Change of internal hydrogen bonding of methyl red upon photoisomerization monitored by forced Rayleigh Scattering

Article abstract of DOI:10.1021/jp9838442

A nonmonotonic, decay-growth-decay type forced Rayleigh scattering (FRS) decay profile was observed in the study of methyl red (o-MR) diffusion in polymer solutions in which the carboxylic hydrogen of o-MR can form a hydrogen bond with polymer chain segments (Lee and Lodge, J. Phys. Chem. 1987, 97, 5546; Lee et al. Macromolecules 1992, 25, 6977). The diffusivities of ground state o-MR (trans form) and of its photoexcited isomer (cis form) are different enough to show the nonmonotoic decay profile, but the origin of the behavior has not been fully understood. In order to elucidate this observation, the dependence of the decay profile shape on polymer concentration as well as on tempreature was investigated by FRS. From the results, we propose that the internal hydrogen-bonding strength between the carboxylic hydrogen and the azo nitrogen in o-MR changes upon photoisomerization from the trans to cis form, which results in the different diffusivities of the two isomers in hydrogen-bonding media. This proposition is supported by ab initio calculations on the molecular structure of o-MR.

Full text of DOI:10.1021/jp9838442

J. Phys. Chem. B 1999, 103, 2355-2360
2355
Change of Internal Hydrogen Bonding of Methyl Red upon Photoisomerization Monitored
by Forced Rayleigh Scattering
Ha Seon Park, Kyung Seok Oh, Kwang S. Kim, and Taihyun Chang*,†
Department of Chemistry, POSTECH, Pohang, 790-784, Korea
Daniel R. Spiegel
Department of Physics, Trinity UniVersity, San Antonio, Texas 78212-7200
ReceiVed: September 24, 1998; In Final Form: January 15, 1999
A nonmonotonic, decay-growth-decay type forced Rayleigh scattering (FRS) decay profile was observed
in the study of methyl red (o-MR) diffusion in polymer solutions in which the carboxylic hydrogen of o-MR
can form a hydrogen bond with polymer chain segments (Lee and Lodge, J. Phys. Chem. 1987, 91, 5546;
Lee et al. Macromolecules 1992, 25, 6977). The diffusivities of ground state o-MR (trans form) and of its
photoexcited isomer (cis form) are different enough to show the nonmonotoic decay profile, but the origin of
the behavior has not been fully understood. In order to elucidate this observation, the dependence of the
decay profile shape on polymer concentration as well as on tempreature was investigated by FRS. From the
results, we propose that the internal hydrogen-bonding strength between the carboxylic hydrogen and the azo
nitrogen in o-MR changes upon photoisomerization from the trans to cis form, which results in the different
diffusivities of the two isomers in hydrogen-bonding media. This proposition is supported by ab initio
calculations on the molecular structure of o-MR.
Introduction
indicates the diffusivities of ground state o-MR and of its
photoproduct become different enough to exhibit the comple-
mentary grating effect6,7,9-16 (assuming a reasonable value for
the refractive index contrast between the complementary gratings
of methyl red, as discussed below). In this paper, we demonstrate
that the difference in internal hydrogen-bonding strength of
o-MR in its two photoisomeric states (i.e., cis and trans form)
is responsible for the nonmonotonic decay profile.
Hydrogen bonding plays an important role in a variety of
physicochemical phenomena.1 In many cases, intermolecular
hydrogen bonding is of concern, but a large number of molecules
have polar groups in positions that allow intramolecular
hydrogen bonding. In certain circumstances, competition of
intra- and intermolecular hydrogen bonding takes place,2 which
leads to intriguing observations. For example, we previously
reported a diffusion study of methyl red in polymer solutions.3
The diffusivity of methyl red was found to be significantly
reduced if there exists hydrogen-bonding interaction between
the dye and polymer chains such as poly(vinyl acetate) or poly-
(methyl methacrylate).3-5 Furthermore, two structural isomers
of methyl red (o-MR vs p-MR; the molecular structures of the
two isomers are shown in the Experimental Section) showed a
significantly different diffusivity in H-bonding media.3,4 The
diffusivity of p-MR was much lower than o-MR in H-bonding
polymer solutions while these two isomers showed a similar
diffusivity in non-H-bonding media. The difference was at-
tributed to the internal H-bonding in o-MR between the
carboxylic hydrogen and the azo nitrogen, which does not exist
in p-MR. The internal H-bonding of the carboxylic hydrogen
in turn weakened the external H-bonding of o-MR to the
polymer chains.3
Forced Rayleigh Scattering. Forced Rayleigh scattering is
a transient optical grating technique which utilizes a spatial
concentration modulation of the photochemical probes as a
diffraction grating.17-19 The concentration modulation is gener-
ated by a brief exposure of the sample containing probe
molecules to a periodic pattern of light intensity (writing
process). Upon exposure, the probe molecules undergo a
photochemical reaction and make a periodic modulation in the
concentration of the photoreaction product. If the photoproduct
of the probe molecule has a refractive index (phase grating) or
absorptivity (amplitude grating) that differs from the original
state, the concentration modulation can act as an optical
diffraction grating. Observing the decay of the diffracted light
(reading process), one can measure the relaxation rate of the
concentration modulation, which contains information on the
diffusivity of the probe molecules in various systems (simple
liquids,20 porous media,21,22 liquid crystals,23,24 polymer solu-
tions, melts and glasses3-5,9,11,12,16,25-28 including block copoly-
mer systems,29,30 supercooled liquids,31,32 and magnetic fluids33).
FRS can also be used to study electrophoretic mobility,34,35
lifetime of the photoproduct,36,37 subsequent photochemical
reactions,38,39 polymer polydispersity40 by thermal diffusion,41,42
near-surface diffusion using evanescent wave,43 and so on. In
this study, our discussion is restricted to the mass diffusion
process, which can be realized by choosing a probe that relaxes
In addition, o-MR was found to show a nonmonotonic,
decay-growth-decay (DGD) type decay profile in H-bonding
media3-8 while a normal single-exponential decay was found
for p-MR as well as for the methyl ester of o-MR.3 The DGD
profile was not observed in polymer matrices which do not
hydrogen bond with o-MR, e.g., polystyrene. The origin of the
DGD profile has not been fully understood, although the profile
† Tel: +82-562-279-2109.Fax: +82-562-279-3399.Email:tc@postech.ac.kr.
10.1021/jp9838442 CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/16/1999

2356 J. Phys. Chem. B, Vol. 103, No. 13, 1999
Park et al.
back to its original state with a sufficiently long photoproduct
lifetime and no subsequent photochemical reactions.
The transient concentration grating formed by the writing
process is in fact a composite of two complementary gratings
of the excess ground state probe and of its photoproduct, which
are 180° out of phase.10,13,18 Therefore, the FRS decay profile
of a phase grating follows the following model function6,7,9-14
Id(t) ) [A1 exp(-t/τ1) - A2 exp(-t/τ2) + Bcoh]2 + Binc (1)
where τ1 and τ2 are the respective decay time constants of two
complementary gratings, A1 and A2 represent the amplitudes of
the optical fields diffracted from each complementary phase
grating, and Bcoh and Binc are the coherent and incoherent
background, respectively. The decay time constant τ is a function
of the probe diffusivity and the lifetime of the photoproduct so
that information on the dynamics of the probe can be obtained.
If the relaxation rates of two complementary gratings are the
same, the FRS signal shows a single-exponential decay.
Figure 1. FRS decay profiles of o-MR, p-MR, and their methyl esters
in 10% PVAc/toluene solution at a grating spacing of 12.1 µm. o-MR
shows a unique nonmonotonic decay profile while the other three dyes
show good single-exponential decay profiles.
phase shifted by π every other pulse eliminates Bcoh in eqs 1
and 2.45 The 488 nm line of an Ar ion laser (Coherent, Model
90-3) is used for the writing beam and the 632.8 nm line of a
He/Ne laser (Melles Griot, 5 mW) for the reading beam. The
temperature of the cell was controlled better than ( 0.1 °C.
Since methyl red does not absorb the reading beam, the
transient diffraction grating created in the sample cell is the
phase grating and we used eqs 1 and 2 for the analysis of the
FRS decay profile. The analysis scheme for the decay profiles
depended on the shape of the signal. For single-exponentially
Id(t) ) [A exp(-t/τ) + Bcoh]2 + Binc
Experimental Section
(2)
The probe dyes, 2-[4′-(dimethylamino)phenylazo]benzoic acid
(methyl red, o-MR) and 4-[4′-(dimethylamino)phenylazo] ben-
zoic acid (p-MR), were acquired from Aldrich and TCI,
respectively, and used without further purification. The methyl
decaying signals, nonlinear regression fit to eq 2 (without Bcoh
)
was used. For decay-growth-decay (DGD) type decay profiles,
it was not possible to obtain a unique result by fitting to eq 1.
It is well-known that a unique multiparameter fit is not possible
when the difference between τ1 and τ2 is not large,3-5,9,11-16
although multiparameter fitting or the stripping analysis method
was practiced when the difference between τ1 and τ2 was large
enough.6,7,34 Instead, the mean decay time constant was deter-
mined by the ∆t method (∆t, time lapse between the dip and
maximum position).14 When the difference between τ1 and τ2
is small, ∆t is an excellent approximation to the mean decay
time constant. Then the diffusivity of MR is obtained from the
slope of 1/τ (single-exponential decay) or 1/∆t (DGD type
decay) vs q2 plot, where q ) 2π/d and d is the spacing of the
optical grating created by crossing the writing beams at the
sample.
ester of p-MR (CH3-p-MR) was prepared through reaction with
methyl alcohol in the presence of sulfuric acid as a catalyst.
The methyl ester of o-MR (CH3-o-MR) was synthesized through
reaction with methyl alcohol after acylation of o-MR with
SOCl2. Poly(vinyl acetate) (PVAc) was obtained from Aldrich
and its Mw and Mw/Mn were determined by gel permeation
chromatography as 87,000 and 2.4 relative to polystyrene
standards, respectively.
Toluene solutions of polymer and probe dye were prepared
gravimetrically, and the polymer concentration is represented
by the volume fraction calculated from the density of PVAc
and toluene at 25 °C. The probe dye concentration was kept at
0.1 mg/mL. We experimentally confirmed that there was no
dye concentration dependence of the dye diffusivity and FRS
decay profile in this concentration range. The solutions were
filtered through 0.2 µm pore PTFE membrane filters (Gelman)
directly to a 5 mm path length spectroscopic quartz cuvette for
the FRS measurements.
The FRS apparatus used in this study was described in detail
elsewhere.44 In order to eliminate the contribution of Bcoh, we
inserted an optical glass window into the path of one of the
two writing beams (crossing at the sample to generate an optical
grating) for every other pulse during the accumulation of the
FRS signal. The tilt angle of the window was adjusted to retard
the phase of the laser beam so that it shifts the optical grating
by a half of its period. This adjustment was done visually by
monitoring the grating projected on a distant screen by use of
a short focal length convex lens. The phase shift of the optical
grating results in a phase shift of the diffracted optical field by
π. Accumulation of the diffracted intensity from the grating
Results and Discussion
Decay-Growth-Decay Profile. FRS decay profiles with four
probe dyes, o-MR, p-MR, and their methyl esters (CH3-o-MR
and CH3-p-MR) in 10% PVAc/toluene solution, are displayed
in Figure 1. They are measured at an identical grating spacing
of 12.1 µm. A nonmonotonic decay is distinct for o-MR, while
the other dyes exhibit single-exponential decay profiles. p-MR
shows a slower decay than CH3-o-MR or CH3-p-MR due to
the H-bonding between p-MR and PVAc.3,4 The nonmonotonic
decay profile of o-MR is unique among the four dyes in PVAc/
toluene solution. In non-H-bonding media, such as either pure
toluene or polystyrene (PS)/toluene solution, all four dyes
including o-MR show single-exponential decay profiles. Their
decay profile shapes and the mean diffusivities determined by
∆t method are summarized in Table 1. We note that the
diffusivities of the four dyes are very close in pure solvent
(toluene) and in a medium in which the probe dye does not
hydrogen bond to the polymer chains (PS/toluene solution). This
indicates that the hydrodynamic sizes of the four dyes are
similar. However, the diffusivities in PVAc/toluene solution

H-Bonding of Methyl Red upon Photoisomerization
J. Phys. Chem. B, Vol. 103, No. 13, 1999 2357
TABLE 1: FRS Decay Profile Shapes and Diffusivities
(×10-6 cm2/s) of o-MR, p-MR, and Their Methyl Esters in
Three Different Media at 25 °Ca
probe
o-MR
CH3-o-MR SE, 12.0 ( 0.2 SE, 8.0 ( 0.1
p-MR SE, 12.0 ( 0.2 SE, 8.1 ( 0.2
CH3-p-MR SE, 12.8 ( 0.6 SE, 8.3 ( 0.3
toluene
10% PS/toluene 10% PVAc/toluene
SE, 12.7 ( 0.2 SE, 8.8 ( 0.3
DGD, 4.4 ( 0.2
SE, 7.9 ( 0.2
SE, 2.7 ( 0.2
SE, 8.4 ( 0.4
a SE: single-exponential decay. DGD: decay-growth-decay.
differ from one another significantly. The H-bonding interaction
between the acid form of methyl red (o-MR or p-MR) and PVAc
chain retards the diffusion of the dyes significantly, while their
methyl esters do not show a significant change. Diffusion of
o-MR is less retarded than p-MR due to the internal H-bonding
in o-MR, which in turn weakens the external H-bonding with
PVAc.3 Also, a nonmonotonic DGD type decay profile is
observed only for o-MR and only when H-bonding interaction
exists between the dye and the polymer chain such as in PVAc
or poly(methyl methacrylate) (PMMA) solution.3-5,44 Therefore,
the origin of the DGD type decay profile found for o-MR is
closely related to the dye’s H-bonding to the polymer chains.
The DGD profile found for o-MR demonstrates that two
complementary gratings, which consist of excess trans and cis
isomers of the dye in the concentration modulation, decay at
different rates.6,7 Since the refractive index of trans form is
known to be larger than that of the cis isomer, and the refractive
indices of both isomers are larger than that of the background
(PVAc/toluene solution), the diffraction efficiency of the trans
isomer rich grating is higher than the cis form rich grating (Atrans
> Acis).13,46 In this circumstance, the relaxation rate of the
concentration grating (i.e., the diffusivity of o-MR) should be
higher for the trans-rich grating than for the cis-rich grating in
order to show a DGD type signal (τtrans < τcis).3,11-13 If the
trans isomer has a lower diffusivity than the cis isomer, a
different type of decay profile (growth decay or non-single-
exponential monotonic decay) would be observed instead of the
DGD type profile.9,11,13 Wang and co-workers also found a lower
diffusivity of cis o-MR relative to its trans form in poly(ethylene
glycol) and poly(caprolactone) diol. They interpreted the
observation as due to a stronger interaction between the cis form
and the polymer host without suggesting the nature of the
interaction.6,7 Here we propose that the higher diffusivity of the
trans form relative to the cis form of o-MR in hydrogen-bonding
media arises from the difference in internal H-bonding strength
between two isomers.
Figure 2. FRS decay profiles of o-MR at three different PVAc
concentrations (from top, 10.0%, 4.8%, and 3.0%) at 25 °C. The grating
spacing is 12.1 µm. The ordinate is normalized to the initial amplitude
for visual aid. The solid lines represent the fit according to eq 1.
Figure 3. The same three FRS decay profiles of o-MR as in Figure 2
displayed with the abscissa normalized to the mean decay time constants
(∆t ) tmax - tdip) so that the relative shapes can be easily compared:
dashed line, 10.0%; dotted line, 4.8%; solid line, 3.0%. It is clear that
the signal intensity at the maximum increases and tdip/∆t becomes
shorter as the PVAc concentration increases, which indicates that τcis/
τtrans increases.
PVAc Concentration Dependence. In Figure 2, FRS decay
profiles of o-MR at three different concentrations (3.0%, 4.8%,
and 10.0%) of PVAc/toluene solutions are displayed. They are
both measured at the same grating spacing of 12.1 µm and at a
temperature of 25.0 °C. The signal is normalized to the initial
signal intensity for visual aid. Judging from the position of the
dip and maximum, the signal decays more slowly at higher
PVAc concentration. It indicates that the diffusion of o-MR is
retarded with PVAc concentration. In addition, it is evident that
the relative intensity at the maximum grows as the concentration
of PVAc is increased. For an easier comparison of the profile
shapes, in Figure 3 the abscissa is normalized by ∆t, which is
a fairly good approximation to the mean decay time constant
of the two complementary gratings.14,15 It is clear that, as the
PVAc concentration increases, tdip/∆t becomes shorter and the
signal intensity at the maximum increases. A simple derivation
using eq 1 leads to the following formula:
tdip ln(Atrans/Acis)
)
(3)
∆t
ln(τcis/τtrans
)
where Bcoh was neglected since it was experimentally eliminated
in this study. Therefore, the trend observed in Figure 3 indicates
that two decay time constants get further apart (τcis/τtrans
increases) as the PVAc concentration is increased, provided the
contrast of the complementary gratings (Atrans/Acis in eq 3)
remains constant. This proviso is reasonable in this system
because the FRS signal of o-MR is due mainly to the refractive
index contrast (phase grating) when the reading beam wave-
length is 632.8 nm,13 and the change of PVAc concentration
does not significantly alter the refractive index of the background
due to the similar refractive indices of PVAc and toluene.46,47

2358 J. Phys. Chem. B, Vol. 103, No. 13, 1999
Park et al.
TABLE 2: Decay Time Constants of Complementary
Gratings at 25 °C in PVAc/Toluene Solutions at Three
Different Concentrations
PVAc (%)
τcis/τtrans
∆t (ms)
D (×10-6 cm2/s)
3.0
4.8
10.0
1.14
1.16
1.18
4.77
5.87
8.63
7.7 ( 0.6
6.6 ( 0.3
4.4 ( 0.2
Figure 5. The same three FRS decay profiles of o-MR as in Figure 4:
dashed line, 15 °C; dotted line, 45 °C; solid line, 75 °C. The abscissa
is normalized to the mean decay time constants (∆t) so that relative
shape, can be easily compared. It is clear that as the temperature is
decreased, the signal intensity at the maximum increases and tdip/∆t
becomes shorter, which indicates that τcis/τtrans increases.
TABLE 3: Decay Time Constants of Complementary
Gratings in 10% PVAc/Toluene Solutions at Three Different
Temperatures
temp (°C)
τcis/τtrans
∆t (ms)
D (×10-6 cm2/s)
15
45
75
1.18
1.16
1.15
10.74
5.26
2.93
3.4 ( 0.1
7.3 ( 0.2
12.7 ( 0.8
consistent with the hypothesis that H-bonding is responsible for
the difference between τtrans and τcis since H-bonding becomes
weaker as the temperature increases.48
Figure 4. FRS decay profiles of o-MR in 10% PVAc/toluene solution
at three different temperatures (from top, 15, 45, and 75 °C). The grating
spacing is 12.1 µm. The ordinate is normalized to the initial signal
amplitude for visual aid. The solid lines represent the fit according to
eq 1.
Putting these experimental observations together, we propose
the following for the origin of the DGD type signal. In general,
the trans isomer of an azobenzene derivative is known to have
a coplanar structure of two phenyl rings and the azo group.49,50
In the cis isomer, however, the two phenyl rings are twisted
out of the plane of the azo group due to the steric hindrance
between phenyl hydrogens.49,50 This effect is observed spec-
troscopically, such that consequent reduction of delocalization
of π electrons results in the hypochromic effect and the
hypsochromic shift of the π-π* transition band.49,50 Since the
coplanar structure of the phenyl rings and the azo group is
expected to be favorable for the internal H-bonding formation
of o-MR via six-membered ring conformation, the internal
H-bonding in the trans form of o-MR would be stronger, which
in turn weakens the external H-bonding with PVAc backbone
more compared to the cis isomer. This leads to a larger reduction
in the diffusivity of the cis isomer relative to the trans form,
which results in a DGD type decay for o-MR in H-bonding
environments. This hypothesis is consistent with the fact that
such a DGD signal is observed only for o-MR, not for its methyl
ester or p-MR, and only in polymer solutions which interact
with o-MR via H-bonding, such as PVAc or PMMA. From the
results we can also note that a small change of decay time
constants, a few % as shown in Tables 2 and 3, can alter the
shape of decay profile substantially, as displayed in Figures 3
and 5. This is an expected result when complementary gratings
play a role.13
These decay profiles are fitted to eq 1 by a nonlinear
regression method and the solid lines in Figure 2 represent the
fit results. For the fitting, the contrast of Atrans/Acis was fixed at
9.4/8.4 using the estimated refractive index of the azo dye in
the system.13 The quality of the fit appears good and the fit
results are summarized in Table 2. The ratio of τcis/τtrans
progressively increases with the concentration of PVAc. In other
words, the difference in the diffusivity between the cis and trans
isomers becomes larger as the concentration of the H-bond
acceptor (PVAc) increases, which makes the DGD type decay
signal more and more pronounced as displayed in Figures 2
and 3. Thus, the observation displayed in Figures 2 and 3 and
Table 2 is consistent with the hypothesis that the trans and cis
isomers of o-MR make hydrogen bonds of different strength
with PVAc; the trans isomer appears to form a weaker external
H-bond with PVAc chains compared to the cis isomer.
Temperature Dependence and the Origin of DGD Profile.
FRS decay profiles of o-MR at three different temperatures (15,
45, and 75 °C) are displayed in Figure 4. They are measured at
the same grating spacing as Figure 2 and at a fixed concentration
of 10% PVAc/toluene solution. The ordinate is again normalized
for visual aid. In Figure 5 the abscissa is also normalized to
∆t. These profiles exhibit essentially the same trend as observed
in PVAc concentration dependence shown in Figure 2. These
decay profiles are fitted to eq 1 by a nonlinear regression method
keeping Atrans/Acis constant at 9.4/8.4. The solid lines in Figure
4 represent the fit results and the fit results are summarized in
Table 3. It is clear that the difference between τtrans and τcis
becomes smaller as the temperature increases. This is again
Ab Initio Calculations. It has been proposed above, based
on the experimental results reported, that the strength of the
internal hydrogen bonding between the carboxylic hydrogen and
the azo nitrogen in o-MR changes upon photoisomerization
between cis and trans forms, resulting in different diffusivities.
To further test this proposal, we performed an ab initio

H-Bonding of Methyl Red upon Photoisomerization
J. Phys. Chem. B, Vol. 103, No. 13, 1999 2359
than Cis-R6′. These results indicate that, owing to the strong
internal hydrogen bonding, Trans-R6 is the most stable con-
former among the trans isomers, while Cis-R6′ is more stable
than Cis-R6 for the cis isomers. This is in full agreement with
our experimental findings.
In summary, we have studied the temperature and polymer
concentration dependence of the tracer diffusion of methyl red
derivatives in polymer solutions by FRS. From the diffusivity
results and FRS decay profile shapes, we found that hydrogen
bonding between o-MR and PVAc is responsible not only for
the retarded diffusion but also for the decay-growth-decay
FRS signal of o-MR. We proposed that the nonmonotonic decay
is caused by internal hydrogen bonding in o-MR. Molecular
structures of trans and cis isomers of o-MR obtained by ab initio
calculations support this proposition.
Acknowledgment. This study was supported in part by the
Basic Science Research Institute Program, Ministry of Education
(BSRI-97-3438). We thank Jae Young Lee and Jungmoon Sung
for the help in the FRS experiments. It is also acknowledged
that most of the calculations were performed using Cray C90
and T3E at SERI. D.R.S. gratefully acknowledges support from
the National Science Foundation (Grant No. CHE-9711426).
Figure 6. Structures and relative energies of conformers of o-MR.
The left insets are conformers with internal hydrogen bonding, while
the right insets (with primes in their codes) are conformers without
internal hydrogen bonding. The relative energies (kcal/mol) in paren-
theses were obtained by HF/6-31G*.
References and Notes
(1) Pimental, G. C.; McClellan, A. L. The Hydrogen Bond; W. H.
Freeman and Co.: San Francisco, 1960.
(2) Nagy, P. I.; Durant, G. J.; Smith, D. A. Modeling the hydrogen
bond; ACS Symp. Ser. 569; Smith, D. A., Ed.; American Chemical
Society: Washington, DC, 1994.
(3) Lee, J.; Park, K.; Chang, T.; Jung, J. C. Macromolecules 1992, 25,
6977.
(4) Park, H. S.; Sung, J.; Chang, T. Macromolecules 1996, 29, 3216.
(5) Lee, J. A.; Lodge, T. P. J. Phys. Chem. 1987, 91, 5546.
(6) Xia, J. L.; Gong, S. S.; Wang, C. H. J. Phys. Chem. 1987, 91,
5805.
(7) Xia, J. L.; Wang, C. H. J. Chem. Phys. 1988, 88, 5211.
(8) Lodge, T. P.; Lee, J. A.; Frick, T. S. J. Polym. Sci. B. 1990, 28,
2607.
(9) Huang, W. J.; Frick, T. S.; Landry, M. R.; Lee, J. A.; Lodge, T.
P.; Tirrell, M. AIChE J. 1987, 33, 573.
(10) Johnson, C. S. Jr. J. Opt. Soc. Am. B 1985, 2, 317.
(11) Spiegel, D. R.; Sprinkle, M. B.; Chang, T. J. Chem. Phys. 1996,
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1996, 29, 5635.
(13) Park, S.; Sung, J.; Kim, H.; Chang, T. J. Phys. Chem. 1991, 95,
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(14) Park, S.; Yu, H.; Chang, T. Macromolecules 1993, 26, 3086.
(15) Park, H. S.; Sung, J.; Lee, H.; Chang, T.; Spiegel, D. R. Bull. Korean
Chem. Soc. 1997, 18, 1006.
(16) Spiegel, D. R.; Marshall, A. H.; Jukam, N. T.; Park, H. S.; Chang,
T. J. Chem. Phys. 1998, 109, 267.
(17) Eichler, H. J.; Gunter, P.; Pohl, D. W. Laser-Induced Dynamic
Gratings; Springer: Berlin, 1986
(18) Nelson, K. A.; Casalegno, R.; Dwayne Miller, R. J.; Fayer, M. D.
conformational study of o-MR. We investigated the possible
conformers of the cis and trans isomers of o-MR with and
without internal hydrogen bonding between the carboxylic
hydrogen and the azo nitrogen. There are two trans conformers
with internal hydrogen bonding forming six- and seven-
membered rings (Trans-R6, Trans-R7), and one cis isomer with
internal hydrogen bonding forming a six-membered ring (Cis-
R6). In addition, there are three corresponding conformers
without internal hydrogen bonding (Trans-R6′, Trans-R7′, Cis-
R6′). Here the primes denote the cases of non-hydrogen-bonded
virtual rings. All the conformers were fully optimized by
Hartree-Fock (HF) theory with GAMESS51 using the 6-31G*
basis set. Structures and relative energies are shown in Figure
6. In the trans isomers, Trans-R6 is the lowest energy conformer,
followed by Trans-R6′, Trans-R7′, and Trans-R7. The predicted
relative energies of Trans-R6′, Trans-R7′, and Trans-R7 with
respect to Trans-R6 are 1.34, 1.96, and 6.56 kcal/mol, respec-
tively. For the cis isomers, Cis-R6 is slightly higher in energy
than Cis-R6′ by 0.95 kcal/mol. The predicted relative energies
of Cis-R6′ and Cis-R6 with respect to Trans-R6 are 16.64 and
17.59 kcal/mol, respectively. The conformational stability can
be understood in terms of molecular structure, in particular the
internal hydrogen bond. Conformers of the trans isomer have a
planar structure (with only a small distortion to reduce the
electron repulsion between the carboxylic group and the azo
group). Conformers with internal hydrogen bonding are expected
to be more stable than conformers without internal hydrogen
bonding. Trans-R6 is highly stabilized by an internal hydrogen
bond whose distance is 1.831 Å in the six-member ring
conformation. Trans-R7 also has an internal hydrogen bond
whose distance is 1.807 Å in the seven-membered ring
conformation. The strain of the seven-membered ring, however,
causes Trans-R7 to be less stable than Trans-R6 as well as the
conformers without internal hydrogen bonding. In the cis
isomers, the phenyl rings are twisted out of the plane of the
azo group due to the steric hindrance between phenyl hydrogen
atoms. Cis-R6 has an internal hydrogen bond whose distance
is 1.916 Å. However, the orientation required for internal
hydrogen bonding is not favorable, making Cis-R6 less stable
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