J. Phys. Chem. A 1999, 103, 7575-7579

7575

ARTICLES
Molecular Dynamics of Auramine O in Low-Viscosity Solutions as Investigated by an
Ultrafast Lensing Effect
Gen Furui,† Kazuki Ito,† Isao Tsuyumoto,‡ Akira Harata,§ and Tsuguo Sawada*,†Department of Applied Chemistry, School of Engineering, UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan, Department of EnVironmental Systems Engineering, Kanazawa Institute of Technology,
7-1 Ohgigaoka, Nonoichi-machi, Ishikawa 921-8501, Japan, and Department of Molecular and Material
Sciences, Graduate School of Engineering Sciences, Kyushu UniVersity, 6-1 Kasugakoen, Kasuga-shi,
Fukuoka 816-8580, Japan
ReceiVed: NoVember 3, 1998; In Final Form: April 23, 1999

Ultrafast change in refractive index after excitation by a femtosecond laser pulse was observed for dilute
Auramine O solutions in low-viscosity solvents by detecting the change as an ultrafast lensing effect (ULE).
The decay of the ULE signals was found to consist of two components, and both their relaxation times
depended on solvents. This is the first reported observation of the faster component by this technique. The
slower component had a similar relaxation time to that of the fluorescence lifetime. Molecular orbital calculation
results attributed the faster to relaxation in the lowest excited singlet state S1 by way of internal rotation andthe slower to the subsequent relaxation from S1 to the ground state. Solvent dependence of the experimentally
determined relaxation time was strongly correlated with the molecular weight of the solvents rather than theirbulk viscosity. A simplified model explaining these results was proposed in which solute and solvent moleculesinteract only via hydrogen bonding, the strength of which is solvent independent.

1. Introduction
Ultrafast dynamics of molecules in solutions has recently
attracted a great deal of interest in association with solvation
structure and energy transfer between molecules. Energy transfer
between solvent molecules and a solute molecule through which
activation occurs is a fundamental process to understandchemical reactions in the liquid phase. Therefore, studying theinteraction between solute and solvent will provide great insight
into the initial process of chemical reactions. Fluorescence lifetime measurements1-4 and transient absorption spectroscopy5-10
have been exclusively performed to observe the interaction
between solute and solvent. Measurements of ultrafast dynamic
Stokes shift of fluorescence can reveal detailed information
about solvation dynamics, and they have been performed for awide variety of systems.2-4 However, the fluorescence technique
has a disadvantage that it is not applicable to nonluminescentsamples. Transient absorption spectroscopy has been used toinvestigate intramolecular vibrational redistribution (IVR), which
gives information about the energy exchange between soluteand solvent. Studies of relaxation in excited electronic statesof dye molecules show that IVR occurs in 200-500 fs.8-10 Thismethod, however, has limited capability owing to its low
sensitivity, which makes application to dilute solutions difficult.
Recently, picosecond IR or picosecond Raman spectroscopy has* To whom correspondence should be addressed. E-mail: tsawada@
mail.ecc.u-tokyo.ac.jp. Fax: +81-3-5841-6037.
† University of Tokyo.
‡ Kanazawa Institute of Technology.
§ Kyushu University.

been applied to the measurement of ultrafast dynamics insolution. Hamm et al.11 investigated the vibrational cooling ofazobenzene after photoisomerization from transient IR spectrausing a 300 fs IR probe pulse. Qian et al.12 reported that transstilbene has the IVR time of 3-5 ps based on picosecond
resonance Raman spectra.
Refractive index distribution induced by a pulsed light focused
into a solution temporally works like a lens element. Thisphenomenon has been used in thermal lens spectroscopy,
applications of which have provided ultrahighly sensitive
detection of nonluminescent species. After a preliminary trial
to combine the same optical configuration as that adopted in athermal lens method with an ultrafast pump and probe technique,13 our research group developed a femtosecond ultrafast
lensing effect (ULE) technique which was used to investigate
ultrafast nonradiative dynamics of β-carotene.14 We demonstrated that this method can be applied to observe nonradiative
processes of nonluminescent samples directly with high sensitivity and with a relatively simple experimental setup. The ULEtechnique might be the only way to observe ultrafast molecular
dynamics of nonluminescent dilute solutions.
In this paper, we focus on ultrafast molecular dynamics ofAuramine O in low-viscosity solutions. This dye molecule is
known as one of the fluorescent species sensitive to environmental viscosity. It has been used to estimate microviscosity inmicelles and membranes.15-17 Oster and Nishijima18 observedthat the fluorescence quantum yield of Auramine O in glycerol
was markedly dependent on temperature. This was explainedin terms of internal rotation of the N,N-dimethylaniline groups,

10.1021/jp9843252 CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/03/1999

7576 J. Phys. Chem. A, Vol. 103, No. 38, 1999

Furui et al.

which contributes effectively to the nonradiative decay to theground state. This indicates that the fluorescent quantum yield
becomes smaller with decreasing viscosity of solvent. It istherefore difficult to observe the dynamics of Auramine O in
low-viscosity solvents, and there are few such reports. The ULEtechnique seems to be a powerful means to investigate such
samples.
Here, we report the first measurement of nonradiative
relaxation of Auramine O in low-viscosity solvents by ULE
technique. We found a previously unknown ultrafast relaxation
whose decay time depends on mean molecular weights formixed hydrogen-bonding solvents, rather than their viscosity.
We propose a simplified model to explain the experimental
results based on observation of absorption and fluorescenceemission spectra and MOPAC molecular orbital calculation.
2. Experimental Section
Principle. When a solution is irradiated by light, a spatial
distribution, typically a Gaussian type, of the light intensity
generates a spatial distribution of the refractive index in thesolution. This distribution gives rise to an optical effect similar
to a lens. This effect has been termed a “thermal lens effect”
since the distribution of the refractive index originates from heat,more specifically it is from a mass density distribution causedby thermal expansion after heat generation. In turn, an ultrafast
laser pulse also generates a similar refractive index distribution
just after irradiation, but its origin is not the mass density
distribution since the mass density cannot change so fast. In
some cases, the change in the refractive index can occur before
the light energy is transformed into heat. We term this an“ultrafast lensing effect” (ULE). Many possible physical origins
of ULEs have been reported.19,20
Apparatus. The experimental setup has been reported previously.14 Only a brief outline follows here. A mode-locked Ti:
sapphire laser (Coherent, Mira 900F) pumped by an Ar ion laser(Coherent, Innova 310) produced femtosecond optical pulses
with a repetition rate of 76 MHz, centered at 780 nm. Eachpulse had an energy of 1 nJ/pulse and 150 fs duration (fwhm)
for the autocorrelation. The output beam was divided into two
by a half mirror. One was used as a probe beam after passing
it through a computer-controlled optical delay line. The otherbeam was frequency-doubled to generate a 390 nm pump beam
after being intensity-modulated by an acousto-optic modulator
at 1.4 MHz. The pump and probe beams were coupled with adichroic mirror, focused by a convex lens (f ) 50 mm) andcollinearly incident into the sample solution of a 0.5 mm optical
path length. On reaching the quartz sample cell, the pump beam
was less than 10 mW in time-averaged intensity and 10 µm in
diameter. Polarization planes of the pump and probe beams wereset parallel to each other. Change in the divergence of the probe
beam was detected as an intensity change at the center of theprobe beam, which was measured with a small-area avalanche
photodiode (0.1 mm diameter). A colored glass filter cut off
the pump beam in front of the photodiode. The photodiodeoutput was passed through a preamplifier and a homemadepassive band-pass filter before being fed into a lock-in amplifier(EG&G Princeton Applied Research, model 5202) used forsensitive phase detection. We employed a flowing system
consisting of a flow cell and a peristaltic tube pump as asampling part. This successfully produced larger signal intensity
because the usual thermal lens effect, which decreases theintensity of the ULE signal, was suppressed owing to the flowof sample solutions.
Absorption spectra were obtained with a UV-visible spectrometer (Jasco, V-570). Fluorescence emission spectra were

Figure 1. ULE signals for Auramine O solutions in water, methanol,and 1-propanol with the concentration of 5 × 10-4M. Auramine O
structural formula.

obtained with F-3010 (Hitachi). The concentrations of solutions
were 1 × 10-4 M for absorption measurements and 5 × 10-6
M for fluorescence measurements.
A molecular orbital calculation was also performed. We
employed the AM1 method with MOPAC (MOPAC93, revision
2)21 HITAC VOS3 in the Computer Center of University ofTokyo.
Auramine O (Schmid GmbH+Co., special grade) was usedwithout further purification. Solvents used were ultrapure water
purified by a Millipore system (18 MΩ/cm conductance),methanol, 1-propanol, ethylene glycol (EG), and acetonitrile(Kanto Chemical Co., Inc.; special grade). The concentrationof the solutions for the ultrafast dynamics study was 5 × 10-4
M, except for the concentration dependence experiment. All themeasurements were carried out at room temperature (22 °C).
3. Results and Discussion
ULE signals of Auramine O solutions in water, methanol,and 1-propanol are presented in Figure 1. Increase in the signal
corresponds to the photoinduced increase in the refractive index
of the solutions and vice versa. Around delay time zero, eachsignal shows an abrupt rise over the baseline followed by an
abrupt fall beneath the baseline. After the initial sharp rise andfall, the signal increases gradually toward the baseline. Thesesignal behaviors were observed for all the Auramine O solutions
we measured. Speed of the signal recovery to the baseline
depended on the solvents as shown in Figure 1.
The signal for pure solvent, e.g., water, showed a symmetricpeak shape, which was fitted to a Gaussian shape of positive
sign with less than 400 fs of full-width at half-maximum (fwhm).
Such a signal with a smaller height was also observed for an
empty cuvette. A negative signal was not observed for any ofthe pure solvents or the empty cuvette. The minimum of thesignal intensity just after the sharp fall increased with increasing
concentration of Auramine O, whereas the maximum value ofthe initial sharp peak remained constant. We concluded that thesignal gradually increasing toward the baseline after the sharp
fall was due to the solute, i.e., Auramine O, while the initial
sharp peak was due to the solvent and the cell window. We
found that the dynamics of the relaxation from the minimum

Molecular Dynamics of Auramine O

J. Phys. Chem. A, Vol. 103, No. 38, 1999 7577

Figure 2. Fitting of the decay part of Auramine O in 1-propanol
solution. The dotted line is the fitting curve by a single-exponential
equation with a time constant of 11.5 ps, and the solid line is by adouble exponential equation with time constants of 5.3 and 23 ps.

TABLE 1: Time Constants of the Relaxation Components of
Auramine O Solutions Obtained by the ULE Techniquesolvent

faster (ps)slower (ps)ethylene glycol
methanol1-propanolacetonitrile
water-ethylene glycol mixture
93:7 (wt %)84:16 (wt %)70:30 (wt %)46:54 (wt %)water-methanol mixture
80:20 (wt %)60:40 (wt %)40:60 (wt %)20:80 (wt %)

7.9
1.5
5.3
0.8

8
23

0.5
1.1
2.4
4.4
0.6
1.0
1.3
1.6

value to the baseline depended on solvents; that is, the ULE of
Auramine O was dependent on solvent molecules.
The initial sharp peak was attributed to the optical Kerr effect(OKE), a third-order nonlinear optical effect, of the solvent andthe quartz cell wall.22 The OKE for water and quartz increases
the refractive index and the OKE temporal evolution follows
cross-correlation between the pump and probe pulses because
the OKE electronic response time dominating the sharp signal
is much faster (∼1 fs) than the pulse width (∼100 fs). Thus,
we regarded the peak center of the ULE signal for pure solvent
as delay time zero. The component before the signal reached
its minimum was neglected in the study on the dynamics ofAuramine O. Since the signals of the pure solvent and the empty
cuvette diminished within several hundred femtoseconds, they
had no influence on the time constants we have determined for
Auramine O solutions (Table 1).
We analyzed the observed ULE signals using a least-squares
fitting method. It has been reported that fluorescence lifetimesof Auramine O in alcohols observed by time-correlated single
photon counting measurement are well explained in terms of
single-exponential decay.23 However, our results for alcohols
were not fitted with a single-exponential decay. As shown in
Figure 2 for the ULE signal of the 1-propanol solution of
Auramine O, it was necessary to assume at least a doubleexponential decay. The curve representing a single-exponential
decay showed obvious systematic displacement from theexperimental data. The double-exponential fitting provided two
relaxation time constants of 5.3 and 23 ps. The fluorescence
lifetime from the first excited singlet state (S1) to the groundstate (S0) has been reported to be 24 ps. Thus, the slower decay
could be attributed to the relaxation process from S1 to S0.
With regard to the faster component, there are two possible
interpretations. The first is reorientation dynamics of solvent

Figure 3. Electronic structure of Auramine O in a vacuum calculated
by AM1 method with MOPAC93. The heat of formation represents
the energy level. The twist angle means the rotational angle of N,Ndimethylaniline groups around C-C bond.

molecules, i.e., the solvation process. However, the solvation
time of Coumarine 153 in propanol has been reported as 18 psfrom a dynamic Stokes shift measurement.24 This is much slower
than 5.3 ps, and thus we think that the faster component is notdue to solvation dynamics. The second interpretation is anonradiative relaxation process in the first excited state. It is
probable that relaxation by way of internal rotation in the S1
occurs more quickly than the relaxation from S1 to S0.
To verify this interpretation, we performed a molecular orbital(MO) calculation using MOPAC93. We calculated energy levelsof S0 and S1 states at each twist angle every 10° for a rotationof an N,N-dimethylaniline group round the C-C bond of the
Auramine O molecule. At the twist angle of 0°, the Auramine
O molecule has planar geometry. For simplicity, the Auramine
O molecule was assumed to be in a vacuum. Each point on the
excited state was calculated by freezing the other degrees of
freedom. As shown in Figure 3, the energy levels of S0 and S1
exhibit different dependence on the twist angle, reflecting thedifference of the electronic structure of each state. The stable
twist angle in S0 is at 30° and that in S1 is at 90°. This suggests
the following process. The geometry of the initial ground stateis at twist angle 30°. After excitation and vibrational relaxationfrom the Franck-Condon state occurs at 30° in the excited state,
the relaxation by way of internal rotation from twist angle 30°
to 90° in S1 occurs as a fast process, followed by the slower
process of the relaxation from S1 to S0. We attribute the fasterprocess (5.3 ps) to relaxation by way of internal rotation in the
S1 state. We might say that the detailed relaxation process of
Auramine O after photoexcitation was first observed by using
the ULE technique.
In this study, we focus on the fast process of vibrational
relaxation within the first excited singlet state, since the latterprocess of relaxation from S(1) to S(2) has been reported
previously. This temporal difference is very significant because
the IVR within S(1) might be expected to occur before breakage
of a hydrogen bond formed with the solvent, whereas deactivation of S(1) to S(0) is probably slower than dissociation of ahydrogen-bonded solvate.
In previous studies, it has been reported that the relaxation
time of Auramine O in solutions depends on only viscosity ofthe solvents. The observed relaxation times of ULE signals inmethanol, 1-propanol, and water-ethylene glycol (EG) mixtures
were plotted as a function of solvent viscosity in Figure 4. In
the water-EG mixtures, the slower component was notobserved, which seemed to correspond to the fluorescenceprocess. Although both plots for monohydric alcohols andwater-EG mixtures showed a good linear correlation, the linear

7578 J. Phys. Chem. A, Vol. 103, No. 38, 1999

Figure 4. Plot of the relaxation time vs solvent bulk viscosity. The
water-ethylene glycol system is depicted by (b), the water-methanol
system by (O), methanol by (9), and 1-propanol by (2). The inset isan expanded view.

Furui et al.

Figure 6. Plot of the relaxation time vs mean molecular weight ofsolvents. The water-ethylene glycol system is depicted by (b), thewater-methanol system by (O), methanol by (9), 1-propabol by (2),and acetonitrile by (×). The inset is an expanded view.

the energy gap between S1 and S0 is 25 000cm-1. The peakshift seems too small to cause a significant change in decay
time. It is unlikely that the solvent polarity affects the decay
time substantially by way of the change of the energy gap. This
result is consistent with the general description that the kineticsof Auramine O is not influenced by solvent polarity.23
We attempted to plot the relaxation time vs molecular weightof solvents instead of bulk viscosity based on the model thatblocking by a solvent molecule affects the relaxation time. Whenthe internal rotation is taken into account, this model seems
reasonable from a microscopic viewpoint. We define mean
molecular weight (MWmean) of mixed solvents by the next
equation,

MWmean )Figure 5. Absorption (a) and fluorescence emission (b) spectra inwater, 1-propanol, and ethylene glycol. The fluorescence emission
spectra were intensity normalized, in which Raman peaks by solventswere eliminated.

correlations themselves were not consistent with each other. For
example, a similar relaxation time was observed between
1-propanol and EG despite the great difference of viscosity
between them. Gautam and Harriman23 have proposed theconcept of “microviscosity”25,26 instead of bulk viscosity, andthey reported a good correlation between the relaxation timeand viscosity in monohydric alcohols. In this study, however,a plot of the relaxation time vs microviscosity did not improvelinearity. This suggests that solvent viscosity cannot well explain
the dependence of the decay time on solvents. This fact has
not been noticed before because ultrafast dynamics of Auramine
O in various low-viscosity solvents has not been investigateduntil now.
Since water and EG are highly polar solvents, solvent polarity
may influence the electronic energy of a molecular orbital, thechange of which affects the relaxation time. To examine theinfluence of solvent polarity on the energy gap, UV-visible
absorption and fluorescence emission spectra of several solutions
were measured (Figure 5). The peak position in the spectra
represents the value of the energy gap between electronic states.
Figure 5 shows that there is only a slight change among solvents
both in absorption spectra and in fluorescence emission spectra.
The peak shift is as small as 10 nm, which is reduced toapproximately 600 cm-1 as a wavenumber, while the peak
position is approximately 400 nm, which means the value of∑i niwi(1)

where i is denotes the solvent and ni is the mole fraction and wi
is the molecular weight of solvent i. The results are shown in
Figure 6 (see experimental data in Table 1). We obtained theinteresting result that the plots of the decay time vs MWmean
for hydrogen-bonding solvents showed a linear correlation.
Moreover, the relaxation time in acetonitrile, a non-hydrogenbonding solvent, was shorter than that in the hydrogen-bonding
solvents. On the basis of our experimental results, we propose
a simple model describing solvent-solute interaction.
In solutions, a hydrogen-bonding solvent molecule is connected with an Auramine O molecule as well as other solvent
molecules by way of hydrogen bonding. We assume that forceof hydrogen bonding between the solute and a hydroxyl groupof a solvent molecule has almost the same strength regardlessof the kind of the molecule. This assumption seems much too
simple, but it is reasonable because the structures of hydrogenbonding solvents studied in this paper are very similar to eachother. On the basis of this assumption, the rotational mobilityof N,N-dimethylaniline groups will be influenced by the massof the solvent molecule (molecular weight) in terms of an inertial
effect. Therefore, larger solvent molecules hinder rotation aroundthe C-C bond in Auramine O more effectively, which resultsin the increase of the relaxation time. On the other hand, theinteraction between solute and a non-hydrogen-bonding solvent
such as acetonitrile is very weak, and such solvent molecules
influence the internal rotation less effectively. This is because
the relaxation time for non-hydrogen-bonding solvent becomesshorter. Thus, this model explains the good correlation in Figure
6 for hydrogen-bonding solvents and the deviation for nonhydrogen-bonding ones. We think these behaviors of solvent
molecules are limited to molecules in the first or second
solvation shells around the solute molecule so that ultrafast
observation of nonradiative relaxation processes is essential toMolecular Dynamics of Auramine O
investigate them. We believe we can provide details by
quantitative discussion using the moment of inertia of thehindered rotor from the measurements of various solvents withdifferent hydrogen-bonding strength at various temperatures.
In conclusion, we investigated the dynamics of Auramine O
in dilute solutions using the ULE technique and found that therelaxation by way of intramolecular rotation was affected bymolecular weight of solvents and hydrogen bonding. We
demonstrated that the ULE technique could be a useful tool toinvestigate interactions between solute and solvent molecules.
We can expect that this technique will be applied to the chemicalsystems which are difficult to measure by other techniques.
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