Solvent Effects on Electronic Structures and Chain Conformations of Î±

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J. Phys. Chem. B 2007, 111, 4128-4136

Solvent Effects on Electronic Structures and Chain Conformations of r-Oligothiophenes in Polar and Apolar Solutions Suci Meng, Jing Ma,* and Yuansheng Jiang Institute of Theoretical and Computational Chemistry, Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing, 210093, People’s Republic of China ReceiVed: NoVember 21, 2006; In Final Form: February 8, 2007

Solvent effects on electronic structures and chain conformations of R-oligothiophenes nTs (n ) 1 to 10) are investigated in solvents of n-hexane, 1,4-dioxane, carbon tetrachloride, chloroform, and water by using density functional theory (DFT) and molecular dynamics (MD) simulations. Both implicit and explicit solvent models are employed. The polarized continuum model (PCM) calculations and MD simulations demonstrate the weak solvent effects on the electronic structures of R-oligothiophenes. The lowest dipole-allowed vertical excitation energies of nTs, obtained from time-dependent DFT/PCM calculations at the B3LYP/6-31G(d) level, exhibit a red shift as the solvent polarity increases, in agreement with experiments. The studied solvents have little impact on the state order of the low-lying excited states provided that the nTs are kept in C2h or C2V symmetry. The MD simulations demonstrate that the chain conformations are distorted to some extent in polar and nonpolar solvents. A qualitative picture of the distribution of solvent molecules around the solvated nTs is drawn by means of radial and spatial distribution functions. The S‚‚‚H-O and π‚‚‚H-O solute-solvent interactions are insignificant in aqueous solution.

Introduction π-Conjugated conducting polymers have attracted extensive experimental and theoretical interest because of their potential applications in organic field-effect transistors, light-emitting diodes, nonlinear optical materials, and lasers.1-4 Among them, thiophene-based oligomers and polymers are the prototypes in many theoretical studies5-24 due to their chemical stability and rich experimental data. However, most of those theoretical studies were restricted in the gas phase. In fact, the surrounding medium affects the electronic structures of π-conjugated conducting polymers to some extent. For instance, it was demonstrated that the solvent molecules can bring about a change in the position, intensity, or even shape of absorption bands,25-34 which was termed solvatochromism.25 Especially, the experimental spectra35,36 have suggested that solvents bring about changes in structures of R-oligothiophenes nTs. Theoretical calculations also play an important role in understanding the solvent effects from microscopic structures. For such purposes, two types of the solvent models have been implemented at different theoretical levels (see, e.g., refs 29-31 and 37-62). In the continuum dielectric approach, the effects of solvent molecules are described in terms of a continuous dielectric constant, .38 In contrast, the explicit solvent model takes the specific short-range interactions into consideration by including the geometrical structures of individual solvent molecules. Recently, a combined discrete/continuum strategy63 has also been employed to describe both specific and bulk effects. All these models can be implemented within the framework of quantum mechanics (QM), molecular mechanics (MM), and their combined scheme (QM/MM). The solvent effects on the properties of thiophene oligomers have been addressed in several theoretical works.64-68 The * Corresponding author. E-mail: [email protected].

geometries and rotation barriers of 2T in solvents of ethyl acetate and acetonitrile were studied by using the Onsager model at the Hartree-Fock (HF) level (with basis sets of 6-31G(d,p) and 6-31+G(d)).64 The torsional potentials and binding energies of 2T in aqueous and acetonitrile solutions were investigated by a simple model with two solvent molecules, a polarized continuum model (PCM), and their combined model, respectively (at the HF/6-311+G(d,p) level).65 Moreover, the solvent effects on the vibrational spectra of thiophene were also studied in the liquid phase and its acetonitrile solution by employing the Onsager model within the frameworks of density functional theory (DFT) and HF with the 6-311++G(d,p) basis set.66 The influence of solvent molecules on the stabilization of π-stacking dimers of singly charged oligothiophenes (2T to 4T) in acetonitrile solution was studied within the isodensity polarized continuum model (IPCM) at the PW91/6-311+G(d,p) level.67 Recently, a unified electrostatic and cavitation model was applied to investigate the thermodynamic and packing structures of positively charged terthiophene and quaterthiophene in dichloromethane, acetonitrile, and aqueous solutions, within the DFT framework.68 However, the solvent effects on the electronic structures and the chain flexibility of nTs as a function of the chain length and solvent polarity are less explored. In the present work, the solvent effects on electronic structures, chain conformations, and short-range solute-solvent interactions of R-oligothiophenes nTs (n ) 1 to 10) are investigated in nonpolar (n-hexane, C6H14: ) 1.89; 1,4dioxane, C4O2H8: ) 2.21; and carbon tetrachloride, CCl4: ) 2.23), less polar (chloroform, CHCl3: ) 4.90), and polar (water, H2O: ) 78.39) solvents. Although n-hexane, 1,4dioxane, and carbon tetrachloride are all nonpolar with the dielectric constants falling in the range of 1.88 < < 2.27,25 the shape and size of these solvent molecules are different from each other. To gain comprehensive understanding of the

10.1021/jp067745x CCC: $37.00 © 2007 American Chemical Society Published on Web 03/31/2007

R-Oligothiophenes in Polar and Apolar Solutions

J. Phys. Chem. B, Vol. 111, No. 16, 2007 4129

SCHEME 1: Selected Solvent Models

influence of polarity and geometry of solvent molecules on the electronic structures of R-oligothiophene nTs, both continuum and discrete solvent models are employed in our work (cf. Scheme 1). PCM, one of the most widely used continuum dielectric methods,38,69 is applied to study the solvent effects on electronic structures, especially the lowest dipole-allowed vertical excitation energies of nTs, owing to the implementation of the time-dependent density functional theory (TD-DFT)/PCM method60,70 in the Gaussian 03 program.71 It has been demonstrated that TD-DFT/PCM calculations can yield reasonable predictions on excitation energies in solutions.72 On the other hand, molecular dynamics (MD) simulations with discrete solvent molecules were also employed to investigate the chain conformations of nTs and the intermolecular interactions between solute and solvent in the studied solutions. Consequently, the qualitative descriptions of the solvent effects on electronic structures of nTs and the packing structures in solutions are presented by combining molecular simulation techniques with quantum chemical methods. Computational Details Ground and Low-Lying Excited States in Dielectric Media. Optimizations of the ground-state geometry of R-oligothiophenes nTs (n ) 2 to 6) were performed in the gas phase and five selected solvents at the B3LYP/6-31G(d) level. The lowest dipole-allowed vertical excitation energies of nTs in various media were then obtained by TD-DFT/PCM nonequilibrium29,42,50,56,61,69,73 calculations. On the basis of the groundstate geometries obtained at the B3LYP/6-31G(d) level, the dipole moments of 2T in the ground and Frank-Condon excited states were then estimated by using a complete active space self-consistent field (CASSCF) method in combination with the PCM model.69,74 The active space was selected as 12 electrons distributed in 10 orbitals consisting of bonding (π) and antibonding (π*) orbitals of the CdC double bonds and p-type lone pair orbitals, n, of S atoms perpendicular to thiophene rings since the lowest singlet excited-state of 2T is assigned to a π f π* transition. The basis set of 6-31G(d) was used. All PCM calculations were carried out by using the Gaussian 03 program.71 Validations of Force Field. The performance of molecular mechanics mainly depends on the selected force fields. In this work, we adopted the polymer consistent force field (PCFF),34,75-77 which had been tested in our previous study of the packing structures of R-oligothiophenes in the amorphous phase.15 To further validate the applicability of PCFF in

describing the intermolecular interactions between nTs and solvent molecules, the potential energy curves of S‚‚‚H-O and π‚‚‚H-O interactions in thiophene-water dimers were tested, as depicted in Figure 1. The interaction energies were estimated at the MP2/6-31+G(d,p) optimized geometries as a function of the distance between the S atom (of thiophene) and the H atom (of water molecule), rS‚‚‚H, or the distance between the H atom (of water) and the geometric center (CEN) (of thiophene), rCEN‚ ‚‚H. The interaction energies correspond to the energy difference between the isolated molecules. The basis set superposition error (BSSE)78 was corrected by the counterpoise method79 for MP2 interaction energies. The interaction potential for C-H‚‚‚Cl of thiophene-carbon tetrachloride dimers was also tested, as shown in Figure S1 (Supporting Information). From the interaction energy curves shown in Figures 1 and S1, one can find that PCFF potentials qualitatively agree with the MP2/6-31+G(d,p) ones near the equilibrium location (2.3 to 2.6 Å) of dimers. In addition, the optimized geometry from PCFF is displayed in the inset of Figure 1b. We can see that PCFF underestimates intermolecular distances by about 0.3 Å, in comparison with the MP2/6-31+G(d,p) results (shown in brackets). To survey the impact of partial charges on the intermolecular interactions, the PCFF interaction energies of the dimers were also calculated with the Mulliken and electrostatic potential (ESP) charges at the MP2/6-31+G(d,p) level, respectively, with the results shown in Figure S2 and Table S1 (Supporting Information). In fact, the relative intermolecular interactions among a set of dimers are rather insensitive to the choice of partial charges. So, it is demonstrated that the PCFF force field can be used to qualitatively describe the packing structures of nTs in solutions. MD Simulations. The MD simulations of a solute molecule nT (n ) 2 to 10) in several solvents were performed in the canonical (NVT) ensemble at 298 K by using an Andersen thermostat.80 The numbers of solvent molecules, depending on the solvent, are tabulated in Table S2 of the Supporting Information. Equations of the motion for systems were integrated using the velocity Verlet algorithm81 with a time step of 1 fs. The periodic boundary condition (PBC) was employed. All MD simulations were carried out on the SGI Origin 200 workstation with the Cerius2 program.82 The cutoff of van der Waals and electrostatic interactions was set to 15.5 Å. To improve the performance of the statistical sampling of the liquid structures, the simulated annealing technique was applied with the temperature reduced gradually from 500 to 298 K. The 400 ps simulation was subsequently carried out after the equilibrium stage had been reached at a temperature of 298 K. Trajectories

4130 J. Phys. Chem. B, Vol. 111, No. 16, 2007

Meng et al. TABLE 1: Dipole Moments (D) and Free Energies of Solvation (kcal/mol) of Ground States of nTs (n ) 2 to 6) in Gas Phase, n-Hexane, and Aqueous Solutionsa ∆Gsol (kcal/mol)

Dipole moments (D) 2T 3T 4T 5T 6T

gas phase

n-hexane

water

n-hexane

water

0.27 0.73 0.46 0.83 0.61

0.30 0.83 0.49 0.94 0.61

0.32 1.05 0.54 1.09 0.60

-0.06 0.86 1.77 2.70 3.61

-3.00 -2.86 -2.73 -2.66 -2.50

a Data were obtained from PCM calculations at the B3LYP/6-31G(d) level.

Figure 1. Evolution of the interaction potentials of (a) S‚‚‚H-O and (b) π‚‚‚H-O interactions as a function of the intermolecular distances, rS‚‚‚H, and rCEN. The interaction energy corresponds to the energy difference between the isolated molecules. The structures of the dimers are obtained by fixing S‚‚‚H and CEN‚‚‚H distances at rS‚‚‚H and rCEN and minimizing other variables at the MP2/6-31+G(d,p) level. The inset of panel b compares the intermolecular distances, rCEN and rCEN‚‚‚O, optimized by using PCFF (upper data) and MP2/6-31+G(d,p) (in brackets). The data are in angstroms.

were collected every 100 fs. Finally, we employed trajectories of the last 200 ps for the statistical analysis of chain conformations. To investigate the possible solute-solvent interactions, simulations of nTs (n ) 1, 3, 9) in 1,4-dioxane and aqueous solutions were carried out. Herein, a relatively longer simulation time (1 ns) and shorter trajectory collection interval (20 fs) were adopted to obtain the radial distribution functions (RDFs) of interatomic or intergroup distances. Results and Discussion Ground States: PCM Study. Geometries. We shall first consider the effect of a dielectric medium on the structures of R-oligothiophene nTs. The optimized geometrical parameters of 2T and 3T in gas phase, n-hexane, 1,4-dioxane, chloroform, and water are displayed in Table S3 (Supporting Information). The optimized geometries in vacuum are in agreement with the experimental data obtained by gas phase electron diffraction.83 The introduction of a dielectric medium seems to lead to insignificant effects on the calculated geometries. The change in the bond lengths and inter-ring dihedral angles of nTs in

solvents is negligible, which is also in accordance with other theoretical findings of 2T in ethyl acetate and acetonitrile solutions calculated by using the Onsager model at the HF/631G(d,p) level.64 The dihedral angles of 2T in solutions (e.g., 160.7° in water) are close to what were predicted by using the PCM method at the HF/6-311+G(d,p) level (in water: 152.4°).65 Moreover, for the short oligomers (like 2T and 3T), there is no apparent twisting of inter-ring bonds with increasing dielectric constant. Free Energies of Solvation. From the viewpoint of the continuum solvent model, the solvent effects are controlled by two factors (i.e., the dielectric constants, , of media and the properties of the solute (such as the dipole moment and the molecular size)). The estimated dipole moments of R-oligothiophenes nTs (n ) 2 to 6) and the total free energies of solvation, ∆Gsol, in various media are listed in Table 1. The small solvent influence on the calculated geometries of nTs may be ascribed to small dipole moments of shorter oligomers. As expected, nTs (n ) 2 to 6) have relatively low dipole moments (0.3 to 1.1 D). In fact, the solvation free energy mainly depends on the chain length of the oligomer and the polarity of the surrounding medium. The ∆Gsol is comprised of several components (i.e., cavitation (∆Gcav), dispersion-repulsion (∆Gdis-rep), and electrostatics (∆Ges)).30,38 The values of ∆Gcav and ∆Gdis-rep are only related to the parameters of solute such as the radius of the cavity, the area of the molecular surface, or the molecular volume.38,84-86 Thus, the nonelectrostatic contributions (∆Gnones ) ∆Gcav + ∆Gvdw) for the fixed nT are nearly invariable in various solutions, whereas the changes in the electrostatic solvation free energies (∆Ges) mainly depend on the dielectric constant of the medium. The electrostatic and nonelectrostatic components of solvation free energies for nTs (n ) 2 to 6) in 1,4-dioxane and aqueous solutions are schematically illustrated in Figure 2. Obviously, the electrostatic contribution increases with increasing dielectric constant of solvent. When R-oligothiophene grows longer, the nonelectrostatic contributions increase rapidly. It may be one of the factors in understanding the decreasing solubility of R-oligothiophenes in n-hexane and n-decane solutions with increasing chain length.87 However, it is worth noting that the dissolution of a solute is a complex process. Solvation free energy cannot be directly applied to account for the solubility of a solute in solution. For example, the calculated value of ∆Gsol is more negative for water than that for n-hexane (or 1,4-dioxane), seemingly in contradiction to the experimental solubilities of R-oligothiophenes in these solvents. The changes in Gibbs free energy of solution to overcome the solute-solute and solventsolvent interactions should be considered.25 In fact, there are already some computer simulations of the enthalpy and entropy changes in addition to the solvation free energies.88,89 The understanding of solubilities of nTs in various solvents is a worthy topic for further investigation.


J. Phys. Chem. B, Vol. 111, No. 16, 2007 4131 TABLE 2: Lowest Singlet Excitation Energies (eV) and Oscillator Strengths (f) for 2T to 6T in Gas Phase (E ) 1.00), n-Hexane (E ) 1.89), 1,4-Dioxane (E ) 2.21), Chloroform (E ) 4.90), and Water (E ) 78.39)a excitation energies (eV) nT

TD-DFT/PCM (f)

expt.

n)2 Gas n-hexane 1,4-dioxane Chloroform Water

4.12 (0.41) 4.03 (0.49) 4.02 (0.49) 4.02 (0.49) 4.00 (0.49)

Gas n-hexane 1,4-dioxane Chloroform Water

3.36 (0.80) 3.25 (0.91) 3.25 (0.91) 3.25 (0.91) 3.25 (0.91)


2.92 (1.20) 2.81 (1.33) 2.81 (1.33) 2.80 (1.33) 2.80 (1.33)


2.64 (1.59) 2.55 (1.73) 2.53 (1.73) 2.53 (1.74) 2.51 (1.74)


2.45 (1.96) 2.35 (2.11) 2.34 (2.11) 2.34 (2.12) 2.32 (2.12)

4.14,b 4.12,c 4.11d 4.10,e 4.05f 4.11,f,g 3.96h n)3 3.54,c 3.55,d 3.51i 3.51,e 3.50f 3.50,f,j 3.61g n)4 3.22,c 3.21d,k 3.17,e 3.17f 3.18,f 3.04,h 3.34,g 3.28,l 3.13m n)5 3.03d 2.98,e 3.00f 2.98,f 3.16g n)6

Figure 2. Electrostatic (∆Ges) and nonelectrostatic (∆Gnones) contributions of solvation free energies in (a) 1,4-dioxane and (b) water as a function of the chain length. The data come from the PCM calculations at the B3LYP/6-31G(d) level.

Excitation Energies in Solvents: TD-DFT and CASSCF Calculations. Red Shifts of Lowest Excitation Energies. The TD-DFT lowest dipole-allowed excitation energies of R-oligothiophenes nTs (n ) 2 to 6) in the gas phase and in various dielectric media, along with available experimental data, are listed in Table 2. It can be seen that the lowest dipole-allowed singlet transitions of nTs (n ) 2 to 6) undergo a weak red shift from n-hexane to water. Moreover, the oscillator strengths become larger with increasing solvent polarity. The simulated spectra of 6T are illustrated in Figure 3 since its red shift in solutions is relatively noticeable (about 7 nm from n-hexane to water) among the studied thiophene oligomers. Herein, a Lorentzian line shape form90 is adopted with the spectral line width set to be 20 nm. The lowest dipole-allowed excitation energies of 2T, 4,4′dimethoxy-bithiophene (DMO44BT), and 3,3′-dimethoxybithiophene (DMO33BT) in n-hexane and acetonitrile were calculated using the TD-DFT/PCM method at the B3LYP/631G(d) level, with the results displayed in Table 3. The absorption spectra of 2T, DMO44BT, and DMO33BT undergo weak red shifts (less than 2 nm) as the solvent polarity increases, in agreement with the experimental observations.28 According to Frank-Condon principle, vertical transitions may correspond to changes in the charge distribution of solutes. Thus, the dipole moment may change upon excitations. For this purpose, the dipole moments of 2T in the ground and first singlet

2.85k 2.85e,f 2.87,f 2.71,h 3.06,g 3.00,n,l 2.92o

a Symmetry of nTs was restricted according to the odd-even number of thiophene rings (C2 for even, Cs for odd), except for 5T in aqueous solution (C1). b Ref 120. c Ref 121. d In methylcyclohexane ( ) 2.02) (ref 26a). e Ref 26a. f Ref 122. g Octyl-substituted nTs (ref 124). h βBlocking derivatives of nTs in dichloromethane ( ) 8.39) (ref 123). i In n-decane ( ) 1.99) (ref 125). j Ref 126. k Ref 127. l Octylsubstituted nTs in tetrahydrofuran ( ) 7.58) (ref 128). m Ref 27. n Octyl-substituted 6T in tetrahydrofuran ( ) 7.58) (ref 129). o Dodecyl-substituted 6T in dichloromethane ( ) 8.39) (ref 130).

Figure 3. Spectra of 6T obtained from TD-DFT/PCM calculations in the gas phase and various dielectric media at the B3LYP/6-31G(d) level.

excited sates were also computed in the gas phase and different dielectric media at the CASSCF(12,10)/6-31G(d) level. As


Meng et al.

TABLE 3: Lowest Singlet Excitation Energies, Eex (eV), and Oscillator Strength (f) for 2T and Bithiophene Deravatives in n-Hexane (E ) 1.89) and Acetonitrile (E ) 36.64) Solutionsa

Eex of 2T n-hexane acetonitrile a

Eex of DMO44BT

TD/PCM (f)

expt.

4.03 (0.49) 4.00 (0.49)

4.12b

TD/PCM (f) 3.86 (0.41) 3.84 (0.41)

expt. 3.93c 3.91 to 3.90c

Eex of DMO33BT TD/PCM (f)

expt.

3.70 (0.51) 3.70 (0.51)

3.89c 3.88 to 3.87c

Data were obtained from TD-DFT/PCM calculations at the B3LYP/6-31G(d) level. b Ref 120. c Ref 28.

Figure 4. (a) Dipole moments and (b) energies of ground and excited states of 2T calculated in the gas phase and in n-hexane, 1,4-dioxane, chloroform, and water, respectively, at the CASSCF(12,10)/6-31G(d) level.

displayed in Figure 4, the magnitude of the dipole moment of the first excited state is slightly larger than that of the ground state. The different extent of solvation of the ground and excited states may rationalize the red shift of the lowest transition energy of nTs in solvents. Figure 5 compares the TD-DFT/PCM excitation energies of nTs in 1,4-dioxane with the corresponding experimental data.26a The average deviation in the lowest vertical excitation energies between TD-DFT/PCM calculations at the B3LYP/6-31G(d) level and the experimental values is 0.39 eV. The underestimation of excitation energies is found here and elsewhere.13,15,91-95 Moreover, the absolute deviation between calculated and experimental excitation energies deteriorates as the R-oligothiophene becomes longer. It was recognized that the increasing charge delocalization leads to inaccuracies in the description of the electronic structure of long π-conjugated chains due to the intrinsic limitation of DFT exchange-correlation functionals.96 On the other hand, in the framework of the PCM model, the simultaneous anisotropic character of solvent medium is ignored. Thus, the effective conjugation length of R-oligothiophenes in liquid solutions may be overestimated to some extent by the PCM method, especially for long chains. Our subsequent MD simulations demonstrate that the backbones of 3T to 6T are slightly distorted in solvents, which will be addressed in the next section. On the basis of the representative conformations collected from MD trajectories, we carried out TD-DFT/PCM calculations of the lowest dipole-allowed excitation energies. Consequently, the average deviation from the experimental values is decreased by 0.03 eV (cf. Figure 5).

Figure 5. Correlation between the excitation energies, Eex (in eV), and the reciprocal chain length (1/n) of R-oligothiophenes in 1,4-dioxane solution. The excitation energies were obtained from TD-DFT/PCM calculations (at the B3LYP/6-31G(d) level) and the TD-DFT calculations on the solvated configurations taken from the MD trajectories (called MD-based), respectively. The experimental data are taken from ref 26a.

State Order. Theoretical attention has also been devoted to the relative order of the low-lying excited states in the gas phase as a function of the number of thiophene rings.21,97-101 Those theoretical studies of the state order of 2T to 6T at various levels give different results, as summarized in refs 26a and 99. Multiconfigurational second-order perturbation theory (CASPT2),


J. Phys. Chem. B, Vol. 111, No. 16, 2007 4133

Figure 6. Probability of end-to-end distance, L (in Å), of nTs and the representative conformations in water. The data come from the statistical analysis of MD simulations.

complete active space self-consistent field (CASSCF), quantum consistent force field/π-electron and configuration interaction singles method (QCFF/PI+CISD), and even the Pariser-ParrPople (PPP) and complete neglect of differential overlap (CNDO)/CISD methods predicted that the 1Bu state was the lowest singlet excited state, whereas the 1Ag state lay above the 1Bu state. However, other semiempirical methods, such as the intermediate neglect of differential overlap (INDO)/multireference determinant-configuration interaction (MRD-CI) method, obtained the opposite conclusions. It was determined from oneand two-photon spectroscopies that the 1Ag state is higher than the 1Bu state.26a,87,102-104 It is interesting to test whether or not the state order will be changed in solutions with increasing solvent polarity. The low-lying excited states of 2T and 3T were calculated in various dielectric media within the symmetry of C2h and C2V. It was found that the lowest dipole-allowed singlet state is the 1B -like state (B or B , depending on the even or odd number u u 2 of thiophene rings), corresponding to the π-π* transitions from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), either in the gas phase or in the studied solvents. The 1Ag-like state lies above the 1Bulike state, in agreement with the experimental observations of 2T in crystalline n-hexane at 77 K, 3T in n-decane at low temperature, as well as 2T to 7T in benzene, methylcyclohexane, ethanol, acetonitrile, and dioxane solutions (from 298 to 77 K).26a,102,104 Therefore, the state order is demonstrated to be insensitive to the polarity of solvents provided that the nTs are kept in C2h or C2V symmetry. Packing Structures in Solutions: MD Study. Distortions of Backbones. It is interesting to investigate the extent of chain distortions caused by various solvents. The MD simulations of nTs (n ) 2 to 10) in n-hexane, 1,4-dioxane, carbon tetrachloride, chloroform, and aqueous solutions were performed to study the statistical distributions of chain conformations within the framework of the explicit solvent model. End-to-end distance, L, is a qualitative parameter to describe the extent of polymer chain distortions.105 Distances between the terminal carbon atoms were analyzed on the basis of trajectories of MD simulations. The populations and representative configurations of nTs in the studied solutions are displayed in Figure 6 (in water) as well as in Figure S3 (in n-hexane), Figure S4 (in 1,4-dioxane), Figure S5 (in carbon tetrachloride), and Figure S6 (in chloroform), in which the populations of endto-end distances, L, are presented by the area under the curves. It can be seen that the chain conformations of nTs (n ) 2 to

10) are distorted in solutions due to the intermolecular interactions, although the extent of chain distortions does not show the specific tendency with increasing solvent polarity or chain length. In addition, the size and shape of the solvent may also affect the chain conformations of nTs. It should be mentioned that the chain distortions for the longer nTs (such as n ) 8, 9, 10) are also affected by other solutes in the neighboring cells under the concentration of 0.1 M. Interactions between Solute and Solvent Molecules. To study the possible short-range interactions in solutions, we investigated the solute-solvent interactions of nTs (n ) 1, 3, 9) in a polar aqueous solution and nonpolar 1,4-dioxane solution. According to previous studies on the intermolecular interactions of hydrocarbons and π-conjugated systems in organic crystals and some complexes,106-117 there may exist two kinds of intermolecular interactions in aqueous solution of thiophene oligomers: (a) linear S‚‚‚H-O and C-H‚‚‚O interactions and (b) π‚‚‚H-O interactions. The simplest model with a set of thiophene-water dimers is investigated at the MP2/6-31+G(d,p) level. As displayed in Figure 1, the intermolecular distances, rCEN, are in the range of 2.3 to 2.6 Å at the minima of the interaction potential curves. The interactions of S‚‚‚H-O and π‚‚‚H-O are very weak. Especially, the S‚‚‚H-O interaction is weaker than the π‚‚‚H-O interaction. The solute-solvent interactions are also evaluated by means of the radial distribution function (RDF), g(r), which gives the probability of finding a given particle (atom or molecule) at a distance r from another given particle relative to the pure solvent. That is to say, the parameter g(r) may provide some information about the distribution of the solvent molecules around the solute. Three series of RDFs, gH‚‚‚O(r), gS‚‚‚H(r), and gCEN‚‚‚H(r), obtained from the 1 ns MD runs in water are presented in Figure 7. The average distances of rHR‚‚‚O, rHβ‚‚‚O, and rS‚‚‚H in the first solvation shell are about 2.80, 2.75, and 3.10 Å, respectively, as represented by the position of the first peaks in RDFs (Figure 7a). The C-H‚‚‚O and S‚‚‚H-O interactions are insignificant since the averaged H‚‚‚O (∼2.80 Å) or S‚‚‚H (3.10 Å) distances are larger than the sum of the van der Waals radii (H + O: 2.72 Å and S + H: 3.00 Å). Similarly, the peak of gCEN‚‚‚H(r) sketched in Figure 7b is located at around 3.90 Å, which is much larger than the equilibrium distance of the π‚‚‚H-O interactions (MP2/6-31+G(d,p): 2.56 Å and PCFF: 2.24 Å). So, the specific π‚‚‚H-O interactions are absent in aqueous solution according to our MD simulations.


Meng et al.

Figure 8. Three-dimensional probability distributions of water around the thiophene oligomers: (a) T, (b) 3T, and (c) 9T. The isosurfaces of SDFs are drawn at thresholds of 3.0, 2.0, and 2.0, respectively. Figure 7. Radial distribution functions: (a) gH‚‚‚O(r) for hydrogen (H in thiophene) to oxygen (O in water) and gS‚‚‚H(r) for sulfur (S in thiophene) to hydrogen (H in water) and (b) gCEN‚‚‚H(r) for geometric center (CEN) of thiophene to hydrogen (H in water). The data come from the statistical analysis of MD simulations, and the statistical error is (0.05 Å.

In addition, the RDFs, gCEN‚‚‚O(r), of nTs (n ) 1, 3, 9) in aqueous solutions are shown in Figure S7 (Supporting Information). The structures of the solution can be also illustrated by the space distribution functions (SDFs).118 SDF gives the visualized pictures of the three-dimensional probability distribution of the atoms around a molecule. In this work, the first solvation shell of SDFs of nTs (n ) 1, 3, 9) is illustrated in Figure 8 by the gOenMol package.119 The water molecules evenly distribute around the center of thiophene without specific orientational distributions in the first solvation shell (r e 5.7 Å, corresponding to the first peak of gCEN‚‚‚O(r) in Figure S7a), as displayed in Figure 8a. On going to the longer oligomers, the population of water molecules around nTs gradually deviates from the spherical distribution. The three-dimensional probability distributions of the water molecules around the central thiophene ring of 3T within the first peak (r e 4.5 Å) are displayed as an example in Figure 8b along the z- and y-axes. One can find that the water molecules in the vicinity (r e 4.5 Å) of the solute populate around two sides of the plane of the thiophene ring. It is worth noting that the longer simulation time is desired for the long nTs to exhaustively sample the conformations. As expected, in 1,4-dioxane solution, the corresponding distances of HR‚‚‚O and Hβ‚‚‚O (3.05 and 3.25 Å, cf. Figure

S8 in the Supporting Information) are larger than the sum of their van der Waals radii (2.72 Å). In summary, there are no specific short-range interactions between studied solvents and thiophene. Therefore, it is not surprising that our PCM calculations can give reasonable predictions on the red shift of the lowest dipole-allowed excitation energies of R-oligothiophene in solutions. Conclusion The solvent effects on the electronic structures, chain distortions of R-oligothiophenes, and short-range solute-solvent interactions have been investigated in n-hexane, 1,4-dioxane, carbon tetrachloride, chloroform, and aqueous solutions by quantum chemistry (TD-DFT/PCM/6-31G(d)) and MD simulations. The lowest dipole-allowed vertical excitation energies of nTs (n ) 2 to 6) as well as bithiophene derivatives exhibit a weak red shift with increasing solvent polarity, which is in agreement with the experimental phenomena.28 Moreover, the state order of the low-lying excited states is insensitive to the medium polarity provided that the nTs are kept in C2h or C2V symmetry. The chain distortions of nTs (n ) 2 to 10) have been investigated in polar and nonpolar solvents by using the MD method. Both the radial distribution (RDFs) and spatial distribution functions (SDFs) are employed to describe the distribution of the solvent molecules around thiophene oligomers. The S‚‚‚H-O and π‚‚‚H-O interactions may be too weak to be identified in aqueous solutions. Other environmental factors such as concentration, temperature, etc. are also very important for further understanding the properties of π-conjugated oligomers and polymers in solutions.

R-Oligothiophenes in Polar and Apolar Solutions Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grants 20433020, 90303020, and 20573050) and the Chinese Ministry of Education (Grant NCET-05-0442). Supporting Information Available: Evolution of interaction potentials of C-H‚‚‚Cl interactions from PCFF and MP2/631+G(d,p) as a function of the intermolecular distance, rH‚‚‚Cl (Figure S1); evolution of interaction potentials between solute and solvent obtained from polymer consistent force field (PCFF) with partial charges taken from PCFF, Mulliken, and ESP, respectively, as a function of the intermolecular distance (Figure S2); populations of distance of terminal carbon atoms as well as the representative configurations of nTs in n-hexane, 1,4dioxane, carbon tetrachloride, and chloroform solutions (Figures S3-S6); RDFs between geometric center of thiophene ring of nTs (n ) 1, 3, 9) and O atom of water molecules in aqueous solutions (Figure S7) and gH‚‚‚O(r) in 1,4-dioxane solution (Figure S8); partial charges from PCFF, Mulliken, and ESP near equilibrium structures of thiophene-water and thiophenecarbon tetrachloride dimers (Table S1); size of solvent models selected in MD simulations (Table S2); and optimized geometry of 2T and 3T in gas phase and solutions using PCM method (Table S3). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kertesz, M. In Handbook of Organic ConductiVe Molecules and Polymers; Nalwa, H. S., Ed.; John Wiley and Sons: New York, 1997; Vol. 4, pp 147-172. (2) Roncali, J. Chem. ReV. 1997, 97, 173. (3) Electronic Materials: The Oligomer Approach; Mu¨llen, K., Wegner, G., Eds.; Wiley-VCH: Weinheim, 1998. (4) Handbook of Conducting Polymers, 2nd ed.; Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds.; Marcel Dekker: New York, 1998. (5) Beljonne, D.; Cornil, J.; Friend, R. H.; Janssen, R. A. J.; Bre´das, J. L. J. Am. Chem. Soc. 1996, 118, 6453. (6) De Oliveira, M. A.; Duarte, H. A.; Pernaut, J.-M. J. Phys. Chem. A 2000, 104, 8256. (7) Ehrendorfer, C.; Karpfen, A. J. Phys. Chem. 1994, 98, 7492. (8) Chadwick, J. E.; Kohler, B. E. J. Phys. Chem. 1994, 98, 3631. (9) Ortı´, E.; Viruela, P. M.; Sańchez-Marıń, J.; Toma´s, F. J. Phys. Chem. 1995, 99, 4955. (10) Jones, D.; Guerra, M.; Favaretto, L.; Modelli, A.; Fabrizio, M.; Distefano, G. J. Phys. Chem. 1990, 94, 5761. (11) Bre´das, J. L.; Street, G. B.; The´mans, B.; Andre´, J. M. J. Chem. Phys. 1985, 83, 1323. (12) Bre´das, J. L.; Silbey, R.; Boudreaux, D. S.; Chance, R. R. J. Am. Chem. Soc. 1983, 105, 6555. (13) Ma, J.; Li, S.; Jiang, Y. Macromolecules 2002, 35, 1109. (14) Hutchison, G. R.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 2005, 127, 16866. (15) Zhang, G.; Pei, Y.; Ma, J. J. Phys. Chem. B 2004, 108, 6988. (16) Cornil, J.; Beljonne, D.; Calbert, J.-P.; Bre´das, J.-L. AdV. Mater. 2001, 13, 1053. (17) Shuai, Z.; Li, Q.; Yi, Y. J. Theor. Comput. Chem. 2005, 4, 603. (18) DiCe´sare, N.; Belleteˆte, M.; Leclerc, M.; Durocher, G. J. Phys. Chem. A 1999, 103, 803. (19) Asaduzzaman, A. M.; Schmidt-D’Aloisio, K.; Dong, Y.; Springborg, M. Phys. Chem. Chem. Phys. 2005, 7, 2714. (20) Zade, S. S.; Bendikov, M. J. Org. Chem. 2006, 71, 2972. (21) Rubio, M.; Merchań, M.; Ortı´, E. Chem. Phys. Chem. 2005, 6, 1357. (22) Fomine, S.; Guadarrama, P. J. Phys. Chem. A 2006, 110, 10098. (23) Zade, S. S.; Bendikov, M. J. Phys. Chem. B 2006, 110, 15839. (24) Hermet, P.; Bantignies, J.-L.; Rahmani, A.; Sauvajol, J.-L. J. Phys. Chem. A 2005, 109, 4202. (25) Reichardt. C. SolVents and SolVent Effects in Organic Chemistry, 3rd ed.; VCH: Weinheim, 2003 and references therein. (26) (a) Becher, R. S.; de Melo, J. S.; Maç anita, A. L.; Elisei, F. J. Phys. Chem. 1996, 100, 18683. (b) de Melo, J. S.; Silva, L. M.; Arnaut, L. G.; Becker, R. S. J. Chem. Phys. 1999, 111, 5427. (c) de Melo, J. S.; Elisei, F.; Becker, R. S. J. Chem. Phys. 2002, 117, 4428. (27) Lee, S. A.; Hotta, S.; Nakanishi, F. J. Phys. Chem. A 2000, 104, 1827.

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Solvent Effects on Electronic Structures and Chain Conformations of Î±

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