Rotational Isomerism of Phenylthiolated Chromophores with Large

Article pubs.acs.org/JPCC

Rotational Isomerism of Phenylthiolated Chromophores with Large Variation of Optical Nonlinearity Jongtaek Kim,†,‡ Ji-Youn Seo,§ Mojca Jazbinsek,∥ Seong-Ji Kwon,∥ Eun-Young Choi,§,¶ Jung-In Seo,† Hoseop Yun,⊥ Yoon Sup Lee,*,† Peter Günter,∥ and O-Pil Kwon*,§ †

Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea Department of Basic Science, Korea Air Force Academy, Cheongju 363-849, Korea § Department of Molecular Science and Technology and ⊥Division of Energy Systems Research and Department of Chemistry, Ajou University, Suwon 443-749, Korea ∥ Rainbow Photonics AG and Nonlinear Optics Laboratory, ETH Zurich, CH-8093 Zurich, Switzerland ‡

S Supporting Information *

ABSTRACT: We design and synthesize various π-conjugated phenyltriene chromophores having different thiolated electron donor groups such as thiol, alkylthiol, and arylthiol groups to systematically investigate the electron-donating ability of sulfur analogues. For comparison, we also investigate the commonly used electron donor groups, oxygen-containing hydroxyl and alkoxy groups, as well as nitrogen-containing dialkylamino groups. Experimental and/or theoretical analysis of the nonlinear optical properties of these chromophores in gas, solution, and solid states suggest that the rotational isomerization of an arylthiolated group acting as electron donor in polyene chromophores induces a large variation of molecular optical nonlinearities. The first hyperpolarizabilities, calculated separately with finite-field (FF) and time-dependent density functional theory (TD-DFT) two-state model methods, indicate that this phenomenon is a consequence of the competitive electronic transitions from the ground state to the first and second excited states modulated by the rotation of thiophenyl group.

1. INTRODUCTION

Electron-rich sulfur is a group 6A element, as is oxygen. The chemical and electronic properties of sulfur-containing compounds are often considered similar to those of the oxygen analogues. However, due to the existence of d orbitals and the unique orbital hybridization, in many cases sulfur-containing compounds behave very differently from oxygen-containing analogues.11 The electron-donating ability of sulfur-containing groups for nonlinear optical chromophores has been rarely investigated.12 In this work, in order to systematically study the electron-donating ability of sulfur analogues, we rationally design and synthesize π-conjugated phenyltriene13−17 chromophores having various thiolated electron donor groups such as thiol, alkylthiol, and phenylthiol groups (see Figure 1). For comparison, the commonly used electron donor groups, oxygen-containing hydroxyl and alkoxy groups, and nitrogencontaining dialkylamino groups are also investigated. The molecular nonlinear optical properties in gas, solution, and crystalline states are investigated experimentally and/or theoretically. We show that rotational isomerization of the arylthiolated group acting as electron donor in the polyene chromophores results in a large variation of the molecular optical nonlinearities.

1,2

High-performance organic nonlinear optical materials are very attractive candidates for future high-speed integrated photonic devices including optical switches and modulators3 and terahertz wave generation and detection.4 Since large macroscopic optical nonlinearity depends on high microscopic molecular nonlinearity, many experimental and theoretical approaches have been focused on molecules with various chemical modifications to enhance molecular nonlinearity.1,2 Push−pull-type π-conjugated molecules with strong electron donors and acceptors exhibit efficient π-electron delocalization and large first hyperpolarizability β.1,5 Commonly used electron acceptor groups are nonionic nitro, sulfonyl, cyano, dicyanovinyl, and tricyanovinyl groups and ionic pyridinium groups.1,2 Recently, many electron acceptors having better electron-withdrawing ability have been reported; for example, 5-membered heterocyclic ring systems such as 4,5,5-trimethyl-2,5-dihydrofuran (TCF) derivatives6 and 3methyl-4-cyano-5-dicyanomethylene-2-oxo-3-pyrrolidine (TCP) derivatives7 and indanedione-based derivatives.8 Commonly used electron donor groups are alkyl- and arylamino groups and alkoxy groups,1,2,9,10 which are based on electronrich atoms such as nitrogen and oxygen atoms, respectively. Contrary to many studies of chemical modifications of electron acceptor groups,1,2,6−8 electron donor groups are relatively less investigated.9,10 © 2012 American Chemical Society

Received: August 14, 2012 Revised: November 4, 2012 Published: November 6, 2012 25034

dx.doi.org/10.1021/jp308070p | J. Phys. Chem. C 2012, 116, 25034−25043

The Journal of Physical Chemistry C

Article

2.1.4. 2-[3-(4-Mercaptostyryl)-5,5-dimethylcyclohex-2enylidene]malononitrile (SH). Yield 19%. 1H NMR (CDCl3): 1.01 (s, 6H, -CH3), 2.46 (s, 2H, -CH2-), 2.60 (s,2H, -CH2-), 4.42 (s,1H, -SH), 6.82 (s, 1H, -CCH-), 6.93−6.97 (d, J = 16.0 Hz, 1H, -CHCH-), 6.98−7.02 (d, J = 16.0 Hz, 1H, -CHCH-), 7.37−7.39 (d, J = 8.8 Hz, 2H, ArH), 7.43−7.45 (d, J = 8.4 Hz, 2H, ArH). 13C NMR (CDCl3) 28.4, 32.4, 39.5, 43.3, 79.1, 112.9, 113.6, 124.0, 128.2, 129.5, 130.1, 134.4, 136.1, 137.2, 153.6, 169.2. Elemental analysis for C19H18N2S (%): calcd C 74.47, H 5.92, N 9.14, S 10.46; found C 71.65, H 5.74, N 8.50, S 10.20. 2.1.5. 2-{5,5-Dimethyl-3-[4-(methylthio)styryl]cyclohex-2enylidene}malononitrile (SM). Yield 27%. 1H NMR (CDCl3) 1.01 (s, 6H, -CH3), 2.46 (s, 2H, -CH2-), 2.51(s, 3H, -SCH3), 2.59 (s,2H, -CH2-), 6.81 (s, 1H, -CCH-), 6.91−6.95 (d, J = 16.0 Hz, 1H, -CHCH-), 6.98−7.02 (d, J = 16.0 Hz, 1H, -CHCH-), 7.21−7.23 (d, J = 8.4 Hz, 2H, ArH), 7.40−7.42 (d, J = 8.4 Hz, 2H, ArH). 13C NMR (CDCl3) 15.6, 28.4, 32.4, 39.6, 43.4, 78.6, 113.0, 113.8, 123.5, 126.3, 128.0, 128.4, 132.3, 136.7, 141.5, 154.0, 169.2. Elemental analysis for C20H20N2S (%): calcd C 74.96, H 6.29, N 8.74, S 10.01; found C 74.70, H 6.28, N 8.66, S 10.04. 2.1.6. 2-{5,5-Dimethyl-3-[4-(phenylthio)styryl]cyclohex-2enylidene}malononitrile (SB1). Yield 37%. 1 H NMR (CDCl3) 1.01 (s, 6H, -CH3), 2.46 (s, 2H, -CH2-), 2.60 (s, 2H, -CH2-), 6.82 (s, 1H, -CCH-), 6.91−6.95 (d, J = 16.0 Hz, 1H, -CHCH-), 6.97−7.01 (d, J = 16.0 Hz, 1H, -CHCH-), 7.22−7.24 (d, J = 8.0 Hz, 2H, ArH), 7.34−7.42 (m, 7H, ArH). 13 C NMR (CDCl3) 169.2, 153.8, 139.7, 136.3, 133.8, 132.8, 129.7, 129.1, 128.3, 123.8, 113.7, 112.9, 43.3, 39.6, 32.4, 28.4. Elemental analysis for C25H22N2S (%): calcd C 78.50, H 5.80, N 7.32, S 8.38; found C 78.50, H 5.81, N 7.33, S 8.37. 2.1.7. 2-{3-[4-(4-Methoxyphenylthio)styryl]-5,5-dimethylcyclohex-2-enylidene}malononitrile (SBOM). Yield 45%. 1H NMR (CDCl3) 1.01 (s, 6H, -CH3), 2.44 (s, 2H, -CH2-), 2.58 (s,2H, -CH2-),3.84 (s, 3H, -OCH3), 6.79 (s, 1H, -CCH-), 6.87−6.91 (d, J = 16.0 Hz, 1H, -CHCH-), 6.94−6.98 (d, J = 16.0 Hz, 1H, -CHCH-), 7.06−7.08 (d, J = 8.0 Hz, 2H, ArH), 7.32−7.34 (d, J = 8.0 Hz, 2H, ArH), 7.43−7.45 (d, J = 8.0 Hz, 2H, ArH). 13C NMR (CDCl3) 169.2, 160.4, 153.9, 142.2, 136.5, 136.4, 132.8, 128.5, 128.0, 127.3, 123.5, 115.3, 113.7, 112.9, 55.7, 43.2, 39.5, 32.3, 28.4. Elemental analysis for C26H24N2OS (%): calcd C 75.70, H 5.86, N 6.79, O 3.88, S 7.77; found C 75.61, H 5.85, N 6.79, S 7.75. 2.2. X-ray Crystallographic Data. The crystal structure of the chromophores was analyzed by single-crystal X-ray analysis; the results are given below. SB1: C25H22N2S, Mr = 382.53, monoclinic, space group P21/ n, a = 5.9245(4) Å, b = 45.446(3) Å, c = 7.7993(6) Å, β = 94.178(2)°, V = 2094.3(2) Å3, Z = 4, T = 290(1) K, μ(Mo Kα) = 0.17 mm−1. Of 16 309 reflections collected in the θ range 3.2−25.0° via ω scans on a Rigaku R-axis rapid S diffractometer, 3674 were unique reflections (Rint = 0.044, completeness = 99.8%). The structure was solved and refined against F2 by use of SHELX97,19 342 variables, wR2 = 0.154, R1 = 0.055 [2444 reflections having Fo2 > 2σ(Fo2)], GOF = 1.17, and max/min residual electron density 0.28/−0.30 e·Å−3. CCDC-824803. OH1,15 CCDC-672263; OM,14 CCDC-278091; DA,14 CCDC-278087.

Figure 1. Molecular structure and crystal symmetry of the investigated polyene chromophores with various electron-donating groups and their abbreviations.

2. EXPERIMENTAL SECTION 2.1. Synthesis. A series of phenyltriene chromophores (see Figure 1) having various electron donor groups were synthesized by Knoevenagel condensation reaction with 2(3,5,5-trimethylcyclohex-2-enylidene)malononitrile including electron acceptor group and corresponding aldehyde according to literature.13−17 4-(Phenylthio)benzaldehyde and 4-(4methoxyphenylthio)benzaldehyde were synthesized according to the literature.18 The synthesis of OH1 (2-[3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene]malononitrile), OM (2-{3-[2-(4-methoxyphenyl)vinyl]-5,5-dimethylcyclohex-2enylidene}malononitrile), and DA (2-{3-[2-(4dimethylaminophenyl)vinyl]-5,5-dimethylcyclohex-2enylidene}malononitrile) chromophores was previously reported.14,15 In 1H and 13C NMR spectra, the chemical shifts are reported in parts per million (ppm, δ) relative to (CH3)4Si (Varian 400 MHz NMR spectrometer). 2.1.1. 4-(Phenylthio)benzaldehyde. Yield 71%. 1H NMR (CDCl3) 9.90 (s, 1H), 7.71 (d, 2H, J = 8.0 Hz), 7.54−7.51 (m, 2H), 7.43−7.41 (m, 2H), 7.23 (d, 2H, J = 8.4 Hz). 13C NMR (CDCl3) 191.1, 147.3, 134.5, 133.7, 131.3, 130.2, 129.9, 129.2, 127.2. 2.1.2. 4-(4-Methoxyphenylthio)benzaldehyde. Yield 98%. 1 H NMR (CDCl3) 3.83 (s, 3H), 6.94 (d, 2H, J = 8.0 Hz), 7.10 (d, 2H, J = 8.0 Hz), 7.45 (d, 2H, J = 8.4 Hz), 7.36 (d, 2H, J = 8.0 Hz), 9.83 (1H, s) 13C NMR (CDCl3) 190.1, 160.7, 148.9, 137.0, 133.1, 130.0, 125.7, 120.6, 115.4, 55.5. 2.1.3. 2-(5,5-Dimethyl-3-styrylcyclohex-2-enylidene)malononitrile (PH). Equimolar amounts of 2-(3,5,5-trimethylcyclohex-2-enylidene)malononitrile (12.5 mmol) and benzaldehyde (12.5 mmol) were dissolved in ethanol (40 mL). The solution was completely dissolved with catalyst, piperidine. The resulting solution was stirred at room temperature for 1 day and the precipitated powder was filtered and washed with ethanol. The other chromophores were obtained in a similar synthetic manner to that used for PH chromophore. Yield: 50% 1 H NMR (CDCl3): 1.01 (s, 6H, -CH3), 2.45 (s, 2H, -CH2-), 2.62 (s, 2H, -CH2-), 6.85 (s, 1H, -CCH-), 6.97−7.01 (d, J = 16.0 Hz, 1H, -CHCH-), 7.05−7.09 (d, J = 16.0 Hz, 1H, -CHCH-), 7.21−7.23 (d, J = 8.4 Hz, 2H, ArH), 7.52−7.53 (d, J = 1.6 Hz, 2H, ArH). 13C NMR (CDCl3) 169.3, 153.8, 137.1, 129.9, 129.3, 129.1, 127.7, 123.7, 113.6, 112.8, 43.3, 39.5, 32.4, 28.4. Elemental analysis for C19H18N2 (%): calcd C 83.18, H 6.61, N 10.21; found C 82.93, H 6.59, N 10.26.

3. RESULTS AND DISCUSSION 3.1. Rational Design of Chromophores. Figure 1 shows the chemical structures of the investigated chromophores. For 25035



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efficient π-electron delocalization,5 we have chosen the widely used phenyltriene bridge as the π-conjugated bridge.13−17 The phenyltriene bridge connected to the dicyanomethylidene electron acceptor has been linked with various electron donor groups as shown in Figure 1. To investigate systematically the electron-donating ability, we have introduced various functional groups having electron-rich O, N, and S atoms as electrondonating groups. Especially for those based on sulfur atoms, we have designed several thiolated electron donor groups, such as thiol, methylthiol, phenylthiol, and methoxyphenylthiol groups for SH, SM, SB1, and SBOM chromophores, respectively (see Figure 1). For comparison, we have investigated chromophore analogues OH1, OM, and DA14,15 with well-known electrondonating groups hydroxyl, methoxy, and dimethylamino, respectively. In addition, we have synthesized the PH chromophore without any electron donor group. The series of phenyltriene chromophores having various electron donor groups were synthesized by Knoevenagel condensation reaction with corresponding aldehyde.13−18 3.2. Molecular Optical Nonlinearity in Gas Phase. To evaluate the microscopic optical nonlinearities, we have calculated the first hyperpolarizability β of the series of phenyltriene chromophores with various electron donors by considering isolated (gas-phase) molecules. We performed quantum chemical calculations by using the Gaussian 03 program20 with density functional theory (DFT) on the hybrid functional B3LYP21 with the 6-311+G(d) basis set.22 The conformation of the electron donor group in the chromophore significantly influences the amplitude of the first hyperpolarizability and its main direction (the main direction of the charge transfer).23,24 The optimized (OPT) molecular conformations of OH, OM, and DA chromophores in gas phase have been obtained by starting optimizations from the molecular conformation in the crystalline state reported previously:14,15 as shown in Figure 2, the H−O−C plane in the OH1 chromophore, the H3C−O−C plane in the OM

chromophore, and (CH3)2N−C plane in the DA chromophore are practically in the same plane as the phenyl ring connected with the triene bridge. In order to obtain optimized structures of thiolated chromophores and PH chromophores without an electron donor, the initial molecular conformations have been constructed by analogy with the molecular conformation of OH and OM molecules in the crystalline state, that is, by assuming the H−S−C plane in the SH chromophore and the C−S−C plane in SM, SB1, and SBOM chromophores to be in the same plane as the phenyl ring connected with the triene bridge. Figure 3 shows molecular conformations of the OPT molecules after optimization from the above initial conformations.

Figure 3. Molecular conformations of optimized (OPT) molecules considered in quantum chemical calculation.

We have calculated the first hyperpolarizability tensor βijk1,2 and the maximal first hyperpolarizability βmax along the chargetransfer direction of optimized molecules by using finite-field (FF) calculations with the applied electric field of 0.001 au to the ground-state molecule .22 The molecular coordinate system xyz is here defined so that the direction of the ground-state dipole moment μg points along the z direction (μg = −μz).22 The results of the FF calculations are listed in Table 1. The static first hyperpolarizability β0 was also calculated via the following two-state model25 with excited states from timedependent density functional theory (TD-DFT) calculations since each of the investigated molecules has a dominant electronic transition:22 β0 =

3Δμge (μge )2 2(Emax )2

(1)

Here, Δμge is the dipole moment difference between the excited and the ground states, μge is the transition dipole moment of the dominant charge-transfer transition, Emax is the resonance energy of the transition, and ε0 is the vacuum permittivity.22 The results of the TD-DFT calculations are listed in Table 2. Note that βmax and β0 both refer to the same parameter (static first hyperpolarizability), but we denote them differently because of the different methods used for calculations. The results of both FF and TD-DFT methods for the OPT molecules show that the maximal first hyperpolarizability βmax

Figure 2. Molecular conformations of (a) OH1, (b) OM, and (c) DA chromophores having conventional electron donor groups in the crystalline state.14,15. 25036



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Table 1. Maximal First Hyperpolarizability (βmax) Resultsa μgb

(= −μZ), D βxxxc (×10−30 esu) βxxyc (×10−30 esu) βxyyc (×10−30 esu) βyyyc (×10−30 esu) βxxzc (×10−30 esu) βxyzc (×10−30 esu) βyzzc (×10−30 esu) βxzzc (×10−30 esu) βyzzc (×10−30 esu) βzzzc (×10−30 esu) βzd (×10−30 esu) βmax (×10−30 esu) θβe (deg)

PH

OH115

OM

SH

SM

SB1

DA22

SBOM

9.5 −0.2 0.1 −0.9 −10.2 −0.9 1.5 16.4 −1.8 −22.0 23.4 38.9 54.2 38.6

11.4 −0.2 −0.4 −1.9 −24.4 −0.3 2.7 33.3 −3.3 −40.7 42.3 75.3 103.6 40.4

12.0 −0.3 −0.5 −1.7 −21.4 −0.6 2.9 34.5 −4.1 −50.0 62.5 96.3 124.4 36.1

10.2 −0.1 0.4 −3.3 −43.3 −1.0 3.8 50.5 −4.0 −54.8 53.7 103.2 146.8 43.1

11.5 −0.1 0.1 −3.3 −43.5 −0.6 4.5 59.0 −5.6 −74.2 85.2 143.7 190.6 39.2

12.1 −0.1 0.3 −3.4 −45.7 −0.7 4.8 64.1 −6.1 −82.8 99.1 162.6 212.4 38.3

14.0 −0.3 −0.1 −0.8 −10.5 −1.0 2.6 31.9 −6.4 −80.5 166.5 197.4 224.4 24.5

13.3 −0.2 −0.9 −8.3 −46.1 0.5 11.0 67.1 −14.6 −88.3 116.6 184.1 232.8 37.2

Calculated by the finite-field (FF) method at B3LYP/6-311+G(d) level for the optimized molecular structures in gas phase. bDipole moments. c Zero-frequency hyperpolarizability tensors. dVector component along the dipole moment direction of the hyperpolarizability tensor. eAngle between μg and βmax. a

Table 2. Static First Hyperpolarizability (β0) Resultsa major contribution Emax[ICT],b eV fosc μged, D Δμgee, D β0, 10−30esu

PH

OH115

OM

SH

SM

SB1

DA22

SBOM

H→L 3.1 (2.9) 1.3 (1.5) 10.5 (11.5) 7.5 (8.4) 48.7 (77.3)

H→L 3.0 (2.7) 1.3 (1.5) 10.7 (11.9) 11.7 (12.3) 87.8 (136.8)

H→L 2.9 (2.7) 1.4 (1.5) 11.0 (12.1) 12.8 (12.8) 104.2 (150.3)

H→L 2.9 (2.6) 1.3 (1.5) 10.9 (12.1) 16.6 (16.4) 139.6 (205.3)

H→L 2.8 (2.5) 1.2 (1.4) 10.9 (12.2) 19.8 (18.7) 179.1 (250.7)

H→ L 2.8 (2.6) 1.4 (1.6) 11.4 (12.6) 18.7 (17.8) 186.9 (253.1)

H→L 2.7 (2.4) 1.3 (1.5) 11.4 (12.9) 17.6 (17.2) 190.5 (298.1)

H→L 2.7 (2.6) 1.4 (1.6) 11.6 (12.7) 19.2 (17.9) 198.9 (262.2)

a

Calculated by TD-DFT at B3LYP/6-311+G(d) level for the optimized molecular structures in gas phase. Values in parentheses are results of the polarized continuum model (PCM) calculations for the gas-phase OPT molecules, with chloroform as solvent. bResonance energy of the main electronic transition. cOscillator strength of the transition. dTransition dipole moments of the dominant charge-transfer transition. eDipole moment difference between the excited and ground states.

and the static first hyperpolarizability β0 for the gas phase without intermolecular interactions are gradually enhanced in the order PH < OH < OM < SH < SM < SB1 < DA < SBOM. Interestingly, with the OPT conformation, the investigated thiolated chromophores exhibit larger molecular nonlinearities than all oxygen-containing analogues and similar or larger nonlinearities compared with the dimethylamino (DA) analogue (see Tables 1 and 2). Therefore, from the gas-phase calculations, the thiolated chromophores are expected to have some advantages over the others by having large optical nonlinearities comparable with that of the DA analogue. 3.3. Molecular Optical Nonlinearity in Solution. Figure 4 shows the absorption spectra of the investigated chromophores in chloroform solution. In contrast to the relatively large difference of the hyperpolarizability β between the thiolated OPT chromophores and the oxygen-containing analogues (OPT) determined by the gas-phase calculations, the experimental wavelengths of maximum absorption λmax in solution are very similar: 421 nm for OH1, 421 nm for OM, 412 nm for SH, 424 nm for SM, 417 nm for SB1, and 428 nm for SBOM. The wavelengths of maximum absorption λmax of the investigated thiolated chromophores are much lower than for the DA chromophore with the dimethylamino donor group

Figure 4. Absoption spectra of the chromophores in chloroform solution (1.24 × 10−5 M): PH (λmax = 394 nm), OH1 (λmax = 421 nm), OM (λmax = 421 nm), SH (λmax = 412 nm), SM (λmax = 424 nm), SB1 (λmax = 417 nm), SBOM (λmax = 428 nm), and DA (λmax = 503 nm). 25037



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(λmax = 503 nm), while the hyperpolarizabilities β are similar or higher in the quantum chemical calculations. Figure 5 shows a plot of the hyperpolarizability vector component along the dipole moment direction βz determined

interacting with solvents. These solvent-independent characteristics of the large SB1 chromophore are quite different from the strong solvent effect on the λmax and β properties in the PCM calculations of, for example, a smaller molecule, p-nitroaniline (PNA), reported in the literature.27 This is because the electron donor amine group and the electron acceptor nitro group of the PNA molecule act as hydrogen-bond donor and acceptor sites and lead to strong hydrogen bonds with solvents.27 Third, in order to investigate this unusual nonlinearity− transparency trade-off behavior, EFISH measurements were performed for the SB1 and DA chromophores in chloroform solution in the same way as in our previous study.14 The SB1 chromophore (μβ z = 0.9 × 10 −66 C·m 5 ·V −1 ) shows experimental values considerably lower than the DA chromophore (μβz = 3.5 × 10−66 C·m5·V−1)14 but almost the same as the OM chromophore (μβz = 0.9 × 10−66 C·m5·V−1).14 Therefore, the reason for the observed discrepancy of the static first hyperpolarizability β between the experimental and theoretical values is not likely due to intermolecular interactions between the molecules and solvents by solvation effect. These results suggest that the possible variation of the molecular geometry23,24 in solution compared to the OPT conformation calculated for the gas phase may be responsible for the remarkably reduced βz and λmax of the SB1 measured in the experiments. The intrinsic structural effects by conformational change of the chromophore may lead to the subsequent violation of the π-electron delocalization, rather than the extrinsic solvent interaction effects. This hypothesis is investigated below by considering the rotational effect of the thiophenyl group on the optical nonlinearity of SB1. 3.4. Rotational Isomerization. Rotational isomerization can strongly influence the optical and the nonlinear optical properties of molecules.23,24 In order to determine the probability for rotation of the phenylthiol group as illustrated in Figure 6a, the total energy differences of rotational isomers of SB1 have been calculated.23 The results are shown in Figure 6b, where the dihedral angle θ denotes rotation of the phenylthiol group with respect to the aforementioned OPT conformation. The maximum total energy difference ΔE between θ = 0° and θ = 90° is relatively small, about 1.53 kcal/mol in the gas-phase calculation and even smaller, 1.47 kcal/mol, in the PCM calculation. In addition, the SB1 conformers with θ = 0° and θ = 180° exhibit lowest total energy with similar values. Therefore, the rotational barrier energies of the thiophenyl group are small enough to induce the rotational isomerization of the SB1 molecule in both gas phase and solution. To investigate the influence of rotational isomerization on their nonlinear optical properties, we have calculated the maximal first hyperpolarizability βmax of the SB1 chromophore by the FF method at the B3LYP/6-31+G(d) level;22 various dihedral angles are considered with respect to the OPT molecule (θ = 0°),28 as shown in Figure 6c. We have found that βmax decreases as the dihedral angle θ increases to 90°. βmax does not substantially decrease up to about θ = 60°, where it stays within 20% of the initial value, whereas it abruptly decreases beyond θ = 60°. To validate the FF results and investigate electronic characteristics concerning this phenomenon, we have further evaluated the static first hyperpolarizability β0 of the SB1 by TD-DFT at B3LYP/6-31+G(d) level for various dihedral angles θ. As shown in Figure 7, the OPT SB1 molecule has two important major and minor electronic transitions, A1 [highest

Figure 5. Transparency−nonlinearity diagram illustrating log βz versus log λmax for polyene chromophores. The βz values (× 10−40 m4·V−1, ○) are for the EFISH measurements of polyene chromophores having conventional alkoxy and dialkylamino electron donors from our previous study.14 The βz values (× 10−30 esu, ●) are for the gas-phase calculations of the chromophores from the present study. MH2 (2-{5methyl-3-[4-(pyrrolidin-1-yl)styryl]cyclohex-2-enylidene}malononitrile) is a DA polyene analogue with a pyrrolidine electron donor, and the calculated βz value of MH2 is obtained from ref 22.

by the electric field-induced second harmonic generation (EFISH) experiments as a function of the wavelength of maximum absorption λmax in chloroform.14 The βz values determined by EFISH were reported previously for polyene chromophores having conventional alkoxy and dialkylamino electron donors (presented with open symbols in Figure 5).14 The conventional polyene chromophores follow the usual nonlinearity−transparency trade-off behavior.1,2 When we add the values of the vector component βz calculated by the FF method into Figure 5, the conventional polyene chromophores (OH1, OM, and DA) show very good agreement with the usual nonlinearity−transparency trade-off behavior. However, the calculated βz values of OPT thiolated chromophores (SM, SB1, and SBOM) are much larger than those of the conventional polyene chromophores at a similar transparency range or λmax and comparable to that of the DA chromophore. In order to explain this discrepancy in hyperpolarizability of the thiolated chromophores, we consider the solvation effect. First, the static first hyperpolarizability β0 has been calculated by the TD-DFT method in solution by use of an implicit solvation model called the polarizable continuum model (PCM).26 The results are listed in Table 2 and compared to the calculation in gas phase without consideration of solvent effect. Also in these calculations the thiolated chromophores show molecular nonlinearity larger than all oxygen-containing analogues and similar to the dimethylamino DA analogue. Second, in the absorption measurement in various solvents having different polarity, the solvation effect is not significant. The wavelengths of maximum absorption λmax of SB1 chromophore in various solvents with differing polarity fall within a small range: 410 nm in THF, 412 nm in dioxane, 407 nm in acetone, 409 nm in methanol, and 418 nm in chloroform. In addition, all conventional and thiolated chromophores considered in this study, except for the OH115 and SH chromophores, do not contain highly sensitive sites 25038



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conjugated triene bridge. The frontier molecular orbital analysis shows lack of electron density in the phenyl group on the thiophenyl group. However, we have found an interesting phenomenon that the rotation of the nonplanar thiophenyl group is able to contribute considerably to the first hyperpolarizability. The results of these calculations are shown in Figure 8. The total hyperpolarizability βtotal is approximated by the sum of the two main contributions β1 and β2 concerning transitions A1 and A2, respectively (see Figure 8a). The β1 contribution dominates for the most stable structure (θ = 0°), whereas the β2 contribution prevails for the most unstable one (θ = 90°) with the magnitude limited to about 50 × 10−30 esu. During the rotation, the β1 contribution decreases while the β2 contribution increases, and hence the total hyperpolarizability βtotal decreases beyond θ = 60°, which is in good agreement with the FF results. This competitive contribution to the total first hyperpolarizability can be understood from the three factors involved in the static first hyperpolarizability calculation (eq 1) by the TD-DFT method. As shown in Figure 8b, the transition energies of A1 and A2, which are both lowered during the rotation, should in general increase the first hyperpolarizability according to the transparency−nonlinearity trade-off. However, the first hyperpolarizability also strongly depends (quadratically) on the transition dipole moment (μge). At θ = 90°, the transition dipole moment associated with the A1 transition (μge1) is almost zero, leading to the disappearance of the β 1 contribution, whereas that of the A2 transition (μge2) has its maximum value, enhancing the β2 contribution as shown in Figure 8c. This phenomenon can be understood from the fact that the S−C bond in the thiophenyl group deviates from the planar molecular framework, leading to a shorter conjugation length. Therefore, the high-energy transition is preferred for the nonplanar S−C bond of the thiophenyl group as the dihedral angle increases. The two transition dipole moments μge1 and μge2 cross at around θ = 60°. This transition dipole moment trade-off explains the observed behavior of βtotal and βmax beyond θ = 60°. The ground- and excited-state dipole moments (μg and μe) and their differences (Δμge) also depend on the rotation as shown in Figure 8d. Over all dihedral angles, the first excited dipole moment (μe1) is larger than the second excited dipole moment (μe2). μe1 has one large peak at θ = 90°, whereas μe2 has small double peaks at θ = 60° and 120°. The behavior of their differences (Δμge1 and Δμge2) with respect to the groundstate dipole moments (μg) resembles dipole moments of the excited states (μe1 and μe2) since the amount of μg change is relatively small during the rotation. Although Δμge1 increases during the rotation, its effect on the increase of β1 is significant only within θ = 60° but rapidly decreases beyond θ = 60° due to the effect of μge1 approaching zero as discussed above. In contrast, β2 gradually increases during the rotation because μge2 increases (see Figure 8a,c). These interpretations lead to the conclusion that the decrease of the β1 contribution during the rotation can be compensated by the increased β2 at θ ≥ 60°. Therefore, it seems that the competitive contribution of the two electronic transitions (i.e., A1 vs A2) accompanying the rotation of thiophenyl group may be responsible for the remarkably reduced β and λmax of the SB1 chromophore in the experiments. 3.5. Solvation Effect with Rotational Isomerization. In order to further investigate the discrepancy of the calculated and experimental results of the hyperpolarizability of thiolated

Figure 6. (a) Molecular geometry of SB1 (OPT conformation) with dihedral angle θ = 0°; the arrows present the normal mode of torsional vibration at 6.62 cm−1. (b) Variation of total energy difference of SB1 as a function of rotation θ of the phenylthiolated group. (c) Maximal first hyperpolarizability βmax calculated by the FF method for SB1 as a function of θ (gas-phase calculations).

Figure 7. Frontier molecular orbitals of the two main electronic transitions A1 and A2 as obtained by the TD-DFT method for SB1 (gas phase).

occupied molecular orbital (HOMO) → lowest unoccupied molecular orbital (LUMO)] and A2 [(HOMO − 1) → LUMO], respectively. In these processes, the electron cloud on the sulfur atom transfers to the acceptor CN group via the 25039



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Figure 8. Calculation results of TD-DFT with rotational isomers of SB1 (gas phase) as a function of the dihedral angles. Total hyperpolarizability βtotal is the sum of the two main contributions β1 and β2 concerning transitions A1 and A2 respectively. μg, μe1, and μe2 are dipole moments of ground, excited state 1, and excited state 2. μge1 and μge2 are transition dipole moments concerning the two main absorption A1 and A2, respectively. Δμge1 and Δμge2 are dipole moment differences of excited states 1 and 2 with respect to the ground state, respectively.

similar to the initial structure with θ = 0° in Figure 6a. The calculated λmax and β0 values in explicit solvation models compared to those in implicit solvation models show only small variation, and the β0 values of SB1 in explicit solvation models are smaller than those of DA by only about 10−20%. Moreover, as seen in Table 3, the calculated λmax values when solvent effects on the SB1 structure are considered are red-shifted up to 492 nm compared with that of the gas-phase calculation (450 nm), in contrast to the blue shift observed in the experiment (417 nm). Therefore, the discrepancy between calculated and experimental results in the hyperpolarizability of thiolated chromophores in solution may be not primarily attributed to solvation effects. In order to expand our hypothesis, we consider the solvation effects together with rotational isomerization by using an SB1 rotational isomer in the explicit solvation model. The initial structure of SB1 isomer with θ = 60°, with either one or two chloroform molecules, was optimized. The resulting optimized geometry does not significantly deviate from the conformer with θ = 0° and is similar to those in Figure 9. Therefore, even when both solvation effects and rotational isomerization are considered, the results are not consistent with the observation in the experiment. Thus, the reduced optical nonlinearity of the SB1 observed in the experiment compared with the calculations is considered in terms of the kinetics rather than the thermodynamics for the rotational isomers. As discussed above, the rotational barrier energies of the thiophenyl group with the maximum total energy difference ΔE of 1.53 kcal/mol are small enough to

chromophores in solution, we have considered solvation effects with rotational isomerization. First, the solvent interaction effect by using both implicit (e.g., PCM) and explicit solvation models in chloroform were considered for the SB1 and DA chromophores. The results are shown in Figure 9 and in Table 3. The geometry optimizations start from the initial structures with θ = 0°. The lone pair of electrons on the sulfur atom can form intermolecular hydrogen bonds with hydrogen atom on chloroform; considering the space, up to two chloroform molecules are used in the explicit solvation models. The resulting molecular conformation of SB1 in all models is very

Figure 9. Optimized molecular conformations of DA and SB1 chromophores via implicit and explicit solvation of chloroform. 25040



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Table 3. Static First Hyperpolarizability (β0) Resultsa gas phase

λmax,d nm Emax [ICT],e eV fosf μge,g D Δμge,h D β0, 10−30esu

implicit solvation (PCM)

DA (gasb/ gasc)

SB1 (gas/ gas)

DA (gas/ PCM)

SB1 (gas/ PCM)

DA (PCM/ PCM)

SB1 (PCM/ PCM)

471 2.63

450 2.76

529 2.35

487 2.55

532 2.33

492 2.52

1.31 11.46 17.55 194.3

1.39 11.52 18.56 189.1

1.49 12.96 16.99 303.1

1.57 12.74 17.35 253.5

1.59 13.42 14.95 289.8

1.61 12.96 16.57 255.5

experiment

λmax, nm Emax [ICT],e eV fosf μge,g D Δμge,h D β0, 10−30esu d

single-crystal structure analysis were grown by slow evaporation method at 30 °C from solution with acetonitrile as solvent. Figure 10a shows a molecular packing diagram with

DAexp

SB1exp

503

417

explicit solvation (PCM/PCM) DA′

SB1′

DA″

SB1″

507 2.45 1.48 12.62 16.98 264.1

480 2.59 1.58 12.69 16.75 236.0

507 2.45 1.48 12.63 16.97 264.3

473 2.62 1.59 12.65 16.09 219.0

a

Calculated by TD-DFT at B3LYP/6-31+G(d) level by considering implicit (PCM) and explicit solvations of the DA and SB1 chromophores in chloroform. bGeometry optimization. cFirst hyperpolarizability calculation. dWavelength of dominant electronic transition. eResonance energy of the main electronic transition. f Oscillator strength of the transition. gTransition dipole moment of the dominant charge-transfer transition. hDipole moment difference between excited and ground states.

induce the rotational isomerization of the SB1 molecule (see Figure 6b). The thermal energy of 0.59 kcal/mol at room temperature is able to populate the SB1 rotamers up to θ = 60°, which is consistent with the results of the theoretical work by Lopes et al.29 It is also noted that the dihedral angle θ and the energy profile concerned with rotation of the thiophenyl group may be regarded as the normal coordinate and the potential energy surface of the vibrational torsion mode of SB1 at 6.62 cm−1, respectively, as shown in Figure 6a. Therefore, the thermal energy sufficiently induces the vibrational excitations of the torsional mode of the SB1 over θ = 60°, leading to the reduction of first hyperpolarizability by disrupting the πelectron delocalization. In addition, the dihedral angle between the planes of two phenyl groups in the biphenyl sulfide moiety (i.e., Ph-S-Ph) of the SB1 rotamers at θ = 90° is about 90°, which is identical with the SB1 rotamers at θ = 0° (or θ = 180°), due to the easy rotation of the thiophenyl group. The SB1 molecule is expected to have substantial population deviated from θ = 0° due to the thermally excited torsional motions and internal rotations, leading to the suppressed optical nonlinearity compared with those of the optimized one and the DA chromophore. This possibility of rotation of the thiophenyl group is also found in the solid-state structure of the SB1 molecule described below. 3.6. Rotational Isomer in the Solid State. Rotation of the thiophenyl group of SB1 can be caused by intermolecular forces as well as thermal energy due to its low rotational barrier energy. In order to confirm the possibility of the existence of a rotational isomer and its influence on the nonlinear optical properties in the solid state, we have investigated the crystal structure of SB1 chromophore. SB1 single crystals for X-ray

Figure 10. (a) Molecular packing diagram of SB1 crystals. The dotted lines present hydrogen bonds of the (sp2)C−H···π interactions with a distance between C−H and C atoms of 2.5−3.0 Å. (b) Molecular conformation of SB1 molecule in crystalline state.

intermolecular interactions and the molecular conformation of the SB1 chromophore in the crystalline state. Main supramolecular interactions are relatively weak hydrogen bonds of the (sp2)C−H···π interactions; in Figure 10a, such bonds with a distance between C−H and C atoms of 2.5−3.0 Å are presented. The weak C−H···π intermolecular and intramolecular interactions between the phenyl groups overcome the small rotational barrier and lead to a rotational isomer with θ = 55.4°, which is considerably deviated from the OPT structure with θ = 0° (see Figure 10b). The first hyperpolarizability of the experimental SB1 molecule (EXP) determined by X-ray analysis is 180.4 × 10−30 esu from the FF calculation, which is in good agreement with our rotational isomerism plot of first hyperpolarizability (see Figures 6c and 8a). Therefore, the rotational isomerism of the nonplanar thiophenyl group in the SB1 chromophore reasonably describes the interesting optical nonlinearities of the SB1 chromophore in solid state as well as in solution, which is quite different from that of the optimized one in gas phase.

4. CONCLUSION We have designed and synthesized various π-conjugated phenyltriene chromophores having different thiolated electron donor groups such as thiol, alkylthiol, and arylthiol groups to 25041



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Photonics; Gupta, M. C., Ballato, J., Eds.; CRC Press: Boca Raton, FL, 2007. (c) Nalwa, H. S.; Watanabe, T.; Miyata, S. In Nonlinear Optics of Organic Molecules and Polymers; Nalwa, H. S., Miyata, S., Eds.; CRC Press: Boca Raton, FL, 1997; Chapt. 4. (d) Dalton, L. R.; Sullivan, P. A.; Bale, D. H. Chem. Rev. 2010, 110, 25−55. (3) (a) Ma, H.; Jen., A. K. Y. Adv. Mater. 2002, 14, 1339−1365. (b) Dalton, L. R. Adv. Polym. Sci. 2002, 158, 1−86. (c) Rezzonico, D.; Jazbinsek, M.; Guarino, A.; Kwon, O. P.; Günter, P. Opt. Express 2008, 16, 613−627. (4) (a) Tonouchi, M. Nat. Photonics 2007, 1, 97−105. (b) Abbott, D.; Zhang, X.-C. T-ray Imaging, Sensing & Retection. Proc. IEEE 2007, 95 (8), 1514−1704 (special issue). (c) Brunner, F. D. J.; Kwon, O. P.; Kwon, S. J.; Jazbinsek, M.; Schneider, A.; Günter, P. Opt. Express 2008, 16, 16496−16508. (5) Marder, S. R.; Cheng, L. T.; Tiemann, B. G.; Friedli, A. C.; Blanchare-Desce, M.; Perry, J. W.; Skindhoj, J. Science 1994, 263, 511− 514. (6) (a) He, M.; Leslie, T. M.; Sinicropi, J. A.; Garner, S. M.; Reed, L. D. Chem. Mater. 2002, 14, 4669−4675. (b) Liu, S.; Haller, M. A.; Ma, H.; Dalton, L. R.; Jang, S. H.; Jen, A. K. Y. Adv. Mater. 2003, 15, 603− 607. (7) Jang, S. H.; Luo, J.; Tucker, N. M.; Leclercq, A.; Zojer, E.; Haller, M. A.; Kim, T. D.; Kang, J. W.; Firestone, K.; Bale, D.; et al. Chem. Mater. 2006, 18, 2982−2988. (8) (a) Ledouxa, I.; Zyss, J.; Barni, E.; Barolo, C.; Diulgheroff, N.; Quagliotto, P.; Viscardi, G. Synth. Met. 2000, 115, 213−217. (b) Schwartz, H.; Mazor, R.; Khodorkovsky, V.; Shapiro, L.; Klug, J. T.; Kovalev, E.; Meshulam, G.; Berkovic, G.; Kotler, Z.; Efrima, S. J. Phys. Chem. B 2001, 105, 5914−5921. (9) Kwon, O.; Barlow, S.; Odom, S. A.; Beverina, L.; Thompson, N. J.; Zojer, E.; Bredas, J. L.; Marder, S. R. J. Phys. Chem. A 2005, 109, 9346−9352. (10) (a) Staub, K.; Levina, G. A.; Barlow, S.; Kowalczyk, T. C.; Lackritz, H. S.; Barzoukas, M.; Fort, A.; Marder, S. R. J. Mater. Chem. 2003, 13, 825−833. (b) Morley, J. O.; Hutchings, M. G.; Zyss, J.; Ledoux, I. J. Chem. Soc., Perkin Trans. 2 1997, 1139−1142. (c) Cheng, Y. J.; Luo, J.; Huang, S.; Zhou, X.; Shi, Z.; Kim, T. D.; Bale, D. H.; Takahashi, S.; Yick, A.; Polishak, B. M.; et al. Chem. Mater. 2008, 20, 5047−5054. (11) (a) Lim, J. S.; Lee, Y. S.; Kim, S, K. Angew. Chem. In. Ed. 2008, 47, 1853−1856. (b) Lim, I. S.; Lim, J. S.; Lee, Y. S.; Kim, S. K. J. Chem. Phys. 2007, 126, No. 034306. (12) Moreno-Manas, M.; Pleixats, R.; Andreu, R.; Garin, J.; Orduna, J.; Villacampa, B.; Levillain, E.; Salle, M. J. Mater. Chem. 2001, 11, 374−380. (13) (a) Shu, C. F.; Tsai, W. J.; Jen, A. K-Y. Tetrahedron Lett. 1996, 37, 7055−7085. (b) Ermer, S.; Lovejoy, S. M.; Leung, D. S.; Warren, H.; Moylan, C. R.; Twieg, R. J. Chem. Mater. 1997, 9, 1437−1442. (14) Kwon, O. P.; Ruiz, B.; Choubey, A.; Mutter, L.; Schneider, A.; Jazbinsek, M.; Gramlich, V.; Günter, P. Chem. Mater. 2006, 18, 4049− 4054. (15) Kwon, O. P.; Kwon, S. J.; Jazbinsek, M.; Brunner, F. D. J.; Seo, J. I.; Hunziker, Ch.; Schneider, A.; Yun, H.; Lee, Y. S.; Günter, P. Adv. Funct. Mater. 2008, 18, 3242−3250. (16) (a) Kwon, O. P.; Kwon, S. J.; Jazbinsek, M.; Seo, J. Y.; Kim, J. T.; Seo, J. I.; Lee, Y. S.; Yun, H.; Günter, P. Chem. Mater. 2011, 23, 239−246. (b) Kwon, O. P.; Jazbinsek, M.; Yun, H.; Seo, J. I.; Seo, J. Y.; Kwon, S. J.; Lee, Y. S.; Günter, P. CrystEngComm 2009, 11, 1541− 1544. (17) Kwon, O. P.; Kwon, S. J.; Jazbinsek, M.; Choubey, A.; Gramlich, V.; Günter, P. Adv. Funct. Mater. 2007, 17, 1750−1756. (18) (a) Dimmock, J. R.; Puthucode, R. N.; Smith, J. M.; Hetherington, M.; Quail, J. W.; Pugazhenthi, U.; Lechler, T.; Stables, J. P. J. Med. Chem. 1996, 39, 3984−3997. (b) Boger, D. L., Patent WO2008/147553, 2008. (19) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112−122. (20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;

systematically investigate the electron-donating ability of sulfur analogues. For comparison, we have also investigated the commonly used electron donor groups, oxygen-containing hydroxyl and alkoxy groups, as well as nitrogen-containing dialkylamino groups. The calculated results of both FF and TDDFT methods of isolated OPT molecules in gas phase show that the first hyperpolarizabilities β are gradually enhanced in the order PH < OH < OM < SH < SM < SB1 < DA < SBOM. Interestingly, the thiolated OPT chromophores show much higher calculated values of the first hyperpolarizabilities β than the conventional polyene chromophores at the similar transparency range or λmax. However, in experimental EFISH measurements in solution, the thiolated polyene chromophore displays molecular nonlinearities considerably lower than the DA chromophore and similar to the OM chromophore. The observed discrepancy of the hyperpolarizability in thiolated chromophores between calculated and experimental values in solution may be caused by a possible variation of the molecular geometry. Consideration of various dihedral angles of the arylthiolated group indicates that the rotational isomerization of arylthiolated group can result in a large variation of molecular optical nonlinearities. Moreover, the possibility for rotational isomerism has been confirmed by the measured conformation of SB1 molecules in the crystalline state.

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ASSOCIATED CONTENT

S Supporting Information *

Complete references 7, 10c, and 20. This material is available free of charge via the Internet at http://pubs.acs.org. Further details of the crystal structure investigation(s) may be obtained from the Cambridge Crystallographic Data Centre [CCDC, 12 Union Rd., Cambridge CB2 1EZ, U.K.; tel. (+44) 1223-336408, fax (+44) 1223-336-033, e-mail [email protected]].

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AUTHOR INFORMATION

Corresponding Author

* E-mail [email protected] (O.-P.K.), [email protected] (Y.S.L.). Present Address ¶

Korea Science Academy of KAIST, Busan 614-822, Korea.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants (2010-0027743, 20090084918, 2007-0056341) and Priority Research Centers Program (2012-0006687) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology. The computational resources were in part provided by KISTI (KSC-2011-C2-18).

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REFERENCES

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Rotational Isomerism of Phenylthiolated Chromophores with Large

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