Orientational and Vibrational Relaxation Dynamics of Perylene and 1-Methylperylene in n-Alcohols: Probing the Balance between van der Waals and Hydrogen-Bonding Interactions

Article abstract of DOI:10.1021/jp9845219

We report on the rotational diffusion and vibrational population relaxation dynamics of perylene and 1-methylperylene in the primary normal alcohols methanol through n-decanol. The rotational diffusion dynamics of the two chromophores are the same to within the experimental uncertainty for n-propanol through n-decanol. We observe a double exponential decay of the induced orientational anisotropy for 1-methylperylene in all of the alcohols and for perylene in n-propanol through n-decanol. These data are consistent with previous literature reports on perylene in high-viscosity solvents and on 1-methylperylene in n-alkanes longer than n-octane. These data also reveal a substantial difference in the behavior of perylene in n-alkanes and n-alkanols. Both chromophores reorient as oblate rotors in the n-alkanols, with the aspect ratio of the oblate ellipsoid describing their motion depending on the solvent aliphatic chain length. The vibrational population relaxation dynamics of the two probe molecules differ significantly, not only because of the difference in the nature of the solvent-solute coupling but also because of subtle differences in the organization of the solvent around the two chromophores. These data reflect the importance of solvent self-association in determining the local environments of these two chromophores.

Full text of DOI:10.1021/jp9845219

J. Phys. Chem. A 1999, 103, 999-1006
999
Orientational and Vibrational Relaxation Dynamics of Perylene and 1-Methylperylene in
n-Alcohols: Probing the Balance between van der Waals and Hydrogen-Bonding
Interactions
S. N. Goldie and G. J. Blanchard*
Michigan State UniVersity, Department of Chemistry, East Lansing, Michigan 48824-1322
ReceiVed: NoVember 23, 1998
We report on the rotational diffusion and vibrational population relaxation dynamics of perylene and
1-methylperylene in the primary normal alcohols methanol through n-decanol. The rotational diffusion dynamics
of the two chromophores are the same to within the experimental uncertainty for n-propanol through n-decanol.
We observe a double exponential decay of the induced orientational anisotropy for 1-methylperylene in all
of the alcohols and for perylene in n-propanol through n-decanol. These data are consistent with previous
literature reports on perylene in high-viscosity solvents and on 1-methylperylene in n-alkanes longer than
n-octane. These data also reveal a substantial difference in the behavior of perylene in n-alkanes and n-alkanols.
Both chromophores reorient as oblate rotors in the n-alkanols, with the aspect ratio of the oblate ellipsoid
describing their motion depending on the solvent aliphatic chain length. The vibrational population relaxation
dynamics of the two probe molecules differ significantly, not only because of the difference in the nature of
the solvent-solute coupling but also because of subtle differences in the organization of the solvent around
the two chromophores. These data reflect the importance of solvent self-association in determining the local
environments of these two chromophores.
Introduction
solvent-solute interactions do not offer the same time-resolution
as the TDFSS measurements, but the phenomena of rotational
Understanding the details of solvent-solute interactions has
received a great deal of attention by the chemistry community
because of the importance of these interactions in determining
properties such as chemical reaction yield and kinetics or the
ability to isolate one compound from another. Interactions
between solutes and their surrounding solvent molecules are
difficult to resolve because, unlike in solids, the spatial
relationships between the molecules are not fixed on time scales
that can be accessed using structural measurements such as
X-ray diffraction or multidimensional NMR spectrometry.
Intermolecular interactions in the liquid phase are more complex
than those in the gas phase because of their characteristic
strength, the property that gives rise to the liquid phase and at
the same time prevents a simple statistical description of
collisional interactions from providing adequate insight. For
these reasons, time domain spectroscopies have found particular
favor in the investigation of solvation processes.
There are essentially three time-resolved spectroscopic ap-
proaches to the measurement of solvent-solute interactions.
These are measurement of the time-delayed fluorescence Stokes
shift (TDFSS) of selected solutes,1-10 rotational diffusion11-31
and vibrational population relaxation.32-44 Of these, TDFSS
measurements have provided the most insight into the early-
time separation of inertial and diffusive contributions to solvent
reorganization about excited solute molecules, and at the same
time these reports have proven to be the most controversial in
terms of their interpretation.45-49 Perhaps the most significant
issue that needs to be addressed regarding TDFSS measurements
is the lack of generality of the effect. Despite the broad
conclusions drawn from these experimental results, only a
handful of chromophores with complicated spectroscopic re-
sponses are known to exhibit resolvable transient Stokes
shifts.2,50 The two other approaches to the measurement of
diffusion and vibrational relaxation are not limited to a few
chromophores; these are general effects, and the characteristic
time scales over which they proceed are consonant with
chemical reaction processes. We concentrate here on the
rotational diffusion and vibrational population relaxation dy-
namics of perylene and 1-methylperylene in the n-alkanols
methanol through n-decanol. We have chosen these probe
molecules because they are relatively well characteri-
zed29-31,39-41,43,44 and because literature reports on their solution-
phase dynamical behavior leave open some significant questions.
Perylene has been studied more extensively than 1-methyl-
perylene. Several reports on the reorientation dynamics of
perylene point to solvents in which a two-component anisotropy
decay is seen30,31 and others where there is only a single decay.29
It is important to understand the fundamental basis for these
results and the data we present here provide some insight into
this matter. Comparing the reorientation behavior of perylene
with that of 1-methylperylene allows us to evaluate whether
there is a measurable contribution from (weak) dipolar solvation
effects in addition to the dominant van der Waals interactions.
We reported previously that the reorientation of perylene in
n-alkanes pentane through hexadecane yielded single-exponen-
tial anisotropy decays29 and the nature of the solvent-solute
frictional interactions changed as the length of the solvent
molecule became similar to that of the solute. For 1-methyl-
perylene in these same solvents, we found that there was a clear
break in the anisotropy behavior at the same point, but the
measured change was not simply a change in the frictional
boundary condition; it was a shift in the functionality of the
anisotropy decay from single to double exponential.42 Such a
change is consistent with a substantial alteration of the way in
which the 1-methylperylene chromophore reorients in the longer
alkane solvents. These data point to a qualitative change in the
nature of solvent-solute interactions that depends on the relative
* To whom correspondence should be addressed.
10.1021/jp9845219 CCC: $18.00 © 1999 American Chemical Society
Published on Web 02/10/1999

1000 J. Phys. Chem. A, Vol. 103, No. 8, 1999
Goldie and Blanchard
lengths of the molecules, but a broader picture of the solvation
of these two molecules has yet to emerge.
in the range 456.7-463.1 nm. The pump wavelength was chosen
to access the 0-0 transition of the chromophore and the probe
V)0
wavelength to stimulate emission from the S1
S0
state to the
We have measured vibrational population relaxation from the
∼1375 cm-1 ring breathing modes of perylene and 1-methyl-
perylene to understand the role of “structural” factors in
mediating solution-phase vibrational excitation transport. We
have found that both the spatial separation between vibrational
donor and acceptor moieties and their relative orientation are
important in mediating vibrational excitation transport.44 Inter-
estingly, it is not the acceptor density that determines the
efficiency of transport. The ability of the donor and acceptor
vibrational coordinates to align with one another mediates
vibrational excitation transport for 1-methylperylene in the
alkanes. In contrast to the reorientation measurements, the
vibrational population relaxation dynamics of these two chro-
mophores cannot be compared directly because the efficiency
of donor-acceptor coupling is determined by the symmetry of
the chromophores.42 Using our two-pulse measurement scheme,
systems that possess a center of inversion, such as perylene,
will necessarily couple to the solvent bath modes according to
the modulation of the solute’s quadrupole moment. For donors
without a center of inversion, such as 1-methylperylene, the
modulation of the solute dipole moment by vibrational motion
mediates excitation transport. These two coupling processes
operate over different length scales (r-8 vs r-6), and thus it is
not possible to compare results directly. Despite the differences
of scale, the solvent-dependent trends observed in the T1 data
for the two molecules provide insight into the average spatial
relationship between donor and acceptor functionalities in
solution.
Comparison of the reorientation and vibrational population
relaxation dynamics we report here for perylene and 1-meth-
ylperylene in the n-alkanols yields significant information on
the nature of the solvent-solute interactions that the chro-
mophores experience. A key difference between the n-alkanes
and n-alkanols is the extent to which solvent self-association
determines their properties.21,51 We find from the reorientation
measurements that self-association of the alcohols determines
the immediate environment of the chromophores to a significant
extent, especially for the longer n-alkanes. The vibrational
population relaxation data suggest that, for the longer n-alkanols,
perylene appears to associate with the nonpolar aliphatic chains
while the more polar 1-methylperylene interacts with the solvent
alcohol functionality. These results are not surprising but do
reveal the point at which solvent size, relative to the chro-
mophore, becomes important for amphiphilic systems.
V)1
state, where the vibrational resonance of interest is the
1375 cm-1 V ) 1 ring breathing mode. These wavelengths were
used for the vibrational relaxation and the reorientation mea-
surements. Our previous work has demonstrated that the ground-
state and excited-state reorientation behavior of perylene are
identical,29 and we assume that this is also the case for
1-methylperylene. The probe laser polarization was set alter-
nately to 0° and 90° relative to the pump laser polarization for
the reorientation measurements and to 54.7° for the vibrational
relaxation measurements. The time resolution of this system,
∼10 ps, is determined by the cross-correlation between the pump
and probe laser pulse trains. Detection of the transient signals
was accomplished using a radio and audio frequency triple-
modulation scheme, with synchronous demodulation detec-
tion.53-55 Each reported time constant is the average of at least
six individual determinations that are themselves the average
of 7-10 time-scans.
Steady-State Spectroscopy. The steady-state absorption
spectra of both chromophores in the n-alkanols were recorded
with 1 nm resolution using a Hitachi U-4001 spectrometer. The
spontaneous emission spectra for the same solutions was
obtained with 1 nm resolution using a SPEX Fluorolog2 model
F111AT spectrometer. These data were used to determine the
appropriate laser wavelengths for each chromophore/solvent
pair. The absorption and emission spectra for perylene and
1-methylperylene in n-pentanol are shown in Figure 1. We
understand the details of these spectral profiles and describe
them in the next section.
Chemicals and Sample Handling. Perylene (99.5%, sub-
limed) was obtained from Aldrich Chemical company and used
as received. 1-Methylperyelene was synthesized by using the
procedure of Peake et al.,56 which is selective for methylation
at the 1-position. Perylene is reacted with CH3Li/LiBr(eth) in dry
THF at -78 °C, followed by quenching with solid I2 and
reduction with Na2S2O3. All chemicals used in the synthesis
were purchased from Aldrich in the highest purity grade
available. THF was dried over Na(s) and distilled prior to use.
Initial purification of the product was accomplished with column
chromatography (SiOx column, 95:5 hexane/ethyl acetate). The
product fraction was recrystallized according to the procedure
specified by Zieger et al.57 The recrystallized product was pure
1
by H NMR, UV-visible absorption spectroscopy, and mass
spectrometry.
Results and Discussion
Experimental Section
The focus of this work is on understanding the solvent-solute
interactions of perylene and 1-methylperylene in the n-alkanols.
Placing our results in the context of previous work on these
same chromophores will allow us to gain insight into the
dominant intermolecular interactions of these probe molecules.
While the majority of the information on the solvation behavior
of these probes is obtained from the time domain measurements,
it is important to understand their steady-state optical responses
because they are reflective of the average environment they
experience.
Steady-State Linear Spectroscopy. Among the reasons for
using perylene and 1-methylperylene for these studies is the
well-behaved optical response of each molecule. The quasimirror
image absorption and emission spectra demonstrate the relatively
small role that vibronic coupling plays in these data,58 and the
Stokes shift of 1-methylperylene compared to that of perylene
Laser System. The picosecond pump-probe laser spectrom-
eter used for both the reorientation and vibrational population
relaxation measurements has been described in detail previ-
ously,52 and we present only a brief outline of its performance
characteristics here. A mode-locked CW Nd:YAG laser (Coher-
ent Antares 76-S) produces 30 W of average power (1064 nm,
100 ps pulses, 76 MHz repetition rate). The output of this laser
is frequency-tripled to produce ∼1.3 W of average power at
354.7 nm. The third harmonic light is used to excite two cavity-
dumped dye lasers (Coherent 702) synchronously. Both lasers
operate with Stilbene 420 laser dye (Exciton). The output of
each laser is ∼100 mW average power at 8 MHz repetition
rate with a pulse that produces a 7 ps fwhm autocorrelation
trace using a three plate birefringent filter. The pump laser
wavelength was set between 429.7 and 438.9 nm, depending
on the chromophore and solvent, while the probe laser was set

Perylene and 1-Methylperylene in n-Alcohols
J. Phys. Chem. A, Vol. 103, No. 8, 1999 1001
Figure 2. Dependence of perylene and 1-methylperylene absorption
maxima on solvent alcohol aliphatic chain length. When normalized
for wavelength shift relative to their absorption spectra in methanol,
the solvent dependencies are identical for both chromophores.
solvent dependences for perylene and 1-methylperylene appear
to be identical and, if plotted in terms of relative shift with
respect to the band position in methanol (not shown), the results
for the two chromophores are identical. We understand these
shifts in the context of the chromophore ground and excited
states being solvated more efficiently with increasing aliphatic
chain length of the alkanol solvent. The excited states of the
chromophores are solvated more efficiently than the ground
states, presumably because van der Waals interactions between
the solvent aliphatic chains and the chromophore π* states are
stronger than those for the ground state. These data are
reminiscent of those for oxazine 725 in the same solvents, except
that we do not see a saturation in the spectral red shift with
increasing aliphatic chain length here. We interpret this differ-
ence in terms of the affinity for the ionic dye oxazine 725 for
the alcohol functionality and the preference of perylene and
1-methylperylene for the aliphatic portion of the solvent. These
data suggest that the balance of interactions between the different
functionalities of these solvents will be important in understand-
ing the transient data we consider next.
Figure 1. (a) Absorption and emission spectra of perylene in
n-pentanol. (b) Absorption and emission spectra of 1-methylperylene
in n-pentanol. Intensities have been normalized for presentation.
points to the presence of a small but finite permanent dipole
moment in 1-methylperylene. PM3 semiempirical calculations
indicate µ ≈ 0.24 D in the S0 and 0.53 D in the S1 for
1-methylperylene.59
Molecular Reorientation. Of the time domain spectroscopies
used in the study of solvent-solute interactions, molecular
reorientation measurements are supported best by a theoretical
framework for the interpretation of the experimental data.62-67
The starting point for relating reorientation data to solvent and
solute properties is the modified Debye-Stokes-Einstein (DSE)
equation,62,63,65-67
The linear responses of solutes can provide significant insight
into the dielectric properties of the solvent. For example, the
absorption maximum of the polar chromophore oxazine 725
exhibits a solvent polarity-dependent red shift in n-alkanol
solvents up to n-heptanol.24 Comparison of those data to the
oxazine 725 absorption maximum in DMSO reveals that at least
one contribution to the polarity-dependent spectral shift is
solvent H-bonding. For the chromophores we use here, H-
bonding almost certainly does not play the same role, but it is
clear that there is a solvent dependence to their absorption
maxima. Before considering this point, we need to account for
the differences in the linear response of perylene and 1-meth-
ylperylene. The absorption maximum of perylene is red shifted
from 1-methylperylene for a given solvent, and the individual
features in the 1-methylperylene spectra are less well-defined.
We understand these differences as arising from the fact that
the methyl group on 1-methylperylene causes the two naph-
thalene moieties to be cocked at ∼20° with respect to one
another.60,61 The resulting slight break in conjugation causes
the blue shift and the reduction in symmetry causes additional
vibrational resonances to become allowed, adding to the greater
width of the absorption and emission spectral features. The
solvent dependence of the absorption maxima of perylene and
1-methylperylene is shown in Figure 2. First, we note that the
ηVf
kBTS
τOR
)
(1)
In eq 1, η is the solvent viscosity, V is the solute hydrody-
namic volume (225 Å3 for perylene and 243 Å3 for 1-meth-
ylperylene),68 f is a frictional interaction term to account for
frictional contributions to solvent-solute interactions, kB is the
Boltzmann constant, T is the temperature, and S is a shape factor
to account for the nonspherical shape of the solute. The DSE
equation is clearly not intended to account for specific molec-
ular-level interactions between molecules, and it is clear that
any description of solvation phenomena based on solvent bulk
properties must be incomplete. These limitations notwithstand-
ing, the modified DSE model provides remarkably close
agreement with many experimental results and it thus serves as
a useful starting point in any discussion of reorientation
measurements.

1002 J. Phys. Chem. A, Vol. 103, No. 8, 1999
Goldie and Blanchard
Experimentally, the time-dependencies of the stimulated gain
signal on the probe beam, polarized parallel (I (t)) and
||
perpendicular (I (t)) to the pump beam, are used to extract
information on the rerandomization of the anisotropic distribu-
tion of chromophores selected by the pump pulse.
I|(t) - I (t)
R(t) )
(2)
I|(t) + 2I (t)
The most common result for reorientation measurements is to
recover an experimental R(t) function that decays as a single
exponential, with the time constant of this decay being taken
as τOR. Such data are often interpreted in the context of eq 1.
There are a number of experiments, however, where R(t) is
found to decay with a multiple exponential functionality,22,30,42,69
or depends on the manner in which the chromophore is
excited.11,31 In these cases, eq 1 is clearly not sufficient. Chuang
and Eisenthal have formulated the relationship between R(t) and
solute properties such as the angle between the excited and
probed one-photon transition moments and the actual anisotropy
in the diffusion constant.64 They found that R(t) can decay with
as many as five exponential components, although the most
common case is that of a single exponential. In cases where
more than one decay component is resolved, it is possible to
interpret the details of the reorientation dynamics in substantial
detail.
We consider that the chromophores have their π-system in
the approximate xy plane, with x being the long, in-plane axis,
and z is perpendicular to the π-system plane. For each
chromophore, our intention is to assign the coordinate system
such that the x-axis coincides with the absorption transition
moment. For perylene, this assignment is coincident with
specific bond axes because of its high symmetry, while for
1-methylperylene it is not, since the π-system for this chro-
mophore is nonplanar. We can relate the experimental reorienta-
tion times to the shape of the volume swept out by the rotating
molecule.64 We describe this volume in terms of either a prolate
ellipsoid, characterized by rotation primarily about its long in-
plane axis, Dx > Dy ≈ Dz, or an oblate ellipsoid, characterized
by rotation about the axis perpendicular to the chromophore
π-system, Dz > Dx ≈ Dy.
Figure 3. (a) Experimental I (t) and I (t) scans for 1-methylperylene
||
in n-pentanol, along with the instrumental response function. These
data are typical of those recorded for reorientation measurements. (b)
Anisotropy function, R(t), for the data shown in (a). The decay is fit to
the function R(t) ) R1(0) exp(-t/τ1) + R2(0) exp(-t/τ2).
and including n-octane.29,42 As indicated by eqs 3 and 4, for
our experimental conditions, a single-exponential decay of R(t)
indicates that the chromophore reorients as a prolate rotor and
a double exponential decay indicates reorientation as an oblate
rotor. It is important to consider both the information content
of the data we report here and how these results augment our
earlier reports on the reorientation of these chromophores in
the n-alkanes.
oblate:
R(t) ) 0.3 exp(-(2Dx + 4Dz)t) + 0.1 exp(-6Dxt) (3)
prolate: R(t) ) 0.4 exp(-6Dzt)
(4)
Using eq 3 to interpret our data, we can obtain the quantities
Dz and Dx. We show the diffusion constants as a function of
solvent aliphatic chain length in Figure 5a and their ratio in
Figure 5b. These data on 1-methylperylene are consistent with
those reported in the n-alkanes because, in both cases, they trend
toward an increasing ellipsoidal aspect ratio with increasing
solvent aliphatic chain length. In our previous work,42 for
n-alkanes octane and shorter we recovered a ratio Dz/Dx < 1,
and in this work, Dz/Dx ranges from ∼4 to ∼8.5. Also, in the
n-alkanes, we see that the ellipsoid aspect ratio saturates at ∼8.5
for sufficiently long alkanes.42 For 1-methylperylene in n-
decanol, Dz/Dx ≈ 8.5. It is not clear that this is a saturation
value because we have not used longer n-alkanols because
n-dodecanol has been shown to form a mesophase between 24
°C and 30 °C.21 Clearly, the data on 1-methylperylene in the
two families of solvents are not identical, but this is not
surprising because of the important role that the alcohol
functionality plays in mediating persistent organization.51,70 The
close correspondence between the two bodies of data, however,
In eqs 3 and 4, we have used the quantity Dx. We have also
assigned the molecular axis along which the transition moments
lie to be the x axis, and for this assignment, the reorientation
measurements will not contain any explicit information on Dx.
In the limit of an oblate rotor, where Dx ) Dy, we could equally
well have made this substitution in eqs 3 and 4. To allow more
obvious comparison with our previous reports, we continue to
use the quantity Dx.
For both perylene and 1-methylperylene in the n-alkanols,
we measure experimental R(t) functions that are fit best by the
sum of two exponential decays (Figure 3) (Table 1). The solvent
dependence of the fitted times is presented in Figure 4. Our
findings are in agreement with several other results reported
for perylene in viscous solvents30,31,69 and for 1-methylperylene
in n-alkanes longer than n-octane.42 Our previous work showed
that perylene exhibits a single-exponential anisotropy decay in
the alkanes n-pentane through n-hexadecane and 1-methyl-
perylene exhibits a single-exponential decay in n-alkanes up to

Perylene and 1-Methylperylene in n-Alcohols
TABLE 1: Reorientation Times and Zero-Time Anisotropies for Perylene and 1-Methylperylene in the n-Alkanolsa
perylene 1-methylperylene
J. Phys. Chem. A, Vol. 103, No. 8, 1999 1003
solvent
R1(0)
τ1 (ps)
R2(0)
τ2 (ps)
R1(0)
τ1 (ps)
R2(0)
τ2 (ps)
CH3OH
0.23 ( 0.06
0.11 ( 0.02
0.20 ( 0.05
0.17 ( 0.05
0.19 ( 0.09
0.19 ( 0.07
0.22 ( 0.05
0.21 ( 0.04
0.20 ( 0.05
0.20 ( 0.05
17 ( 2
30 ( 6
16 ( 4
27 ( 8
21 ( 7
28 ( 6
35 ( 7
32 ( 5
33 ( 15
37 ( 7
0.16 ( 0.04
0.25 ( 0.05
0.28 ( 0.03
0.19 ( 0.05
0.19 ( 0.06
0.21 ( 0.04
0.23 ( 0.03
0.18 ( 0.02
0.16 ( 0.04
0.21 ( 0.02
11 ( 2
17 ( 2
18 ( 2
17 ( 3
21 ( 7
23 ( 7
29 ( 4
29 ( 5
28 ( 5
40 ( 6
0.06 ( 0.01
0.07 ( 0.03
0.09 ( 0.03
0.12 ( 0.04
0.12 ( 0.05
0.13 ( 0.04
0.14 ( 0.02
0.14 ( 0.02
0.17 ( 0.03
0.11 ( 0.02
34 ( 5
53 ( 13
62 ( 6
69 ( 14
81 ( 27
107 ( 26
139 ( 15
133 ( 15
132 ( 20
206 ( 21
C2H5OH
C3H7OH
C4H9OH
C5H11OH
C6H13OH
C7H15OH
C8H17OH
C9H19OH
C10H21OH
0.10 ( 0.03
0.06 ( 0.03
0.11 ( 0.03
0.11 ( 0.03
0.10 ( 0.03
0.11 ( 0.01
0.14 ( 0.04
0.15 ( 0.02
59 ( 17
101 ( 39
82 ( 16
110 ( 22
168 ( 29
157 ( 15
146 ( 23
217 ( 45
a The data are the best-fit results of the data to the function R(t) ) R1(0) exp(-t/τ1) + R2(0) exp(-t/τ2). Times are given in picoseconds and the
uncertainties listed are standard deviations ((1σ) for at least six determinations of each quantity.
Figure 4. Reorientation time constants plotted as a function of solvent
viscosity for perylene (b) and 1-methylperylene (9). The reorientation
time constants are identical to within the experimental uncertainty for
the two chromophores in the solvents used. The fast reorientation time
is essentially solvent viscosity independent and the dependence of the
slow reorientation time does not depend linearly on viscosity.
suggests that the same fundamental physical processes are
operative in both systems.
The data on perylene in the alkanes and alkanols provide less
in the way of direct correspondence. For n-alkanes up to
n-hexadecane29 and in methanol and ethanol, we recover a
single-exponential decay, and for n-alkanols propanol through
decanol, we find a two-component anisotropy decay. Perylene
reorients as a prolate rotor in alkanes and as an oblate rotor in
the alkanols n-propanol and longer. In methanol and ethanol,
we recover what appears to be a single exponential decay, and
note that our ability to distinguish between a fast second
component and a single decay is limited by temporal resolution
and signal-to-noise ratio. The apparently discontinuous change
in Dz for perylene in ethanol and n-propanol (Table 2) suggests
that we have not resolved the fast second component in R(t)
for the smaller solvents. We measure Dz/Dx ≈ 8.5 for perylene
Figure 5. (a) Cartesian components of the rotational diffusion constant
as a function of solvent aliphatic chain length for perylene (Dz ) b,
Dx ) O) and 1-methylperylene (Dz ) 9, Dx ) 0) in the n-alkanols.
(b) The ratio Dz/Dx as a function of solvent chain length for perylene
(b) and 1-methylperylene (9). These data indicate that the environment
experienced by both chromophores is essentially identical and the
structural anisotropy in the solvent cage increases with solvent aliphatic
chain length.
in n-decanol and Dz/Dx < 1 for perylene in n-decane. This
remarkable difference in aspect ratios for one chromophore in
the different solvents underscores the important role that the
presence of the alcohol group plays in the organization of the
solvent.
It is interesting to note that, for both chromophores, the
increasingly anisotropic environment formed with increasing
solvent aliphatic chain length is characterized by a strong
dependence of Dz on solvent identity and a much weaker solvent
dependence of Dx (Figure 5a). The dominant constraint imposed
on the chromophores is for reorientation out of plane, suggesting
a layered or quasilamellar environment. This finding is consis-
tent with our results on 1-methylperylene in the n-alkanes, and
the fact that we observe this phenomenon in all solvents argues
for persistent local organization over a length-scale that is at
least on the order of the chromophore (vide infra). We note
also that this result is in excellent agreement with a recent report

1004 J. Phys. Chem. A, Vol. 103, No. 8, 1999
Goldie and Blanchard
TABLE 2: Cartesian Components of the Rotational
Diffusion Constant, D, for Perylene and 1-Methylperylene in
the n-Alkanolsa
TABLE 3: Vibrational Population Relaxation Times, T1, for
Perylene and 1-Methylperylene in the n-Alkanolsa
perylene
1-methylperylene
perylene
1-methylperylene
Dz/Dxb Dx (GHz) Dz (GHz) Dz/Dxb
4.9 ( 0.7 20.3 ( 0.7 4.1 ( 0.2
solvent
T1 (ps)
T1 (ps)
solvent Dx (GHz) Dz (GHz)
CH3OH
209 ( 68
374 ( 109
443 ( 111
389 ( 119
397 ( 109
180 ( 52
439 ( 109
54 ( 24b
40 ( 18
31 ( 7
CH3OH
C2H5OH
9.8 ( 1.2 <1
5.6 ( 1.1 <1
C2H5OH
C3H7OH
C4H9OH
C5H11OH
C6H13OH
C7H15OH
C8H17OH
C9H19OH
C10H21OH
3.2 ( 0.8 13.1 ( 1.5 4.2 ( 0.3
48 ( 11
126 ( 20
36 ( 11
60 ( 11
41 ( 12
160 ( 41
214 ( 25
99 ( 38
C3H7OH 2.8 ( 0.8 14.2 ( 3.6 5.0 ( 0.4 2.7 ( 0.3 12.5 ( 1.4 4.7 ( 0.1
C4H9OH 1.7 ( 0.6 8.4 ( 2.5 5.1 ( 0.5 2.4 ( 0.5 13.5 ( 2.4 5.6 ( 0.3
C5H11OH 2.0 ( 0.4 10.9 ( 3.6 5.4 ( 0.4 2.1 ( 0.7 10.9 ( 3.6 5.3 ( 0.5
C6H13OH 1.5 ( 0.3 8.2 ( 1.8 5.4 ( 0.3 1.6 ( 0.4 10.1 ( 3.1 6.5 ( 0.4
C7H15OH 1.0 ( 0.2 6.7 ( 1.3 6.7 ( 0.3 1.2 ( 0.1 8.0 ( 1.1 6.7 ( 0.2
C8H17OH 1.1 ( 0.1 7.3 ( 1.1 6.9 ( 0.2 1.3 ( 0.1 8.0 ( 1.4 6.4 ( 0.2
C9H19OH 1.1 ( 0.2 7.0 ( 3.2 6.1 ( 0.5 1.3 ( 0.1 8.3 ( 1.5 6.6 ( 0.2
C10H21OH 0.8 ( 0.2 6.4 ( 1.2 8.3 ( 0.3 0.8 ( 0.1 5.8 ( 0.9 7.2 ( 0.2
72 ( 16b
185 ( 74b
a Relaxation times are determined from at least six individual data
sets, and the uncertainties are standard deviations. Times are values
from fits of the experimental data to eq 5 and are given in ps. b Results
of fits to eq 6.
a These quantities and their uncertainties are derived from the fitted
time constants shown in Table 1. b The uncertainties in the quantities
Dz/Dx were calculated from the standard deviations in the Dz and Dx
data according to the formula σD /D ) (Dz/Dx) (σD /Dx)2+(σD /Dz)2.
z
x
x
x
z
As we had mentioned above, the alkanol solvents likely exist
primarily as H-bonded trimers, and such a configuration will
likely give rise to a microheterogeneous environment. With this
picture in mind, we present our T1 data (Table 3).
by the Johnson group on two-photon excitation-induced ani-
sotropy decays, where the form of the R(t) function was found
to depend measurably on the excitation polarization conditions.31
It is clear that the reorientation behavior of these two
chromophores depends sensitively on the solvent in which they
are dissolved. With only the reorientation data, it is possible to
draw correlations between bulk solvent properties and micro-
scopic reorientation behavior, but the extent to which these
correlations can provide real insight into the relevant intermo-
lecular interactions is limited. For this reason, we have also
studied the vibrational population relaxation behavior of these
chromophores in the same alkanol solvents. Those data, in
conjunction with the reorientation measurements, provide ad-
ditional insight into the average organization of solvents about
the solutes.
We have focused on the vibrational relaxation behavior of
the perylene 1375 cm-1 and the 1-methylperylene 1370 cm-1
ring breathing modes.39-44 We have chosen these modes because
the terminal CH3 group on the alkanol exhibits a rocking motion
at ∼1378 cm-1, essentially degenerate with the solute donor
mode. In this manner, measurement of T1 as a function of
alkanol identity can provide information on the spatial proximity
and relative orientation of the donor and acceptor vibrational
coordinates. The details of how the experimental technique
operates have been presented elsewhere39,71,72 and we forego a
discussion of this matter here. The vibrational population
relaxation time constants we report here are significantly longer
for perylene than for 1-methylperylene in methanol through
n-heptanol. We understand this effect in the context of the nature
of the coupling between the donor and acceptor.40,42 For
molecules possessing a center of inversion, the lowest order
multipole moment that can be modulated using our excitation
scheme is the quadrupole moment. Assuming an IR-active
acceptor mode, the intermolecular coupling will decay with
Before we discuss our vibrational population relaxation data,
we consider what is known about the solvent alcohols. NMR
experiments have shown that, at room temperature, the dominant
form of the n-alkanols is the trimer,51 which is believed to exist
in a ring-bound configuration.21 This finding indicates that the
solvent bath mode is heterogeneous on the length scale sensed
by the solvent. Such local heterogeneity can, in principle, be
sensed by vibrational population relaxation as we discuss below.
Another significant implication of the presence of trimers in
solution is that it makes the dominant form of the solvent have
a hydrodynamic volume 3 times larger than that predicted from
calculations of monomer solvent.68 This is true in the limit of
the characteristic lifetime of the solvent trimers being on the
same order as the dynamics we measure. For the alkanols, the
longitudinal relaxation time, τL, ranges from tens to hundreds
of picoseconds as the aliphatic chain length increases.70 With
the larger effective solvent species, it is reasonable to expect
more restriction of the chromophores, consistent with our
findings of an effective oblate rotor shape in the n-alkanols and
in the longer n-alkanes for 1-methylperylene.
Vibrational Population Relaxation Measurements. Vibra-
tional population relaxation measurements provide information
that is complementary to reorientation measurements. While
these two measurements sense fundamentally different phe-
nomena, both depend on the properties of the chromophore local
environment. The reorientation data we present here indicate
that, in the n-alkanols, both perylene and 1-methylperylene exist
in environments that are restricted in such a way as to allow
chromophore rotation primarily about an axis perpendicular to
the π-system plane. The type of solvent local organization most
consistent with these findings is that of a quasilamellar medium.
donor-acceptor spacing as r-8 40
For a donor that does not
.
possess a center of inversion, the excited vibrational resonance
will modulate the dipole moment and the donor-acceptor
coupling will depend on intermolecular distance as r-6 42
We
.
have synthesized and use 1-methylperylene precisely because
it is of lower symmetry than perylene, and we have shown that
the coupling between the ring breathing mode and the solvent
bath will be stronger than it is for perylene.
Because the theoretical framework for the interpretation of
vibrational population relaxation in liquids is not well developed,
we do not attempt to extract solvent-solute interaction informa-
tion from single T1 data points. Rather, we are interested in the
solvent-dependent behavior of these time constants (Figure 6).
We observe a break in the T1 trend for both chromophores at
n-octanol. For 1-methylperylene, we see an abrupt increase in
T1 with increasing alkanol chain length. We interpret this effect
as a change in the proximity of the solvent acceptor mode
relative to the solute donor mode. This finding suggests the
association of the 1-methylperylene chromophore with the
solvent alcohol functionalities. We can rationalize this assertion
based on the fact that 1-methylperylene has a nonzero dipole
moment and dipolar interactions must therefore contribute to
the solvent-solute interactions at some level. When the alkanol
chain becomes long enough that the terminal CH3 group can,

Perylene and 1-Methylperylene in n-Alcohols
J. Phys. Chem. A, Vol. 103, No. 8, 1999 1005
but for the longer alkanols the signal functionality changes to
be a sum of exponentials (Figure 7b).
S(t) ) a exp(-t/τste) + b exp(-t/τ2)
(6)
where τ2 is the fast decay time constant and τste is the stimulated
emission decay time constant. We have encountered this solvent-
dependent changeover previously for tetracene in n-hexadecane43
and understand it in the context of the solvent-solute interac-
tions either providing or mediating an additional nonradiative
decay pathway. In the previous work, we conjectured that
intersystem crossing was responsible for the observed solvent-
dependent change in the functionality of S(t) because tetracene
has a relatively low fluorescence quantum yield, φfl, and a
43,73,74
modest intersystem crossing quantum yield, φISC
.
For
perylene, φfl is ∼0.94, and this quantity depends only weakly
on the identity of the solvent.75 Intersystem crossing is thus not
a likely explanation for the results we present here. Because
the origin of this new relaxation pathway is not known, there
is necessarily ambiguity in assigning the physical significance
of the recovered time constant. We therefore will not attempt
to cloud the discussion presented here with speculative inter-
pretations of these data. The fact that we see a change in the
interactions between perylene and its immediate environment
at the same point that we observe a change for 1-methylperylene
indicates that perylene is sensing either the same local environ-
ment as 1-methylperylene or its complement. For 1-methyl-
perylene, the T1 data are consistent with a measurable contribu-
tion from dipole-dipole solvent-solute interactions, but for
perylene, there is no permanent dipole moment, precluding the
contribution of such interactions. The complement to the
environment sensed by 1-methylperylene would be for perylene
to interact most strongly with the aliphatic portion of the solvent,
and this suggestion is in intuitive agreement with the dominance
of van der Waals interactions between (symmetric) PAHs and
aliphatic solvents. Indeed, we observe this same behavior for
tetracene only in long-chain aliphatic solvents.
Figure 6. Vibrational population relaxation times for perylene (b)
and 1-methylperylene (0) as a function of solvent aliphatic chain length.
Taken as a whole, the reorientation and vibrational population
relaxation data for perylene and 1-methylperylene in the alkanols
point to the importance of solvent self-organization in determin-
ing solvent-solute interactions. Alcohols are known to form
persistent H-bonded networks with the lifetime of the network
being proportional to the aliphatic chain length.51,70 NMR data
point to the dominance of H-bonded trimers in solution,51 and
with the assumption that the characteristic time constant of the
H-bonded network is the same as the average lifetime for the
trimers, it is likely that these species dominate the solvent-
solute interaction dynamics we sense. The reorientation data
suggest that both chromophores are constrained to reorient in
an environment that is best described as quasilamellar.42 The
presence of solvent trimers implies structural heterogeneity on
the length scale of the solvent molecules, with relatively
hydrophobic and hydrophilic microenvironments in close spatial
proximity. The vibrational population relaxation measurements
suggest that 1-methylperylene associates with the more closely
with polar portions of the solvent, and this segregation becomes
pronounced for sufficiently large alkanol solvents. For perylene,
it is likely that the chromophore associates with the less polar
environment, although it would be necessary to assign some
specific physical meaning to the second exponential decay seen
for n-octanol through n-decanol to augment this assertion.
Figure 7. Magic-angle data for perylene in (a) n-pentanol and (b)
n-decanol. Note the difference in the functionality of the population
decay. See text for a discussion.
on average, be relatively isolated from the 1-methylperylene
chromophore, the donor-acceptor coupling weakens, accounting
for our experimental findings.
For perylene, the situation is not directly comparable. This
is because the functionality of the population relaxation dynam-
ics data changes for n-octanol through n-decanol. For solvents
methanol through n-heptanol, we recover the expected form of
the signal for a coupled three-level system (Figure 7a). We have
reported on the details of this experiment previously39,71,72 and
forego a detailed discussion here.
Conclusions
We have studied the rotational diffusion and vibrational
population relaxation dynamics for perylene and 1-methyl-
S(t) ) a exp(-t/τste) - b exp(-t/T1)
(5)

1006 J. Phys. Chem. A, Vol. 103, No. 8, 1999
Goldie and Blanchard
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Acknowledgment. We are grateful to the National Science
Foundation for support of this project through Grant CHE 95-
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