J. Phys. Chem. 1993,97, 1085-1096
1085
.rr-Stacking and Aggregation of Pyridinium-Substituted Indolizines Phillip W. Carter? and Stephen C. DiMagnot Department of Chemistry, University of California, Berkeley, Berkeley, California 94720
John D. Porter' and Andrew Streitwieser' Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, and Lawrence Berkeley Laboratory, Berkeley, California 94720 Received: September 23, 1992
The tendency for pyridinium-substituted indolizinesto aggregate in aqueous and acetonitrile solution is dominated by solute-solvent interactions and by the amphiphilic nature of the cations. In solution, these compounds exhibit an unusual sequence of discrete, red-shifted fluorescence bands whose intensities are a function of concentration over the range 10-5-10-3 M. It is proposed that .Ir-stacking of the indolizine residues results in weak electronic interactions (0 0.13 eV) which are sufficient to result in the delocalization of bound excitonic states over the aggregate. Fluorescence from the aggregate occurs by radiative annihilation of the delocalized exciton. Stepwise dimerization (K2) and trimerization ( 4 ) equilibrium constants for the cations calculated from the emission spectra range from about 400 to 65 000. Solution conductivity measurements indicate that cation aggregation does not require ion pairing. However, where significant ion-pairing does occur, K2 and K3 are increased by a t least an order of magnitude. X-ray crystallography was used to determine the structures of cation dimers which form in the solid state, and these structures were used as working models for the geometry of the solution aggregates. Electrostatic and dispersion interactions calculated on the basis of those geometries account for some of the free energy of aggregate formation, but solvent entropic effects are believed to provide the strongest driving force for indolizine aggregation in solution. N
I. Introduction Because of their high electronic polarizability, extended r systems are particularly susceptibleto inter- and intramolecular electrostatic perturbations. We are interested in studying this type of interaction because it may be possible to use molecularlevel electrostatic interactions to tailor the physical and electronic properties of materials made from extended r systems. In addition, new and unusual properties may be observed for materials in which these interactions are taken to extremes. We have investigated the physical and electronic consequences that result from covalently binding positive charge, in the form of pyridinium moieties, onto indolizine, a fused-ring heteroaromatic system. Through spectroscopy and molecular orbital calculations we have confirmed that pyridinium substitution of indolizines modifies the indolizine ?r wave functions and energies and modifies the dynamics of indolizine electronic excited states. In addition, we have discovered that the presence of covalently bound positive charge in these species stabilizes the formation of cation dimers and higher aggregates in solution and in the solid state. These aggregate species have interesting electronic structures, reflected in the unusual fluorescence spectra of the compounds. One important advantage of the indolizine systems is that X-ray structures of pyridinium-substituted indolizine dimersare available.' By using the solid-state structures to model the geometry of the aggregates, we were able to estimate the magnitudes of several different intermolecular interactions and assess the relative importance of each in favoring the formation of aggregates. The aggregation of charged dyes in solution has long been of interest due to applications in photographic sciences2 and laser sciences.3 Factors that have been identified as having an effect
' Present address: Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455. 8 Present address: Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104. Authors for correspondence. 0022-3654/58/2097-1085$04.00/0
on the structures and physical properties of dye2v3and drug4qs aggregates include solvent permittivity,&*the identity of substituents, particularly those that yield amphiphilic propertie~:.S~~'3 and interactions with counteri~ns.'~J~ One of the advantages of studying the pyridinium-substituted indolizinesis that it is possible to study quite unambiguously the relationshipsbetween physical structure, electronic structure, ion pairing, and cation aggregation because independentexperimentalmeasurementsof these factors can be made. The two physical reactions of pyridinium-substituted indolizines that are of primary interest here are the stepwise aggregation of solvated indolizine cations and the ion pairing with gegenions in solution. Stepwise aggregation can be represented
where n is the aggregation number and z is the charge number of the solvated indolizine cation Izz+(sol). Ion pairing between anionic gegenions and the cationic indolizine aggregates can be represented IZ,Xj-,
(nz-j+ I )+(sol)
+ x-(sol) e Iz>p'-j)+(sol);
j = 1 , 2 , 3 , ... (2) wherej is the ion pairing number. Only monovalent anions Xhave been used in the present study, as shown in eq 2. In the present study, equilibrium constants for reactions 1 and 2 were determined using optical and conductance measurements, respectively. The important issues concerning aggregation of extended T systems which we were able to address using the pyridinium-substituted indolizine systems concern the relative importance of ion pairing to the process of aggregation, the role of the solvent in these processes, the relative importance of dispersion and electrostatic interactions in determining aggregation and the electronic consequences of *-stacking in the aggregates. Experimental methods are outlined in section 11. In section 0 1993 American Chemical Society
Carter et al.
1086 The Journal of Physical Chemistry, Vol. 97, No. 5, 1993
CI
tl
synthesis of neutral indolizine derivatives has been presented previously by Wadsworth and o o - ~ o r k e r s . ~Purification ~J~ of the cationic indolizines using silica and alumina columns and standard chromatographic techniques proved to be difficult because of the high charge density of the compounds. However, cation-exchange methods, cellulose-based methods, and polyacrylamide gel electrophoresis were employed as purification techniqueswith some success. The most versatile and satisfactory method for purifying the substituted indolizines was found to be the following: Crude ionic reaction products were subjected to polyacrylamide gel filtration chromatography (Bio-Rad Bio-Gel P-2 polyacrylamide, 200400 mesh, 5 cm diameter X 20 cm length column, 0.1 M KCI eluant at 8CL150 mL/h) followed by precipitation with an appropriate gegenion and recrystallization. Purified products were characterized routinely by elemental analysis, UV-vis spectrophotometry, ' H and I3C NMR spectroscopy, and also X-ray crystallography in the case of compound 211.16
Figure 1. Structures of the pyridinium-substituted indolizines and the parent compound: 1, 2-(pyridinium-l-yl)-l,3-dichloroindolizinea chloride, X- = CI-,b hexafluorophosphate, X- = PFs-, c t e t r a phenylborate, X-= BPhd-. 2,1,2-bis(pyridinium-l-yl)-3-chloroindolizine a dichloride, X- = CI-,b dihexafluorophosphste, X- = PFs-, c ditetraphenylborate, X = BPh4-. 3, 1,2-bis(4-terr-butylpyridinium-1yl)-3-chloro-7-tert-butylindolizine a dichloride, X- = CI-,b dihexafluorophosphate, X-= PF6-,c ditctraphenylborate, X-= BPhd-. 4, indolizine and numbering scheme.
111, we present experimental results concerning the physical and electronic structure of the indolizines. The X-ray structure data, the results of molecular orbital calculations, the results of absorption and fluorescence spectroscopy relevant to these issues, and the results of solution conductivity measurements are summarized there. In section IV we discuss the aggregation of pyridiniumsubstituted indolizines in solution. The spectroscopic evidence for cation aggregation in these systems is presented and discussed with reference to the solid-state data and molecular orbital calculations. Fluorescence data are used to calculate stepwise aggregation constants and to provide information about the electronic structure of the aggregates. The thermodynamic parameters related to aggregation are also calculated from the fluorescence data and are discussed in this section with reference to the electrostatic factors affecting the formation of aggregates. The conductivitydata are analyzed in section V. Ionic transport parameters calculated from the data provide information about the size of the aggregates and their solvation in aqueous and acetonitrile solution. Ion pairing equilibrium constants are calculated from the conductivity data for the pyridiniumsubstituted indolizines with three different anions to determine the parameters controlling this process. The relative importance of ion pairing in cation aggregation is discussed by comparing the results of the fluorescence and conductivityexperiments. Finally, we present a summary of our conclusions in section VI. 11. Experimental Section
The cationic indolizine compounds la, 2a, and 3a (numbering and lettering scheme given in Figure 1) were synthesized using the methods described previouslyl,'6 and then purified. The
Fluorescencespectroscopy and conductivitymeasurementswere performed on aqueous and acetonitrile solutions of the purified indolizines. Water was purified in a five-stage Millipore ion exchange/filtration system to a measured conductivity (4-6)over the same concentration range with the uncertainty in the maximum value of n reflecting the uncertainty in the spectral deconvolution process. TemperaturedependencesofKz(lb) and K2(3b) in acetonitrile solution were analyzed over the temperature range 20-60 OC to obtain the standard enthalpy and entropy of the dimerization reaction. The results obtained from the fluorescence spectra are AHzo(lb) = -1 8 kJ/mol, ASz0( lb) = 9 J/(K mol) for compound 1 and AH2"(3b) = -25 kJ/mol, ASz0(3b) = 30 J/(K mol) for compound 3. The standard reaction enthalpies are modestly exothermicand similar to those obtained for other charged, polar dye systems in ~ a t e r . It ~ ?is ~clear that the high values of K2 in the pyridinium-substituted indolizines are primarily due to the positive reaction entropies associated with solvent structure breaking.3 The high value of ASz0(3b)for the heavily alkylated speciesis particularly striking. Hence,the pyridinium-substituted indolizines can be considered to be amphiphilic species and solvophobic interactionsare a major driving force for aggregation of these speciesinpolar solvents. Again, the behavior of TCNQ'in aqueoussolution offers an interesting contrast.39 The enthalpy measured for dimerizationof the lithium salt of TCNQ-in water is 4 4 W/mol, considerably larger than for the indolizines, while the reaction entropy is -82 J/(K mol), large and negative. Apart from the differences in size and dipole moment, part of the differencecould also be due to ion pairing in the case of LiTCNQ, since Li+ is such a well-known structure-making ion in aqueous solution.33~40 Electronic Structure of Indolizine Aggregites. The observed shifts in the fluorescence curves of the aggregates are too large and are of the wrong sign to be explained by a localized molecular-exciton model such as the model described above in eq 4. Consequently, weconsidered a simple perturbativeelectronic
-0.8
,
,
-0.6
,
,
-0.4
,
,
-0.2
,
1 -0.0
cos[(pi.n)/(n+ l)] Figure 7. Photon energy of the peak emission for soluble aggregates of compound 2 as a function of the aggregation number, n, analyzed according to eq 5. Points plotted on they axis are for crystallinesamples (see Table
I).
model for the aggregates which allowed for significant electronic interaction between contacting, degenerate indolizine centres in the r-stack, thus leading to delocalized bound-exciton states in the cluster. A simple one-dimensional, perturbative, extendedHiickel formalism41describing the mixing of degenerate r and T* molecular states in the cluster leads to the following expression for the energy of the lowest "mr*-to-rd' transition of the cluster as a function of aggregation number:
AE,,= AE,+ 4 8 cos ( ( m ) / ( n + 1)) where AE,, is the SI+& emission energyof a cluster of aggregation number n and fl is the "TU" interaction energy between adjacent stacked indolizine T systems. Since the aggregates are closedshell systems, there is no net electronic binding energy in the ground state due to these interactions according to this simple formalism. Experimental fluorescence data for compounds 1-3 were analyzed according to eq 5, and the results for compound 2 are shown in Figure 7. There was significantly larger curvature in the plot when other aggregation schemes were used to define n. Also included are emission energies for the crystalline samples, plotted as if n for those samples. As expected,the estimated values of fl for these "*a-like" interactions in the solvated aggregates are similar for the different compounds, and they are small compared to the values of 8 appropriate to covalent "p+' interactions, justifying the use of the perturbative approach. We calculate 8 = 0.12 eV for compound 1,fl = 0.15 eV for compound 2, and fl = 0.13 eV for compound 3. The calculated values of correspond to the emission energy of the exciton bound on an infinite one-dimensional solvated indolizine aggregate. These values are AE- = 2.39 eV for compound 1, AE, = 2.30 eV for compound 2, and AE- = 2.44 eV for compound 3. Note that these values do not necessarily correlate well with the emission energies of the crystalline samples, which is reasonable since the solvated aggregatesare expected to have a different band structure from the crystalline materials. Recent calculations of the electronic structure of aromatic radicals and polymer radicals42 indicate that there may be interesting electronic properties in the reduced or oxidized indolizine clusters if they can be prepared. Covalent indolizine dimersl' and oligomers43have been prepared and they show interesting absorption and pH-dependent prop erties. Intermolecular Interactions Influencing Aggregation. The thermodynamic data derived for cation dimerization in solution indicate that entropicsolventeffects are the primary driving force for aggregation but that net aggregation enthalpies are still significant. The electronic structures of the pyridinium-substituted indolizines are such that intermolecularinteractionsshould be dominated by electrostatic terms which are straightforward toestimateusing simplemodels. This isanother attractive feature of the indolizine systems. Here we consider the relative importance of dispersion, dipole-dipole, ion-ion, and ion-dipole
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1092 The Journal of Physical Chemistry, Vol. 97, No. 5, 1993
interactions in determining the tendency to form aggregates in solution and the structure of the aggregates. We can estimate the relative importance of these different types of intermolecular interactions given the results of the molecular orbital calculations and the proposed model of the aggregate geometry based upon the X-ray structure data. Dispersive van der Waals interactions between the indolizines would favor maximum planar overlap44of the ?r systems. The geometry of the solid-state dimers is consistent with maximum *-overlap under the steric constraints imposed by the pyridinium and chloride substituents. An order-of-magnitude estimate of the attractive dispersion interactions between stacked indolizine dimers in the gas phase can be obtained from the London equation:45
where &is, is the dispersion energy (in joules), a0 is the electronic polarizability of the interacting residues (in C2 m2/J), I is the ionization potential of the residue (in joules), e0 is the permittivity of vacuum (8.854 X 10-12 F/m), and r is the distance between the residues (in meters). The electronic polarizability of the indolizine *-system was estimated by interpolation from the values45of benzene (a0 = 1.14 X 1 0-39C2 m2/J) and naphthalene (ao= 1.94 X 10-39 Cz m2/J) to be 1.8 X C2 m2/J. Using the experimental value46of I = 7.26 eV for indolizine gives &is,, -100 kJ/mol. Taking into account the increased ionization potential of the cations with respect to the neutral parent compoundswould raise thisvalue slightly, but taking intoaccount the lack of complete *-system overlap in the dimer would lower thevalueslightly. This is a sizeable driving force for dimerization in vacuo. However, this net disperison energy would be reduced45 by a factor of 25-30 if the dimer pair were surrounded by a dielectric medium of refractive index 1.3-1.4, for example if the dimer were dissolved in water or acetonitrile. This would make the net contribution of dispersion interactions about -3 to -4 kJ/mol for the solvated aggregates. The semiempiricalmolecular orbital calculations revealed that sizeabledipole moments are induced in the plane of the indolizines by the pyridinium substituents. The centers of *-electron density were calculated to be near the N-C9 bond in both the mono- and bissubstituted cations. The dipole-dipole interaction energy for a paraxial, coplanar pair of identical point dipoles is45
-
Edd = {-$/(4.ecd3)){2 cos2 0 - sin2 8 cos 4)
(7) where Edd is the dipole-dipole interaction energy (in joules), p is the dipole moment of each dipole (in C m), e is the static relative permittivity of the medium, eo is the permittivity of vacuum, r is the center-to-center distance between the dipoles (in meters), and 8 and 4 are angles defined in Figure 6. Assuming that the N-C9 bond defines the center of the induced dipole, then 0 67O for the solid-state dimers and 4 = 180' for compound 1, 4 = 124' for compound 2. Estimates of Edd are calculated from the results of the MNDO calculations to be Edd -108 kJ/mol for compound 1 and Edd -120 kJ/mol for compound 2 in vacuo. The dipole-dipole interactions are large, exothermic interactions because of the high induced dipole moments of the molecules. In vacuo, they are comparable in magnitude to dispersion interactions. Attractive dipole-dipole interactions would also favor cofacial stacking of the polar x systems if steric hindrance prevents close end-to-end dipole alignment. However, according to eq 7, the attractive dipole-dipole interactions between pyridinium-substituted indolizinesare reduced in media of high static dielectric constant such as water (e = 79.8) or acetonitrile (e = 36.5) to between -1.5 and -3.5 kJ/mol. Other electrostatic interactions which should be considered in the formation of dimers and higher aggregates are the repulsive electrostatic interactions between pyridinium groups on different pyridinium-substitutedindolizinesand the ion-dipole interactions
-
-
-
between pyridinium groups and the polarizable indolizinemoiety of the opposite cation. The electrostatic interaction between two pyridinium groups will raise the energy of the aggregates and is, of course
E,, = e2/(47ree,,r) where E,, is the interaction energy between point unit charges (in joules), e is the elementary charge (1.602 X C), r is the effectivedistance between 'point" pyridinium charges (in meters), and e is the static relative permittivity of the surrounding medium. The molecular orbital calculations indicated that the pyridinium charge was quite delocalized in these systems. Consequently, a value of r > r", where "r is the nitrogen-nitrogen distance between pyridinium groups, is appropriate for estimating E, in this case. This puts an upper bound on E,, for compound 1 in water of E,, C 2 kJ/mol and E,, C 4 kJ/mol in acetonitrile. For compounds 2 and 3 the corresponding values are C8 and C16 kJ/mol. These repulsive interactionsstrongly favor an anticofacial arrangement of the pyridinium groups in the dimers and higher aggregates. The ion-dipole interaction is also repulsive for the dimer geometry shown in Figure 2:45
Ed = -(ep cos ~ ) / ( ~ T c c / )
(9) where EMis the point charge-point dipole interaction energy (in joules). In this case, 0 = 150° and E* for compound 1 is
