7188
J. Phys. Chem. 1995,99, 7188-7192
Monopole-Dipole Model for Symmetrical Solvatochromism of Hemicyanine Dyes Peter Fromherz Max-Planck-Institute for Biochemistry, Department of Membrane- and Neurophysics, 0-82152 MartinsriedWiinchen, Gennany Received: January 26, 1995; In Final Form: February 28, 1999
The absorption spectrum of zwitterionic hemicyanine dyes is shifted to the blue and their fluorescence spectrum is shifted to the red by solvents of high polarity. This symmetrical solvatochromism is rationalized by a Born-Marcus-type theory of reversible charging for the positive chromophore alone which is described by an electrical point charge and an electrical point dipole in the center of a sphere.
Introduction The absorption and emission spectra of many organic dyes depend on the polarity of the solvent.' This solvatochromism is assigned generally to a change of the molecular dipole moment upon electronic excitation and an interaction of the dipole moment with the polarizable environment. The effects are usually described by theories based on Onsager's reaction field.2-5 In the present paper, a set of hemicyanine dyes is considered where a straightforwardapplication of these theories fails. The zwitterionic hemicyanine dyes consist of a positively charged chromophore and a negative counterion which is bound by a spacer as shown in Figure 1. Their solvatochromism exhibits a striking symmetry: an increasing polarity of the solvents gives rise to a blue shift of the absorption spectrum and a red shift of the emission spectrum. The opposite spectral shift has an almost identical value for solvents of all polarities-from chloroform to water. This symmetrical solvatochromism is rationalized on the basis of the Bom-Marcus theory of reversible which is applied to the positively charged chromophore of the dyes alone, disregarding the negative sulfonate.
Materials and Methods Dyes. All seven hemicyanine dyes (Figure 1) are zwitterionic amphiphiles. The positive charge of the chromophore is compensated by a negative sulfonate. In the group of the proper hemicyanines RH364, RH160, and RH237, aniline and pyridinium are connected by one, two, and three double bonds.I0." In di4ANJZPPS, a naphthylamine is connected to pyridinium by one double bond.I2 In the group of the biaryl hemicyanines BABP, BNBP, and BNBIQ, aniline and naphthylamine, respectively, are joined directly to pyridinium and isoquinolinium, re~pectively.'~.'~ The dyes RH237 and di4ANEPPS were obtained from Molecular Probes (Junction City, OR). All others were synthesized as described in the literature. Spectra. Stock solutions of the dyes (about 1 mM) were prepared in ethanol. They were diluted by solvents of highest available purity (Merck, Darmstadt, and Nuka, Neu-Ulm). Chloroform, decanol, dichloromethane,octanol, benzyl alcohol, hexanol, pentanol, cyclohexanol, butanol, cyclopentanol, propanol, acetone, ethanol, methanol, acetonitrile, dimethylformamide, ethylene glycol, and water were used. The solvents are described by their static dielectric constant 6 at 25 "C and their refractive index n D at 589 nm.I5 The absorption spectra were 'Abstract published in Advance ACS Abstracts, April 15, 1995.
0022-365419512099-7188$09.0010
measured with a Varian Cary 219 spectrophotometer at 25 "C, using final concentrations of about 10 pM (with 1% ethanol). Corrected fluorescence spectra were recorded in a SpexFhorolog fluorometer at 25 "C at a spectral width of 2.3 nm. Solutions of 3-10 pM were used. Some of the data are taken from the 1iterat~re.I~ Computation. The quantum chemical parametrization AM1 of the program package MOPAC was used.I6 A geometry of minimal energy was determined for homologs with two methyl substituents at the amino group and a third methyl group at the ring nitrogen of the chromophores. The electrical charge on all atoms was determined in the ground state and in the first excited singlet state at constant geometry. From those data the intramolecular displacement 6 of the elementary charge eo was determined with respect to the direction of the single bonds between the aryl moieties, as caused by electronic excitation.
Results The wavenumbers VABS and VEM of the maxima of absorption and emission in 18 solvents are shown in Figure 2 for the proper hemicyanines RH364, RH160, and RH237, for the biaryl hemicyanines BABP, BNBP, and BNBIQ, and for di4ANEPPS. The solvents are characterized by the polarity function Fl(n,c) according to Brunschwig, Ehrenson, and S ~ t i n .It~depends on the refractive index n and the static dielectric constant E , according to eq 1. The parameter ci-an intramolecular dielectric constant-is chosen as c1 = 2 (cf. ref 9). The choice of this particular polarity function to plot the data will become clear in the discussion.
There is a common trend for all dyes: the absorption is shifted to the blue and the emission is shifted to the red with increasing polarity of the solvent. This solvatochromism is almost symmetrical,with a constant average of the wavenumbers (PABS f VEM)/2 d l over the range of polarity as marked in Figure 2. The solvatochromism is due solely to a change of the Stokes shift VABS - VEM. The values of (VABS - VEM)/2 are plotted versus the polarity function F l ( n , ~in ) Figure 3. The Stokes shift is more sensitive to polarity for larger chromophores within the classes of the proper hemicyanines (Figure 3a) and of the biaryl hemicyanines (Figure 3b). For chromophores of equal size, it is more sensitive with a lower number of free double bonds (Figure 3c). In all cases, the Stokes shift increases fairly linearly with the polarity function Fl(n,c). 1995 American Chemical Society
Solvatochromism of Hemicyanine Dyes
J. Phys. Chem., Vol. 99, No. 18, 1995 7189
\ r BNBP
BABP
c~Q3-y)+-(%,,, C4H9
di4ANEPs
Figure 1. Zwitterionic hemicyanine dyes: at the top, the proper hemicyanines RH364, RH160, and RH237; in the center, the biaryl hemicyanines BABP, BNBP, and BNBIQ; at the bottom, di4ANEPPS.
Discussion At fist, the framework of the Bom-Marcus theory of solvatochromism is summarized according to Brunschwig, Ehrenson, and S ~ t i n .The ~ theory is found to contradict the experiments if applied to the whole neutral dye molecules. It is applied then to the positively charged chromophore alone. It is found that a simple model with a monopole and a dipole in the center of a sphere accounts for all crucial features of the experiments. Finally, the role of the counterion is considered. Spectra and Solvation Energy. The energies EABSand EEM of absorption and emission are partitioned into (a) the change of Gibbs energy AI$ by electronic excitation in vacuum, (b) the intramolecular reorganization energy AIN, (c) the (negative) work functions W G ( E ) and W E ( € ) to transfer the ground state and the excited state from the vacuum into a solvent with dielectric constant E , and (d) the reorganization energy AouT(~,E)of the solvent with refractive index n and dielectric constant E
These expressions hold for identical reorganization energies in the ground state and in the excited state. The reorganization energy AO"T(~,E) is given by the work to transfer a virtual molecular state E-G, defined by the difference of the charge distribution of excited state and ground state?q8 from a relaxed solvent (effective dielectric constant E ) to a nonrelaxed solvent with an effective dielectric constant n2
From sum and difference of the experimental energies of absorption and emission, we obtain the total change of the Gibbs energy by electronic excitation in the relaxed solvent and the total reorganization energy in the given solvent, respectively,
Symmetry of Solvatochromism. The experimental data in Figure 2 show that the average (EABS E E M ) /of~ the energies of absorption and emission is almost constant over the whole range of polarity for all hemicyanine dyes. It follows then from eq 5 that the total change of the Gibbs energy by electronic excitation does not change significantly in relaxed solvents. In
+
other words, the work function of solvation of the ground state and of the excited state is similar in all solvents with W E ( € ) = W G ( E ) . This is possible only if the charge distribution in the ground state and in the excited state of the molecules is equivalent with respect to the polarization of the solvent. To specify this intriguing result, a minimal model for the charge distribution is considered. Monopole-Dipole Model. The work functions W E ( E E ~ ) , W O ( E E ~ )or , W E - G ( E E ~ to ) transfer a molecule in a statej = E, G, or E-G from the vacuum into a medium with an effective dielelectric constant E E =~ E or n2 can be evaluated for an arbitrary charge distribution in a sphere of radius a with an intemal dielectric constant cia9 In terms of a formal multipole expansion, the most elementary model for a charge distribution is the superposition of a point charge qj and a point dipole ,iij in the center of the sphere. With the permittivity of the vacuum EO, the work of solvation is
+
We have seen that the average (EABS E E M ) of / ~ the energy of absorption and emission reflects an identity WE(€) = W G ( E ) of the work of solvation of the ground state and of the excited state in a relaxed solvent. Comparing eq 7 with that result, we conclude that the absolute values of charge and of dipole moment must be identical in the ground state and in the excited state with 1qE1 = (f@I and Ii'i~l= Iii~lto account for an equivalence of the charge distribution with respect to the polarization of the solvent. Zwitterionic Molecule. The monopole term of eq 7 disappears with respect to an electrically neutral dye molecule, of course. In terms of the remaining dipole model, a symmetrical solvatochromismimplies that the absolute value of the molecular dipole moment remains unchanged upon excitation. This conclusion would be drawn equally well from the Onsager-type theories as discussed, e.g., by L i p t a ~ . ~ An identical absolute value of the dipole moment in the ground state and in the excited state of the zwitterionic hemicyanine dyes is rather unlikely. The positive charge eo of the chromophore is displaced upon excitation from the pyridinium or isoquinolinium moiety toward the anilino or naphthylamine moiety. The displacements 6 along the molecular axis as obtained from quantum chemical computations are
Fromherz
7190 J. Phys. Chem., Vol. 99, No. 18, 1995 C 24wO
24MM
--
22m
-
20000
-
5
f2 t
a
s
i
-
'ea
I
maam s a
"o. . .- . o.-o-.Ix& m
16m
c
22m
a
I
I
RH237
RH160
. RH364
*
a
2 m oL
i
4
- .. -
Qse0...0..? .
a
aa,a
14MM:
a
12m -
L
lZow 0.10
0.15
0,25
0,20
0,30
.ea
12m 015
010
polarity function F,
e
I
BNBP
1
2
4
,
,
,
0.10
0.15
0.20
0.25
1
020
8 025
1
030
polanty function F,
24000
1
**a-
24MM
f
J
BNBIQ
- t 14000
I
0.30
0.10
0.15
0.20
0,25
0.30
polarity function F,
polarity function F,
2
m
0.10
0,15
0.20
0,25
I
0,30
polarity function F,
g
I
di4ANEPPS
0.10
0.15
0.20
0.25
0.30
polarity function F,
Figure 2. Wavenumbers of the maxima of absorption VABS (upper dots) and of emission FEM(lower dots) for the proper hemicyanines RH364, E )18 solvents. RH160, and RH237, for the biaryl hemicyanines BABP, BNBP, and BNBIQ, and for di4ANEPPS versus the polarity function F I ( ~ , of F ] ( ~ , Edepends ) on the dielectric constant E and the refractive index n according to eq 1 with an intramolecular dielectric constant = 2. The average wavenumbers ( F ~ 4s DEM)/2 are marked by open circles. They are fitted by horizontal dashed lines. The linear relations of the wavenumbers BABS and BEM indicated by dotted lines are symmetrical with respect to (VABS -k &M)/2. Their parameters are taken from the fit of the Stokes shift in Figure 3. (The data in water at the highest value of the polarity function are not very reliable due to low solubility.)
summarized in Table 1. They are between 0.187 and 0.416 nm. Therefore, we expect an increase of the molecular dipole moment by 8.9 to 19.8 D. It is difficult to see how the excited state and the ground state can be equivalent with respect to the polarization of the solvent. We come to the conclusion that the dipole model of solvatochromism-applied to the zwitterionic molecule-is not compatible with the experimental data. Chromophore Alone. To resolve the apparent discrepancy of experiment and theory, it is postulated that the sulfonate moiety of the zwitterionic dyes does not contribute to the solvatochromism. The postulate implies that the solvation shell of the sulfonate remains unchanged upon excitation and that all resolvation refers to the positively charged chromophore of the hemicyanines alone. We assign then the monopole-dipole model to the charged
chromophore as follows: a sphere of radius a is thought to be located around the center of the chromophore. In the ground state, an elementary charge eo is located eccentrically at a distance +6/2 toward the pyridiniumhoquinolinium. In the excited state, the charge is located eccentrically at a distance -6/2 toward the anilinehaphthylamine. These two charge distributions may be represented as superpositions of a constant quadrupole with two charges ed2 at positions f6/2 and of a dipole with charges +ed2 at position +6/2 and -ed2 at position -6/2 in the ground state and of a reversed dipole in the excited state with exchanged positions of the charges. If the off-center displacement of the charges is small with respect to the radius of the sphere with 6
