Conformational switching and exciton interactions in hemicyanine-based bichromophores

Article abstract of DOI:10.1021/ja010199v

Conformational changes in two hemicyanine-based bichromophores were demonstrated by varying the polarity as well as temperature of the medium. Dramatic changes in the ground and excited singlet state properties were observed upon folding of the bichromoph

Full text of DOI:10.1021/ja010199v

J. Am. Chem. Soc. 2001, 123, 7859-7865
7859
Conformational Switching and Exciton Interactions in
Hemicyanine-Based Bichromophores
S. Zeena and K. George Thomas*
Contribution from the Photochemistry Research Unit, Regional Research Laboratory (CSIR),
TriVandrum 695 019, India
ReceiVed January 23, 2001. ReVised Manuscript ReceiVed May 17, 2001
Abstract: Conformational changes in two hemicyanine-based bichromophores were demonstrated by varying
the polarity as well as temperature of the medium. Dramatic changes in the ground and excited singlet state
properties were observed upon folding of the bichromophores, due to the formation of intramolecular aggregates
of H-type. These aspects were studied, in detail, using steady-state absorption and time-resolved fluorescence
spectroscopy. Time-resolved fluorescence studies indicate that both the bichromophores exhibit a monoex-
ponential decay, with a short lifetime, in mixed toluene-CH2Cl2 solvents having lower proportions of toluene.
Interestingly, biexponential decay with short and long-lived species was observed at higher proportions of
toluene, due to the presence of unfolded and folded forms. Folding results in the intramolecular stacking of
the chromophores which restrict their torsional dynamics, leading to a longer lifetime. Upon laser excitation,
the folded form of the bichromophore undergoes rapid conformational changes, due to photoinduced thermal
dissociation.
Introduction
phenylacetylene-based oligomers into ordered helical structures7
and (ii) the donor-acceptor interaction of aromatic groups,
leading to pleated structures.8 Conformational changes and
molecular motions in photoactive molecular and supramolecular
systems can be modulated by chemical, photochemical, or
electrochemical methods.1,2 Such changes when translated to
optical as well as electronic properties can form the basis of
switching devices.1 We have now designed two nonconjugated
bichromophores (1 and 2 in Scheme 1), which can fold and
unfold by varying the solvent polarity or by the application of
external stimuli such as heat or light.
Earlier studies on nonconjugated bichromophores of cya-
nines,12 squaraines,13 and porphyrins14 mainly deal with the
interaction between the chromophores. In contrast with aromatic
compounds which form molecular associates in their excited
states (e.g., pyrene15), chromophoric dyes have a unique ability
to self-organize into aggregates in their ground state. Interaction
between the chromophores in their ground state has been fairly
well explained by McRae and Kasha16a in terms of exciton
coupling theory, in which the excited state of the dye aggregate
splits into two energy levels (Davydov splitting). The transition
to the upper excited state is allowed in the case of face-to-face
The design and study of molecular as well as supramolecular
photoactive systems have been actively pursued in recent years,
due to their potential applications in optoelectronic devices1-6
(for e.g., molecular switches,2-4 sensors,3 transducers,5 and
information processing and storage devices6). Of particular
interest is the design of molecular systems which undergo
conformational changes,7-9 analogous to the folding of pro-
teins.10 Synthetic molecular systems and polymers which can
fold into well-defined conformation in solution (foldamers11),
through noncovalent interactions, have been reported. These
include (i) solvophobically driven conformational folding of
* Address correspondence to this author: (phone) 91-471-515249; (fax)
91-471-490186; (e-mail) georgetk@md3.vsnl.net.in.
(1) (a) Lehn, J.-M. Supramol. Photochem.: Concepts PerspectiVes 1995.
(b) Balzani, V.; Moggi, L.; Scandola, F. In Supramolecular Photochemistry;
Balzani, V., Ed.; Reidel: Dordrecht, The Netherlands, 1987; pp 1-28. (c)
Molecular Electronics; Jortner, J., Ratner, M., Eds.; Blackwell: Oxford,
UK, 1997.
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M.; Stoddart, J. F. Angew. Chem., Int. Ed. 2000, 39, 3348.
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A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997,
97, 1515. (b) Fabbrizzi, L.; Poggi, A. Chem. Soc. ReV. 1995, 24, 197.
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Gunnlaugsson, T.; Maxwell, P. R. S.; Rice, T. E. J. Am. Chem. Soc. 1999,
121, 1393. (c) Rathore, R.; Magueres, P. L.; Lindeman, S. V.; Kochi, J. K.
Angew. Chem., Int. Ed. 2000, 39, 809. (d) Ashton, P. R.; Balzani, V.; Becher,
J.; Credi, A.; Fyfe, M. C. T.; Mattersteig, G.; Menzer, S.; Nielsen, M. B.;
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Soc. 1999, 121, 3951. (e) Fernasndez-Acebes, A.; Lehn, J. M. Chem. Eur.
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Yokozawa, T.; Prince, R. B.; Moore, J. S. J. Am. Chem. Soc. 1999, 121,
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2000, 122, 10298. (b) Sakamoto, S.; Obataya, I.; Ueno, A.; Mihara, H.
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10.1021/ja010199v CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/24/2001

7860 J. Am. Chem. Soc., Vol. 123, No. 32, 2001
Scheme 1. Synthesis of the Bichromophores 1 and 2
Zeena and Thomas
Figure 1. Absorption spectra of bichromophore 1 and model compound
3 in dichloromethane. Inset shows the absorption spectra of 1 and 3 in
methanol.
Table 1. Absorption and Emission Properties of Compounds 1-3
in Dichloromethane
absorption
fluorescence
λ
max(em.)a Φfb
compd
λ
max, nm (ꢀ, M-1cm-1
)
∆ν1/2, cm-1
1
2
3
540 (104940)
540 (107000)
553 (89563)
2979
2979
2283
600
600
597
0.01
0.0098
0.018
a Solutions were excited at 500 nm and emission monitored in the
region 520-700 nm for estimating φf; b error limit ( 5%.
(parallel) dimers and to the lower state for head-to-tail (linear)
dimers.16 The former type of dimer is characterized by a
hypsochromically shifted absorption band (H-aggregate) and the
latter one by a bathochromically shifted absorption band (J-
aggregate), as compared to the isolated monomer. Recently,
some interesting observations have been made in the case of
nonconjugated dimers of a few merocyanines12a and squaraines,13
tethered by methylene groups. For both these systems, an
extended conformation is preferred in nonpolar solvents and a
folded one in polar solvents, leading to J- and H-aggregates,
respectively. In contrast to the squaraine dimers, which do not
fluoresce, an excimer-type fluorescence was observed from the
folded form of merocyanine dimers. The studies on bichro-
mophores, reported so far, deal mainly with chromophoric
interactions, and it is possible to design bistable systems by
modulating their optical properties. In the present study, we
report the dynamics of folding-unfolding processes of two
nonconjugated bichromophores containing hemicyanine units
(hemicyanines are polymethines containing heterocyclic and
nonheterocyclic groups on either end) linked together by a poly-
(ethylene glycol) chain (Scheme 1). The conformational switch-
ing in these molecular systems led to substantial changes in
their singlet excited-state properties, due to the formation of
intramolecular aggregates.
Results and Discussion
Synthesis. Bichromophores 1 and 2, containing two units of
hemicyanines ((aminostyryl)benzothiazolium chromophores),
were synthesized through the pathways shown in Scheme 1.
Synthesis and spectroscopic characterization of the bichro-
mophores are given in the Experimental Section and the details
of the intermediate compounds are included in the Supporting
Information. The model compound 3 (Scheme 1) was prepared
1
by following a reported procedure.17 On the basis of the H
NMR studies, it is concluded that 1 and 2 adopt an “all-trans”
geometry, which minimizes the steric interactions in the
bichromophores.
UV/Vis Absorption and Conformational Switching. The
absorption spectra of bichromophore 1 and model compound 3
in dichloromethane and methanol are presented in Figure 1. The
spectral properties of 1-3 in dichloromethane are summarized
in Table 1. Both the bichromophores exhibit a broad band
centered around 540 nm in CH2Cl2. The absorbance of this band
follows Beer Lambert law and the spectral shape remains
unaffected with concentration, ruling out the possibility of strong
intra- or intermolecular interactions. Thus, the broad band,
absorbing around 540 nm, may correspond to the monomeric
form of the bichromophores. Similar results were obtained for
model compound 3 and bichromophores, in polar solvents such
as CHCl3, CH3CN, CH3OH, etc. The origin of this structureless
absorption band is attributed to a donor-acceptor charge-transfer
(CT) transition (Supporting Information). Recent X-ray diffrac-
tion studies18 as well as absorption spectral studies19a have also
suggested an intramolecular charge-transfer resonance between
(12) (a) Lu, L.; Lachicotte, R. J.; Penner, T. L.; Peristein, J.; Whitten,
D. G. J. Am. Chem. Soc. 1999, 121, 8146. (b) Ushakov, E. N.; Gromov, S.
P.; Fedorova, O. A.; Pershina, Y. V.; Alfimov, M. V.; Barigelletti, F.;
Flamigni, L.; Balzani, V. J. Phys. Chem. 1999, 103, 11188. (c) Katoh, T.;
Inagaki, Y.; Okazaki, R. J. Am. Chem. Soc. 1998, 120, 3623. (d) Chibisov,
A. K.; Zakharova, G. V.; Gorner, H.; Sogulyaev, Y. A.; Mushkalo, I. L.;
Tolmachev, A. I. J. Phys. Chem. 1995, 99, 886.
(13) Liang, K.; Frahat, M. S.; Perlstein, J.; Law, K. Y.; Whitten, D. G.
J. Am. Chem. Soc. 1997, 119, 830.
(14) (a) Harriman, A.; Heitz, V.; Sauvage, J. P. J. Phys. Chem. 1993,
97, 5940. (b) Tamiaki, H.; Holzwarth, A. R.; Miyatake, T.; Tanikaga, R.;
Shaffner, K. Angew. Chem., Int. Ed. Engl. 1996, 35, 772.
(15) Forster, T. Angew. Chem., Int. Ed. Engl. 1969, 8, 333.
(16) (a) Rae, E. G. M.; Kasha, M. In The Molecular Exciton Model, in
Physical Processes in Radiation Biology; Augenstein, L., Mason, R.,
Rosenberg, B., Eds.; Academic Press: New York, 1964; pp 23-42. (b)
Valdes-Aguilera, O.; Neckers, D. C. Acc. Chem. Res. 1989, 22, 171.
(17) Hamer, F. M. J. Chem. Soc. 1956, 1480.
(18) Alfimov, M. V.; Churakov, A. V.; Federov, Y. V.; Federova, O.
A.; Gromov, P.; Hester, R. E.; Howard, J. A. K.; Kuzmina, L. G.; Lednev,
K.; Moore, J. N. J. Chem. Soc., Perkin. Trans. 2 1997, 2249.

Exciton Interations in Hemicyanine-Based Bichromophores
J. Am. Chem. Soc., Vol. 123, No. 32, 2001 7861
Scheme 2. (A) Pictorial Representation of the Aggregation
of Bichromophore (Type 1, card pack arrangement; Type 2,
intramolecular folding) and (B) an Illustration of the
Folding-Unfolding Process of Bichromophores Based on the
Photophysical Studies
Figure 2. (a) Visible absorption spectra of 1 at 25 °C in mixed
toluene-CH2Cl2 solvents of the following proportions (toluene:CH2-
Cl2): (a) 0:100, (b) 70:30, (c) 75:25, and (d) 95:5. (b) Visible absorption
spectra of 1 at various temperatures in a (4:1) mixture of toluene and
CH2Cl2 (a) 15, (b) 20, (c) 24, (d) 28, (e) 32, (f) 36, and (g) 38 °C.
indicating that an equilibrium process is involved. Similar results
are observed for the bichromophore 2 (Supporting Information).
Cyanines and squaraines derived from benzothiazolium units
are known to form face to face or parallel dimers (H-aggregates),
characterized by the appearance of a blue-shifted absorption
band.20 In the present case, the new high-energy band, observed
at 420 nm, may be assigned to an H-aggregate, arising from
the parallel stacking of the hemicyanine chromophores, on the
basis of analogy. Parallel stacking is possible in two different
ways: (i) through an intermolecular interaction between two
or more bichromophores (aggregation number g2) in a card
pack arrangement or (ii) through an intramolecular interaction
between two hemicyanine units of the bichromophore, as a result
of folding of the molecule (aggregation number ) 1). A
simplified representation of these possibilities is shown in
Scheme 2A. Additional information concerning the nature of
stacking was obtained by investigating the aggregation properties
of the bichromophores. The aggregation numbers of 1 (Figure
3) and 2 were determined by studying the effect of concentration
of the bichromophores on the absorption spectrum. Monomer
and aggregate bands are well separated and the spectral overlap
at the absorption maximum is negligible (Figures 2 and 3). The
extinction coefficients of 1 and 2 were estimated at their peak
maxima in a 7:3 mixture of toluene and CH2Cl2 (aggregation
is not observed in this solvent composition at 25 °C). The
concentrations of the monomer and aggregate were estimated21
and they followed a linear dependence for both the bichro-
mophores. The plot of log[monomer] versus log[aggregate] was
found to be linear and the aggregation number was estimated
the nitrogen atoms in hemicyanine dyes. The half-width of the
absorption band (υ1/2) of 3 in CH2Cl2 is 2200 cm-1 and a
spectral broadening was observed with increase in polarity of
the solvent (Supporting Information). There are several bonds
in hemicyanines, which may undergo torsional motions. It is
suggested that the torsional motions, coupled with solvent
reorganization may lead to spectral broadening of hemicyanines,
in polar solvents.19b Compared to 3, both the bichromophores
1 and 2 exhibit a broad absorption band and υ1/2 is not influenced
by solvent polarity (υ1/2 ∼ 3000 cm-1). The chromophoric units
in 1 and 2 are linked together by a flexible poly(oxoethylene)
chain. Depending on the nature of the solvent, the poly-
(oxoethylene) bridging unit can adopt various conformations,9c
which may result in weak through-space interactions between
the two hemicyanine units, in 1 and 2. These factors along with
the torsional motions of the hemicyanines may lead to the
broadening of the absorption spectrum.
Addition of varying amounts of toluene to a solution of 1 in
CH2Cl2 led to a gradual hypsochromic shift in its absorption
band (Figure 2a). Interestingly, on increasing the toluene content
in a solution of 1 in CH2Cl2 (25 °C), a decrease in the intensity
of the monomer band was observed, accompanied by the
formation of a sharp band at 420 nm (traces “c” and “d” in
Figure 2a). The intensity of the 420 nm band is markedly higher
at lower temperatures (Figure 2b). A complete reversal to the
monomeric form was observed with an increase in temperature,
(20) (a) Seifert, J. L.; Corner, R. E.; Kushon, S. A.; Wang, M.; Armitage,
B. A. J. Am. Chem. Soc. 1999, 121, 2987. (b) Khairutdinov, R. F.; Serpone,
N. J. Phys. Chem. B 1997, 101, 2602. (c) Das, S.; Thomas, K. G.; Thomas,
K. J.; Madhavan, V.; Liu, D.; Kamat, P. V.; George, M. V. J. Phys. Chem.
1996, 100, 17310.
(19) (a) Thomas, K. J.; Thomas, K. G.; Manojkumar, T. K.; Das, S.;
George, M. V. Proc. Ind. Acad. Sci. (Chem. Sci.) 1994, 106, 1375. (b) Cao,
X.; McHale, J. L. J. Chem Phys. 1998, 109, 1901.
(21) The equilibrium constants (K ) [F]/[UF]) were estimated from the
concentrations of the unfolded form ([UF] ) absorbance of UF/ꢀ of UF)
and the folded form ([F] ) total concentration - [UF]).

7862 J. Am. Chem. Soc., Vol. 123, No. 32, 2001
Zeena and Thomas
intermolecular dimerization constants of merocyanines, which
are mainly driven by electrostatic dipole-dipole interactions.23a
The free energy for the formation23b (-∆G24°C) of the intramo-
lecular aggregates of 1 and 2 was estimated as 0.35 and 3.1 kJ
mol-1, respectively. The low value of the equilibrium constant
and free energy of formation of the homoaggregates are
advantageous in the present case, since they permit the inter-
conversion in a smaller temperature range (15-35 °C). Forma-
tion of intermolecular aggregates of 3 was not observed under
identical conditions.
Steady-State Emission Properties. Recent investigations24
of the excited state properties of hemicyanine dyes suggest that
(i) the singlet excited state of the chromophore deactivates
mainly through bond twisting (rotamerism24a,b) and rigidization
by structural modifications24c prevent the nonradiative deactiva-
tion and (ii) the substitution of the dialkylamino group in
hemicyanines markedly reduces the trans-cis isomerization.24d
The model compound 3 in CH2Cl2 has a broad emission
spectrum with a maximum centered around 597 nm. The
quantum yield of fluorescence (0.018 in CH2Cl2 at 25 °C) was
found to be independent of the excitation wavelength. Viscosity-
dependent studies of 3 indicate that the singlet excited state of
the chromophore deactivates through competing radiative and
nonradiative channels (Supporting Information). In the present
case an enhancement in the fluorescence quantum yield (Φf)
and lifetime (τs) was observed for 3 in viscous medium.
Rigidization of the molecule in viscous medium may retard the
nonradiative decay channels such as bond twisting in the excited
state.
Figure 3. Visible absorption spectra of 1 at different concentrations
(0.56-2.84 µM) in a 17:3 mixture of toluene and CH2Cl2 at 25 °C.
The inset shows the plot of log [aggregate] vs log [monomer].
as 1, from the slope. A representative example is shown in the
inset of Figure 3. These results indicate the formation of
intramolecular aggregates of Type II (Scheme 2A), wherein the
bichromophoric system undergoes a folding process. Thus, as
shown in Scheme 2B the bichromophores can exist in two
extreme conformations, folded and unfolded, depending on the
polarity and temperature. The sharp band with absorption at
420 nm probably corresponds to a folded conformation. The
long wavelength band observed in the 450-600 nm region
corresponds to an unfolded geometry where the bichromophores
can exist in several disordered conformations.
Solvent-dependent conformational changes in poly(oxoeth-
ylene) linkers were investigated by theoretical as well as
experimental methods.9c,22 It is reported that the carbon-carbon
bond of the -OCH2CH2O- unit exists predominantly in the
gauche conformation in a high polar medium, but shifts to the
trans conformation in a low polar medium.22 Solvent-dependent
conformational changes in poly(oxoethylene)-linked binuclear
complexes were investigated recently by probing the photoin-
duced energy transfer.9c In polar solvents, the unfolded con-
formation of bichromophores may be more stable due to the
presence of the gauche conformation. The polarity-dependent
conformational changes of the bridging unit and the solvopho-
bicity of the dye in nonpolar medium help in the folding of 1
and 2, overcoming the Coulombic repulsion between the
chromophores. Solvophobically driven folding processes were
recently reported for substituted phenylacetylene oligomers7 and
merocyanine dimers.12a,20b In the present case, both the molec-
ular systems prefer a well-ordered folded conformation in
nonpolar medium with parallel stacking of the chromophores
(Scheme 2B), since such an arrangement can minimize the polar
chromophoric surface exposed to the nonpolar solvent.
Conformational switching of 1 and 2 was studied by steady
state and time-resolved fluorescence techniques. The unfolded
forms of both bichromophores in CH2Cl2 emit around 600 nm
(Table 1), with almost identical quantum yields (Φf ∼ 0.01). A
substantial lowering of emission yield was observed for both
bichromophores on increasing the toluene content. The relative
emission intensities of 1 (excited at the isosbestic point) in
mixtures of dichloromethane and toluene are shown in Figure
4. Lowering of the emission yield is attributed to the formation
of intramolecular aggregates, resulting from the folding of the
bichromophores. The folded form of the bichromophores (in a
19:1 mixture of toluene and CH2Cl2) was further selectively
excited at 420 nm, where the unfolded form does not absorb.
An emission spectrum having a similar spectral shape as that
of the unfolded form with an extremely low yield (φf < 10-4
)
was observed in the case of the folded forms of 1 and 2. It has
been reported that intermolecular aggregates of cyanine dyes
possess lower fluorescence yields and longer singlet lifetimes
than their corresponding monomers.20a On the basis of the
exciton theory, the transition to the upper energy level is allowed
for the H-aggregates, which rapidly deactivate to the lower
energy level via internal conversion. The emission from the
lower energy level is theoretically forbidden16 and hence the
singlet excited state of the bichromophoric aggregate gets
trapped in a low emissive state. Schematic representation of
the energy level of the unfolded and the folded forms of the
bichromophores is proposed in Scheme 3. The effect of addition
of toluene on the quantum yield of fluorescence of 3 in CH2Cl2
is shown in the inset of Figure 4 for comparison. A slight
increase in φf was observed for 3 on increasing the proportions
One of the interesting features of these bichromophores is
their ability to undergo temperature-dependent conformational
changes. The folding-unfolding process in bichromophores is
highly sensitive to the temperature of the medium (for e.g.,
Figure 2b) and the interconversions are possible in a smaller
temperature range. The equilibrium constants (K24°C) for the
formation of intramolecular aggregates of 1 and 2 through the
folding are estimated as 1.2 and 3.5, respectively. These values
are orders of magnitude lower than those reported for the
(23) (a) Wurthner, F.; Yao, S. Angew. Chem., Int. Ed. 2000, 39, 1978.
(b) Standard free energy change, ∆G ) -RT lnK.
(24) (a) Ephardt, H.; Fromherz, P. J. Phys. Chem. 1989, 93, 7717. (b)
Kim, J.; Lee, M. J. Phys. Chem. 1999, 103, 3378. (c) Rocker, C.; Heilemann,
A.; Fromherz, P. J. Phys. Chem. 1996, 100, 12172. (d) Gorner, H.; Gruen,
H. J. Photochem. 1985, 28, 329.
(22) Bjorling, M.; Karlstrom, G.; Linse, P. J. Phys. Chem. 1991, 95,
6706.

Exciton Interations in Hemicyanine-Based Bichromophores
J. Am. Chem. Soc., Vol. 123, No. 32, 2001 7863
Figure 5. Excitation spectra of 1 (6 µM) in mixtures of toluene and
CH2Cl2 (toluene:CH2Cl2): (a) 17:3 and (b) 1:1. The inset shows the
excitation spectra of 3 (6 µM) in the same solvent mixtures (emission
monitored at 580 nm for 1 and 3).
Figure 4. Emission spectra of 1 at 25 °C in mixtures of toluene and
CH2Cl2 (toluene:CH2Cl2): (a) 3:7, (b) 1:1, and (c) 4:1. The solutions
were excited at the isosbestic point (450 nm). The inset shows the effect
of addition of toluene on the quantum yield of fluorescence of 3 in
CH2Cl2. The solutions were isoabsorptive at the excitation wavelength
(500 nm).
Scheme 3. Schematic Representation of the Energy Levels
of the Unfolded and Folded Forms of the Bichromophores
Figure 6. Relative distribution of the folded and unfolded forms of 1
at 20 °C, on addition of toluene to a solution of 1 in CH2Cl2.
Table 2. Fluorescence Lifetimesa-c and Fractional Contributionsd,e
of 2 in Mixed Toluene-CH2Cl2 Solvents at 20 °C
of toluene and is attributed to the decrease in polarity of the
medium. The lack of strong φf dependency of 3 in mixtures of
toluene-CH2Cl2 further supports the formation of intramolecular
H-aggregates of the bichromophores.
toluene:CH2Cl2
τ1, ns (ø1, %)
τ2, ns (ø2, %)
1:1
3:1
17:3
19:1
0.37 (100)
0.33 (56)
0.33 (35)
0.76 (21)
3.08 (44)
3.76 (65)
4.20 (79)
The origin of the emission of the folded and unfolded forms
was confirmed by recording the excitation spectra of the
bichromophores as a function of solvent composition (Figure
5). The excitation spectrum of 1, recorded in a 1:1 mixture of
toluene and CH2Cl2 (25 °C), possesses a broad band around
540 nm, which closely matches with the absorption character-
istics of the unfolded bichromophore (Figure 2a). In a 17:3
mixture of toluene and CH2Cl2, the excitation spectrum of 1
(trace a) exhibits a band at 420 nm, apart from the long
wavelength band, and matches with the corresponding absorp-
tion spectrum in spectral shape (Figure 2). The intensity of the
long wavelength band is higher in the excitation spectrum due
to the higher emission yield of the monomer, compared to the
aggregated form. These results further confirm that the emission
of the folded form originates from the 420 nm band and that of
the unfolded form originates from the 540 nm band. The
excitation spectrum of 3, in both the solvent mixtures, is shown
in the inset of Figure 5 for comparison and the 420 nm band is
absent.
a τ1and τ2. b The quality of the fit is judged by means of the usual
statistical parameters such as ø2 (1.10-1.29); error limit (5%. c Excited
at 440 nm and emission followed at 600 nm. d ø1 and ø2. e Fractional
contributions of 1 are presented in Figure 6.
of 1 and 2 was achieved by investigating the singlet lifetimes
as a function of solvent composition and temperature. Both the
bichromophores exhibited monoexponential decay with a short
lifetime (∼350 ps) in mixed toluene-CH2Cl2 solvents, with
lower proportions of toluene (<50%) at 20 °C. Interestingly,
both the bichromophores exhibited biexponential decay, with a
short-lived (τ1) and long-lived (τ2) component (Figure 6 and
Table 2) on increasing the proportions of toluene (>70%) at
20 °C. Gradual increase in singlet lifetimes was observed, for
both forms, with a decrease in the temperature of the medium
(Table 3). Biexponential behavior, observed for both bichro-
mophores in CH2Cl2 containing a high composition of toluene,
is due to the presence of folded and unfolded forms. The short-
Fluorescence Lifetimes. Characterization of the singlet
excited states of the folded form as well as the unfolded form

7864 J. Am. Chem. Soc., Vol. 123, No. 32, 2001
Zeena and Thomas
Table 3. Fluorescence Lifetimesa-c and Fractional Contributionsd
of 1e as a Function of Temperature
temp (°C)
τ1, ns (ø1, %)
τ2, ns (ø2, %)
17.5
21
25
27
33
0.51 (25)
0.41 (38)
0.42 (49)
0.33 (51)
0.27 (72)
4.23 (75)
3.69 (62)
3.72 (51)
3.15 (49)
3.06 (28)
a τ1and τ2. b The quality of the fit is judged by means of the usual
statistical parameters such as ø2 (0.88-1.16); error limit (5%. c Excited
at 440 nm and emission followed at 600 nm. d ø1 and ø2. e 75% (v/v)
toluene/CH2Cl2.
lived species is assigned as the unfolded form and the long-
lived component as the folded form of the bichromophore.
Intramolecular folding of 1 and 2 may result in the parallel
stacking of the two chromophores, which can restrict the
torsional dynamics of the hemicyanines, leading to longer
lifetimes.20a Similar results were obtained for the model
compound in viscous medium, in which the singlet excited state
exhibits a longer lifetime, due to restricted motion (Supporting
Information).
The interconversion between the folded and unfolded forms
was further investigated by plotting their fractional contributions
of lifetimes as a function of solvent polarity (Figure 6) and
temperature. The population of the long-lived species increased
substantially for both the bichromophores on (i) increasing the
toluene content (Figure 6) and (ii) decreasing the temperature
of the medium (Table 3). A complete reversal in the fractional
contribution of lifetimes was observed on increasing the
temperature of the medium.
Laser Flash Photolysis Studies. We have examined the
photoprocesses in the folded and unfolded forms of bichro-
mophores employing the laser flash photolysis technique. The
folded form was excited using an excimer pumped dye laser
(420 nm) and the unfolded form using a Nd:YAG laser (532
nm).
Figure 7. (a) Transient absorption spectrum measured following laser
pulse excitation (532 nm, ∼120 mJ/pulse) of the unfolded form of 1
in a 1:1 mixture of toluene and CH2Cl2. The inset shows the transient
decay at 420 nm. (b) Transient absorption spectrum measured following
laser pulse excitation (420 nm, ∼10 mJ/pulse) of the folded form of 1
in a 9:1 mixture of toluene and CH2Cl2.
In polar solvents, such as methanol, both bichromophores
exist in the unfolded form and no transient was observed in the
nanosecond time scale. Interestingly, a transient absorption was
observed around 420 nm for 1, in a 1:1 mixture of toluene and
CH2Cl2 (Figure 7a). The transient decay at 420 nm (inset of
Figure 7a) consists of a major short-lived (k1 ) 1.07 × 104
s-1) and a minor long-lived component (k2 ) 6.32 × 103 s-1).
The major component is quenched by O2, ferrocene, and
â-carotene.25 On the basis of these results and the similarity of
spectral behavior to those reported for other hemicyanines,24d
the major transient species is assigned as a triplet. In less polar
solvents, the bichromophores may be existing as a contact ion
pair and the heavy atom induced intersystem crossing leads to
the formation of a triplet. A transient species with similar
properties, observed on addition of I- to a methanolic solution
of bichromophore, further confirmed the heavy atom induced
triplet formation. The yield of the minor component (k2) is less
than 10% and hence no attempt was made to establish its
identity.
depletion of the ground state of the folded form. The band
observed at 450-600 nm corresponds to the ground-state
absorption of the unfolded form and it does not decay up to
100 µs (100 µs is the highest detection limit of our equipment).
The ground state absorption spectra recorded before and after
the laser flash photolysis remained unchanged, indicating that
the refolding process is completely reversible and occurs in the
time scale longer than 100 µs. Intermolecular aggregates of
cyanines were reported to disrupt under laser excitation through
photoinduced thermal dissociation,20b whereas in the case of
squaraines a photodissociation mechanism20c has been suggested.
In the present case, laser excitation of the folded forms of 1 as
well as 2 may cause a local temperature jump, leading to the
unfolding of bichromophores.
Laser pulse excitation of the folded form of bichromophores
results in their unfolding. The transient spectrum of the folded
form of the bichromophore 1 in a 9:1 mixture of toluene and
CH2Cl2, observed immediately after the laser pulse, is shown
in Figure 7b. The unfolding process is prompt and we could
not observe it in the nanosecond time scale. The transient formed
was not quenched by O2. The negative absorbance observed in
the spectral region of 375-450 nm is due to the laser-induced
Conclusions
The newly designed hemicyanine-based bichromophores
possess a unique property of folding-unfolding and their
interconversion was achieved by varying the polarity of the
medium or temperature. Both the bichromophores prefer a well-
defined folded conformation in nonpolar medium and the
solvent-dependent conformational changes of the poly(oxoet-
hylene) linker group as well as the solvophobicity of the
(25) Carmichael, I.; Hug, G. L. J. Phys. Chem. Ref. Data 1986, 15, 1.

Exciton Interations in Hemicyanine-Based Bichromophores
J. Am. Chem. Soc., Vol. 123, No. 32, 2001 7865
Bichromophore 1. Elution of the column using a mixture (2:5) of
methanol and chloroform gave 0.2 g (43%) of 1 as a solid. Mp 217-
218 °C dec; IR (KBr) νmax 1776, 1739, 1668, 1534, 1457, 1392, 1273,
chromophores in nonpolar solvents drive the folding process,
overcoming the Coulombic repulsion. The laser flash excitation
of the folded form results in fast conformational changes due
to photoinduced thermal dissociation. The conformational
switching in such bichromophores could lead to the design of
a novel type of molecular switching devices. Also, similar
strategies of chromophoric interactions can be employed for
probing conformational changes in macromolecular systems.
1
1183, 815, 758 cm-1; H NMR (CDCl3 + DMSO-d6) δ 8.23 (d, J )
7.75 Hz, 2H), 8.03 (d, J ) 8.28 Hz, 2H), 7.94 (d, J ) 15.2 Hz, 2H),
7.83 (d, J ) 8.54 Hz, 4H), 7.71 (t, J ) 7.67 Hz, 2H), 7.61 (t, J ) 7.52
Hz, 2H), 7.49 (d, J ) 15.33 Hz, 2H), 6.79 (d, J ) 8.6 Hz, 4H), 4.17
(s, 6H), 3.2-3.9 (m, 12H), 3.04 (s, 6H); 13C NMR (CDCl3 + DMSO-
d6) δ 153.62, 151.23, 141.44, 133.99, 128.84, 127.15, 126.84, 123.98,
121.85, 114.87, 112.25, 107.55, 105.22, 98.27, 96.09, 71.07, 68.53,
52.13, 29.65; HRMS calcd for C40H44N4O2S2I2 [M - I]+ 803.1950,
found 803.1964 (FAB high-resolution mass spectroscopy).
Experimental Section
All melting points are uncorrected and were determined on an
Aldrich melting point apparatus. IR spectra were recorded on a Perkin-
Elmer Model 882 IR spectrometer and the UV-visible spectra on a
Shimadzu UV-3101PC UV-vis-NIR scanning spectrophotometer. 1H
and 13C NMR spectra were recorded on a Bruker DPX-300 MHz
spectrometer. Emission spectra were recorded on a SPECTRACQ
spectrofluorimeter and corrected using the program supplied by the
manufacturer. The quantum yield of fluorescence was determined by
a relative method using optically dilute solutions of the dyes (OD of
0.1 at the excitation wavelength) using Rhodamine 6G in ethanol (Φf
) 0.9) as standard. Fluorescence lifetimes were measured using a
Tsunami Spectra Physics single photon counting system. A Ti Sapphire
laser, having a fundamental wavelength of 880 nm, was used as an
excitation source. The average output power is 680 mW with a pump
power of 4.5 W. The pulse width of the laser is <2 ps. The flexible
harmonic generator (FHG) gives the second harmonic (440 nm) output
from the Tsunami laser system. The fluorescence was detected using
a two-stage microchannel plate photomultiplier (MCP-PMT R38094).
The fluorescence decay measurements were further analyzed using the
IBH software library, which includes an iterative shift of the fitted
function as part of ø2 goodness of the fit criterion. Laser flash photolysis
of the unfolded form was carried out in an Applied Photophysics model
LKS-20 laser kinetic spectrometer using the second harmonic (532 nm)
of a Quanta Ray GCR-12 series pulsed Nd:YAG laser with a pulse
duration of 10 ns band energy of 120 mJ/pulse. Laser flash photolysis
experiments of the folded form were carried out using a Lambda Physik
Lextra 50 excimer laser pumped scanmate 2 dye laser. The excitation
wavelength was 420 nm. All solutions for laser flash photolysis were
deaerated by bubbling argon for 15 min unless otherwise indicated.
General Method for Synthesis of Bichromophores (1 and 2). A
mixture of 2-methylbenzothiazoliummethiodide (1 mmol) and the
appropriate N-methyl(p-formyl)aniline-tethered glycol (0.5 mmol) was
refluxed in methanol (50 mL) in the presence of piperidine (2 drops)
for 72 h under argon atmosphere. The solvent was removed under
reduced pressure and the crude product was chromatographed over
neutral alumina.
Bichromophore 2. Elution of the column using a mixture (1:9) of
methanol and chloroform gave 0.22 g (45%) of 2 as a solid. Mp 143-
145 °C dec; IR (KBr) νmax 1587, 1534, 1519, 1435, 1389, 1343, 1268,
1183, 1116, 1031, 985, 813, 758, 717, 638, 519 cm-1; 1H NMR (CDCl3
+ DMSO-d6) δ 8.27(d, J ) 7.83 Hz, 2H), 8.07(d, J ) 8.37 Hz, 2H),
7.96 (d, J ) 15.26 Hz, 2H), 7.85 (d, J ) 8.54 Hz, 4H), 7.74 (t, J )
7.77 Hz, 2H),7.64 (t, J ) 7.61 Hz, 2H), 7.54 (d, J ) 15.32 Hz, 2H),
6.83 (d, J ) 8.73 Hz, 4H), 4.20 (s, 6H), 3.2-3.9 (m, 16H), 3.07 (s,
6H); 13C NMR (CDCl3 + DMSO-d6) δ 171.65, 153.29, 150.38, 142.33,
133.33, 129.29, 127.85, 127.22, 124.22, 121.93, 116.36, 112.45, 106.55,
104.79, 70.48, 70.39, 68.36, 51.68, 36.08; HRMS calcd for C42H48N4-
O3S2I2 847.2213 [M - I]+, found 847.2186 (FAB high-resolution mass
spectroscopy).
Acknowledgment. The authors thank the Council of Sci-
entific and Industrial Research (CSIR), the Regional Research
Laboratory (CSIR) Trivandrum, and the Department of Science
and Technology (DST Grant No. SP/S1/G-21/97), Government
of India for financial support. We also thank Professor P.
Natarajan and Dr. P. Ramamurthy, National Centre for Ultrafast
Processes, and the Department of Inorganic Chemistry, Uni-
versity of Madras for allowing access to the Picosecond Time
Correlated Single Photon Counting and Laser Flash Photolysis
facilities. This is contribution No. RRLT-PRU 132 from the
Regional Research Laboratory, Trivandrum, India.
Supporting Information Available: Details of the synthesis
of the intermediate compounds 5-8, aggregation properties of
2, and the absorption and emission properties of 3 (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA010199V