. Since this was not the case, we corrected measured fluorescence intensities Iy and IF according to r,(t)=l.l6Z;f”“(t) andZ,(t)=ly(t) where the factor of 1.16 compensates for the different reflectivities of the sample cell surface for the two different excitation polarizations. Examples of rotation-free fluorescence intensity curves of LDS 751 in EtOH at room-temperature shown in fig. 3 present the time-dependence of the fluorescence at five different wavelengths. The
The fluorescence intensity curves shown in fig. 3 clearly indicate that the kinetics are wavelength dependent and quite complicated. For wavelengths in the blue part of the fluorescence spectrum a bi-exponential function (with a rapid component < 10 ps and a slow one with a much longer characteristic time) is needed to describe the decay in a satisfactory way. From 700 nm and all the way to the red tail of the fluorescence spectrum, on the other hand, the decay does only exhibit the “slow” component. This slow component, which is common for all wavelengths, may be attributed to the relaxed excited state. Moreover, the rise-time is practically instantaneous in the blue but becomes increasingly slower when going to the red. It is important, however, to note that neither the rise time nor the rapid decay may be interpreted in terms of any “true” kinetics. They should rather be seen as the phenomenological result of the dynamic Stokes shift, which takes place on a very short time-scale.
4. Data treatmentand analysis In order to characterize and quantisize the temporal evolution of the fluorescence spectrum, we have used the method described by Maroncelli and Fleming [ 20 1. The experimental fluorescence intensity curves were first fitted with model functions F(t) constructed from the convolution of a fluorescence impulse response function Z,(t) with the apparatus functionA [8,21], co F(t)=
j I,(t’)A(t-t’) -00
dt’.
(2)
In order to describe the observations, as outlined in paragraph 3, we need three exponential functions, one for the rise time rl, and two for the decay, r2 and 7j. The impulse response function used was consequently 347
ZF(f)=C[~exp(-f/72)+(I-01)
exp(-t/r3)
-Bed-t/71)1.
(3)
C and (Yare only normalizing constants, and p allows forl,(t)#O at r=O (OX/?< 1). The slower decay r3 corresponds to the relaxed fluorescence lifetime (see below), but r1 and r2, on the other hand, depend strongly on the observation wavelength. This latter fact is indeed expected since they are not related to any simple relaxation phenomenon, but rather to the dynamic Stokes shift. For the apparatus function we used a best tit of the “autocorrelation” of the laser pulse obtained using the sum-frequency generation setup. This was done by mixing the gating pulse and laser light diffused from the sample cell when filled with a micro-emulsion. It was found that the apparatus function could be well described by a sech’ function with fwhm 2.5 ps. We consider that this allows us an effective time resolution of ~0.5 ps after the deconvolution procedure. The model functions F(t) were fitted to the observed fluorescence intensity curves by standard nonlinear regression methods [ 22 1, each wavelength being treated separately. Fitted values of /I were approximately 0.8 for all wavelengths and r, varied between 0.9 ps at 640 nm up to 3.8 ps at 775 nm. The relaxed fluorescence lifetime z, was in all cases fixed to an average value obtained separately by fitting only the long time tails with mono-exponential Fitted fluorescence response functions
functions. I,(t) were
finally properly scaled by using the time-integrated value of the function and the steady-state fluorescence intensity measured at a given wavelength with
a fully corrected conventional spectrofluorometer. Time-resolved spectra could then be constructed from the intensities at different wavelengths for fmed time. Since the number of different wavelength values was limited to ten wavelengths, the spectra thus obtained were fitted with log-normal distribution functions [ 231, see fig. 4, as described by Maroncelli and Fleming [ 20 1.
5. Results The dynamic Stokes shift can be evaluated by 348
8 October 1993
CHEMICALPHYSICSLETTERS
Volume213,number 3,4
13000
14000
Wave number
15000
16000
(cm-‘)
Fig. 4. Time-resolved spectra (after deconvolution) of LDS 75 1 in ethanol at 2 ( 0 ) and 32 ( V ) psafter excitation.Corresponding fitted log-normal functions are shown as solid curves.
Time (ps)
Fig. 5. The dynamic Stokesshift of LDS 751 in ethanol as obtained by studying the time-evolution of different “characteristic” frequencies of the fluorescence spectrum; ( l ) the peak frequency and the mean frequency (0 ).
studying some “characteristic” frequency of the fitted log-normal function as a function of time. However, the choice of such a parameter is not evident. In fig. 5 we show two different choices of characteristic frequency; the peak frequency and the mean (first moment ) frequency as defined by (4) As can be seen in fig. 5, the resulting curves are very different, displaying the important change in asymmetry of the fluorescence spectrum with time. In the following, we consider only the mean frequency as defined by eq. (4). The mean frequency can be used as the time-dependent variable r(t) in the correlation function c(t) defined by [20]
Volume213, number 3,4
Y(C)-P(a) Y(O)-f(m)
c(t)= _
CHEMICALPHYSICSLETTERS
-
(5)
In eq. (5), P(1), r(O) and P(co) are the characteristic frequencies at t, t= 0 and t = 03. The correlation function c(t) can be related to solvent dynamics [ 24,25 1, which may be described by a mono-, multior non-exponential decay, depending on the solvent properties, see the discussion below. We find that the correlation function c(t) of LDS 751 in ethanol at room-temperature is well described by a single exponential with a 1/e time of 5 + 1 ps, and the total spectral shift to be about 20 t 3 nm. The uncertainties in these values are estimated from the indirect way used to construct the time-dependent fluorescence spectra, but it should also be noted that these values are only valid for t> 0.5ps, due to our limited time resolution; see the discussion below.
6. Discussion According to simple continuum models, solvent relaxation around a newly created charged species or dipole can be described using the solvent longitudinal dielectric relaxation time z,= ( E,/E~)z,, [ 2628], where to and t, are the static and infinite frequency dielectric constants respectively, and To is the Debye relaxation time of the solvent [29,301.For associative solvents like normal alcohols, Garg and Smyth [ 3 1] proposed that there are three dielectric dispersion regions, each with its characteristic dielectric constants and relaxation time. In that case there are also three different longitudinal relaxation times given by t z~,~=- w,i rDD,+i= 1,2,3.
(6)
6.i
Barthel et al. [ 321 recently confirmed the existence of three dispersion regions in ethanol and gave accurate values for the dielectric constants and relaxation times at 20°C. However, they disputed Garg and Smyth’s original interpretation of the different relaxation times. According to Barthel et al. the fastest relaxation time ( Q_,~)is related to the formation and breaking of hydrogen bonding, the intermediate relaxation time (Q) to monomer reorientation and the slowest one (TV,,) to the motion of aggregates.
8 October 1993
Since the monomer concentration of ethanol at ambient temperature is supposed to be very low [ 33,341, one can expect the solvent relaxation to be dominated by the slow cooperative motion (z~,~) with perhaps a minor contribution from the fast hydrogen-bonding dynamics ( qs ). According to Barthel et al. [ 321, in ethanol, the three longitudinal relaxation times are rL,l= 30 ps, rL2=7.6 ps and 7L,s= 1.3 ps. Our observed value for the LDS 75 1 fluorescence spectral relaxation time, ‘t= 5 2 1 ps, does not compare properly with either zL,, or ‘Tag. In particular, our value is significantly smaller than 7LL,L. To our knowledge, this is the first time a TDFSS study of the LDS 75 1 molecule has been undertaken. For the similar dye molecule LDS 750 (styryl 7) in methanol and n-butanol, Fleming and co-workers [ 121 reported fluorescence spectral shifts with observed relaxation time constants of 3.3 and 66 ps, respectively. Our value of 5 f 1 ps for LDS 75 1 in ethanol is slightlybigger than the value found by these authors for LDS 750 in methanol, which seems normal in view of the respective viscosities of methanol and ethanol. More recently [ 17,18,35], Fleming and co-workers studied the solvation dynamics of LDS 750 and coumarin 153 in acetonitrile and methanol with femtosecond time resolution, and observed very rapid kinetics ( < 100 fs) assigned to inertial effects of the first solvent shell. However, our time resolution, x0.5 ps, does not allow us to show the existence or non-existence of such fast kinetics. For the same reason, our observed spectral shift of x 20 nm is not inconsistent with the 50 nm spectral shift observed by Fleming and co-workers in the case of LDS 750. It is possible that very rapid inertial relaxation is present also in our case but that it remains undetected due to the limited time resolution. Blanchard [ 361 studied the temporal evolution of stimulated emission of LDS 750 in butanols with a resolution of 10 ps. He observed structured spectra which he interpreted as due to inhomogeneous relaxation kinetics. The origin of these complex kinetics was tentatively given as an end-group photoisomerization (“TICT” state formation). We have absolutely no evidence for such a process regarding LDS 751 in ethanol. We are presently pursuing a large investigation of the LDS 751 photophysics using steady state ab-
sorption and fluorescence spectroscopy, solvato349
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CHEMICAL PHYSICS LETTERS
chromism, HPLC, NMR and nanosecond transient absorption spectroscopy [ 37 ] in order to clarify the relaxation dynamics of its molecular structure and solvent cage.
References [I] J.D. Simon, Accounts Chem. Res. 21 (1988) 128. [2] P.F. Barbara, Accounts Chem. Res. 21 ( 1988) 195. [3] M. Maroncelli, J. MacInnis and G.R. Fleming, Science 243 (1989) 1674. [4] G.R. Fleming and P.G. Wolynes, Physics Today, 36 (May 1990). [ 51P.F. Barbara and W. Jarzeba, Advan. Photochem. 15 ( 1990) [6] k. Mahrand M.D. Hirsch, Opt. Commun. 13 (1975) 96.
[ 71 L.A. Hallidy and M.R. Topp, Chem. Phys. Letters 46 ( 1977) 8. [S] G.S. Beddard, T. Doust and G. Porter, Chem. Phys. 61 (1981) 17. [9] K. Ding, S.J. Courtney, A.J. Strandjord, S. Flom, D. Friedrich and P.F. Barbara, J. Phys. Chem. 87 (1983) 1184. [lo] R.J.D. Miller, M. Pierre and M.D. Fayer, 1. Chem. Phys. 78 (1983) 5138. [ 1 I] A.B. Myers, P.L. Holt, M.A Pereira and R.M. Hochstrasser, Chem. Phys. Letters 132 (1986)585. [ 121 E.W. Castner Jr., M. Maroncelb and G.R. Fleming, J. Chem. Phys. 86 (1987) 1090. [ 131A. Mokhtari, J. Chesnoy and A. Lauberau, Chem. Phys. Letters 155 (1989) 593. [ 141J. Shah, IEEE J. Quantum Electr. 24 (1988) 276. [ 151M.A. Kahlow, W. Jarzeba, T.P. DuBrnil and P.F. Barbara, Rev. Sci. Instrum. 59 (1988) 1098. [ 161P. Hebert, G. Baldacchino, T. Gustavsson and J.-C. Mialocq, in: Ultrafast phenomena, Vol. 8, eds. M. Migus and M. Martin, Proceedings, Antibes (Springer, Berlin, 1992). [ 171S.J. Rosenthal, X. Xie, M. Du and G.R. Fleming, J. Chem. Phys. 95 (1991) 4715.
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[ 181M. Cho, S.J. Rosenthal, N.F. Scherer, L.D. Ziegler and G.R.Fleming, J. Chem. Phys. 96 ( 1992) 5033. [ 191P. Hkbert, S. Marguet, T. Gustavsson and J.-C. Mialocq, Opt. Commun. 90 (1992) 85. [ 201 M. Maroncelli and G.R. Fleming, J. Chem. Phys. 86 (1987) 6221. [2 1] M.D. Hirsch, M.A. Marcus, A. Lewis, H. Mahr and N. Frigo, Biophys. I. 16 (1976) 1399. [22]W.H. Press, B.P. Rannery, S.A. Teukolsky and W.T. Vetterling, Numerical recipes (Cambridge Univ. Press, Cambridge, 1989). [23] D.B. Siano and D.E. Metzler, J. Chem. Phys. 51 (1969) 1856. [24] B. Bagchi, D.W. Oxtoby and G.R. Fleming, Chem. Phys. 86 (1984) 257. [25] G. van der Zwan and J.T. Hynes, J. Phys. Chem. 89 (1985) 4181. [26] H. Frolich, Theory of dielectrics, 2nd Ed. (Oxford Univ. Press, Oxford, 1949). [27] A. Mozumder, J. Chem. Phys. 50 (1969) 3153,3162. [ 281 J.H. Baxendale and P. Wardman, Nature 230 (1971) 449. [29] R. Schiller, in: Excess electrons in dielectric media, eds. C. Ferradin and J.-P. Jay-Germ, (CRC Press, Boca Raton, 1991) ch. 4, p. 105. [ 301 M.D. Newton and H.L Friedman, J. Chem. Phys. 88 (1988) 4460. [31]S.K.GargandC.P.Smyth, J.Phys.Chem.69 (1965) 1294. [32] J. Barthel, K. Bachuber, R. Buchner and H. Hetzeaauer, Chem. Phys. Letters 165 (1990) 369. [33] Y. Sakai, Y. Sadaoka and T. Yamamoto, Bull. Chem. Sot. Japan 46 ( 1973) 3575. [34] G. Kabisch and hf. Klose, Z. Physik. Chem. (Leipzig) 266 (1985) 687. [35] S.J. Rosenthal, N.F. Scherer, M. Cho, X. Xie, M.E. Schmidt and G.R. Fleming, in: Ultrafast phenomena, Vol. 8, eds. M. M&us and M. Martin, Proceedings, Antibes (Springer, Berlin, 1992). [36] G.J. Blanchard, J. Chem. Phys. 95 (1991) 6317. [37 ] P. Htbert, G. Baldacchino, T. Gustavsson and J.-C. Mialocq, to be published.
CHEMICAL PHYSICS LETTERS
8 October 1993
Sub-picosecond fluorescence study of the LDS 75 1 dye molecule in ethanol P. Hkbert ‘,
G.Baldacchino, T. Gustavsson and J.-C. Mialocq
CEA, Centre d’Etudes de Saclay, D.%WDRECAM/scM/URA33I
CNRS, 91 I91
Gifwr- YvetteCedex, France
Received 28 April 1993;in final form 28 July 1993
A timedependent fluorescence Stokes shift study of the LDS 751 (styvl8) dye molecule in ethanol solution is presented. Fluorescence intensity curves were recorded with sub-picosecond time resolution at various wavelengths using the fluorescence upconversion method. From the experimental curves, time-resolved fluorescence spectra have been reconstructed, allowing the calculationofthespectmlshiftco~elationfunctionc(t)=[Y(t)-~(m)]/[Y(O)-Y(o3)].Wefindc(f)ofLDS751inethanolat room temperature to be well described by a monoexponential function with a I/e time of 5 f I ps.
1. Introduction A great deal of attention is currently given to the role of solvation in chemical reactions involving electron- or proton-transfer and trans-cis photoisomerization [ l-5 1. The experimental data are in most cases obtained by time-resolved emission- or absorption-spectroscopy. However, in non-viscous solvents and at ambient temperature the above-mentioned processes are extremely rapid, typically on the order of some picoseconds or even shorter, i.e. hundreds or tens of femtoseconds. For this reason, detailed studies of these phenomena became technically possible only with the development of ultrashort (pica- and femto-second) laser pulses. Today, such lasers are readily available in photochemistry laboratories, presenting a wide choice of wavelength, pulse energy and repetition rate. Unfortunately, the potentially high temporal resolution offered by such systems is often marred by the electronics used in the detection system (time-correlated single-photon counting, streak camera etc. ) . One way to get around this problem is to use the method of sum-frequency generation (or “fluorescence upconversion” when applied to a laser-induced emission spectrum ). The first “photochemical” application of this method was made in 1975 by Mahr and Hirsch
[6], who attempted to study the time-evolution of the luminescence from a rhodamine 6G dye jet pumped by a mode-locked argon-ion laser. Rapidly, several research groups adopted this new technique for ultrafast emission spectroscopy [ 7- 131. Detailed technical descriptions of the sum-frequency generation technique can be found in refs. [ 14,151. In this Letter we present a time-resolved study of the fluorescence Stokes shift of the unsymmetrical cyanine dye molecule LDS 75 1 (scheme 1) in ethanol solution, based on data obtained using the fluorescence upconversion technique. Part of this material has been presented recently [ 161. It should be mentioned here that a nice study of the time-dependent fluorescence Stokes shift (TDFSS) of a related molecule, LDS 750 (styryl7) has already been reported by Fleming and co-workers [ 12,17,181.
’ DRECAM/SPAM 0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
Scheme 1.
345
Volume 2 13, number 3,4
CHEMICAL PHYSICS LETTERS
2. Experimental A1.7x10-4MsolutionofLDS751 (styryl8; 2-(4(4-dimethylaminophenyl)-1,3-butadienyl)-3-ethylbenzothiazolium perchlorate) (Lambda Physik) was prepared in absolute ethanol (Merck, pro analysi). Steady-state absorption and emission spectra recorded with a CARY 1E spectrophotometer and a SPEX Fluorolog 2Fl llA1 spectrofluorometer respectively are shown in fig. 1. Time-resolved fluorescence intensity curves were obtained by the fluorescence upconversion technique. The laser source was a hybridly mode-locked dye laser running with rhodamine 6G and pinacyanol chloride as gain/saturable absorber couple and pumped by a cw mode-locked and frequency-doubled Nd: YAG laser. This cavity-dumped laser system has been described in detail elsewhere [ 19 ] and has been shown to provide pulses as short as 650 fs, but, due to the weak fluorescence and low peak power of the laser pulses we were forced to increase the pulse duration in order to maximize the pulse energy. The typical pulsewidth was x 1.6 ps (fwhm supposing a sech* pulse form). We tuned the laser to 610 nm, 35 nm to the red of the LDS 75 1 absorption maximum, (A,= 575 nm), in order to excite mainly the v=O vibrational level of the first excited singlet state S,. The laser pulse was split in two halfs by a bearn-splitter, one half serving as a gating pulse and one half exciting the sample. The gating pulse passed through a @ plate, turning the polarization by 90” to horizontal, and a delay line consisting of a comer cube (Precision Lapping) mounted on a motorized translation stage (Microcontrdle). The exciting pulse passed also through a f4 plate, which could be ad-
0
400
500
600
Wavelength
700
800
(nm)
Fig 1. Steady-state absorption and emission spectra of LDS 75 1 in ethanol.
346
8 October 1993
justed for either vertical or horizontal excitation before being focalized into the sample. The fluorescence from the sample cell was collected by a 1: 1 imaging system consisting of two f=S cm achromatic lenses. The solid angle obtained with this system was 0.12 sr. The fluorescence was focalized at small angle with the gating pulse in a 0.5 mm typeII urea crystal (INRAD) cut at 72”. By choosing the polarization of the excitation pulse, we could record the fluorescence polarized either parallel I,,= Zvv) or perpendicular (1, =Zuv) to the excitation (the suffixes denote excitation/detection polarizations in the laboratory) pulse, thus allowing for the construction of the rotation-free fluorescence intensity curves as described below. The sum-frequency generated in the UV around 320 nm, according to the fluorescence wavelength examined, was passed through a 0.25 m focal-length monochromator (Instruments SA HR25) and a Schott UG5 UV bandpass filter. The resolution for the sum-frequency at 320 nm was set to about 0.5 nm corresponding to 2 nm for the red fluorescence, limited by the laser bandwidth. The filtered UV light was detected by a UV-sensitive photomultiplier-tube (RTC 20204) coupled to a lockin photon counter (Stanford SR400). The combined use of a type II crystal and a spectral filtering was found necessary in order to diminish the contribution at 305 nm from the SHG of the gating pulse. During optimization of the signal the excitation pulse was modulated by a mechanical chopper and phase-sensitive detection used in order to suppress the residual SHG-signal. However, once optimized, the chopper was eliminated, and the full excitation intensity was used. The experimental setup is shown in fig. 2. Fluorescence intensity curves for both vertical and horizontal excitation were recorded at ten different wavelengths between 650 and 780 nm, thus covering the main part of the steady-state fluorescence spectrum (see fig. 1) . For each wavelength, both a “short” and a “long” time interval were recorded with steps of 667 fs and 3.333 ps, respectively (covering 50 and 250 ps, respectively), in order to get information regarding the fast rise time on the one hand, and the relaxed fluorescence decay on the other. In order to compensate for fluctuations in the laser intensity, several tens of recordings were averaged before further treatment. The rotation-free fluorescence decay
Volume 213, number 3,4
CHEMICAL PHYSICS LETTERS Delay-line
I
I
8 October 1993
curves, normalized to unity, have been given an arbitrary time-origin in order to coincide at half-maximum (for display purpose only).
3. Observations
Fig 2. The experimental setup: M: monochromator; PM: photomultiplier tube.
,z 1.0
z 0.8 al
-2 ;
0.6
It] : 0.4 i 2
750
nm
700 660 665 650
nm nm nm nm
0.2 0.0
Fig 3. Examples of rotation-free fluorescence intensity curves of LDS 751 in ethanol (610 nm excitation). The decays are arbitrarily normalized to unity at maximum intensity.
G(t) can be constructed from Z,( t) and I, (t ) using the relation
G(O=J,(Q+2Z,(O,
(1)
but this implies that I,,(t) and I, (t) have been recorded under exactly the same experimental conditions. Since this was not the case, we corrected measured fluorescence intensities Iy and IF according to r,(t)=l.l6Z;f”“(t) andZ,(t)=ly(t) where the factor of 1.16 compensates for the different reflectivities of the sample cell surface for the two different excitation polarizations. Examples of rotation-free fluorescence intensity curves of LDS 751 in EtOH at room-temperature shown in fig. 3 present the time-dependence of the fluorescence at five different wavelengths. The
The fluorescence intensity curves shown in fig. 3 clearly indicate that the kinetics are wavelength dependent and quite complicated. For wavelengths in the blue part of the fluorescence spectrum a bi-exponential function (with a rapid component < 10 ps and a slow one with a much longer characteristic time) is needed to describe the decay in a satisfactory way. From 700 nm and all the way to the red tail of the fluorescence spectrum, on the other hand, the decay does only exhibit the “slow” component. This slow component, which is common for all wavelengths, may be attributed to the relaxed excited state. Moreover, the rise-time is practically instantaneous in the blue but becomes increasingly slower when going to the red. It is important, however, to note that neither the rise time nor the rapid decay may be interpreted in terms of any “true” kinetics. They should rather be seen as the phenomenological result of the dynamic Stokes shift, which takes place on a very short time-scale.
4. Data treatmentand analysis In order to characterize and quantisize the temporal evolution of the fluorescence spectrum, we have used the method described by Maroncelli and Fleming [ 20 1. The experimental fluorescence intensity curves were first fitted with model functions F(t) constructed from the convolution of a fluorescence impulse response function Z,(t) with the apparatus functionA [8,21], co F(t)=
j I,(t’)A(t-t’) -00
dt’.
(2)
In order to describe the observations, as outlined in paragraph 3, we need three exponential functions, one for the rise time rl, and two for the decay, r2 and 7j. The impulse response function used was consequently 347
ZF(f)=C[~exp(-f/72)+(I-01)
exp(-t/r3)
-Bed-t/71)1.
(3)
C and (Yare only normalizing constants, and p allows forl,(t)#O at r=O (OX/?< 1). The slower decay r3 corresponds to the relaxed fluorescence lifetime (see below), but r1 and r2, on the other hand, depend strongly on the observation wavelength. This latter fact is indeed expected since they are not related to any simple relaxation phenomenon, but rather to the dynamic Stokes shift. For the apparatus function we used a best tit of the “autocorrelation” of the laser pulse obtained using the sum-frequency generation setup. This was done by mixing the gating pulse and laser light diffused from the sample cell when filled with a micro-emulsion. It was found that the apparatus function could be well described by a sech’ function with fwhm 2.5 ps. We consider that this allows us an effective time resolution of ~0.5 ps after the deconvolution procedure. The model functions F(t) were fitted to the observed fluorescence intensity curves by standard nonlinear regression methods [ 22 1, each wavelength being treated separately. Fitted values of /I were approximately 0.8 for all wavelengths and r, varied between 0.9 ps at 640 nm up to 3.8 ps at 775 nm. The relaxed fluorescence lifetime z, was in all cases fixed to an average value obtained separately by fitting only the long time tails with mono-exponential Fitted fluorescence response functions
functions. I,(t) were
finally properly scaled by using the time-integrated value of the function and the steady-state fluorescence intensity measured at a given wavelength with
a fully corrected conventional spectrofluorometer. Time-resolved spectra could then be constructed from the intensities at different wavelengths for fmed time. Since the number of different wavelength values was limited to ten wavelengths, the spectra thus obtained were fitted with log-normal distribution functions [ 231, see fig. 4, as described by Maroncelli and Fleming [ 20 1.
5. Results The dynamic Stokes shift can be evaluated by 348
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13000
14000
Wave number
15000
16000
(cm-‘)
Fig. 4. Time-resolved spectra (after deconvolution) of LDS 75 1 in ethanol at 2 ( 0 ) and 32 ( V ) psafter excitation.Corresponding fitted log-normal functions are shown as solid curves.
Time (ps)
Fig. 5. The dynamic Stokesshift of LDS 751 in ethanol as obtained by studying the time-evolution of different “characteristic” frequencies of the fluorescence spectrum; ( l ) the peak frequency and the mean frequency (0 ).
studying some “characteristic” frequency of the fitted log-normal function as a function of time. However, the choice of such a parameter is not evident. In fig. 5 we show two different choices of characteristic frequency; the peak frequency and the mean (first moment ) frequency as defined by (4) As can be seen in fig. 5, the resulting curves are very different, displaying the important change in asymmetry of the fluorescence spectrum with time. In the following, we consider only the mean frequency as defined by eq. (4). The mean frequency can be used as the time-dependent variable r(t) in the correlation function c(t) defined by [20]
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Y(C)-P(a) Y(O)-f(m)
c(t)= _
CHEMICALPHYSICSLETTERS
-
(5)
In eq. (5), P(1), r(O) and P(co) are the characteristic frequencies at t, t= 0 and t = 03. The correlation function c(t) can be related to solvent dynamics [ 24,25 1, which may be described by a mono-, multior non-exponential decay, depending on the solvent properties, see the discussion below. We find that the correlation function c(t) of LDS 751 in ethanol at room-temperature is well described by a single exponential with a 1/e time of 5 + 1 ps, and the total spectral shift to be about 20 t 3 nm. The uncertainties in these values are estimated from the indirect way used to construct the time-dependent fluorescence spectra, but it should also be noted that these values are only valid for t> 0.5ps, due to our limited time resolution; see the discussion below.
6. Discussion According to simple continuum models, solvent relaxation around a newly created charged species or dipole can be described using the solvent longitudinal dielectric relaxation time z,= ( E,/E~)z,, [ 2628], where to and t, are the static and infinite frequency dielectric constants respectively, and To is the Debye relaxation time of the solvent [29,301.For associative solvents like normal alcohols, Garg and Smyth [ 3 1] proposed that there are three dielectric dispersion regions, each with its characteristic dielectric constants and relaxation time. In that case there are also three different longitudinal relaxation times given by t z~,~=- w,i rDD,+i= 1,2,3.
(6)
6.i
Barthel et al. [ 321 recently confirmed the existence of three dispersion regions in ethanol and gave accurate values for the dielectric constants and relaxation times at 20°C. However, they disputed Garg and Smyth’s original interpretation of the different relaxation times. According to Barthel et al. the fastest relaxation time ( Q_,~)is related to the formation and breaking of hydrogen bonding, the intermediate relaxation time (Q) to monomer reorientation and the slowest one (TV,,) to the motion of aggregates.
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Since the monomer concentration of ethanol at ambient temperature is supposed to be very low [ 33,341, one can expect the solvent relaxation to be dominated by the slow cooperative motion (z~,~) with perhaps a minor contribution from the fast hydrogen-bonding dynamics ( qs ). According to Barthel et al. [ 321, in ethanol, the three longitudinal relaxation times are rL,l= 30 ps, rL2=7.6 ps and 7L,s= 1.3 ps. Our observed value for the LDS 75 1 fluorescence spectral relaxation time, ‘t= 5 2 1 ps, does not compare properly with either zL,, or ‘Tag. In particular, our value is significantly smaller than 7LL,L. To our knowledge, this is the first time a TDFSS study of the LDS 75 1 molecule has been undertaken. For the similar dye molecule LDS 750 (styryl 7) in methanol and n-butanol, Fleming and co-workers [ 121 reported fluorescence spectral shifts with observed relaxation time constants of 3.3 and 66 ps, respectively. Our value of 5 f 1 ps for LDS 75 1 in ethanol is slightlybigger than the value found by these authors for LDS 750 in methanol, which seems normal in view of the respective viscosities of methanol and ethanol. More recently [ 17,18,35], Fleming and co-workers studied the solvation dynamics of LDS 750 and coumarin 153 in acetonitrile and methanol with femtosecond time resolution, and observed very rapid kinetics ( < 100 fs) assigned to inertial effects of the first solvent shell. However, our time resolution, x0.5 ps, does not allow us to show the existence or non-existence of such fast kinetics. For the same reason, our observed spectral shift of x 20 nm is not inconsistent with the 50 nm spectral shift observed by Fleming and co-workers in the case of LDS 750. It is possible that very rapid inertial relaxation is present also in our case but that it remains undetected due to the limited time resolution. Blanchard [ 361 studied the temporal evolution of stimulated emission of LDS 750 in butanols with a resolution of 10 ps. He observed structured spectra which he interpreted as due to inhomogeneous relaxation kinetics. The origin of these complex kinetics was tentatively given as an end-group photoisomerization (“TICT” state formation). We have absolutely no evidence for such a process regarding LDS 751 in ethanol. We are presently pursuing a large investigation of the LDS 751 photophysics using steady state ab-
sorption and fluorescence spectroscopy, solvato349
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chromism, HPLC, NMR and nanosecond transient absorption spectroscopy [ 37 ] in order to clarify the relaxation dynamics of its molecular structure and solvent cage.
References [I] J.D. Simon, Accounts Chem. Res. 21 (1988) 128. [2] P.F. Barbara, Accounts Chem. Res. 21 ( 1988) 195. [3] M. Maroncelli, J. MacInnis and G.R. Fleming, Science 243 (1989) 1674. [4] G.R. Fleming and P.G. Wolynes, Physics Today, 36 (May 1990). [ 51P.F. Barbara and W. Jarzeba, Advan. Photochem. 15 ( 1990) [6] k. Mahrand M.D. Hirsch, Opt. Commun. 13 (1975) 96.
[ 71 L.A. Hallidy and M.R. Topp, Chem. Phys. Letters 46 ( 1977) 8. [S] G.S. Beddard, T. Doust and G. Porter, Chem. Phys. 61 (1981) 17. [9] K. Ding, S.J. Courtney, A.J. Strandjord, S. Flom, D. Friedrich and P.F. Barbara, J. Phys. Chem. 87 (1983) 1184. [lo] R.J.D. Miller, M. Pierre and M.D. Fayer, 1. Chem. Phys. 78 (1983) 5138. [ 1 I] A.B. Myers, P.L. Holt, M.A Pereira and R.M. Hochstrasser, Chem. Phys. Letters 132 (1986)585. [ 121 E.W. Castner Jr., M. Maroncelb and G.R. Fleming, J. Chem. Phys. 86 (1987) 1090. [ 131A. Mokhtari, J. Chesnoy and A. Lauberau, Chem. Phys. Letters 155 (1989) 593. [ 141J. Shah, IEEE J. Quantum Electr. 24 (1988) 276. [ 151M.A. Kahlow, W. Jarzeba, T.P. DuBrnil and P.F. Barbara, Rev. Sci. Instrum. 59 (1988) 1098. [ 161P. Hebert, G. Baldacchino, T. Gustavsson and J.-C. Mialocq, in: Ultrafast phenomena, Vol. 8, eds. M. Migus and M. Martin, Proceedings, Antibes (Springer, Berlin, 1992). [ 171S.J. Rosenthal, X. Xie, M. Du and G.R. Fleming, J. Chem. Phys. 95 (1991) 4715.
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[ 181M. Cho, S.J. Rosenthal, N.F. Scherer, L.D. Ziegler and G.R.Fleming, J. Chem. Phys. 96 ( 1992) 5033. [ 191P. Hkbert, S. Marguet, T. Gustavsson and J.-C. Mialocq, Opt. Commun. 90 (1992) 85. [ 201 M. Maroncelli and G.R. Fleming, J. Chem. Phys. 86 (1987) 6221. [2 1] M.D. Hirsch, M.A. Marcus, A. Lewis, H. Mahr and N. Frigo, Biophys. I. 16 (1976) 1399. [22]W.H. Press, B.P. Rannery, S.A. Teukolsky and W.T. Vetterling, Numerical recipes (Cambridge Univ. Press, Cambridge, 1989). [23] D.B. Siano and D.E. Metzler, J. Chem. Phys. 51 (1969) 1856. [24] B. Bagchi, D.W. Oxtoby and G.R. Fleming, Chem. Phys. 86 (1984) 257. [25] G. van der Zwan and J.T. Hynes, J. Phys. Chem. 89 (1985) 4181. [26] H. Frolich, Theory of dielectrics, 2nd Ed. (Oxford Univ. Press, Oxford, 1949). [27] A. Mozumder, J. Chem. Phys. 50 (1969) 3153,3162. [ 281 J.H. Baxendale and P. Wardman, Nature 230 (1971) 449. [29] R. Schiller, in: Excess electrons in dielectric media, eds. C. Ferradin and J.-P. Jay-Germ, (CRC Press, Boca Raton, 1991) ch. 4, p. 105. [ 301 M.D. Newton and H.L Friedman, J. Chem. Phys. 88 (1988) 4460. [31]S.K.GargandC.P.Smyth, J.Phys.Chem.69 (1965) 1294. [32] J. Barthel, K. Bachuber, R. Buchner and H. Hetzeaauer, Chem. Phys. Letters 165 (1990) 369. [33] Y. Sakai, Y. Sadaoka and T. Yamamoto, Bull. Chem. Sot. Japan 46 ( 1973) 3575. [34] G. Kabisch and hf. Klose, Z. Physik. Chem. (Leipzig) 266 (1985) 687. [35] S.J. Rosenthal, N.F. Scherer, M. Cho, X. Xie, M.E. Schmidt and G.R. Fleming, in: Ultrafast phenomena, Vol. 8, eds. M. M&us and M. Martin, Proceedings, Antibes (Springer, Berlin, 1992). [36] G.J. Blanchard, J. Chem. Phys. 95 (1991) 6317. [37 ] P. Htbert, G. Baldacchino, T. Gustavsson and J.-C. Mialocq, to be published.