Molecular Engineering of Benzothiazolium Salts with Large Quadratic

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Molecular Engineering of Benzothiazolium Salts with Large Quadratic Hyperpolarizabilities: Can Auxiliary Electron-Withdrawing Groups Enhance Nonlinear Optical Responses? Peter Hroba´rik,*,†,‡ Ivica Sigmundova´,§ Pavol Zahradnı´k,§ Peter Kasa´k,| Vladimir Arion,⊥ Edith Franz,# and Koen Clays# Institut fu¨r Physikalische und Theoretische Chemie, Julius-Maximilians-UniVersita¨t Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany, Institute of Inorganic Chemistry, SloVak Academy of Sciences, Du´braVska´ Cesta 9, SK-84536 BratislaVa, SloVakia, Department of Organic Chemistry, Faculty of Natural Sciences, Comenius UniVersity, Mlynska´ dolina, SK-84215 BratislaVa, SloVakia, Polymer Institute, SloVak Academy of Sciences, Du´braVska´ Cesta 9, SK-84541 BratislaVa, SloVakia, Institute of Inorganic Chemistry, UniVersity of Vienna, Wa¨hringer Strasse 42, A-1090 Vienna, Austria, and Department of Chemistry, UniVersity of LeuVen, Celestijnenlaan 200D, B-3001 LeuVen, Belgium ReceiVed: September 9, 2010; ReVised Manuscript ReceiVed: October 26, 2010

A series of push-pull chromophores comprising a dimethylamino or diphenylamino electron-donating functionality and a cationic benzothiazolium acceptor with an additional electron-withdrawing group (EWG ) NO2 or CN) at various positions of the heterocyclic benzene ring have been synthesized and comprehensively investigated for their linear and quadratic nonlinear optical (NLO) properties by means of UV-visible spectroscopy and hyper-Rayleigh scattering, as well as by quantum-chemical calculations at different levels of theory (B3LYP, CAM-B3LYP, MP2, and RI-CC2). In general, all chromophores under study display large static quadratic hyperpolarizabilities β0, comparable to or in most cases even larger than their conventional stilbazolium-containing analogues, which makes these systems attractive for practical NLO applications. The introduction of an auxiliary EWG into the heterocyclic benzene ring causes a substantial red shift of the intramolecular charge-transfer band. Still, at the same time, this modification of the structure reduces the β0 values in systems with longer π-conjugated spacers. The unexpected negative impact of the EWG substitution pattern on the quadratic NLO activity is rationalized by quantum-chemical calculations as well as by experimentally determined one-photon absorption characteristics and is discussed in detail. Furthermore, computational studies revealed that push-pull benzothiazolium dyes with a “reverse” polarity with respect to the commonly used one would be a more worthwhile target for synthesis, because of their enhanced NLO response due to the positive effect of an auxiliary electron-withdrawing group. Introduction Materials exhibiting large optical nonlinearities are still the subject of intense studies because of their potential application in modern communication technologies involving optical data processing, transmission, or storage, where they gradually replace classical electronic devices.1,2 Besides the technological importance of nonlinear optical (NLO) materials, there is also a scientific interest in their use in bioimaging.3 For instance, second harmonic generation (SHG), the younger and complementary method of two-photon excited fluorescence (TPEF), can in conjunction with a suitable NLO-phore produce highresolution images from deep inside biological tissues. In the last two decades, a considerable effort has been particularly focused on the development of organic molecules with enhanced second-order NLO properties.4,5 As compared to conventional inorganic crystals, organic-based materials offer several advantages, such as (a) a higher second-order susceptibility χ(2) * To whom correspondence should be addressed. E-mail: peter.hrobarik@ savba.sk. † Julius-Maximilians-Universita¨t Wu¨rzburg. ‡ Institute of Inorganic Chemistry, Slovak Academy of Sciences. § Comenius University. | Polymer Institute, Slovak Academy of Sciences. ⊥ University of Vienna. # University of Leuven.

through high molecular hyperpolarizability β; (b) a lower dielectric constant, and thus a very fast response; (c) easier fabrication; and (d) the possibility of fine-tuning of NLO properties through systematic chemical modifications of chromophores. Structure-property relationships based on the theoretical and experimental studies revealed that high molecular quadratic hyperpolarizabilities β arise from a combination of strong electron-donating and electron-withdrawing groups (EWGs) positioned at the extremities of a suitable π-conjugated spacer.6 In these so-called push-pull systems, the presence of the donor and acceptor moieties ensures the ground-state charge asymmetry, whereas the polarizable π-conjugated bridge provides a pathway for the redistribution of the electron density under the influence of external electric fields. The nature of the conjugation path is of critical importance: While polyene chains are superior in the efficient charge transfer (CT), their low chemical, photochemical, and thermal stabilities have shifted the researchers’ interest toward more stable and easily delocalizable heteroaromatics.7 In this respect, electron-rich and electron-poor heterocycles gained a special place of their own due to their ability to act as auxiliary electron-donating and electronaccepting moieties, respectively. Having chromophores comprising an electron-deficient azole or benzazole type heteroaromatic ring, considerable improve-

10.1021/jp108623d  2010 American Chemical Society Published on Web 11/24/2010

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Figure 1. Schematic structure of push-pull 3-metylbenzothiazolium salts with a polyenic π-bridge: [1]+ (n ) 1), [2]+ (n ) 2), [3]+ (n ) 3), [4]+ (n ) 4), and an additional EWG. The EWG substituent (NO2, CN) together with its position (x) is indicated as a prefix before a label of the root compound. The dimethylamino group is assumed as a donor substituent by default; otherwise, diphenylamino (NPh2) functionality is indicated as the Ph suffix appended to the label of the corresponding compound.

ment of molecular quadratic hyperpolarizabilities can be achieved by quaternization of the heterocyclic nitrogen. This modification leads to charged organic salts that exhibit NLO activity often a magnitude higher than their neutral analogues. Next, aspects of a particular interest in organic salts are their inherently greater stabilities and higher chromophore number densities than in alternative poled NLO-polymer materials.8 Moreover, the use of a counterion variation allows one to control the alignment of ionic chromophores in noncentrosymmetric bulk structures, which are essential for the quadratic NLO effect.9 Among such compounds, stilbazolium type salts such as 4-(4-dimethylaminostyryl)-1-methylpyridinium tosylate (DAST), whose crystals have been commercialized recently for use in terahertz (THz) wave generation via nonlinear frequency mixing,10 are particularly attractive because of their large NLO responses.11 Recently, Coe et al. reported the results of hyper-Rayleigh scattering (HRS) measurements for 3-methylbenzothiazoliumderived push-pull salts [1]PF6-[4]PF6 (cf. Figure 1) in acetonitrile solution.12 Measurements revealed that the benzothiazolium salts exhibit larger NLO responses than their 1-methylpyridinium analogues, and the static first hyperpolarizability β0 increases with the length of the polyene chain. On the basis of this finding, it is anticipated that the benzothiazolium moiety might be a more effective electron-accepting subunit in NLO chromophores than a pyridinium cation. Although many theoretical and experimental investigations on neutral NLO-phores involving the benzothiazole as an auxiliary acceptor have been reported to date,13-16 a detailed understanding of the “molecular structure-NLO activity” relationships for the closely related benzothiazolium-containing systems is lacking, despite the growing interest in their use.12,17,18 Up to the present time, all studies have been devoted specifically to the influence of the π-bridge length and its extension by electron-rich heteroaryl moieties.15,17 Still, no particular attention has been paid to structural modifications on the positively charged heterocyclic ring itself, and no other positions than C-2 were assumed for linking a donor functionality to the benzothiazolium moiety. In view of large quadratic NLO responses of the first four members of the series [1]+-[4]+, we wondered whether an increase of electron-withdrawing capability of the benzothiazolium moiety by an addition of the acceptor group (NO2 or CN) would lead, if ever, to any increases in static first

Hroba´rik et al. hyperpolarizabilities β0. To explore this possibility, we present here the synthesis of a series of benzothiazolium salts with an auxiliary EWG attached to the heterocyclic benzene ring (cf. Figure 1) and a comprehensive study of linear and NLO properties of these systems by means of UV-visible spectroscopy and HRS, respectively. To understand experimentally observed trends and to establish structure-activity relationships useful for engineering large quadratic NLO responses in benzothiazolium-derived chromophores, quantum-chemical calculations of intramolecular charge-transfer (ICT) excitation properties and static first hyperpolarizabilities β0 at different levels of theory [density functional theory (DFT), timedependent DFT, MP2 and RI-CC2] have been carried out. Besides the initial molecular design strategy, where an auxiliary EWG is introduced into the heterocyclic benzene ring, the effects of the replacement of a dialkylamino electron-donating functionality by a diarylamino one and a relative orientation of the benzothiazolium subunit in push-pull chromophores are discussed as well. Experimental Section Synthetic Procedures. The starting 2-methylbenzothiazole, 4-(N,N-dimethylamino)benzaldehyde, and (E)-3-[4-(N,N-dimethylamino)phenyl]propenal were obtained commercially and used as supplied. The 4-(N,N-diphenylamino)benzaldehyde was prepared by the procedure described in ref 19 and (E)-3-[4(N,N-diphenylamino)phenyl]propenal as follows. Synthesis of (E)-3-[4-(N,N-Diphenylamino)phenyl]propenal. The reaction mixture of 4-(N,N-diphenylamino)benzaldehyde (400 mg, 1.48 mmol), (triphenylphosphanylidene)acetaldehyde20 (540 mg, 1.78 mmol), and tetrabutylammonium chloride (TEBA-Cl; 34 mg, 0.15 mmol) as a phase-transfer catalyst in dry toluene (20 mL) was heated at 90-100 °C for 24 h under a nitrogen atmosphere. After it was cooled, the mixture was poured into water (50 mL), and an aqueous layer was extracted with toluene (2 × 30 mL). The combined organic extracts were dried over Na2SO4, and the solvent was removed under vacuum. The crude product was purified by column chromatography on silica gel (eluent hexane/EtOAc ) 95:5) giving the title compound as a dark yellow viscose oil (310 mg, 70% yield). 1H NMR (300 MHz, CDCl3): 9.64 (d, J ) 7.8 Hz, 1H, CHO), 7.41-7.29 (m, 4H, C6H5), 7.38 (d, J ) 15.8 Hz, 1H, CH), 7.27 (d, J ) 8.7 Hz, 2H, HPh), 7.15-7.08 (m, 6H, HPh), 7.01 (d, J ) 8.7 Hz, 2H, HPh), 6.59 (dd, J ) 15.8, 7.8 Hz, 1H, CH). 13C NMR (75 MHz, CDCl3): 193.67, 152.65, 150.77, 146.54, 129.81, 129.57, 126.62, 125.98, 125.70, 124.44, 120.99. Anal. calcd (%) for C21H17NO: C, 84.25; H, 5.27; N, 4.68. Found: C, 84.10; H, 5.39; N, 4.55. Quaternization of 2-Methylbenzothiazoles. General Procedure. A mixture of 2-methylbenzothiazole (5 mmol), respectively, its nitro- or cyano-substituted derivative (cf. Supporting Information), and iodomethane (2.13 g, 15 mmol) in methanol (4 mL) was irradiated in a microwave apparatus at MW power of 60 W for 40 min (the temperature inside the reactor increased to 115 °C). After the mixture was cooled, the resulting precipitate was collected and washed with cold methanol to yield the corresponding 2,3-dimethylbenzothiazolium iodide as a white crystalline solid. 2,3-Dimethyl-5-nitrobenzothiazolium Iodide. mp 255-257 °C. 1 H NMR (300 MHz, DMSO-d6): 9.19 (d, J ) 2.4 Hz, 1H, H-4), 8.70 (d, J ) 9.0 Hz, 1H, H-7), 7.71 (dd, J ) 9.0, 2.4 Hz, 1H, H-6), 4.31 (s, 3H, N-CH3), 3.23 (s, 3H, CH3). 13C NMR (75 MHz, DMSO-d6): 181.9, 147.9, 141.8, 135.0, 126.2, 122.3,

Molecular Engineering of Benzothiazolium-Containing NLO-Phores 112.9, 36.9, 17.8. Anal. calcd (%) for C9H9IN2O2S: C, 32.16; H, 2.70; N, 8.33; S, 9.54. Found: C, 32.09; H, 2.69; N, 8.34; S, 9.44. 2,3-Dimethyl-6-nitrobenzothiazolium Iodide. mp 260-261 °C. 1 H NMR (300 MHz, DMSO-d6): 9.43 (d, J ) 2.1 Hz, 1H, H-7), 8.68 (dd, J ) 9.0, 2.4 Hz, 1H, H-5), 8.51 (d, J ) 9.0 Hz, 1H, H-4), 4.25 (s, 3H, N-CH3), 3.24 (s, 3H, CH3). 2,3-Dimethyl-7-nitrobenzothiazolium Iodide. mp 179-180 °C. 1 H NMR (300 MHz, DMSO-d6): 8.79 (dd, J ) 8.4, 0.6 Hz, 1H, H-4), 8.68 (dd, J ) 8.4, 0.6 Hz, 1H, H-6), 8.19 (t, J ) 8.4 Hz, 1H, H-5), 4.30 (s, 3H, N-CH3), 3.27 (s, 3H, CH3). 13C NMR (75 MHz, DMSO-d6): δ 181.0, 143.1, 142.2, 130.4, 124.6, 124.0, 123.9, 37.1, 17.2. Anal. calcd (%) for C9H9IN2O2S: C, 32.16; H, 2.70; N, 8.33; S, 9.54. Found: C, 32.07; H, 2.71; N, 8.33; S, 9.58. 6-Cyano-2,3-dimethylbenzothiazolium Iodide. mp 250-251 °C. 1H NMR (300 MHz, DMSO-d6): 8.95 (d, J ) 1.2 Hz, 1H, H-7), 8.50 (d, J ) 8.7 Hz, 1H, H-4), 8.36 (dd, J ) 8.7, 1.2 Hz, 1H, H-5), 4.22 (s, 3H, N-CH3), 3.21 (s, 3H, CH3). 13C NMR (75 MHz, DMSO-d6): 181.6, 144.0, 132.3, 129.7, 129.5, 118.2, 117.7, 110.4, 36.3, 17.8. Anal. calcd (%) for C10H9IN2S: C, 37.99; H, 2.87; N, 8.86; S, 10.14. Found: C, 37.69; H, 2.84; N, 8.74; S, 10.54. The target compounds [1]I, [2]I, and [3]I were synthesized as described previously.21 All other push-pull benzothiazolium salts were prepared analogously according to the following general procedure: Reaction of 2,3-Dimethylbenzothiazolium Iodide DeriVatiVes with Aldehydes. A mixture of 2,3-dimethylbenzothiazolium iodide, respectively, its nitro- or cyano-substituted derivative (2 mmol), and the corresponding aldehyde (2 mmol) in methanol (5 mL) was heated under reflux for 6-15 h in the presence of basic catalyst (pyridine or piperidine, 0.2 mmol). In the case of [2-Ph], no basic catalyst was used, since its employment led to undesirable byproduct and thus low yield. The reaction mixture was allowed to cool slowly to room temperature, and a dark purple microcrystalline solid was filtered off, washed with cold methanol and then with diethyl ether, and dried. If further purification was required, the resulting dark solid was recrystallized from methanol. All new compounds were fully characterized by 1H and 13C NMR spectroscopies and found to be analytically pure by elemental analyses (see the Supporting Information). Metathesis. General Procedure. To a methanol/acetonitrile solution (20 mL, v/v ) 1:1) of the corresponding benzothiazolium iodide salt (0.2 mmol) was added dropwise a concentrated solution of KPF6 (147 mg, 0.8 mmol) in distilled water, and the reaction mixture was stirred for 4 h at room temperature. Consequently, a large excess of distilled water (100 mL) was added to the mixture, and the precipitated hexafluorophosphate salt was centrifuged (3000 rpm, 15 min), washed with diethylether, and dried in vacuo. Yields ranged from 95 to 98%. Characterization and Spectral Properties. The 1H and 13C NMR spectra were recorded on a Varian Gemini 300 spectrometer, operating at 300.07 MHz (1H) and 75.46 MHz (13C) using TMS as an internal standard. 15N chemical shifts were measured indirectly, via 1H-15N long-range correlation on a VARIAN INOVA 600 MHz NMR spectrometer operating at 599.78 (1H) and 60.78 MHz (15N) using the same setup and conditions as described in ref 22. Unified chemical shift scale was used for 15N with nitromethane as a secondary standard (δ ) 0.0 ppm; Ξ ) 10.1367670).23 All NMR measurements were performed in DMSO-d6 solution. Chemical shifts δ are referred to in terms of parts per million, and J-coupling constants are

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given in Hertz. Abbreviations for multiplicity are as follows: s (singlet), d (doublet), t (triplet), q (quadruplet), and m (multiplet). Electronic absorption spectra were measured on a HewlettPackard 8452A diode array spectrophotometer, and fluorescence measurements were done on a Hitachi F-2000 fluorescence spectrophotometer. Concentrations of samples never exceeded 1 × 10-5 M to avoid molecule aggregation and photochemical dimerization processes. All solvents were dried and freshly distilled before use. Combustion analysis was performed on a Carlo Erba Science 1106 instrument. The melting points were measured using a Kofler apparatus and are uncorrected. HRS. The apparatus and experimental procedures used for the femtosecond HRS24 studies were the same as described previously.25 All measurements were performed by using the vertically polarized 800 nm fundamental of a regenerative modelocked Ti3+:sapphire laser (Spectra Physics). Samples and the external reference compound, crystal violet (the octupolar βzzz,800 value ) 338 × 10-3 esu), were dissolved in methanol. Dilute solutions (10-5-10-4 M) were used to ensure linear dependence of I2ω/I2ω2 on solute concentration, precluding the need for Lambert-Beer correction factors. An absence of demodulation, that is, constant values of β versus frequency, showed that no fluorescence contributions to the HRS signals were present at 400 nm. This situation may indicate (i) a lack of fluorescence, (ii) spectral filtering out of fluorescence, or (iii) a fluorescence lifetime too short for its demodulation to be observed within the bandwidth of the instrument. The reported β values are the averages taken from measurements at different amplitude modulation frequencies. No polarization selection was used in HRS intensity measurements on CT chromophores, and the resulting major βzzz tensor component along the molecular CT z-axis is reported as β. X-ray Crystallography. Suitable crystals of the salt [6-NO21]PF6 · (CH3)2CO were obtained by dissolving [6-NO2-1]PF6 in acetone and slow evaporation of the solvent. X-ray diffraction measurements were performed on a Bruker X8APEX II CCDdiffractometer. A single crystal was coated with Parathone-N oil, mounted at room temperature on the tip of glass fiber, and cooled under a stream of cold N2 maintained by a KRYOFLEX low-temperature apparatus. The crystal was positioned at 40 mm from the detector, and 2044 frames were measured, each for 30 s over 1° scan width. The data were processed using SAINT software. The structure was solved by direct methods and refined by full-matrix least-squares techniques. Nonhydrogen atoms were refined with anisotropic displacement parameters. H atoms were placed at calculated positions and refined as riding atoms in the subsequent least-squares model refinements. The isotropic thermal parameters were estimated to be 1.2 times the values of the equivalent isotropic thermal parameters of the atoms to which hydrogens were bonded. Used were the following computer programs and tables: structure solution, SHELXS-97;26 refinement, SHELXL-97;27 molecular diagrams, ORTEP;28 and scattering factors.29 Quantum-Chemical Calculations. The ground-state structures of all benzothiazolium systems were fully optimized in vacuo at the DFT level by using the B3LYP exchangecorrelation functional30 and the TZVP basis set.31 With the aim of assessing the quality of DFT optimized structures, selected systems were also optimized using the RI-MP2 method.32 All of these calculations were carried out with the Turbomole 6.0.33 Vertical excitation energies (Emax), oscillator strengths (fosc), and adiabatic dipole moment changes (∆µ01) between the ground state and the first excited state were computed at both timedependent DFT and the second-order approximate coupled-

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cluster (CC2)34 level. DFT calculations with the B3LYP or the Coulomb-attenuated CAM-B3LYP35 functional and the 6-31G(d) basis set were performed with the Gaussian 09 program package,36 whereas CC2 calculations were done using the Turbomole within the resolution-of-the-identity (RI) approximation for the evaluation of the electron-repulsion integrals (RICC2) in conjunction with the TZVP basis set. The latter two methods were used as it is known that the B3LYP functional cannot guarantee a correct description for CT excitations due to the improper treatment of the long-range exchange potential.37 In the case of TD-DFT calculations, solvent effects were simulated by employing a polarizable continuum model (PCM).38 The static quadratic hyperpolarizabilities β0 were computed using the finitefield numerical derivative method at the DFT as well as at the second-order Møller-Plesset (MP2) perturbation level of theory in the Gaussian 09. The finite-field results are reported as the magnitude of the β tensor (total hyperpolarizability), defined as follows:

β)

(∑β )

2 1/2

i

(i ) x, y, z)

(1)

i

TABLE 1: Crystallographic Data and Refinement Details for [6-NO2-1]PF6 · (CH3)2CO [6-NO2-1]PF6 · (CH3)2CO empirical formula MW crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Fcalcd (g cm-3) T (K) µ (mm-1) crystal size (mm3) final R1a wR2 [I > 2σ(I)]b GOFc

C21H24F6N3O3PS 543.46 triclinic P1j 9.4565(2) 11.8283(3) 11.8421(3) 73.340(2) 67.1790(10) 76.8330(10) 1159.42(5) 2 1.557 100 0.287 0.24 × 0.12 × 0.10 0.0334 0.0866 1.053

a R1 ) Σ||Fo| - |Fc||/Σ|Fo|. b wR2 ) {Σ[w (Fo2 - Fc2)2]/Σ[w(Fo2)2 ]}1/2. GOF ) {Σ[w(Fo2 - Fc2)2]/(n - p)}1/2, where n is the number of reflections and p is the total number of parameters refined.

c

where

1 βi ) 3

∑ j

1 (βijj + βjij + βjji) ) βiii + 3

∑ (βijj + 2βjji) j*i

(2) It is worth noting that the dominant contribution to the total hyperpolarizability in the systems considered here originates from the zzz component of the hyperpolarizability tensor, which allows us to compare these values with experimental data. All finite-field computations employed an electric field of 0.001 au. The atomic charges in the ground state and the first excited state were calculated by the natural population analysis (NPA),39 using the built-in NBO3.1 subroutines of the Gaussian. Results and Discussion Synthesis and Characterization. A key step in the synthesis of the target push-pull benzothiazolium salts was a Knoevenagel type condensation of 2,3-dimethylbenzothiazolium iodide, respectively, its EWG-substituted derivatives, with corresponding carbaldehydes. The EWG-substituted benzothiazole derivatives have been synthesized as follows (cf. Supporting Information): A direct nitration of the commercially available 2-methylbenzothiazole with fuming nitric acid afforded 2-methyl-6-nitrobenzothiazole.19 This was consequently transformed into the corresponding amine by a sonochemical reduction with powdered iron and hydrochloric acid in ethanol. Diazotation of the obtained 6-amino-2-methylbenzothiazole, followed by a Sandmeyer reaction with CuCN and KCN, afforded the 2-methylbenzothiazole-6-carbonitrile.14 The 2-methyl-5-nitrobenzothiazole was prepared in a ring-forming reaction from N-(2bromo-5-nitrophenyl)acetamide. The obtained 5-nitro-substituted benzothiazole was also utilized for the synthesis of 2-methyl4-nitrobenzothiazole using the following sequence of reactions: The nitro group was reduced, followed by protection of the obtained amine with acetyl chloride, nitration to the C-4 position, removal of the acetyl group, and finally deamination. A similar strategy was used to prepare 2-methyl-7-nitrobenzothiazole, starting from 6-amino-2-methylbenzothiazole. The corresponding 2,3-dimethylbenzothiazolium iodides were prepared by a

quaternization of the 2-methylbenzothiazoles with methyl iodide under microwave irradiation, which has significantly shortened the reaction times, while the yields remained similar to those obtained by conventional heating. The methylation of 2-methyl4-nitrobenzothiazole did not provide the expected quaternary salt (only unreacted starting materials were recovered), presumably due to the steric hindrance of the nitro group, which is in close proximity of the heterocyclic nitrogen. Crystallographic Study. The aim of this study is to investigate the molecular second-order NLO properties in solution. However, from a practical point of view, the investigation of bulk NLO response is of major importance. Attempts to obtain suitable crystals large enough for X-ray analysis from concentrated solutions of benzothiazolium iodides were not successful. Therefore, selected iodide salts were metathesized to the corresponding hexafluorophosphates, which in our experiences form appropriate crystals more readily. In this case, we were able to get desirable crystals of [6-NO2-1]PF6 · acetone, whose crystal structure is reported herein (cf. Table 1 and Figure 2). Unfortunately, like its analogues derived from unsubstituted [1]+, [6-NO2-1]PF6 crystallized in triclinic centrosymmetric space group P1j. The packing of [6-NO2-1]PF6 appears to be dominated by both the pairwise π-stacking between the dimethylamino-substituted phenyl ring and the benzothiazolium moiety and the formation of an apolar environment for the PF6counterion. The same motif of mutual π-stacking was observed also in previously published charged push-pull benzothiazolium salts.12,17 This structural preference may indeed explain why these salts crystallize in a centrosymmetric fashion that prevents them from displaying appreciable bulk NLO responses. Nevertheless, comparison of the crystal structures of [1]PF6 and [6-NO2-1]PF6 allows us directly to compare the effect of an auxiliary EWG group on the molecular structure (cf. Table 2). As expected for systems with considerable donor-acceptor π-coupling, the backbone of cation in [6-NO2-1]PF6 is nearly planar. Interestingly, a nitro functionality attached to the heterocyclic benzene ring is twisted about 9.8° with respect to the benzothiazolium fragment, most likely due to the crystal packing effects. Like its analogue [1]PF6, the cation in [6-NO21]PF6 shows some degree of ground-state charge separation, as

Molecular Engineering of Benzothiazolium-Containing NLO-Phores

Figure 2. (a) ORTEP view of the cation in [6-NO2-1]PF6 · (CH3)2CO with atom-labeling scheme (the thermal ellipsoids are drawn at 50% probability level); (b) intermolecular π-π stacking in the unit cell.

evidenced by the partially quinoidal structure of its dimethylamino-substituted phenyl ring and adjacent ethenylene spacer. Comparing selected bond lengths in [1]PF6 and [6-NO2-1]PF6 listed in Table 2, one can see that no significant changes in the molecular structure are induced upon introduction of an auxiliary EWG. This is in agreement with DFT (B3LYP/TZVP) and MP2 (RI-MP2/TZVP) optimized structures of these compounds. Because structural parameters obtained by these two independent methods are virtually identical, a less expensive DFT method was used for all structure optimizations, if not stated otherwise. Apart from 4-nitro-substituted derivatives, where a NO2 group is twisted from a plane of the attached benzene ring (about 39°) due to the steric repulsion with the heterocyclic N-methyl group, and those comprising nonplanar diphenylamino functionality, all other DFT-optimized minima take up a fully planar arrangement (if hydrogen atoms are not assumed). Electronic Absorption Spectra. The UV-visible absorption characteristics of all benzothiazolium salts under study and


corresponding spectra of selected compounds measured in methanol are presented in Table 3 and Figure 3, respectively. In general, the electronic spectra are dominated by very intense low-energy bands in the visible spectral region with absorption maxima λmax exceeding 500 nm in all cases. These bands are associated with the ICT from the electron-donating dimethylamino or diphenylamino edge substituent to the electronwithdrawing benzothiazolium moiety and are accompanied with a large change of the dipole moment upon photoexcitation (vide infra). The extent of this CT depends crucially on the strength of the donor and acceptor substituents and the length of the π-conjugated bridge and is reflected in the intensity and position of the absorption maximum. Furthermore, less intense absorption bands are observed in the ultraviolet spectral region and correspond to the π-π* excitations (cf. Supporting Information, Table S1). It is worth noting that an exchange of the iodide anion for hexafluorophosphate in selected benzothiazolium salts had practically no influence on the measured UV-visible spectral characteristics. In an orbital picture, the lowest energy excitation (ICT band) corresponds predominantly to the transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), with a small admixture of the HOMO f LUMO+1 and HOMO-1 f LUMO transitions (cf. Figure 4 and Supporting Information, Table S2). The HOMO is mainly constrained to the donor part, whereas the LUMO has its largest atomic orbital coefficients at the atoms of benzothiazolium moiety and adjacent double bond. However, other carbon atoms in a π-conjugated bridge also contribute to both orbitals, leading to their extensive overlap, and thus to relatively large oscillator strengths. The ICT characteristics calculated by means of time-dependent DFT in vacuo and with the inclusion of bulk solvent effects via a PCM are reported in Tables 4 and 5, respectively. In general, ICT energies computed in vacuo for systems with a shorter π-bridge (n ) 1, 2) are overestimated by ca. 0.3-0.5 eV at the B3LYP level and somewhat largely by using the CAM-B3LYP (by ca. 0.4-0.6 eV). The effect of the solvent shifts Emax considerably to lower energies and thus leads to a better agreement with the experimental data. To assess the reliability of TD-DFT calculations, more accurate and expensive resolution-of-identity secondorder approximate coupled-cluster (RI-CC2) calculations have been performed as well (Table 6). In this case, much better

TABLE 2: Comparison of Selected DFT and MP2 Optimized Structural Parameters with the Crystal Structure Data for [1]+ and [6-NO2-1]+ a,b [1]+ parameters N(1)-C(2) N(1)-C(18) S(1)-C(2) C(2)-C(8) C(8)-C(9) C(9)-C(10) C(10)-C(11) C(11)-C(12) C(12)-C(13) C(13)-N(2) N(1)-C(2)-C(8)-C(9) C(8)-C(9)-C(10)-C(11) C(12)-C(13)-N(2)-C(16) a

B3LYP 1.354 1.465 1.759 1.408 1.375 1.418 1.418 1.370 1.425 1.353 179.6 179.7 0.0

MP2 1.352 1.468 1.732 1.419 1.371 1.425 1.415 1.379 1.422 1.357 179.9 179.8 0.1

[6-NO2-1]+ crystal [1]PF6 bond lengths 1.348(4) 1.469(3) 1.725(3) 1.412(4) 1.361(4) 1.429(4) 1.411(8) 1.366(6) 1.417(8) 1.357(4) dihedral angles 177.4(11) 178.5(13) 5.2(25)

Bond lengths are given in Ångstroms. b Crystal data for [1]PF6 taken from ref 12.

B3LYP 1.359 1.466 1.757 1.401 1.380 1.412 1.420 1.367 1.427 1.350 179.9 179.9 0.0

MP2 1.354 1.467 1.737 1.413 1.375 1.419 1.417 1.377 1.423 1.354 179.9 179.9 0.0

crystal [6-NO2-1]PF6 1.3441(17) 1.4712(17) 1.7351(14) 1.4130(18) 1.3674(19) 1.4294(18) 1.4092(19) 1.3735(19) 1.4126(19) 1.3636(17) 177.58(12) 178.92(13) 10.2(2)

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TABLE 3: ICT Absorption Characteristics and HRS Data for Benzothiazolium Saltsa,b system

λmax (nm)

Emax (eV)

εmax (dm3 mol-1 cm-1)

[1]+ [2]+ [3]+ [4]+ [5-NO2-1]+ [6-NO2-1]+ [7-NO2-1]+ [5-NO2-2]+ [6-NO2-2]+ [7-NO2-2]+ [6-CN-1]+ [6-CN-2]+ [1-Ph]+ [2-Ph]+ [6-NO2-1-Ph]+ [6-CN-1-Ph]+

523 (520) 562 (558) 580 (576) (580) 547 562 556 612 632 620 551 618 512 537 551 541

2.37 (2.38) 2.21 (2.22) 2.14 (2.15) (2.14) 2.27 2.21 2.23 2.03 1.96 2.00 2.25 2.01 2.42 2.31 2.25 2.29

62400 (61200) 58200 (54200) 46300 (54400) (44200) 76900 80700 52800 51700 45100 54100 86500 70800 36800 52800 34200 49300

foscc

β800 (× 10-30 esu)

β0 (× 10-30 esu)

1.07 1.24 1.33

415 ( 23 (425 ( 15) 1536 ( 45 (1040 ( 35) 1438 ( 76 (1100 ( 30) (2100 ( 100) 330 ( 13 428 ( 43 303 ( 13 1211 ( 62 740 ( 55 920 ( 40 400 ( 30 1175 ( 85 672 ( 20 1100 ( 85 504 ( 22 1085 ( 90

165 ( 10 (170 ( 6) 758 ( 22 (510 ( 17) 752 ( 40 (570 ( 15) (1100 ( 50) 153 ( 6 211 ( 11 146 ( 6 674 ( 34 416 ( 31 516 ( 23 190 ( 15 657 ( 48 253 ( 8 485 ( 37 238 ( 10 490 ( 40

0.80 1.02 0.76 0.73 0.68 0.82 1.05 1.17 0.62 1.06 0.31 0.61

a All data, except those in parentheses, are given for benzothiazolium iodides in methanol solution. b Data in parentheses are taken from ref 12 (hexafluorophosphate salts in acetonitrile solution). c Oscillator strength obtained from (4.32 × 10-9 M)A, where A is the integrated extinction coefficient.

Figure 3. UV-visible absorption spectra of selected benzothiazolium salts at 298 K in methanol.

accord with experimental Emax values has been achieved, even in vacuo. Because the predicted trends are in line with experimental observations in most cases, the results of quantumchemical calculations are used to discuss some structure-property relationships. Furthermore, we do not restrict ourselves only to the systems synthesized herein but perform calculations also on molecules with longer π-conjugation length (n ) 3, 4) and an additional EWG to generalize the experimental results. Elongation of the π-Bridge. This influence was already studied for the series [1]PF6-[4]PF6 in ref 12, and here, we summarize only briefly the main features. The position of ICT band moves predictably to lower energy with increasing the π-conjugation length, with the difference in Emax of ca. 0.24 eV when passing from [1]+ to [4]+ (cf. Table 3). A decrease of the ICT energy within this series is exponential and approaches a plateau already for n ) 4. Extension of the π-conjugated system leads to red shifting of the ICT maxima also within the series containing an auxiliary EWG or a diphenylamino functionality instead of a dimethylamino group. A similar but less obvious decrease in Emax is observed also for π-π* excitations (cf. Supporting Information, Table S1). Interestingly, the experimentally observed leveling of excitation energies Emax with the length of a polyene π-bridge is faster than that predicted by both TD-DFT and RI-CC2 method. This can be presumably

Figure 4. Relevant molecular orbitals (isosurface (0.03 au) in the benzothiazolium salt [6-NO2-1]+.

attributed to the not perfectly planar structure of dyes with a longer polyenic π-bridge in solution (molecular dynamic effects), trans-cis isomerization in the excited state,40 and/or limited description of CT processes for these systems by the methods employed herein.41



TABLE 4: Calculated ICT Characteristics and Static First Hyperpolarizabilities β0 (in Vacuo) TD-DFT (B3LYP)

TD-DFT (CAM-B3LYP) a

cation

Emax (eV)

fosc

[1]+ [2]+ [3]+ [4]+ [4-NO2-1]+ [5-NO2-1]+ [6-NO2-1]+ [7-NO2-1]+ [4-NO2-2]+ [5-NO2-2]+ [6-NO2-2]+ [7-NO2-2]+ [6-NO2-3]+ [6-NO2-4]+ [6-CN-1]+ [6-CN-2]+ [6-CN-3]+ [6-CN-4]+ [1-Ph]+ [2-Ph]+

2.75 2.50 2.25 2.07 2.66 2.75 2.69 2.63 2.41 2.47 2.43 2.39 2.22 2.05 2.71 2.45 2.23 2.06 2.39 2.11

1.48 1.97 2.45 2.89 1.35 1.61 1.56 1.09 1.90 2.13 2.08 1.61 2.59 3.06 1.63 2.14 2.63 3.11 1.38 1.75

a

β0 ∆µ01 (D) (× 10-30 esu) Emax (eV) 5.9 8.3 9.2 11.2 11.5 6.2 8.7 18.4 11.3 7.2 9.1 17.5 9.0 9.6 5.4 6.6 7.1 7.4 15.5 18.2

129 323 606 1124 256 147 215 342 472 313 404 632 653 1059 137 298 517 819 480 1043

2.88 2.57 2.32 2.12 2.82 2.86 2.83 2.85 2.52 2.55 2.53 2.55 2.28 2.09 2.83 2.53 2.28 2.09 2.66 2.35

B3LYP

CAM-B3LYP

MP2

a

fosc 1.56 2.09 2.60 3.09 1.61 1.67 1.68 1.63 2.16 2.22 2.22 2.16 2.73 3.23 1.70 2.24 2.74 3.23 1.74 2.24

β0 β0 β0 β0 ∆µ01 (D) (× 10-30 esu) (× 10-30 esu) (× 10-30 esu) (× 10-30 esu) 5.7 6.7 7.3 8.0 5.9 5.5 5.7 6.2 6.1 5.8 5.8 6.5 5.5 5.4 5.2 5.6 5.4 5.1 9.6 11.2

114 257 470 796 129 121 130 135 253 241 245 264 391 590 120 240 384 558 272 592

75 140 235 393 95 90 111 108 161 147 180 182 265 396 74 126 193 277 239 460

107 217 390 676 114 112 122 121 204 198 210 219 322 488 108 194 309 456 238 493

197 453 900 1685 188 186 192 199 364 368 365 403 588 938 187 373 638 980 386 892

Estimated from the two-state model.

TABLE 5: Calculated ICT Characteristics and Static First Hyperpolarizabilities β0 (in CH3OH Employing the PCM Solvation Model) TD-DFT (B3LYP)


cation

Emax (eV)

fosc

[1]+ [2]+ [3]+ [4]+ [4-NO2-1]+ [5-NO2-1]+ [6-NO2-1]+ [7-NO2-1]+ [4-NO2-2]+ [5-NO2-2]+ [6-NO2-2]+ [7-NO2-2]+ [6-NO2-3]+ [6-NO2-4]+ [6-CN-1]+ [6-CN-2]+ [6-CN-3]+ [6-CN-4]+ [1-Ph]+ [2-Ph]+

2.63 2.33 2.09 1.91 2.40 2.55 2.44 2.30 2.18 2.29 2.20 2.11 2.00 1.83 2.57 2.28 2.06 1.88 2.42 2.12

1.61 2.10 2.57 3.03 1.01 1.06 1.41 0.57 1.63 2.02 1.98 0.99 2.54 3.04 1.75 2.25 2.73 3.21 1.66 2.08

a

β0 ∆µ01 (D) (× 10-30 esu) Emax (eV) 8.1 9.3 10.0 11.1 15.1 10.9 15.6 21.9 13.2 9.9 13.6 18.3 12.0 11.4 8.9 9.0 8.8 8.4 12.5 14.6

222 476 863 1474 344 216 462 317 647 516 779 593 1175 1730 284 523 849 1249 455 980

2.77 2.45 2.19 2.00 2.67 2.74 2.69 2.71 2.36 2.41 2.37 2.39 2.11 1.92 2.71 2.38 2.13 1.93 2.68 2.36

B3LYP

CAM-B3LYP

MP2

a

fosc 1.67 2.18 2.66 3.14 1.71 1.79 1.80 1.73 2.24 2.31 2.32 2.25 2.80 3.28 1.81 2.32 2.80 3.28 1.84 2.33

β0 β0 β0 β0 ∆µ01 (D) (× 10-30 esu) (× 10-30 esu) (× 10-30 esu) (× 10-30 esu) 5.9 6.4 6.6 7.0 6.2 5.8 6.5 6.6 5.9 5.8 6.1 6.4 5.5 5.2 5.9 5.8 5.5 5.0 7.4 8.0

142 293 515 842 171 155 186 177 309 296 330 322 503 740 167 306 491 706 220 435

300 658 1279 2370 413 397 561 459 807 745 1018 883 1670 2687 377 718 1231 1961 602 1377

376 898 1877 3618 432 421 490 444 929 904 1005 955 1842 3248 436 933 1782 3111 549 1276

601 1572 3560 7182 679 661 732 670 1592 1573 1658 1616 3148 5580 689 1622 3239 5667 832 2024


Effect of an Auxiliary EWG. An additional EWG attached to the benzothiazolium ring is supposed to enhance its electronaccepting capability. This is manifested in bathochromic shifts, which are even much more pronounced than those observed upon elongation of the π-bridge. Specifically, introducing a nitro group into the C-6 position of [1]I causes a red shift of 0.16 eV, and a larger bathochromic shift of 0.25 eV is observed going from [2]I to [6-NO2-2]I (cf. Table 3). The red shifting of the ICT band can be mainly addressed to a stabilization of the LUMO orbital by an auxiliary EWG (cf. Supporting Information, Table S2) and depends on the strength and the position of EWG. Comparing the Emax values of 6-nitro- and 6-cyanosubstituted analogues, a more distinct effect of a stronger acceptor (NO2) can be noticed (cf. Table 3).

Position of an Auxiliary EWG. The most effective CT between an electron-donating functionality and the benzothiazolium moiety, resulting in the lowest energy gap Emax, is experimentally observed for systems where an auxiliary EWG occupies the C-6 position. Although the variation of Emax values among the positional isomers is modest, relative trends are in line with the excitation energies calculated by the CAM-B3LYP and RI-CC2 method. Specifically, Emax decreases in the following order: 5-NO2 > 7-NO2 > 6-NO2 (cf. Table 3 and Tables 4-6). It is worth noting that the B3LYP method somehow overestimates an electrostatic interaction between the partially negatively charged extremities of the NO2 group and the partially positively charged sulfur atom in 7-nitro-substituted derivatives, which results in their lowest predicted ICT energy and oscillator

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TABLE 6: RI-CC2 Calculated ICT Characteristics and Estimated First Hyperpolarizabilities for Selected Chromophores cation +

[1] [2]+ [3]+ [4]+ [4-NO2-1]+ [5-NO2-1]+ [6-NO2-1]+ [7-NO2-1]+ [4-NO2-2]+ [5-NO2-2]+ [6-NO2-2]+ [7-NO2-2]+ [6-NO2-3]+ [6-NO2-4]+ [6-CN-1]+ [6-CN-2]+ [5]+ [6]+ [7]+ [8]+ [2-CN-5]+ [2-CN-6]+ [2-CN-7]+ [2-CN-8]+

Emax (eV)

fosc

∆µ01 (D)

β0 (× 10-30 esu)

2.45 2.14 1.89 1.71 2.40 2.43 2.40 2.42 2.10 2.12 2.11 2.12 1.87 1.68 2.41 2.10 2.05 1.87 1.72 1.60 1.34 1.18 1.06 0.96

1.66 2.22 2.74 3.26 1.73 1.77 1.78 1.73 2.32 2.36 2.37 2.32 2.91 3.45 1.80 2.38 0.82 1.11 1.43 1.75 0.94 1.25 1.59 1.96

5.6 7.0 8.2 9.9 4.2 4.0 4.0 4.6 4.5 4.4 4.2 5.2 4.2 4.6 4.1 4.6 24.4 27.8 30.9 34.0 22.4 23.7 24.2 24.2

195 492 1024 1994 165 154 160 172 354 338 331 389 581 1038 181 360 718 1458 2682 4488 2690 5534 10063 16664

π-conjugated bridge. The underestimated ICT energies predicted by quantum-chemical calculations could be then ascribed to the following points: (a) an overestimated CT toward the benzothiazolium unit and thus reduced delocalization in the diarylamino fragment (that in turn should have an effect of suppressing the main flow of molecular CT) and (b) overestimated planarity in diphenylamino end-capped systems optimized by the DFT method. The structures of [1]+, [2]+, [1-Ph]+, and [2-Ph]+ were therefore also optimized at the MP2 level of theory and compared with those obtained using the B3LYP. While geometries of [1]+ and [2]+ remained essentially unchanged, the MP2 structures of diphenylamino-containing systems exhibited a sizable deviation from the planar π-bridge configuration, with inter-ring torsion angles between the benzothiazolium fragment and the 4-donor substituted phenyl ring of 26 and 18° for [1-Ph]+ and [2-Ph]+, respectively. Using the MP2 geometries, a somewhat smaller difference between computed ICT energies of dimethylamino and diphenylamino-substituted derivatives has been achieved, although the hypsochromic shift was not reproduced exactly (cf. Supporting Information, Table S4). Emission Spectra. Emission spectra measured in methanol exhibit negligible fluorescence with quantum yields not exceeding the value of 0.005. This can be ascribed to an enhanced ICT interaction, which often results in fluorescence quenching due to the enhanced probability of nonfluorescent twisted ICT state population in polar solvents. Indeed, this may be of practical importance, particularly when nonradiative relaxation is required. The evidence of the relaxed twisted ICT state in the commercially available benzothiazolium-derived laser dye, which is structurally related to [2]I, was recently confirmed by means of femtosecond time-resolved fluorescence measurements.43 Solvatochromism. Solvatochromic studies have been carried out in a range of common organic solvents, and the results are summarized in Table 7. All benzothiazolium salts exhibit a negative solvatochromism, that is, a blue shift of the absorption maxima with increasing solvent polarity, albeit the shifts are often relatively small. In a traditional picture used to explain the solvatochromic behavior of dipolar systems through different solvation of the ground and Franck-Condon excited state, the negative solvatochromism results from better stabilization of the ground state as compared to the excited state by polar solvents. This would in turn imply for systems studied here a larger dipole moment in the ground state than in the CT excited state. However, this traditional and simplified approach, in which one only considers the equilibrium solvation of the ground and excited states, often fails for charged push-pull dyes.44 For instance, it cannot explain opposite shifts in absorption and fluorescence spectra of most hemicyanine dyes, including those closely related to the compounds studied in this work.45 Therefore, the conclusion about the polarity of the ground and

strength. The superiority of the C-6 position can be rationalized on the basis of the electronic structure of the parent 2,3dimethylbenzothiazolium cation. Here, the largest LUMO coefficients are almost equally located at atomic pz orbitals of C-6 and C-4, while contributions of C-5 and C-7 are approximately one-third of them (cf. Supporting Information, Table S3). Hence, better stabilization of the LUMO, and thus greater lowering of Emax, is anticipated for positions C-4 and C-6. Replacement of NMe2 with NPh2. The replacement of a dimethylamino group with a diphenylamino one in [1]I leads to the hypsochromic shift of 0.05 eV and is even more pronounced for the couple [2]I/[2-Ph]I (0.10 eV). Surprisingly, this is in contrast with both calculated excitation energies (cf. Tables 4-5) and usually observed bathochromic shifts in neutral systems.42 The unexpected behavior, based on which the diphenylamino functionality seems to be a “less efficient” donor than dimethylamino group in spite of its π-excessive nature, can be rationalized as follows: While substitution of an alkyl with a phenyl increases the electron density at nitrogen by the positive inductive effect (+I), at the same time this substitution decreases the electron density at nitrogen by resonance delocalization (-M effect). The net electron-donating effect depends on the ability of the rest of the molecule to attract “loosely fastened” π-electrons from the diphenylamino fragment and, in principle, can be altered by the nature and planarity of the

TABLE 7: ICT Data for Pertinent Benzothiazolium Salts in Various Common Organic Solvents λmax(ICT) (nm) salt

dioxane (εr ) 2.2)

CHBr3 (εr ) 4.4)

CHCl3 (εr ) 4.8)

THF (εr ) 7.4)

CH2Cl2 (εr ) 8.9)

acetone (εr ) 20.7)

CH3OH (εr ) 32.6)

CH3CN (εr ) 37.5)

[1]I [2]I [6-NO2-1]I [6-NO2-2]I [6-CN-1]I [6-CN-2]I [1-Ph]I [2-Ph]I

543 578 576 671 565 650 525 558

560 622 604 694 586 676 563 612

550 612 596 680 582 664 554 598

541 572 572 642 558 622 522 543

552 628 591 685 580 673 554 608

522 557 562 627 551 614 510 532

520 562 562 628 551 618 508 537

520 557 563 630 551 612 507 529



maximum. The static hyperpolarizability β0 is independent of the used fundamental wavelength and in the two-state model is related to simple linear spectroscopic parameters (vide supra):

β0 )

Figure 5. Dependence of the absorption maximum for the ICT bands of pertinent benzothiazolium salts on dielectric constant of the solvent.

excited state on the basis of solvatochromic studies is no longer obvious. Instead, a more complex picture has to be taken into account. Here, the solvent configuration, which was in equilibrium with the charge distribution of the ground state molecule, is out of equilibrium with the new charge distribution of the Franck-Condon excited state. It is worth noting that a moderate negative solvatochromism in these systems is rather well reproduced by time-dependent DFT calculations using the nonequilibrium PCM solvation model, even though the computed dipole moment of the ground state is substantially smaller than that of the excited state (cf. Supporting Information, Table S5).46 The plots of experimental Emax values as a function of the solvent polarity, represented by a dielectric constant εr, for [1]I, [2]I, and their 6-nitro-substituted analogues are depicted in Figure 5. Hence, it is obvious that the bathochromic shifts observed with decreasing solvent polarity are somewhat prominent in halogenated solvents. This is particularly evident by comparing the ICT bands in dichloromethane and tetrahydrofuran. Although the εr value of THF (εr ) 7.4) is very close to that of CH2Cl2 (εr ) 8.9), positions of ICT maxima differ more than 10 nm and appear even at longer wavelengths for more polar dichloromethane. Assuming these two solvents, the most pronounced effect (up to 56 nm) is observed for systems with longer π-conjugated bridge ([2]I and derivatives thereof). The anomalous behavior hints at an increased ICT character and/or some specific interactions presented in halogenated solvents. Similar trends were observed also for some pyridinium salts.47 However, this phenomenon was not satisfactorily clarified to date, and a more detailed study is beyond the scope of the present paper. NLO Properties. The β800 values for all push-pull benzothiazolium salts in methanol solution have been determined using femtosecond HRS with an 800 nm laser. To extract the resonance-enhancement effect, the static β0 values were derived on the basis of the Oudar’s two-state model48 (eq 1):

|(

β0 ) βλ 1 -

λmax2 λ2

)(

1-

4λmax2 λ2

)|

(3)

where βλ is the dynamic hyperpolarizability measured at the wavelength of the incoming laser beam (in this case λ ) 800 nm) and λmax stands for the wavelength of the ICT absorption

3∆µ01(µ01)2 3∆µ01fosc ∝ 2 (Emax) (Emax)3

(4)

Although this model is too simplified and may fail in certain cases, it is widely used for rationalization of observed β0 values in terms of one-photon excitation characteristics. This allows not only an insight into the origin of the hyperpolarizabilities on a more fundamental level but can be also helpful in a systematic improvement of β0 values within the given series. The results of DFT calculations on the static molecular quadratic hyperpolarizabilities are summarized in Tables 4 and 5. Aware of the serious drawback of the DFT in hyperpolarizability evaluation, calculations also have been performed at the MP2 level of theory. The latter method has been shown to provide reliable β0 values for related benzazolo-oxazolidines systems, which could act as NLO switches.49,50 Comparing the finite-field results with the β0 values derived from the two-state model (eq 4), it is evident that the relative trends obtained by these two approaches are parallel. That supports the appropriateness of applicability of the two-state model for these compounds. Thus, the relative error that would be introduced taking this simplified model is conjectured to be approximately identical for all chromophores considered here. Elongation of the π-Bridge. As shown previously, increasing the length of a polyene chain within the series [1]+-[4]+ is accompanied by the decrease in Emax, along with increases in fosc and ∆µ01, which leads to the enhanced NLO response.12 In keeping with expectations, a similar behavior also has been found for benzothiazolium-based chromophores, where a conjugated system was extended via a heteroaryl moiety (furan, thiophene, or N-methylpyrrole).15 However, a steady increase in β0 values with the length of π-bridge predicted by quantum chemical calculations has not been experimentally confirmed. The discrepancy could be attributed to the same reasons as mentioned previously concerning the electronic absorption spectra. Effect of an Auxiliary EWG. In contrast to ICT energies, the static hyperpolarizabilities β0 are less affected by the presence of an auxiliary EWG than by the length of the π-conjugated bridge. Although an EWG substitution pattern considerably lowers Emax, and thus could be a driving force to the enhanced NLO response, this modification of the structure yields reduced β0 values in most cases. The only exceptions are [6-CN-1]I, [6-NO2-1]I, and [6-CN-1-Ph]I, where a modest increase of the first hyperpolarizability as compared to the parent compounds can be noticed. Indeed, these results indicate that an enhanced NLO response cannot be necessarily expected on increased λmax values within the series. On the contrary, a stronger acceptor is introduced to [2]+, and a smaller β0 is observed ([6-NO2-2]I vs [6-CN-2]I). This finding can be satisfactorily rationalized by the results of quantum chemical calculations and experimentally determined linear optical properties. Comparing ICT characteristics within the series [1]+-[4]+ and those for their 6-nitro-substituted derivatives in vacuo, one can notice a smaller degree of CT, represented by a change in dipole moments between the ground state and the excited state ∆µ01, in the latter series (cf. Tables 4 and 6). The larger the polyene chain extension, the more pronounced is the difference between ∆µ01 values of benzothiazolium salts [1]+-[4]+ and

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Figure 6. Calculated ICT characteristics and first hyperpolarizabilities (derived from the two-state model) of NLO-phores [1]+-[4]+ in comparison with their 6-nitro-substituted analogues (RI-CC2/TZVP).

Figure 7. Benzenoid and quinoid resonance forms of 6-acceptor-substituted push-pull benzothiazolium salts.

TABLE 8: NPA Charge Analysis in Pertinent Benzothiazolium Salts [1]+

[6-NO2-1]+

fragment

q(S0)

q(S1)

∆q

NMe2 Ph π-spacer benzothiazolium EWG

0.112 0.194 0.063 0.631

0.295 0.365 -0.059 0.398

NMe2 Ph π-spacer benzothiazolium EWG

0.085 0.181 0.074 0.659

0.206 0.377 -0.026 0.444

q(S0)

q(S1)

[5]+

[2-CN-5]+

q(S0)

q(S1)

∆q

q(S0)

q(S1)

∆q

0.183 0.172 -0.122 -0.233

B3LYP/6-31G(d)/PCM(CH3OH) 0.132 0.321 0.189 0.045 0.228 0.431 0.203 0.073 0.076 0.031 -0.045 0.037 0.823 0.625 -0.198 0.845 -0.259 -0.408 -0.149

0.294 0.443 0.140 0.123

0.249 0.370 0.103 -0.722

0.075 0.124 0.065 0.747 -0.011

0.319 0.487 0.182 0.182 -0.171

0.245 0.363 0.117 -0.565 -0.159

0.120 0.195 -0.101 -0.215

CAM-B3LYP/6-31G(d)/PCM(CH3OH) 0.103 0.219 0.116 0.020 0.213 0.399 0.186 0.061 0.086 0.001 -0.084 0.038 0.849 0.652 -0.197 0.881 -0.251 -0.272 -0.021

0.151 0.318 0.077 0.454

0.131 0.257 0.039 -0.427

0.036 0.086 0.054 0.812 0.012

0.171 0.394 0.205 0.337 -0.108

0.136 0.308 0.151 -0.475 -0.120

their corresponding 6-nitro-substituted analogues. The dependence of ICT characteristics and resultant static first hyperpolarizabilities derived from the two-state model with increasing the π-bridge length (n) is depicted in Figure 6. Hence, it is obvious that the decrease in ∆µ01 is more dramatic than both a decrease in ICT energies and an increase in oscillator strengths and is thus responsible for reduced β0 values in vacuo. This behavior can be intuitively understood as follows: The electronic ground-state structure of push-pull benzothiazolium dyes studied here can be viewed as a compromise between the benzenoid and the quinoid resonance forms, where a positive charge is delocalized to some extent (cf. Figure 7). From the CT point of view, the resonance forms A and B represent the electronic structure of the ICT state. Because the electronwithdrawing effect of the benzothiazolium fragment is amplified

∆q

by addition of an auxiliary acceptor group, increased groundstate CT from the donor part toward the benzothiazolium moiety with an additional EWG should occur. This assumption is supported by NPA analysis (cf. Table 8) and 15N NMR measurements (cf. Table 9), which revealed both more shielded benzothiazolium nitrogen and more deshielded dimethylamino nitrogen in nitro end-capped derivatives. Furthermore, assuming that an auxiliary acceptor does not directly participate in the π-delocalization, thus taking only the resonance form A into account, smaller change of the dipole moment upon photoexcitation is anticipated in derivatives with an additional EWG. The situation can be, however, more complicated, particularly in polar media, which would tend to stabilize the excited state described by the resonance form B. Shifting the mesomeric equilibrium toward this form can result in somewhat larger



TABLE 9: Experimentally Determined 15N NMR Chemical Shifts in Pertinent Benzothiazolium Saltsa

[1]I [6-NO2-1]I [2]I [6-NO2-2]I [3-Me-2-Styryl-Btz]Ic

δ(15N+-CH3) (ppm)

δ(15NMe2) (ppm)

∆δ (ppm)b

-207.7 -210.4 -206.7 -209.4 -196.4

-311.2 -304.6 -318.6 -313.8

103.5 94.2 111.9 104.4

a Measured in DMSO-d6 solution at 298 K (cf. Experimental Section). b ∆δ ) δ(15NMe2) - δ(15N+-CH3). c 3-Methyl-2-styrylbenzothiazolium iodide (analogue of [1]I, where NMe2 group is replaced by H).

change of the dipole moment ∆µ01 and a decrease of the oscillator strength (due to a smaller overlap between electron and hole wave functions) in comparison with those of [1]+-[4]+. This scenario is predicted by the B3LYP calculations simulating bulk solvent effects via the PCM model (cf. Table 8 and Table 5). From Table 3, one can notice that the experimentally determined oscillator strengths in EWG end-capped systems are somewhat smaller than those of [1]I and [2]I, and the difference between them becomes larger with increasing n. Specifically, going from [6-NO2-1]I to [6-NO2-2]I, the oscillator strength decreases by 35%. This can be addressed to a stabilization of the resonance form B by a polar methanol and by moving the positive centers apart from each other when going from [1]+ to [2]+. In contrast to the gas phase, the reduced β0 values in methanol solution could be then ascribed mainly to the reduced fosc values. These results indeed indicate an important role of the solvent effects and a limitation of DFT methods to describe complex CT processes in these systems (cf. Figure 7). A similar trend associated with EWG substitution also has been observed very recently for related systems comprising a cationic indoleninium acceptor.50 However, in that case, the unexpected behavior could be attributed to a resonance enhancement (two-photon scattering frequency accidentally lay within the ICT band) as noted by the authors. Interestingly, the presence of an auxiliary EWG in neutral push-pull benzothiazole derivatives has been shown to result in enhanced NLO response.13 This indeed corroborates that the strength of the electron donor and electron acceptor has to be optimized for the specific system to achieve the maximum hyperpolarizability. Position of the EWG. The experimentally observed variation of β0 values among the positional isomers of nitro-substituted benzothiazolium salts derived from [1]I and [2]I is relatively modest, and from the perspective of nonlinear optics, no general conclusion about the efficiency of given positions can be drawn. For instance, the C-6 position is the most efficient one in benzothiazolium salts derived from [1]+, but this position seems to be the worst one in chromophores with an elongated π-conjugated bridge (n ) 2). While the former observation is in line with β0 values computed using a solvation model, the latter trend is better reproduced by calculations in vacuo. However, because the calculated β0 values of positional isomers are in a relatively narrow range of numerical values (except those obtained using the B3LYP method), the ordering of the structures according to them must not be necessarily consistent with the sequence determined experimentally. The observation that the decrease of β0 values in systems with n ) 2, where an additional EWG occupies C-5 or C-7 position, is not as dramatic as in the case of 6-NO2 substitution, can be qualitatively explained by the fact that the former positions are not able to couple with the benzothiazolium cation in mesomeric notation.

Figure 8. Schematic structure of benzothiazolium salts with a reverse polarity and an additional EWG.

This results in somewhat larger oscillator strengths for 5-NO2and 7-NO2-substituted derivatives and consequently in larger hyperpolarizabilities as compared to that of [6-NO2-2]I (cf. Table 3). Replacement of NMe2 with NPh2. The chromophores [1-Ph]I and [2-Ph]I show different trends in experimentally determined β0 values with respect to those of parent [1]I and [2]I, respectively. While in the former case ([1]I/[1-Ph]I) the substitution results in a sizable enhancement of the first hyperpolarizability (by ca. 1.5 times), a considerable lowered NLO response of [2-Ph]I as compared to that of [2]I can be noticed. Again, this indicates a controversial nature of the diphenylamino unit as a more efficient NLO donor in comparison with the dimethylamino group (vide supra). The reduced β0 value in [2-Ph]I could be attributed to the pronounced hypsochromic shift and smaller oscillator strength, both arising presumably from a modest deviation of the π-bridge from planarity. Comparing computed hyperpolarizabilities for the MP2-optimized geometries of [2-Ph]+ and [2]+, smaller disparity is predicted in NLO response between these two systems, although [2-Ph]+ remains superior (cf. Supporting Information, Table S4). Nevertheless, the substitution of a dimethylamino group for a diphenylamino one affords enhanced NLO response for all dyes with a shorter polyenic π-bridge ([1]I and derivatives thereof). ReWerse Polarity. A question that arises in the context of this study is whether benzothiazolium-derived systems cannot really benefit from the presence of an auxiliary EWG. Looking again at the LUMO coefficients in the parent 3-methylbenzothiazolium cation, one can notice the largest contribution coming from pz atomic orbitals located at the C-2 position (cf. Supporting Information, Table S3). Thus, positioning of an additional EWG to this site could lead to even greater stabilization of the LUMO and most likely to a more pronounced decrease of Emax in comparison with derivatives, where an auxiliary acceptor occupies the heterocyclic benzene ring. This indeed requires a change of the commonly used polarity (relative position of donor and acceptor moieties) in push-pull benzothiazolium salts. To address the effect of reverse polarity, a series of benzothiazolium-containing chromophores with and without 2-acceptor substituent are considered herein (cf. Figure 8; we assume only the C-6 position for a donor in further studies). To the best of our knowledge, the setup, where an electrondonating functionality is linked via π-conjugated spacer to a benzene ring of the benzothiazole/-ium moiety, is unusual, and only few neutral systems containing the aforementioned motif have been synthesized very recently.14,19 The results of quantum-

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TABLE 10: Calculated ICT Characteristics and Static First Hyperpolarizabilities β0 (in Vacuo) TD-DFT (B3LYP)


cation

Emax (eV)

fosc

[5]+ [6]+ [7]+ [8]+ [5-CN]+ [6-CN]+ [7-CN]+ [8-CN]+

1.76 1.61 1.49 1.40 1.55 1.46 1.38 1.31

0.43 0.55 0.69 0.83 0.64 0.82 1.01 1.20

a

β0 ∆µ01 (D) (× 10-30 esu) Emax (eV) 31.0 34.8 38.6 42.6 24.7 27.1 29.8 32.8

749 1415 2496 3997 1297 2219 3531 5330

2.51 2.29 2.12 1.94 1.91 1.75 1.62 1.51

B3LYP

CAM-B3LYP

MP2

a

fosc 0.80 1.12 1.46 1.71 0.84 1.11 1.40 1.72

β0 β0 β0 β0 ∆µ01 (D) (× 10-30 esu) (× 10-30 esu) (× 10-30 esu) (× 10-30 esu) 29.5 32.2 34.5 34.4 30.9 33.5 35.7 37.7

453 929 1636 2465 1154 2135 3639 5773

473 811 1283 1916 556 815 1246 1870

358 692 1205 1869 780 1351 2163 3275

393 770 1367 2230 1167 2310 4186 7089


TABLE 11: Calculated ICT Characteristics and Static First Hyperpolarizabilities β0 (in CH3OH Employing PCM Solvation Model) TD-DFT (B3LYP)


cation

Emax (eV)

fosc

[5]+ [6]+ [7]+ [8]+ [5-CN]+ [6-CN]+ [7-CN]+ [8-CN]+

2.23 2.07 1.93 1.82 1.57 1.45 1.35 1.27

0.60 0.85 1.14 1.47 0.73 0.93 1.16 1.40

a

β0 ∆µ01 (D) (× 10-30 esu) Emax (eV) 31.3 33.1 34.3 35.1 33.9 36.8 39.5 42.0

518 975 1675 2649 1987 3460 5677 8774

3.09 2.85 2.64 2.43 2.22 2.05 1.92 1.81

B3LYP

CAM-B3LYP

MP2

a

fosc 1.34 1.90 2.44 2.73 0.98 1.28 1.61 1.96

β0 β0 β0 β0 ∆µ01 (D) (× 10-30 esu) (× 10-30 esu) (× 10-30 esu) (× 10-30 esu) 16.7 15.3 14.1 13.3 25.4 26.1 26.5 26.7

234 384 574 776 705 1187 1871 2735

812 1544 2668 4159 3143 5676 9562 15231

383 699 1153 1717 1478 2654 4456 7040

323 580 956 1466 1242 2211 3668 5711


chemical calculations for these systems are presented in Tables 6, 10, and 11. Starting with results computed in vacuo for derivatives without an additional EWG ([1]+-[4]+ vs [5]+-[8]+), one can notice that all methods used in this work show almost identical behavior in predicted β0 values. Namely, structures with the reverse polarity [5]+-[8]+ are predicted to be somewhat more efficient NLO chromophores than their corresponding analogues [1]-[4]+ with the same π-conjugation length. Calculations with the inclusion of bulk solvent effects (assuming polar methanol as a solvent) revealed an opposite trend, with larger static hyperpolarizabilities β0 within the series [1]+-[4]+. However, these results should be taken with caution, since hyperpolarizabilities computed by using the PCM approach for [1]+-[4]+ and their derivatives are in all cases considerably overestimated with respect to those determined experimentally and calculated in vacuo as well. However, and more importantly, irrespective of whether calculations were performed in the gas phase or by using the PCM solvation model, it is evident that an additional acceptor occupying the C-2 position brings considerable improvement in β0 values. Here, the positive effect of an auxiliary EWG is largely driven by a favorable decrease of the ICT energy, which is more pronounced than that within a series of push-pull dyes with the commonly used polarity, together with a relatively small change in the product ∆µ01 fosc. A substantially larger change of the dipole moment ∆µ01 predicted by all methods in [5]+-[8]+ and derivatives thereof can be intuitively rationalized by (a) a larger distance between an electron-donating functionality and the quaternized heterocyclic nitrogen in comparison with that distance in [1]+-[4]+ and (b) an experience that a benzene ring placed into π-conjugation pathway tends to suppress the ground-state CT (cf. Table 8). The answer to the title question is thus positive, even though a change in the relative position of the donor and auxiliary acceptor groups on the benzothiazolium moiety is required.

Conclusions We have synthesized a series of push-pull benzothiazolium salts bearing an auxiliary electron-withdrawing substituent (NO2 or CN) at various positions of the heterocyclic benzene ring. These systems were investigated for their linear and quadratic NLO properties both experimentally and theoretically. In general, the introduction of EWG into the benzene ring of the benzothiazolium moiety leads to a pronounced red shift of the absorption maxima and depends on the strength and the position of EWG, with the most effective CT in 6-nitro-substituted derivatives. However, the impact of the EWG substitution pattern on NLO properties is not straightforward. From the β0 values determined via HRS measurements in methanol, it is evident that an additional EWG bound to the heterocyclic benzene ring unexpectedly reduces the β0 values in most cases. The only exceptions are benzothiazolium dyes with a short π-conjugated bridge (n ) 1) and EWG occupying the C-6 position. The observed negative impact on the quadratic NLO activity can be primarily attributed to reduced oscillator strengths in polar media, arising from a more complex nature of the ICT state. Our studies also revealed that the substitution of a dimethylamino group for a diphenylamino one results in enhanced NLO response for all dyes with a short spacer (n ) 1), whereas this substitution does not seem to be favorable for systems with longer polyenic π-bridge. Nevertheless, all benzothiazolium chromophores prepared herein display large static quadratic hyperpolarizabilities β0 of similar magnitude or in most cases even larger than that of the DAST, making these systems attractive for practical NLO applications. Furthermore, quantum-chemical calculations show that not only the linking of a strong donor group and/or electron-rich heteroaryl moieties to the C-2 position but also, and even more so, a change of the commonly used polarity in push-pull benzothiazolium salts together with a 2-acceptor substitution

Molecular Engineering of Benzothiazolium-Containing NLO-Phores may result in larger first hyperpolarizabilities. Synthesis of this new class of benzothiazolium salts is thus highly desirable. The results obtained in the present study could provide a useful guideline to the design of novel NLO efficient benzothiazoliumcontaining dyes, and it is hoped that this knowledge also may be applied to related benzoxazolium and benzimidazolium salts. Acknowledgment. This work has been supported by the Slovak Grant Agencies APVV (No. 0259-07), VEGA (No. 1/4470/07), and COMCHEM (Contract No. II/1/2007). E.F. and K.C. acknowledge the Fund for Scientific Research Flanders (FWO-V, G.0312.08) and the University of Leuven (GOA/2006/ 03) for financial support. P.H. is indebted to the Alexander von Humboldt Foundation for a research fellowship. Supporting Information Available: Synthesis of 2-methylx-nitrobenzothiazoles (x ) 4, 5, 7), complete characterization (1H, 13C NMR, elemental analysis) of push-pull benzothiazolium salts, UV-visible absorption data, more detailed quantumchemical analysis, and CIF file with the crystallographic parameters and structural data for [6-NO2-1]PF6 · (CH3)2CO. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Zyss, J., Ed. Molecular Nonlinear Optics: Materials, Physics and DeVices; Academic Press: Boston, 1994. (2) Papadopoulos, M. G., Leszczynski, J., Sadlej, A. J., Eds. Nonlinear Optical Properties of Matter: From Molecules to Condensed Phases; Springer: Dordrecht, the Netherlands, 2006. (3) Reeve, J. E.; Anderson, H. L.; Clays, K. Phys. Chem. Chem. Phys. 2010, 12, 13484–13498. (4) Marder, S. R. Chem. Commun. 2006, 131–134. (5) Nalwa, H. S., Miyata, S., Eds. Nonlinear Optics of Organic Molecules and Polymers; CRC Press: Boca Raton, FL, 1997. Bosshard, C., Sutter, K., Preˆtre, P., Hulliger, J., Flo¨rsheimer, M., Kaatz, P., Gu¨nter, P., Eds. Organic Nonlinear Optical Materials: AdVances in Nonlinear Optics; Gordonand Breach Publishers: Amsterdam, the Netherlands, 1995. (6) Kanis, D. R.; Ratner, M. A.; Marks, T. J. Chem. ReV. 1994, 94, 195–242. (7) Dirk, C. W.; Katz, H. E.; Schilling, M. L.; King, L. A. Chem. Mater. 1990, 2, 700–705. Moylan, C. R.; Miller, R. D.; Twieg, R. J.; Betterton, K. M.; Lee, V. Y.; Matray, T. J.; Nguyen, C. Chem. Mater. 1993, 5, 1499– 1508. Moylan, C. R.; Twieg, R. J.; Lee, V. Y.; Swanson, S. A.; Betterton, K. M.; Miller, R. D. J. Am. Chem. Soc. 1993, 115, 12599–12600. Miller, R. D.; Lee, V. Y.; Moylan, C. R. Chem. Mater. 1994, 6, 1023–1032. Moylan, C. R. J. Phys. Chem. 1994, 98, 13513–13516. Albert, I. D. L.; Morley, J. O.; Pugh, D. J. Phys. Chem. 1995, 99, 8024–8032. Varanasi, P. R.; Jen, A. K. Y.; Chandrasekhar, J.; Namboothiri, I. N. N.; Rathna, A. J. Am. Chem. Soc. 1996, 118, 12443–12448. Brasselet, S.; Cherioux, F.; Audebert, P.; Zyss, J. Chem. Mater. 1999, 11, 1915–1920. Jen, A. K. Y.; Liu, Y. Q.; Zheng, L. X.; Liu, S.; Drost, K. J.; Zhang, Y.; Dalton, L. R. AdV. Mater. 1999, 11, 452–455. Wang, Y. K.; Shu, C. F.; Breitung, E. M.; McMahon, R. J. J. Mater. Chem. 1999, 9, 1449–1452. Breitung, E. M.; Shu, C. F.; McMahon, R. J. J. Am. Chem. Soc. 2000, 122, 1154–1160. (8) Abbotto, A.; Beverina, L.; Bozio, R.; Bradamante, S.; Ferrante, C.; Pagani, G. A.; Signorini, R. AdV. Mater. 2000, 12, 1963–1967. Coe, B. J. Acc. Chem. Res. 2006, 39, 383–393. Ruiz, B.; Yang, Z.; Gramlich, V.; Jazbinsek, M.; Günter, P. J. Mater. Chem. 2006, 16, 2839–2842. Guieu, V.; Payrastre, C.; Madaule, Y.; Garcia-Alonso, S.; Lacroix, P. G.; Nakatani, K. Chem. Mater. 2006, 18, 3674–3681. (9) Wong, M. S.; Bosshard, C.; Günter, P. AdV. Mater. 1997, 9, 837– 842. (10) Taniuchi, T.; Ikeda, S.; Okada, S.; Nakanishi, H. Jpn. J. Appl. Phys. 2 2005, 44, L652–L654. Schneider, A.; Neis, M.; Stillhart, M.; Ruiz, B.; Khan, R. U. A.; Günter, P. J. Opt. Soc. Am. B 2006, 23, 1822–1835. (11) Marder, S. R.; Perry, J. W.; Yakymyshyn, C. P. Chem. Mater. 1994, 6, 1137–1147. Coe, B. J.; Harris, J. A.; Asselberghs, I.; Clays, K.; Olbrechts, G.; Persoons, A.; Hupp, J. T.; Johnson, R. C.; Coles, S. J.; Hursthouse, M. B.; Nakatani, K. AdV. Funct. Mater. 2002, 12, 110–116. Kaino, T.; Cai, B.; Takayama, K. AdV. Funct. Mater. 2002, 12, 599–603. Coe, B. J.; Harris, J. A.; Asselberghs, I.; Wostyn, K.; Clays, K.; Persoons, A.; Brunschwig, B. S.; Coles, S. J.; Gelbrich, T.; Light, M. E.; Hursthouse, M. B.; Nakatani, K. AdV. Funct. Mater. 2003, 13, 347–357. Kim, H. S.; Lee, S. M.; Ha, K.; Jung, C.; Lee, Y. J.; Chun, Y. S.; Kim, D.; Rhee, B. K.; Yoon, K. B. J. Am.


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JP108623D

Molecular Engineering of Benzothiazolium Salts with Large Quadratic

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