The Electronic Spectra of the Sandwich Stacked PFBT: A Theoretical

Jun 1, 2011 - Jiande Gu,*. ,†,‡ and Jerzy Leszczynski*. ,†. †. Interdisciplinary Nanotoxicity Center, Department of Chemistry, Jackson State U...
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The Electronic Spectra of the Sandwich Stacked PFBT: A Theoretical Study Jing Wang,† Jiande Gu,*,†,‡ and Jerzy Leszczynski*,† † ‡

Interdisciplinary Nanotoxicity Center, Department of Chemistry, Jackson State University, Jackson, Mississippi 39217, United States Drug Design & Discovery Center, State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 201203 P. R. China ABSTRACT: Stacked models that include 9,90 -bis(600 -N,N,N-trimethylammonium)hexyl]fluorene-co-alt-4,7-(2,1,3-benzothiadiazole)dibromide (F(BT)F) monomer sandwiched between two stacked 2,1,3-benzothiadiazole (BT) units were explored using theoretical approaches. Molecular structures and the optical characteristics of the investigated species were investigated at the M06-2X/6-311G(d,p)//TD-M06-2X/6-311G(d,p) level of theory. In all models, the electronic excitation to the lowest singlet ππ* excited state (S1(ππ*)) is governed by the highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO f LUMO) transitions. The obtained results suggest that stacking interaction might have only minor effects on the transition energy for both absorption and emission processes. Instead, the reduction in the excitation energy of the stacked complexes should be attributed to the dipoledipole interaction. The larger the interaction energy of the stacked models, the bigger the observed differences between absorptionemission energies. The presence of the solvation medium with small dielectric constant may increase the absorptionemission energy differences. It is expected that the largest absorptionemission shift can be observed in the benzene solution.

1. INTRODUCTION Due to the existence of their π-conjugated mainframe, conjugated polymers (CPs) can form electronically delocalized structures that exhibit the characteristics of semiconducting and photon harvesting.1,2 Attached to the π-conjugated backbone of CPs, specific functional groups may be ionized in polarizing media. CPs are easily manipulated in solutions and are suitable for collective response and optical amplification of fluorescent signals.3,4 Therefore, CPs have been commonly applied in designing and developing new biosensors. Interestingly, biosensors containing optically amplifying CPs have been used to determine single nucleotide polymorphism.5 The interpolyelectrolyte complexes of CPs and DNA have been explored for the development of multicolor biosensors.6 In addition, cationic polythiophenes have been developed in biomedical applications to optically detect DNA and proteins.7 One of the nonlinear cationic CPs (CCPs) with a range of backbone regiochemistries was synthesized, and fluorescence resonance energy transfer (FRET) experiments indicate that flexible structural polymers are good donors to fluorescein-labeled doublestranded DNA (dsDNA).8 The excitation process has also been investigated by Bazan’s group to reveal the FRET transferring from a CCP to an intercalated DNA dye.9 The sensory assays of poly[9,90 -bis(600 -N,N,N-trimethylammonium)hexyl]fluoreneco-alt-4,7-(2,1,3-benzothiadiazole)dibromide] (PFBT) have been developed and incorporated into DNA chips for the strandspecific DNA detection.10 r 2011 American Chemical Society

The electronic properties of CPs are determined by their molecular structures. Studies show that the optical properties of an acceptor/donor/acceptor (A/D/A) chromophore o-dichlorobenzene (o-DCB) can be tuned by combining with a Lewis acid B(C6F5)3.11 X-ray photoelectron spectroscopy experiments suggest that the changes in the size of the counteranion (CA) alter the interchain contacts and aggregation and, therefore, cause a substantial alteration of photoluminescence (PL) quantum yields. The optoelectronic properties of PFBT are affected by the counterions attached to the parent conjugated backbone.12 In PFBT, the π-delocalized backbone containing phenylenefluorene segments is copolymerized with 2,1,3-benzothiadiazole (BT) units and charged pendant groups. The corresponding cationic conjugated polyelectrolytes have been designed, synthesized, and characterized to determine the concentrations of dsDNA.13 However, the mixtures of various nanosized CPs (including the blue-emitting polyfluorene (PF) doped with three different green-, yellow-, and red-emitting CP acceptors) do not exhibit the features of aggregation. Only little improvement in fluorescence quantum yield has been observed in the energy transfer mediated fluorescence measurement.14 Computational approaches provide alternative ways to explore the relationship between the electronic properties of CPs Received: February 3, 2011 Revised: May 5, 2011 Published: June 01, 2011 6376

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Figure 1. Structural scheme of sandwich stacked F(BT)F (9,90 -bis(600 N,N,N-trimethylammonium) hexyl]fluorene-co-alt-4,7-(2,1,3-benzothiadiazole)dibromide) by two 2,1,3-benzothiadiazoles (BT1 and BT2) (hydrogens are not shown in the scheme).

and their molecular structures. The developments of the computational chemistry methods enable a step-by-step examination of the influence of the all components of surroundings, characterized by increased complexity, on the optoelectronic properties of the CP segments. The study of the monomer PFBT by time-dependent density functional theory (TDDFT) indicates that the torsion angles between the fluorene and benzothiadiazole units of the CCP PFBT-X may influence both the spatial occupancy and electronic properties of PFBT.15 Our previous theoretical study revealed that the TDDFT method with the moderate basis sets (6-311G(d,p) or 6-311þþG(d,p)) can reasonably simulate the absorption and emission spectra of the monomer of PFBT.16 Previous TDDFT study of the stacking effects of the fluorene (F) and BT units of the monomer of F(BT)F reveals that the stacking of F or BT units upon the monomer F(BT)F lowers the absorption and emission energy of F(BT)F.17 Thus, the stacking effects of the CPs of PFBT might be critical on their optoelectronic properties. To enhance the knowledge of the effects of multilayer stacking interactions of the neighboring segments, various models of different stacking patterns with the electron-acceptor (BT) on both sides of the F(BT)F unit have been investigated in the present study. Figure 1 shows the details of the studied stacking models. TDDFT approaches have been applied to explore the optical properties of the different species constructed from the above models.

2. COMPUTATIONAL DETAILS Density functional theory (DFT) with the hybrid meta exchange-correlation functional M06-2X18 was applied in all the calculations. The M06-2X functional represents a highnonlocality functional that includes double the amount of nonlocal exchange (2X). It shows good performance for ππ stacking interactions.1921 The standard valence triple-ζ basis set, augmented with d-type polarization functions for heavy elements and p-type polarization functions for H, namely 6-311G(d,p),22 was used. To improve the reliability of the energy predictions of the stacked complexes, BSSE contributions in binding energies have been taken into account through the counterpoise correction of Boys and Bernardi.23 The ground state geometries of the stacked fragment/monomer/fragment systems were fully optimized at the above-mentioned theoretical

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Figure 2. Definition of vectors of the stacking models. (The geometric center of the six-member ring of BT is adopted as the original point of the BT subunit, which are denoted as O0 for F(BT)F, O1 for BT1, and O2 for BT2, respectively. The vector v0 originated on O0 points to the sulfur atom (S) of F(BT)F. Similarly, v1 and v2 are the vectors pointing from O1 to the S atom in BT1 and from O2 to S in BT2, respectively.).

level by analytical gradient techniques. The structures corresponding to the local minima on the potential energy surface were verified by the harmonic frequency analysis. For a better description of the stacking interactions, single point calculations with basis sets including the diffuse functions 6-311þG(d,p) and 6-311þþG(d,p) were also performed based on the optimized structures obtained at the M06-2X/6-311G(d,p) level of theory. The TDDFT method has become widely applied in studying electronic transitions because of its remarkably low computational cost and the accuracy of sophisticated quantum chemical methods for the valence-excited states.2426 In the present study, TDDFT (with the functional M06-2X and with basis sets 6-311G(d,p), 6-311þG(d,p), and 6-311þþG(d,p)) was employed to predict the electronic vertical singlet transition energies of the studied species. To evaluate the solvent effects on the optical properties of the studied models, the polarizable continuum model (PCM) selfconsistent reaction field of Tomasi and co-workers27 was integrated along with the TDDFT method for all gas-phaseoptimized structures to simulate the absorption and emission spectra. Four solvents were considered for the PCM/TDDFT calculations: water, chloroform, benzene, and n-hexane with dielectric constants of 78.36, 4.71, 2.27 and 1.88, respectively. All the calculations were carried out using the Gaussian 0928 package of programs.

3. RESULTS AND DISCUSSION Validation of the Method. The molecular structures of the monomer F(BT)F in singlet and triplet states were optimized at the M06-2X/6-311G(d,p) level of theory. The corresponding optical properties were studied by using the TD-M062X approach. Compared to the results of our previous studies with different functionals (B3LYP and BH&H),16,17 the geometrical parameters of the optimized structures predicted by the M06-2X method are very close to those calculated by the B3LYP (and the BH&H) method. However, the optical properties of F(BT)F evaluated by the TD-M06-2X approach seems to be better than those estimated by the other functionals (TD-B3LYP and TDBH&H). Specifically, the adsorption-emission energy difference determined by the TD-M06-2X approach amounts to 145 nm, about 11 nm smaller than those computed by the TD-B3LYP (156 nm) and TD-BH&H (156 nm) methods. As a comparison, the corresponding experimental value is 135 nm. Therefore, the 6377

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Table 1. Parameters of the Initial Modelsa model

j1

j2

M1

180°

180°

M2

90°

90°

M3

270°

M4 M5

0° 180°

Table 2. Geometrical Parameters for the Optimized Stacking Models in the Ground State Calculated at the M06-2X/ 6-311G(d,p) Levela models

R1

R2

R1

R2

θL

θR

j1

j2

90°

M1

3.277

3.248

1.7

1.7

32.7

37.0

136

174

0°

M2

3.186

3.242

4.5

7.2

31.6

42.2

76

67

0°

M3

3.282

3.315

3.3

9.7

23.8

42.5

297

68

M4 M5

3.235 3.253

3.323 3.300

2.4 1.3

1.1 1.5

31.1 43.6

36.6 40.7

11 177

7 4

j1 is the angle between the vectors v0 and v1; j2 is the angle between v0 and v2. a

a

R1 is the distance between the stacked layers of BT1 and F(BT)F in angstroms; R2 is the distance between the stacked layers of BT2 and F(BT)F in angstroms; R1 (or R2) is the angle in degrees between the planes of the subunit BT in the F(BT)F layer and the upper BT1 (or BT2); θL and θR are the angles between the planes of segment F and the plane of the subunit BT for the F(BT)F layer in degrees.

Figure 3. Optimized M2 structure for ground state at the M06-2X/ 6-311G(d,p) level of theory. (The plane vectors of t1 and t2 represent the planes of BT1 and BT2.).

M06-2X functional performs better than the other functional, such as B3LYP and BH&H, in the TD-DFT predictions of the optical properties of the PFBT systems. Models. One F(BT)F monomer intercalates into two stacked BT segments: (BT1 and BT2, see Figure 1), forming a stacked three-layer complex. In the present study, our attention is focused on the effects of the BT moiety of the F(BT)F sandwiched between the two BT units. As depicted in Figure 2, the geometric center of the six-member ring (C3aC4C5 C6C7C7a) of BT is adopted as the original point of the BT subunit. These centers are denoted as O0 for F(BT)F, O1 for BT1, and O2 for BT2, respectively. The vector v0 which originate on O0 points to the sulfur atom (S) of F(BT)F. Similarly, v1 and v2 are the vectors pointing from O1 to the S atom in BT1 and from O2 to S in BT2, respectively. The orientation patterns of the stacked layers are represented through the angles j1 (the angle between the vectors v0 and v1) and j2 (the angle between v0 and v2). To evaluate the effects of various stacking interactions on the spectra of F(BT)F, five typical stacking patterns feasible in the studied complex were constructed as initial models M1, M2, M3, M4, and M5, based on the values of j1 and j2 (Table 1). Geometries and Energies of the Stacked Complexes in the Ground State. The models representing the stacked complexes in the ground state were fully optimized at the M06-2x/ 6-311G(d,p) level of theory. The optimized structure of M2 are depicted in Figure 3 and the essential parameters are summarized in Table 2. Three layers of the stacked BT subunits in the optimized complexes are essentially parallel to each other with the face-to-face pattern. The angles (R) between the plane vectors of the BT subunits (i.e., the vector perpendicular to the BT plane, see definition in Figure 2) are less than 10° for all the stacked complexes. The distance between the stacked layers (R1 and R2, defined as the distance between geometric centers of the six-member rings of BTs, O0, O1, and O2) ranges from 3.19 to

Figure 4. Optimized structure of F(BT)F in the ground state at the M06-2X/6-311G(d,p) level of theory. (The plane vectors tL, tR, and t0 represent the planes of the two segments F and the subunit BT, respectively.).

3.32 Å. These parameters indicate that all the optimized complexes are stabilized through the ππ stacking interaction. Among the five optimized complexes the M1 species is the most stable one. The stacking energy of M1 amounts to 27.40 kcal/mol. Two other complexes, M2 and M3, are found to have only slightly lowered the stacking energies (26.86 kcal/ mol for M2, and 26.22 kcal/mol for M3, respectively). Meanwhile, relatively weaker stacking interactions are predicted for the M4 and M5 complexes. The corresponding stacking energy amounts to 22.21 kcal/mol for M4 and 21.35 kcal/ mol for M5, respectively. By examining the geometric parameters of M1, one can see that the direction of the up-layer of  BT (v1 of BT1) is about 136 away from the vector v0 of F(BT)F, while the v2 of BT2 is almost antiparallel to the v0 of F(BT)F. In this way, the orientation of the v1 of BT1 is about  40 away from the v2 of BT2. This phenomenon is unique among the optimized models. In the other optimized complexes, the v1 and v2 are either nearly parallel or antiparallel to each other (see Table 2). The optimized geometry of the F(BT)F monomer in the ground state shows a nearly symmetric structure. The bending of the F segment toward the BT center is similar at the both sides. The angle between the plane of the segment F (represented by the plane vector tL or tR, see Figure 4) and the plane of the subunit BT (represented by t0), θL (or θR), is 39°. This symmetric feature of F(BT)F is not present in the stacked complexes. Due to the stacking interactions, θL decreases to 31.6° in M1 and 23.8° in M2 while θR increases to 42.2° in M1 and 42.5° in M2. On the other hand, both θL and θR decrease in the complexes M3 (31.1° for θL and 36.6° for θR) and M4 (32.7° for θL and 37.0° for θR). In general, stacking on the BT moiety of F(BT)F seems to flatten the F(BT)F species. 6378

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Figure 5. SOMOs for stacking models in absorption excitation.

However, this is not true for the M5 complex, in which both θL and θR increase slightly (θL amounts to 43.6° and θR amounts to 40.7°). These changes in the sandwiched form of BTF(BT)F BT complexes suggest that the F moieties are easy to rotate around the C2C40 bond or the C200 C70 bond in F(BT)F. The First Excited States and the Vertical Singlet Transition Energies. Based on the optimized geometry of the models considered for the ground state, the electronic vertical singlet excitation energies of the complexes were calculated at the TDM06-2X/6-311G(d,p) level of theory. The principal absorption band of the complexes investigated in this study is found to correspond to the first excited state (with oscillator strength stronger than 0.51). Much smaller oscillator strengths are found for higher excited states, signifying that higher excitations are minor contributions to the absorption bands. It is important to note that the first excited state (S1(ππ*)) of the monomer F(BT)F is dominated by the electron transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO).16,17 The singly occupied molecular orbitals (SOMOs) of the first excited state of the stacked complexes (Figure 5) reveal that the excitation is largely located on the F(BT)F unit. The SOMOs of the stacked complexes resemble the HOMO and LUMO of the F(BT)F monomer. This feature is similar to our previous results revealed in the studies of the F(BT)FBT and F(BT)FF systems.17 The excitation energies of the sandwiched F(BT)F complexes are summarized in Table 3. Compared to the monomer (with the first excitation energy of 3.30 eV or 376 nm), the first excitation energy of M1 (3.14 eV or 394 nm) is by about 0.16 eV lower. Similar red-shift of the excitation energy is found for M4 (3.17 eV, 391 nm). On the other hand, the excitation energy of the M5

Table 3. The Stacking Energies and the Absorption Spectra Properties of the Ground State Stacked Models (M06-2X/ 6-311G(d,p) Level)a ΔE

Eexcitation

wavelength

oscillator strengths

(kcal/mol)

(eV)

(nm)

(f)

F(BT)F

3.30(3.25)

376(381)

0.82(0.77)

M1

27.40(27.41)

3.14(3.11)

394(399)

0.52(0.49)

M2

26.86(26.91)

3.23(3.20)

383(388)

0.60(0.57)

M3

26.22(26.24)

3.21(3.17)

386(391)

0.57(0.54)

M4 M5

22.21(22.91) 21.35(21.85)

3.17(3.14) 3.30(3.26)

391(394) 376(380)

0.61(0.59) 0.60(0.57)

a

The values in the parentheses represent single point calculation results at the M06-2X/6-311þþG(d,p) level based upon the optimized structures obtained at the M06-2X/6-311G(d,p) level.

complex (3.30 eV) is essentially the same as that of the monomer. Notice that the parallel pattern of the BT units of the upper and lower layers in M1 and M4 results in a reinforced dipole moment along the v0 direction, while the antiparallel arrangement of the BTs in the upper layer and bottom layer cancels the dipole moment. Since the excited electron occupies the molecular orbital that spreads around the BT segment of F(BT)F, the parallel pattern of the BT units of the upper and lower layers seems to reduce the excitation energy through the dipoledipole interactions. Moreover, less significant red-shift of the excitation energy is observed in the stacked M2 (3.23 eV) and M3 (3.21 eV) complexes. In these two cases, both v1 and v2 are almost perpendicular to v0. Therefore, the influence of the dipoledipole interaction on the excitation transition is less significant. 6379

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Figure 6. SOMOs for stacking models in emission excitation.

Table 4. Geometrical Parameters for the Optimized Stacking Models in the Triplet State Calculated at the M06-2X/ 6-311G(d,p) Level models

R1

R2

R1

R2

θL

θR

j1

j2

M1

3.214

3.198

0.4

1.1

25.3

25.6

163

173

M2

3.268

3.192

0.2

2.9

17.6

30.5

58

75

M3

3.265

3.190

3.4

2.8

15.7

30.4

296

74

M4

3.245

3.274

1.5

0.9

29.0

29.6

3

6

M5

3.275

3.318

1.3

3.1

28.5

27.2

173

4

This elucidation can be further justified by the reference to the two-layer stacked models in the previous study which concluded that interaction of F (or BT) unit with F(BT)F facilitates excitation by lowering the excitation energy by 0.100.14 eV. Since the dipole moment of BT (or F) cannot be eliminated in the two-layer systems, this reduction of the excitation energy should be attributed to the dipoledipole interaction. Geometries and Energies of the Stacked Complexes in the First Excited State. Previous studies16 suggest that it is reasonable to apply the fully optimized triplet state structure to approximate the local minimum structure of the open-shell singlet first-excited state. The validity of this approximation lies on the fact that the first excitation of the systems is dominated by the HOMO f LUMO transition. The SOMOs of the complexes in their first excited state depicted in Figure 5 ensure that the same approximations are applicable in the present systems. The geometries of the stacked complexes optimized in their triplet state are depicted in Figure 6 and the main geometrical parameters are listed in Table 4. In agreement with the results of the previous studies, the optimized F(BT)F monomer in the first

excited state shows a more planar structure than that in the singlet ground state. The angle between the plane of the F segment and the plane of the BT subunit, θL (or θR), is 22.7 in  the first excited state, about 16 smaller than that of the monomer in the ground state (39°). Interactions with the BT subunits on both upper and bottom layers result in a slight twists of the F(BT)F in the sandwiched form in the M1, M4, and M5 models. The angles between the plane of the F segment and the plane of the BT subunit are 25.3°25.6° for M1, 29.0°29.6° for M4, and 28.5°27.2° for M5. On the other hand, uneven alterations  are detected for M2 and M3. θL amounts to 17.6 for M2 and 15.7° for M3, which is about 5°7° smaller than that for the monomer, while θR is 30.5° for M2 and 30.4° for M3, which is about 8° larger than that for F(BT)F in the monomer form. Three layers of the stacked BT subunits in the optimized complexes in their first excited state are arranged in well stacked form. The angles (R) between the plane vectors of the BT subunits are less than 4° for all the stacked complexes in the excited state. The distance between the stacked layers varies from 3.21 Å to 3.32 Å. The values of the calculated parameters suggest that these complexes in their first excited state are more efficiently stacked as compared to their ground state. The ππ stacking energy amounts to 28.75 kcal/mol for M1, 28.85 kcal/mol for M2, 29.32 kcal/mol for M3, and 24.45 kcal/mol for M4. This is about 13 kcal/mol larger than the stacking energy of the corresponding models in their ground state. One exception is the M5 complex, for which the stacking energies are almost the same for both excited state (21.39 kcal/ mol) and ground state (21.35 kcal/mol). This uniformity of stacking energy values is consistent with the similarity of their geometric parameters. The angles between the plane vectors of the BT subunits are R1 = 1.3° and R2 = 3.1° in the excited state, 6380

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Table 5. The Emission Spectra Characteristics of the Triplet State Stacked Models (M06-2X/6-311G(d,p))a Eexcitation

wavelength

oscillator

Δλ

(eV)

(nm)

strengths (f)

(nm)

F(BT)F M1

2.38(2.33) 2.26(2.22)

521(531) 549(557)

0.86(0.82) 0.57(0.54)

145(150) 155(158)

M2

2.32(2.28)

535(543)

0.53(0.51)

152(155)

M3

2.31(2.27)

537(545)

0.56(0.54)

151(154)

M4

2.33(2.30)

533(539)

0.57(0.55)

142(145)

M5

2.38(2.35)

520(528)

0.64(0.61)

144(148)

a

The values in the parentheses represent single point calculation results at the M06-2X/6-311þþG(d,p) level based upon the optimized structures obtained at the M06-2X/6-311G(d,p) level.

Table 6. The AbsorptionEmission Energy Differences (Δλ) of the Models in the Presence of the Polarizable Mediums (nm) at the M06-2X/6-311G(d,p) Level water

chloroform

benzene

n-hexane

F(BT)F

149

154

157

155

M1

151

157

161

160

M2

152

155

157

156

M3

152

155

158

156

M4

144

147

148

148

M5

149

152

153

151

which are very close to those in the ground stat (R1 = 1.3° and R2 = 1.3°). The distances between the stacked layers are R1 = 3.27 Å and R2 = 3.32 Å, about 0.02 Å longer than those in the ground state. The Emission Properties of the Stacked Models. The relative emission spectra are predicted through the TD-M062X method based upon the optimized structure of the first excited state (Table 5). The emission energy of the monomer F(BT)F is calculated to be 2.38 eV, 0.92 eV lower than the absorption energy. The absorptionemission energy difference amounts to 145 nm. It is interesting to note that the stacked complex M5 has the same emission energy as the monomer F(BT)F. Therefore, both M5 and F(BT)F have almost the same absorptionemission energy difference (144 nm vs 145 nm). Stacking interaction seems to have no crucial effects on the transition energy for both absorption and emission processes. On the other hand, the dipoledipole interaction in the other stacked models reduces the corresponding emission energy from 0.05 eV (M4: 2.33 eV) to 0.12 eV (M1: 2.26 eV) as compared to that of the monomer. The corresponding absorptionemission energy differences of the stacked complexes are 155 nm for M1, 152 nm for M2, 151 nm for M3, and 142 nm for M4. The trends revealed for the interaction energies follows the absorption emission energy differences, that is, the larger the interaction energy, the bigger the absorptionemission energy difference. The AbsorptionEmission Energy Difference in the Presence of the Polarizable Medium. To evaluate the solvent effects on the optical properties of the studied models, the PCM model with different dielectric constants was combined with the TD-M06-2X method to evaluate the absorption and emission spectra. Four dielectric constants were selected, which correspond to water (78.36), chloroform (4.71), benzene (2.27), and n-hexane (1.88). Table 6 summarizes the absorptionemission

energy differences (Δλ) of the models in the presence of the polarizable medium. It is clear that for all the models, Δλ in aqueous solution is close to that in the gas-phase. The largest difference in Δλ between the gas-phase and aqueous solution is 5 nm (M5). Notable increase in Δλ can be seen in the chloroform solution, where the most crucial changes in Δλ amount to 8 nm (M5) and 10 nm (F(BT)F). With benzene as solvent, the Δλ of all the models increases significantly. Compared to the gasphase, the increase in Δλ varies from 5 nm (M2) to 12 nm (F(BT)F monomer) in benzene solution.

4. CONCLUSIONS The molecular structures and the optical properties of the F(BT)F monomer sandwiched between two stacked BT units in different orientations have been explored at the M06-2X/ 6-311G(d,p)//TD-M06-2X/6-311G(d,p) level of theory. The electronic excitation to the lowest singlet ππ* excited state (S1(ππ*)) of all models is dominated by the HOMO f LUMO orbital configuration. The results suggest that stacking interactions have no crucial effects on the transition energy for both absorption and emission process. Instead, the reduction in the excitation energy of the stacked complexes should be attributed to the dipoledipole interaction. The trends of the interaction energies have been found to follow the absorptionemission energy differences: the larger the interaction energy, the bigger the absorptionemission energy difference. The presence of the polarizable medium with small dielectric constant may increase the absorptionemission energy differences. It is expected that the largest absorptionemission energy difference can be observed in the benzene solution. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: (J.G.) [email protected]; (J.L.) [email protected].

’ ACKNOWLEDGMENT This work was financially supported by the NSF-PREM program (Grant No. 0611539) in the U.S. In China, it was supported by National Science & Technology Major Project ‘Key New Drug Creation and Manufacturing Program’, China (Number 2009ZX09301-001). We would like to thank the Mississippi Center for Supercomputing Research for a generous allotment of computer time. ’ REFERENCES (1) Pinto, M. R.; Schanze, K. S. Synthesis 2002, 9, 1293–1309. (2) Bazan, G. C. J. Org. Chem. 2007, 72, 8615–8635. (3) Wilson, W. D. Science 2002, 295, 2103–2105. (4) Aldaz-carroll, L.; Tallet, B.; Dausse, E.; Yurchenko, L.; Toulme, J. J. Biochemistry 2002, 41, 5883–5893. (5) Gaylord, B. S.; Massie, M. R.; Feinstein, S. C.; Bazan, G. C. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 34–39. (6) Liu, B.; Bazan, G. C. J. Am. Chem. Soc. 2004, 126, 1942–1943. (7) Ho, H. A.; Najari, A.; Leclerc, M. Acc. Chem. Res. 2008, 41, 168–178. (8) Liu, B.; Wang, S.; Bazan, G. C.; Mikhailovsky, A. J. Am. Chem. Soc. 2003, 125, 13306–13307. (9) Wang, S.; Gaylord, B. S.; Bazan, G. C. J. Am. Chem. Soc. 2004, 126, 5446–5451. 6381

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