Solvation Dynamics of Coumarin 480 in Sol-Gel Matrix
Samir Kumar Pal, Dipankar Sukul, Debabrata Mandal, Sobhan Sen, and
Kankan Bhattacharyya*
Physical Chemistry Department, Indian Association for the CultiVation of Science,
JadaVpur, Calcutta 700 032, India
ReceiVed: September 29, 1999; In Final Form: December 15, 1999
Solvation dynamics of Coumarin 480 (C-480) in a tetraethyl orthosilicate (TEOS) sol-gel matrix has been
studied using time-resolved emission spectroscopy. In the macroscopically solid TEOS matrix the solvation
dynamics of C-480 is described by a major (85%) component of 120 ( 20 ps and a minor (15%) component
of 800 ( 100 ps. These components are substantially slower than the solvation dynamics in bulk water. The
rotational relaxation time of C-480 in this matrix is found to be very short (<80 ps).
1. Introduction
Water molecules in confined environments play a crucial role
in many natural processes and control the structure, function,
and dynamics of many biomolecules. Recently, several groups
studied the dynamics of water and other small molecules in a
wide variety of confined environments.
1-11
These include water
surface,
1,4
proteins,
2
cyclodextrin,
3
microemulsion,
5
sol-gel
matrix,
6,7
zirconia particle,
8
polymer hydrogel,
9
lipid,
10
and
micelles.
11
The dielectric relaxation time of pure water is 10
ps.
12
In bulk water, solvation dynamics occurs on the subpico-
second time scale.
2,13
However, in most of these organized and
confined media both solvation dynamics
3,5,10-11
and dielectric
relaxation
2,14
of water are found to be slower by 3-4 orders of
magnitude. In the case of a microporous solid (e.g., sol-gel
matrix
6,7
or polymer hydrogel
9
) the bulk viscosity is very high
and, hence, a very slow relaxation dynamics is expected.
However, recently it is reported that both the sol-gel
6
and
polymer hydrogel
9
exhibit very fast solvation dynamics and
rotational relaxation. The fast dynamics is attributed to the
presence of large pores in these media.
15,16
Large biomolecules
can pass through these pores, and hence, the dynamics of small
water molecules or fluorescent probes is very fast in these media.
The inorganic sol-gel composite obtained from the hydrolysis
of tetraalkyl orthosilicate acts as a good host for many biological
materials.
17
Many enzymes such as glucose oxidase,
17a
aspart-
ase,
17b
urease,
17c
and nonenzymes such as bacteriorhodopsin
17d
can be encapsulated in biologically active form for a very long
period in a sol-gel glass. Such sol-gel glasses doped with
biomolecules have potential applications as chemical sensors.
It is obviously interesting to find out the dynamics occurring
in such an interesting material. However, there have been
relatively few studies on relaxation dynamics in sol-gel
matrices. Bright et al.
6
studied relaxation of acrylodan-labeled
BSA in a sol-gel matrix using phase fluorometry and reported
that the protein molecule remains highly mobile in this matrix.
Pant and Levinger employed femtosecond upconversion to study
the solvation dynamics of coumarin-343 (C-343) adsorbed to
zirconia particles in water.
8
Fourkas and co-workers
7
carried
out a careful study on the optical Kerr effect (OKE) of methyl
iodide and acetonitrile in sol-gel glasses of different pore sizes.
They observed that for both the liquids the decay of the OKE
signal in a sol-gel glass is multiexponential with a major
component similar to that in bulk liquid and an additional
component, which is about 4 times slower. The ratio of the
amplitudes of the fast (bulk) and the slow component increases
with the pore size.
7
In this work, we report on a picosecond
time dependent Stokes shift (TDSS) study of solvation dynamics
of water molecules trapped in a tetraethyl orthosilicate (TEOS)
sol-gel matrix using coumarin 480 (C-480, Scheme 1) as a
probe.
2. Experimental Section
TEOS (Aldrich, 99%) and C-480 (Exciton) were used as
received. To 1 mL of a 3 × 10
-4
M solution of C-480 in neat
liquid TEOS taken in a quartz tube was added 1 mL of a 2.5 ×
10
-3
N aqueous HCl solution drop by drop. The mixture is
vortexed for 30 min and then allowed to stand at 40 °C for
several hours to facilitate hydrolysis. Once the gel is formed,
the gel was kept exposed to air for nearly 2 months to allow
the alcohol formed to escape and to complete aging of the gel.
The reported pore size of the sol-gel glass prepared at pH )
3 and H
2
O/Si ratio ≈ 12, is 10-20 Å.
18
The absorption (against
water as reference) and emission and excitation spectra of the
gel were recorded periodically. It is observed that after 1 month
the spectra display no further change. All steady state and time-
resolved studies were made using a sufficiently aged gel (2
months old). The sol-gel matrix was found to cause scattering.
At the wavelength of excitation (300 nm), due to scattering
transmittance of the sol-gel sample without C-480 is 50%. The
sol-gel matrix, however, exhibited no emission.
Steady-state absorption and emission spectra were recorded
on JASCO 7850 and Perkin-Elmer 44B instruments, respec-
tively. Quantum yields (φ
f
) were measured using the reported
19
quantum yield (0.66) of C-480 in water. The sample was excited
at 300 nm with the second harmonic of a cavity-dumped
* E-mail: pckb@mahendra.iacs.res.in. Fax: (91)-33-473-2805.
SCHEME 1: Structure of Coumarin-480
2613J. Phys. Chem. B 2000, 104, 2613-2616
10.1021/jp993484y CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/07/2000
Rhodamine 6G dye laser (Coherent 702-1) pumped by a
continuous wave (cw) mode-locked Nd:YAG laser (Coherent,
Antares). The emission was collected at magic angle polarization
for lifetime measurement by a Hamamatsu MCP photomultiplier
(2809-U). The typical system response at 300 nm excitation is
about 80 ps. For rotational relaxation studies, the emission
intensity at perpendicular (I
⊥
) and parallel (I
|
) polarizations were
collected alternatively for 100 s. For a typical anisotropy decay
a peak count of 10 000 counts was collected at parallel
polarization. The r(t) is then calculated using the relation
The G factor of the setup was determined using a dye whose
rotational relaxation time is very short (e.g., nile red in
methanol).
3. Results
3.1. Steady-State Spectra. In neat liquid TEOS, C-480
exhibits an absorption maximum at 365 nm and an intense
emission with emission maximum at 430 nm (Figure 1) and
quantum yield φ
f
) 0.55. The position of the maximum of C-480
in TEOS is intermediate between those reported
19
in cyclohex-
ane (λ
max
abs
) 360 nm and λ
max
em
) 410 nm) and acetonitrile
(λ
max
abs
) 380 nm and λ
max
em
) 450 nm). In the TEOS sol-
gel matrix, the emission spectrum of C-480 exhibits a marked
red shift to 480 nm (Figure 1) with φ
f
) 0.55 and the absorption
spectrum (Figure 2) also exhibits a red shift to 390 nm. The
excitation spectrum of C-480 in the gel (Figure 2) remains
identical to the absorption spectrum, which rules out the
presence of any impurity. It is evident that the absorption and
emission maxima of C-480 in a gel are very similar to those in
water
19
(λ
max
abs
) 395 nm and λ
max
em
) 490 nm). This indicates
that in the TEOS gel C-480 experiences a highly polar and protic
environment presumably due to the presence of a large amount
of entrapped water.
3.2. Time-Resolved Study. In neat liquid TEOS, C-480
exhibits a single-exponential decay with lifetime 3.1 ns (Figure
3) and the decay is independent of the emission wavelength.
The lifetime of C-480 in neat liquid TEOS is close to that
reported
19
in acetonitrile (3.3 ns). However, in the TEOS gel,
the emission decays of C-480 are found to be wavelength
dependent. At the red end the decay is preceded by a growth,
while at the blue end a fast decay is observed (Figure 4). For
example, the decay at the red end (570 nm) is fitted to a
biexponential with a growth component of 200 ps and a decay
component of 6.1 ns while at the blue end (435 nm) a
biexponential decay with an average lifetime of 3.8 ns is
observed. Such a decay at the blue end and growth at the red
end is typical of systems undergoing solvation dynamics.
20,21
From the decays the time-resolved emission spectra (Figure 5)
were constructed by following the method discussed by Ma-
roncelli and Fleming.
20
The solvation dynamics is described by
the decay of the solvent correlation function, C(t), defined as
where ν(0), ν(t), and ν(∞) are respectively the emission
frequencies at time 0, t, and ∞. The decay of C(t) is shown in
Figure 6, and the decay parameters are summarized in Table 1.
It is readily seen that C-480 displays a fast solvation dynamics
Figure 1. Emission spectra of C-480 in (i) neat TEOS (- - -), (ii) TEOS
gel (s), and (iii) water (‚‚‚).
Figure 2. Absorption spectra of C-480 in (i) neat TEOS (s) and (ii)
TEOS gel (‚‚‚) and (iii) excitation spectrum of C-480 in TEOS gel
(-‚-).
Figure 3. Fluorescence decay of C-480 in neat TEOS.
Figure 4. Fluorescence decays of C-480 in TEOS gel at (i) 550 nm,
(ii) 480 nm, and (iii) 435 nm.
C(t) )
ν(t) - ν(∞)
ν(0) - ν(∞)
r(t) )
I
|
(t) - GI
⊥
(t)
I
|
(t) + 2GI
⊥
(t)
2614 J. Phys. Chem. B, Vol. 104, No. 12, 2000 Pal et al.
with a major (85%) component of 120 ( 20 ps and a minor
(15%) component of 800 ( 100 ps, leading to an average
solvation time 〈τ
s
〉 ) a
1
τ
1
+ a
2
τ
2
) 220 ( 30 ps.
3.3. Rotational Relaxation. While solvation dynamics arises
due to the motion of the trapped water molecules in the TEOS
gel, the motion of the probe C-480 gives rise to the time
dependent optical anisotropy. It is observed that in the TEOS
gel the rotational relaxation of C-480, i.e., the decay of r(t), is
very fast and occurs in a time scale <80 ps.
4. Discussion
It is evident that the emission properties of C-480 in TEOS
gel are substantially different from those in neat liquid TEOS.
In the macroscopically solid gel, the probe C-480 molecule
experiences a very polar environment, as indicated by the
absorption and emission maxima, the quantum yield of emission,
and the long component of decay of about 6 ns, which is similar
to the lifetime of C-480 in water (5.9 ns
19
). The polar
environment within the TEOS matrix may be attributed to the
presence of trapped water. It is apparent that even in the rigid
sol-gel matrix, the solvation dynamics is very fast. The average
solvation time in the gel is 220 ( 30 ps. This is slower than
the solvation time in bulk water (0.3 ps).
3,13
However, the
dynamics in the gel is much faster than the nanosecond
dynamics observed in cyclodextrin,
3
microemulsions,
5
mi-
celles,
11
or lipids.
10
The rotational relaxation study also suggests
that the probe C-480 remains highly mobile within the sol-gel
matrix. It may be recalled that steady state anisotropy in a titania
gel indicates that though the bulk viscosity is extremely high,
the local microviscosity is extremely low.
22
A previous phase
fluorometry study in the sol-gel matrix
6
and a TDSS study in
the polyacrylamide hydrogel
9
also suggest high mobility of
organic fluorescent probes inside the gel. According to NMR
23a
and simulation
23b
studies the diffusion coefficient of water in a
hydrogel is lower than that in bulk water only by a factor of 2.
Similar high mobility in the polyacrylamide hydrogel is also
reported by Moerner et al.
16
Using fluorescence microscopy,
they demonstrated that almost all (98%) of the probe molecules
(nile red) remain highly mobile in the polyacrylamide hydro-
gel.
16
As discussed by Fee and Maroncelli, a large part of the
ultrafast component of solvation is missed in a picosecond
setup.
21
If the sample is excited near its absorption maximum
(about 400 nm in this case), the amount of solvation missed
may be calculated using the procedure described by Fee and
Maroncelli.
21
Unfortunately, using the second harmonic of our
Rhodamine 6G/DODCI dye laser, we excited the sample at 300
nm, which is at the blue end of its absorption spectrum. For
the wavelength of excitation in the present study (300 nm) the
several stages of curve fitting suggested by Fee and Maroncelli
21
is unlikely to give a good estimate of the missed part of the
Stokes shift. However, according to Fleming et al. for C-480
about 50% of the estimated Stokes shift is missed even in a
femtosecond upconversion setup.
3b
Using a femtosecond setup
they determined ∆ν for C-480 to be 1340 cm
-1
in water.
3b
In
the sol-gel matrix ∆ν for C-480 is found to be 400 cm
-1
, which
is one-third of that in water. For C-343, the femtosecond
upconversion study carried out by Pant and Levinger
8
indicates
∆ν to be 150 cm
-1
in the zirconia particle, which is nearly one-
fifth of that (800 cm
-1
) in water.
The most significant observation of this work is the observa-
tion of a very long component of solvation in the sol-gel matrix.
The solvation time of C-480 in the sol-gel matrix (220 ( 30
ps) is nearly 700 times slower compared to that in bulk water
(0.3 ps
3b
) and is about 100 times slower than that (0.24 ps) in
Figure 5. Time-resolved emission spectra of C-480 in TEOS gel at
(i)0ps(4), (ii) 200 ps (b), (iii) 2000 ps (3).
Figure 6. Decay of response function, C(t), of C-480 in TEOS gel.
The points denote the actual values of C(t) and the solid line denotes
the best fit to a biexponential decay.
TABLE 1: Decay Characteristics of the Solvent Response
Function, C(t), of C-480 in TEOS Gel
∆ν (cm
-1
) a
1
τ
1
a
(ps) a
2
τ
2
b
(ps) 〈τ
s
〉
c
(ps)
400 0.85 120 0.15 800 220
a
(20 ps.
b
(100 ps.
c
〈τ
s
〉 ) a
1
τ
1
+ a
2
τ
2
; (30 ps.
Solvation Dynamics of Coumarin 480 in Sol-Gel Matrix J. Phys. Chem. B, Vol. 104, No. 12, 2000 2615
zirconia particles.
8
It should, however, be emphasized that we
are missing a large part of the Stokes shift, which continues to
occur on a subpicosecond time scale, and are observing only a
long-lived component of the overall solvation dynamics. Fourkas
et al.
7
reported a very fast and almost bulklike OKE relaxation
in the sol-gel glass. For instance, at 290.6 K for CH
3
Iina
sol-gel glass of pore size 24 Å, they found that the contribution
of the bulklike component is 87% while that of a 4 times slower
component is only 13%.
7b
The slow component of solvent
relaxation and the fast rotational dynamics, reported in the
present study and the fast OKE relaxation in the sol-gel glass
7
may be reconciled as follows. The pore size of the glasses used
by Fourkas et al.
7
are much bigger than the size of the small
CH
3
IorCH
3
CN molecules, and hence, dynamics of the latter
is largely unhindered in the porous glass. However, in our case,
the pores (10-20 Å)
18
are only slightly bigger than the probe
C-480 molecule. Since the probe C-480 fills up the pore, the
movement of the water molecules becomes highly hindered.
The other possible cause of retardation of motion of water
molecules is hydrogen bonding between the water molecules
and the silicate network. It appears that like solvation dynamics,
a major part of the rotational dynamics remains bulklike and
very fast in the gel. This fast component presumably obscures
observation of any slow component of rotational dynamics. It
seems that the relatively free rotational movement and presence
of a large amount of water within the TEOS gel allows the
biomolecules to attain their native or biologically active
structure.
17
5. Conclusion
The present work shows that C-480 dye molecules experience
a highly polar and protic environment within the solid TEOS
sol-gel matrix, as evidenced by the position of the absorption
and emission maxima, quantum yield of emission, and long
emission lifetime. The TDSS study indicates that the average
solvation time of C-480 in the sol-gel glass of pore size 10-
20 Å,
18
is 220 ( 30 ps. This component is about 700 times
slower than that in bulk water
3
and about 100 times slower than
that in a zirconia particle
8
but is about 1 order of magnitude
faster than the slow solvation dynamics reported in cyclodex-
trin,
3
microemulsions,
5
lipids,
10
and micelles.
11
The rotational
relaxation of C-480 in TEOS gel is observed to be very fast
(<80 ps). A major part of solvation and rotational dynamics of
C-480 in the gel, however, continue to be very fast and missed
by a picosecond setup. The high rotational mobility of the probe
and the presence of trapped water molecules may be responsible
for the biological activity of entrapped biomolecules in the sol-
gel matrix.
Acknowledgment. Thanks are due to Council of Scientific
and Industrial Research (CSIR), Government of India, for
generous research grants. S.K.P., D.S., D.M., and S.S. thank
CSIR for awarding fellowships.
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