Article pubs.acs.org/JPCA
Fluorescence Properties of (E,E,E)‑1,6-Di(n‑naphthyl)-1,3,5-hexatriene (n = 1, 2): Effects of Internal Rotation Yoriko Sonoda,*,† Yukihiro Shimoi,‡ Midori Goto,§ Norimitsu Tohnai,∥ and Masatoshi Kanesato† †
Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Higashi 1-1-1, Tsukuba, Ibaraki 305-8562, Japan ‡ Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Umezono 1-1-1, Tsukuba, Ibaraki 305-8568, Japan § IBEC Center, National Institute of Advanced Industrial Science and Technology (AIST), Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, Japan ∥ Department of Material and Life Science, Graduate School of Engineering, Osaka University, 2-1, Yamadaoka, Suita, Osaka 565-0871, Japan S Supporting Information *
ABSTRACT: The fluorescence spectroscopic properties of (E,E,E)-1,6-di(n-naphthyl)-1,3,5-hexatrienes (1, n = 1; 2, n = 2) have been investigated in solution and in the solid state. In solution, the absorption maxima (λa) of the lowest-energy band (1, 374 nm; 2, 376 nm in methylcyclohexane) were similar for 1 and 2, whereas the fluorescence maxima (λf) (1, 545 nm; 2, 453 nm) and quantum yields (ϕf) (1, 0.046; 2, 0.68) were very different regardless of the solvent polarity. The fluorescence spectrum of 1 was independent of the excitation wavelength (λex), whereas the spectrum of 2 was weakly λexdependent. In the solid state, the spectroscopic properties of 1 and 2 were similar (λa = 437−438 nm, λf = 496−505 nm, ϕf = 0.04−0.07). The origins of emission are both considered to be mainly monomeric. With the help of single-crystal X-ray structure analysis and ab initio quantum chemical calculation, we conclude that the red-shifted and weak emission of 1 in solution originates from a planar excited state having small charge transfer character, reached from a twisted Franck−Condon state by the excited-state geometrical relaxation accompanied by the internal rotation around the naphthalene (Ar)−CH single bond. The similar fluorescence properties of 1 and 2 in the solid state can be attributed to the restriction of the geometrical relaxation. The effects of the Ar−CH rotational isomerism on the fluorescence properties in solution, for 2 in particular, are also discussed.
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INTRODUCTION
of the most important examples would be the red-shifted fluorescence from “TICT” (twisted intramolecular charge transfer) molecules in polar solvents.6−9 (E,E,E)-1,6-Diphenyl-1,3,5-hexatriene (DPH) is a highly fluorescent molecule with a one-dimensional polyenic structure and is commercially available as a fluorescence probe in biological membrane studies. The emission properties of unsubstituted DPH in solution have long attracted considerable attention because of its dual fluorescence from the first two singlet excited states, S1 (2Ag) and S2 (1Bu) to the ground state S0 (1Ag).10−12 Further, it has been shown that the photophysical8,9,13−16 and photochemical17,18 properties of DPH can effectively be controlled by the introduction of appropriate ring substituents.
Fluorescent organic molecules are of great interest not only in molecular science but also in material science due to their potential use as photoactive materials such as light emitting diodes (LEDs)1−3 and sensors.4,5 Owing to the intensive and extensive studies, the relationship between molecular structure and spectroscopic properties is now fairly well understood in solution. In the solid state, however, the structure−property relationship is more complicated and still much remains to be clarified. The fluorescence spectra in the solid state are often red- or blue-shifted compared to those in solution, and the shifts can mainly be attributed to (i) the restriction of intramolecular geometrical change such as internal rotation (rigidification), and (ii) intermolecular interactions in the solid state. Internal rotation around single bonds in organic molecules is a very common phenomenon in solution chemistry. The geometrical relaxation accompanied by internal rotation in the excited states heavily affects their photophysical properties. One © 2012 American Chemical Society
Received: June 20, 2012 Revised: December 10, 2012 Published: December 21, 2012 566
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Chart 1. Chemical Structures and Atom Numberings of 1 and 2
structures of molecules in solution and in the solid state and investigated the effects of internal rotation around the Ar−CH single bonds on the spectroscopic properties.
1,6-Di(1-naphthyl)-1,3,5-hexatriene (1) and its 2-naphthyl isomer (2) (Chart 1) are expected to have structures of different molecular planarity at least in the ground state, due to the different steric hindrance around the naphthalene (Ar)− CH single bonds. On the other hand, the geometry of the triene would be less affected by the substitution site of the naphthalene ring. Therefore, the internal rotation around the Ar−CH bonds in the ground and excited states of 1 and 2 should directly affect their spectroscopic properties in solution. We can also expect that the introduction of naphthalene rings in 1 and 2 will increase the molecular crystallinity due to the rigid hydrocarbon framework like other acenes, which possibly leads to the formation of single crystals suitable for X-ray analysis. As evidenced by numerous examples of crystal structure analysis, the naphthalene-containing molecules often pack not in stacks but in a herringbone pattern in the solid state. This prevents the formation of excimers, which results in the considerable decrease in fluorescence emission efficiency of crystalline materials. For 1 and 2, the absorption and fluorescence spectra in THF solution, and the absorption spectra in the solid state, have been measured.19,20 However, the details of the spectroscopic properties in solution and the fluorescence properties in the solid state are unknown at present. As for the related molecules of 1 and 2, the photoproperties of 1,2-di(n-naphthyl)ethylenes (1-DNE, n = 1; 2-DNE, n = 2)19−22 and naphthylphenylethene (styrylnaphthalene)23−26 and its polyene analogs27,28 have been investigated for a few decades. In particular, the effects of Ar−CH rotational isomerism on the spectroscopic properties of 2-DNE have thoroughly been studied in solution.21 Also for DNEs, however, the solid-state properties have attracted less attention.20,29 In this study, the fluorescence properties of 1 and 2 were studied in diluted solution and in the solid state. Crystals 1 and 2 were all photochemically stable. We performed ab initio calculations and single-crystal X-ray analysis to clarify the
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EXPERIMENTAL AND THEORETICAL METHODS Materials. Compounds 1 and 2 were synthesized from naphthaldehydes and (E)-2-butene-1,4-bis(triphenylphosphonium chloride) (TCI) by the Wittig reaction. For the purification of 2, a Japan Analytical Industry LC-908 preparative GPC was used. IR spectra were recorded on a Mattson Infinity Gold FT-IR spectrometer. 1H and 13C NMR spectra were recorded on a Varian Gemini-300 BB spectrometer (300.1 and 75.5 MHz, respectively) with tetramethylsilane (TMS) as an internal reference. All solvents used in the measurements of absorption and fluorescence spectra were of spectroscopic grade (Dojin). (E,E,E)-1,6-Di(1-naphthyl)-1,3,5-hexatriene (1). To a solution of 1-naphthaldehyde (WAKO) (1.56 g, 10 mmol) and the bisphosphonium salt (3.25 g, 5 mmol) in ethanol (20 mL) was slowly added dropwise a solution of sodium ethoxide (0.60 M, 16.7 mL, 10 mmol) at 45 °C under nitrogen atmosphere in the dark. After stirring for 45 h, ethanol (20 mL) was added to the reaction mixture. The resulting yellow solid was filtered off and washed with aqueous ethanol (60% v/v, 20 mL). The crude product was a mixture of ZEZ and EEE isomers (ZEZ: EEE = 7: 3) as shown by 1H NMR.30 To induce Z-to-E isomerization, the product was dissolved in toluene (Tol) and refluxed for 8 h in the dark with a catalytic amount of iodine. After the solvent was evaporated under the reduced pressure, the resulting solid was recrystallized from Tol to give single crystals of 1 suitable for X-ray analysis. Yield: 15%. The purity was checked by HPLC. Mp: 224−226 °C (decomp) (lit.19 217−219 °C). νmax (KBr): 1586, 1325, 1003, 978, 952, 930, 880, 865, 849, 798, 774, 756, and 734 cm−1. 1H NMR (CDCl3): δ 8.19 (2H, d, J 8.4, arom), 7.86 (2H, dd, J 7.4 and 1.9, arom), 7.78 (2H, d, J 8.2, arom.), 7.74 (2H, d, J 7.4, arom.), 567
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Table 1. Absorption and Fluorescence Data of 1 in Solution solvent methylcyclohexane toluene tetrahydrofuran dichloromethane acetonitrile methanol
λa (nm) 374, 279, 382e 381, 281, 381, 282, 378, 279, 376, 279,
λf (nm)a
245
480, 479, 479, 476, 482, 475,
246 247 244 244
513, 513, 513, 514, 513, 511,
545, 547, 547, 547, 548, 547,
591 589 591 591 595 597
(sh) (sh) (sh) (sh) (sh) (sh)
d
ΔEss (cm−1)b
ϕf
8389 7896 7965 7965 8207 8314
0.046 0.063 0.050 0.061 0.039 0.032
τs (ns)c (χ2) 2.82 3.02 3.04 3.27 2.93 2.78
(1.17) (1.36) (1.20) (1.19) (1.22) (1.37)
a λex = underlined λa. bCalculated from the underlined values of λa and λf. cλex = 340 nm, λem = underlined λf. dsh: shoulder. eDue to the solvent absorption, peaks at shorter wavelengths than 280 nm were not able to be observed.
Table 2. Absorption and Fluorescence Data of 2 in Solution methylcyclohexane toluene tetrahydrofuran
dichloromethane
acetonitrile
methanol
λf (nm)a
ΔEss (cm−1)b
ϕf
τs (ns)c (χ2)
404, 424, 453, 480
4521
0.68
7.12 (1.02)
411, 426, 455, 482
4200
0.79
5.06 (1.22)
406, 428, 455, 479
4407
0.71
6.25 (1.14)
362, 343 (sh) 291, 282
408, 430, 458, 483
4482
0.75
6.20 (1.29)
358, 289, 255 356, 288, 255
338 (sh) 280
403, 424, 456, 482
4737
0.50
6.28 (1.20)
337 (sh) 278
403, 425, 453, 476
4735
0.47
7.17 (1.25)
λa (nm)
solvent 398, 313, 405, 316, 401, 314, 265, 402, 315, 265, 396, 312, 269, 395, 311, 269,
376, 301, 382, 303, 379, 302, 256 380, 302, 256 375, 300, 262, 373, 300, 262,
360, 289, 364, 292, 361, 291,
341 (sh)d 279e 345 (sh) 283e 342 (sh) 282
a λex = underlined λa. bCalculated from the underlined values of λa and λf. cλex = 340 nm, λem = underlined λf. dsh: shoulder. eDue to the solvent absorption, peaks around 260 nm were not able to be observed.
7.48−7.57 (6H, m, arom.), 7.43 (2H, d, J 14.6, triene), 7.03 (2H, apparently (app) ddd, J 15.1, 7.1 and 3.0, triene), 6.74 (2H, app dd, J 7.1 and 3.0, triene). 13C NMR (CDCl3): δ 134.6, 134.1, 133.8, 131.8, 131.1, 129.5, 128.6, 128.0, 126.1, 125.8, 125.6, 123.5, and 123.2. UV−vis λmax (dichloromethane (DCM)): 381 nm (ε = 52 400 M−1 cm−1). (E,E,E)-1,6-Di(2-naphthyl)-1,3,5-hexatriene (2). Compound 2 was synthesized from 2-naphthaldehyde (TCI) and the phosphonium salt according to a method similar to that for 1 described above. The crude product (Z,E mixture) was obtained in 67% yield. After purified by GPC (chloroform eluent), the product was refluxed with a trace amount of iodine in Tol, and then recrystallized three times from the same solvent to give single crystals of 2 suitable for X-ray analysis. The purity was checked by HPLC. Mp: 250−253 °C (decomp) (lit.19 251−262 °C (decomp)). νmax (KBr): 1591, 1368, 1274, 1002, 968, 955, 930, 903, 866, 846, 818, 771, 750, and 741 cm−1. 1H NMR (CDCl3): δ 7.78−7.82 (8H, m, arom), 7.65 (2H, dd, J 8.6 and 1.5, arom), 7.40−7.49 (4H, m, arom), 7.05 (2H, app ddd, J 15.5, 6.9 and 3.1, triene), 6.79 (2H, d, J 15.3, triene), 6.63 (2H, app dd, J 7.0 and 2.9, triene). 13C NMR data were not able to be obtained due to low solubility. UV−vis: λmax (DCM) 380 nm (ε = 91 400 M−1 cm−1). Photochemical Stability of 1 and 2. Compounds 1 and 2 were decomposed on prolonged irradiation in solution at room temperature in air. The fluorescence spectra in solution were therefore recorded by minimum exposure to light to avoid any photoreactions during the measurements. In contrast, 1 and 2 were photostable in the solid state. No Z−E isomerization,
oxidation, nor cycloaddition was observed from the E,E,E isomers. Measurements of Absorption Spectra. The absorption spectra of 1 and 2 in solution were measured at room temperature in air using a Shimadzu UV-3150 spectrometer. All solutions were highly diluted ((1−2) × 10−5 M). The absorption spectra in the solid state were obtained by Kubelka−Munk conversion of diffuse reflectance spectra. The reflectance spectra were recorded on a Shimadzu UV-3150 spectrometer equipped with an integrating sphere accessory (model ISR-3100). The sample solids were placed between quartz plates (40 × 10 mm2). Measurements of Fluorescence and Fluorescence Excitation Spectra, Fluorescence Quantum Yields, and Lifetimes. The corrected fluorescence and fluorescence excitation spectra of 1 and 2 in solution and in the solid state were measured at room temperature in air using a SPEX Fluorolog-3 spectrometer. The excitation and emission (monitor) wavelengths (λex and λem, respectively) in the measurements were shown in Tables 1−3. For the measurements in solution, concentration of the sample was (1−2) × 10−6 M. The Fluorescence quantum yields (ϕf) in solution were determined using a solution of quinine sulfate in 1 N H2SO4 as a standard (ϕf = 0.546).31 The spectra of the solid samples were recorded using the front face geometry. The sample solids were placed between quartz plates (40 × 10 mm2) on the sample holder. The measurements of the solid-state ϕf were performed at Osaka University by using a JASCO FP-6500 spectrofluorometer with a fluorescence 568
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Table 3. Absorption and Fluorescence Data for Crystals 1 and 2
a sh: shoulder. bThe maximum wavelength in the fluorescence excitation spectrum; λem = 524 nm for 1, 537 nm for 2. cλex = 390 nm. dCalculated from the underlined values of λa and λf. eλex = 400 nm for 1, 440 nm for 2. fλex = 337 nm, λem = underlined λf.
integrate sphere unit (model ISF-513). The sample crystals were encapsulated in a quartz cell (30 × 30 × 0.3 mm3) under deoxygenated conditions. The fluorescence decay curves in solution and in the solid state were obtained by the time-correlated single-photon counting (TCSPC) method, using a HORIBA NAES 700 equipped with a subnanosecond nitrogen laser (λex = 337 nm), a HORIBA FluoroCube 3000-U and a Hamamatsu Photonics QuantaurusTau equipped with UV LEDs (λex = 340 nm). For the solid-state absorption and fluorescence measurements, the samples were in the microcrystalline form in all cases. Single Crystal X-ray Structure Analysis. Single crystal Xray diffraction measurements were performed at 183 K using a Bruker SMART CCD area-detector diffractometer with graphite monochromated MoKα radiation (λ = 0.710 73 Å). Data collection, reduction, and empirical absorption correction were carried out using SMART (Bruker 2001), SAINTPLUS (2001) and SADABS (2001).32 The structure was solved by direct methods using SIR9233 and refined by full matrix least-squares on F2 with SHELXTL.34 The non-hydrogen atoms were refined anisotropically. The hydrogen atoms were placed in geometrically calculated positions and refining a riding model. Computational Method. Ab initio quantum chemical calculations were performed for the isolated molecules of 1 and 2. Their geometries and torsional potentials at the electronic ground state were calculated by the second order Møller− Plesset (MP2)35 perturbation method with the 6-311G** basis set. The time-dependent Hartree−Fock (TDHF)36 and configuration interaction singles (CIS)37 methods with the 6311G** basis set were applied to excited states. The calculations were carried out with the Gaussian 09 program.38
Figure 1. Absorption and fluorescence spectra of (a) 1 and (b) 2 in methylcyclohexane. λex = 374 nm for 1, 376 nm for 2.
398 nm (0−0) with its vibrational progressions at 376 (0−1), 360 (0−2), and 341 nm (sh) (0−3). Among them the 0−1 peak was the strongest in intensity. The energy spacings were ca. 1400 cm−1, corresponding to the CC and C−C stretches of conjugated trienes. For the (b) band, λa was observed at 313 (0−0) nm with the progressions at 301 (0−1), 289 (0−2), and 279 (0−3) nm. The spacings were ca. 1300 cm−1, somewhat smaller than those for the (a) band. Thus, the positions of λa for the three lowest energy bands (a)−(c) were similar for 1 and 2. On the other hand, the shapes of the spectra of 1 and 2 were clearly different. For both molecules, the positions of λa showed only a small dependence on the solvent polarity (Tables 1 and 2). Also, the spectral shapes did not change greatly (Figures S1 and S2 in the Supporting Information). The small solvent dependence of λa suggests that the effects of solvent polarity on the strength of the solvent−solute intermolecular interaction are similar in the ground and excited states. 1.2. Fluorescence Properties. Fluorescence Spectra. In the fluorescence spectrum of 1 in MCH, the emission maximum (λf) was observed at 545 nm, largely red-shifted by 171 nm from λa at 374 nm. The Stokes shift (ΔEss) was 8389 cm−1, and the overlap of the absorption and fluorescence spectra was almost none (Figure 1a). Different from the broad shape of the absorption spectrum, the emission spectrum was weakly structured with the spacings of ca. 1300 cm−1. No mirrorimage relationship was observed between the absorption and fluorescence spectra, showing that the excited state(s) responsible for the fluorescence emission was clearly different from the Franck−Condon state. In the spectrum of 2, λf was
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RESULTS AND DISCUSSION 1. Absorption and Fluorescence Properties in Solution. Figure 1 shows the absorption and fluorescence spectra of (a) 1 and (b) 2 in solution. Tables 1 and 2 summarize the absorption and fluorescence data in various kinds of solvents with different polarity. 1.1. Absorption Properties. In the absorption spectrum of 1 in methylcyclohexane (MCH), three broad bands were observed with the absorption maxima (λa) at (a) 374, (b) 279, and (c) 245 nm. On the other hand, two structured bands were observed around (a) 376 and (b) 289 nm in 2. Although the band corresponding to that of (c) in 1 was not able to be observed in 2 in MCH and Tol, it was seen in other solvents, as shown in Table 2. For the main band (a) in 2, λa was found at 569
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observed at 453 nm, red-shifted from λa by 77 nm. The value of ΔEss was relatively large and the absorption−fluorescence spectral overlap was relatively small (Figure 1b). The fluorescence spectrum exhibited vibrational structures as the absorption spectrum and the observed spacing was ca. 1300 cm−1. However, the mirror-image relationship was not strictly observed between them. It is therefore considered that the fluorescence emission of 2 also occurs from the excited state(s) different from the Franck−Condon state. The position of λf for 1 located at longer wavelength by 92 nm (3726 cm−1) relative to that for 2 in MCH. This is in sharp contrast to the fact that λa were very similar for 1 and 2. Consequently, ΔEss for 1 was much larger than the value for 2. The positions of λf for 1 and 2 showed only a small dependence on the solvent polarity (Tables 1 and 2). This suggests that the solvent effects on the magnitude of solvent− solute interaction are similar in the ground state and the excited state responsible for the fluorescence emission. It also suggests that the emissive states of the molecules have only small charge transfer (CT) character. In the fluorescence spectrum of 2, a small peak was observed at 404 nm in MCH (Figure 1b). The position and intensity of this small peak relative to those of the main peak at 453 nm, and thus the spectral shape, were somewhat dependent on the solvent polarity (Figure S2, Supporting Information). In 1, such a small peak was not clearly seen, and the spectral shape was less dependent on the solvent polarity (Figures 1a and S1, Supporting Information). For the absorption spectrum of 2 in MCH, the Huang−Rhys factor (S)39 is estimated to be 1.3 from the equation S = I1←0/ I0←0, where I0←0 and I1←0 are the intensities of the 398 nm (0− 0) and 376 nm (0−1) peaks, respectively. For the fluorescence spectrum, on the other hand, the S value may be very roughly estimated to be 1.6 from the intensity ratio of the two peaks at 424 and 453 nm, neglecting the weak peak at 404 nm. If we accept this, then we see the S values obtained from the absorption and emission spectra are similar, although the mirror-image relationship was not strictly observed as mentioned above. The fluorescence spectrum of 1 was independent of λex, indicating that fluorescence from 1 originates basically from only one kind of emitting species. In contrast, the spectrum of 2 was weakly λex-dependent. Figure 2 shows the fluorescence spectra of 2 at different λex in MCH. The results were similar in acetonitrile (AN). As λex moved to longer wavelengths, λf
shifted to longer wavelengths. The observation suggests that the fluorescence from 2 originates from more than one kind of emissive species. The fluorescence excitation spectrum of 1 was independent of λem and was essentially the same as the absorption spectrum. The excitation spectrum of 2 was also considerably similar to the absorption spectrum; however, it showed a small λem dependence. Figure 3 shows the excitation spectra of 2 at
Figure 3. Fluorescence excitation spectra of 2 in methylcyclohexane at different emission wavelengths.
different λem in MCH. Although the peak positions in the spectrum did not significantly depend on λem, the spectrum became less structured as λem shifted to longer wavelengths. The results were similar in AN. Fluorescence Lifetimes. The fluorescence lifetimes (τs) of 1 and 2 in several kinds of solvents are listed in Tables 1 and 2. Tables S1 and S2 in the Supporting Information summarize τs data at different λem in MCH and AN. The fluorescence decay curves of 1 were able to be analyzed by monoexponential function to give τs of 2.8−3.1 ns in low polar (MCH) and highly polar (AN) solvents. No strong λem dependence of τs was observed. This is consistent with the fact that the fluorescence from 1 is due to only one kind of emitting species. The decay curve of 2 is displayed in Figure 4. In AN, the curves were able to be fully analyzed by monoexponential function to give τs of 6.2−6.4 ns at all λem examined. Also in MCH, the curve analysis gave single-component τs of 7.0−7.2 ns at all λem examined other than 404 nm, although τs of 0.75 ns was obtained as a minor component (8%) at λem = 404 nm. The observation of single-component τs of 2 seems to be inconsistent with the weak λex dependence of the fluorescence spectrum and the λem dependence of the excitation spectrum described above. This can be understood by assuming that the τs for the different emissive species are very similar, and the differences are not able to be detected by our TCSPC apparatus with the time resolution of 50−100 ps. Alternatively, the results suggest that one of the emitting species predominantly exists as a major component in solution. Fluorescence Quantum Yields. Molecule 1 was only weakly fluorescent, whereas 2 was highly fluorescent in solution. The dependence of ϕf on the solvent polarity was relatively small (Tables 1 and 2). The radiative rate constants (kf = ϕf/τs) are calculated to be 1.9 × 107 s−1 for 1 and 1.2 × 108 s−1 for 2 in DCM. Using these values, we obtain the nonradiative rate constants (knr = kf[(1/ϕf) − 1]) of 2.9 × 108 s−1 for 1 and 4.0 × 107 s−1 for 2 in the same solvent. The relatively large knr for 1
Figure 2. Fluorescence spectra of 2 in methylcyclohexane at different excitation wavelengths. 570
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Figure 4. Fluorescence decay curve of 2 in methylcyclohexane. λex = 340 nm, λem = 453 nm. χ2 = 1.02. Time calibration: 5.66 × 10−2 ns/ch.
suggests that the molecule has an efficient nonradiative decay (NRD) route in solution, which is absent for 2. Obviously, the large difference in kf (= ϕf/τs) for 1 and 2 is mainly attributed to the large difference in ϕf. On the other hand, kf values estimated by the Strickler−Berg method40 (kf (SB)) using the integrated intensity of the absorption41 are 5.6 × 108 s−1 for 1 and 7.2 × 108 s−1 for 2 in DCM; the values are not greatly different for 1 and 2. We should note here that kf(SB) is significantly larger than kf (=ϕf/τs) for each molecule. This can at least partially be attributed to the possible violation of the assumption underlying the Strickler−Berg method that the geometry of the molecule stays the same as it goes from the ground to the excited state.40,42 The larger difference between kf(SB) and kf (=ϕf/τs) for 1 than for 2 suggests a larger geometrical change upon photoexcitation for 1. 2. Absorption and Fluorescence Properties in the Solid State. Figure 5 shows the absorption, fluorescence, and fluorescence excitation spectra of (a) 1 and (b) 2 in the solid state. The absorption and fluorescence data are summarized in Table 3. 2.1. Absorption Properties. In the solid-state spectra of 1 and 2, two bands were observed around 437−438 and 350− 352 nm. The positions of λa were very similar, as in solution. On the other hand, the shapes of the absorption spectra were very weakly structured (essentially broad) in 1 and more structured in 2. The spectral features are thus fundamentally the same in solution and in the solid state. When we consider the shoulder at 461 nm observed in the spectrum of 2 to be the progression of the peak at 438 nm, the energy spacing is calculated to be ca. 1100 cm−1. The value can be compared with that of 1400 cm−1 for the main band (a) in MCH. 2.2. Fluorescence Properties. Fluorescence Spectra. In the spectrum of 1, λf were observed at 524 and 496 nm. The redshift from λa (437 nm) to λf (496 nm) was 59 nm. The relatively small ΔEss (2722 cm−1) corresponds to a considerably large absorption-fluorescence spectral overlap (Figure 5a). Similar to the absorption spectrum, the fluorescence spectrum was very weakly structured with the spacing of ca. 1100 cm−1. The absorption and fluorescence spectra show a fairly good mirror-image relationship. The fluorescence spectrum of 2 was distinctly structured as the absorption spectrum (Figure 5b). In the spectrum, λf was observed at 505 nm with clear vibrational progressions. The spacing of 1300 cm−1 was similar to the value in the absorption spectrum. A good mirror-image relationship was observed for the absorption and fluorescence spectra. The
Figure 5. Absorption, fluorescence, and fluorescence excitation spectra of crystals (a) 1 and (b) 2. λex = 390 nm for 1 and 2. λem = 524 nm for 1, 537 nm for 2.
red-shift from λa (438 nm) to λf (505 nm) was 67 nm. Corresponding to the relatively small ΔEss (3029 cm−1), the absorption−emission overlap was very large. Thus, in contrast to the observation in solution, the difference in the solid-state λf for 1 (496 nm) and 2 (505 nm) was only 9 nm. As the difference in λa was similarly small, this led to similar ΔEss for 1 and 2 in the solid state. Unlike in solution, the fluorescence spectra of 1 and 2 both showed no λex dependence in the solid state. The fluorescence excitation spectrum of 1 exhibited no λem dependence and was fundamentally the same as the absorption 571
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spectrum (Figure 5a). Also, the excitation spectrum of 2 was essentially independent of λem. The peaks at 438 and 459 nm in the excitation spectrum correspond probably to those at 438 and 461 nm in the absorption spectrum (Figure 5b) (Table 3). However, we should note here that the excitation spectrum of 2 was not completely the same as the absorption spectrum; the small peak at 483 nm in the absorption spectrum was only weakly observed as a broad band centered at 492 nm in the excitation spectrum (Figure 5b). Considering that no clear fluorescence spectrum was obtained at λex = 483 nm and no λem dependence was observed in the excitation spectrum, the absorption at 483 nm is possibly due to nonfluorescent species or structural defects in the solid sample.43 Fluorescence Lifetimes. Table S3 in the Supporting Information summarizes the τs data of 1 and 2 in the solid state. Although the perfect analysis of the fluorescence decay curves of 1 and 2 was difficult due to the intrinsic inhomogeneity of the solid samples, the main components of τs were roughly estimated to be 0.7−1.0 ns at all λem examined. The values of ca. 1 ns are typical for the single-molecule (monomeric) fluorescence of small organic dyes in the solid state.44 Similar τs were obtained for other DPH derivatives which we investigated previously.9,14,15 The results of τs measurements are thus fundamentally similar for 1 and 2, although the origin of the fluorescence having longer τs of 2−5 ns, detected as a minor component at most of λem examined, is unclear at present. Fluorescence Quantum Yields. In the solid state, ϕf of 1 and 2 were similarly small (Table 3). If we consider the values of kf also in the solid state as in solution, they are calculated to be 4.2 × 107 for 1 and 7.1 × 107 for 2, using τs of 0.98 ns. 3. Crystal Structures. 3.1. Molecular Structures. Table 4 shows the single crystal data of 1 and 2 obtained by X-ray
Table 5. Major Geometrical Parameters for the X-ray and the Ground-State Optimized Structures of 1 (Conformer A) molecule X C7C1 (r1) C1C2 (r2) C2C3 (r3) C3C4 (r4) C4C5 (r5) C5C6 (r6) C6C17 (r7) BLAc C8C7C1C2 (ϕ1) C18C17C6 C5 (ϕ2)
formula formula weight crystal color, habit crystal size (mm3) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalc (g/cm3) T (°C) mp (°C) R1 (I > 2σ(I))
2
C26H20 332.44 pale yellow, rectangular 0.25 × 0.15 × 0.05 triclinic P1 6.950(5) 7.400(5) 17.571(12) 92.119(12) 96.424(13) 90.191(11) 897.3(10) 2 1.230 −90 226 0.0828
C26H20 332.44 pale yellow, rectangular 0.30 × 0.10 × 0.05 monoclinic P 21 6.5560(14) 7.2720(15) 18.452(4) 90.00 90.519(4) 90.00 879.7(3) 2 1.255 −90 253 0.0483
molecule Y
b
A1 (C2)
A2 (Ci)
Bond Length (Å) 1.484(9) 1.491(10) 1.321(10) 1.346(9) 1.422(9) 1.451(10) 1.338(10) 1.323(10) 1.427(10) 1.478(10) 1.340(10) 1.303(9) 1.458(9) 1.482(10) 0.115 0.152 Torsion Angle (deg) 141.0(7) −147.0(7)
1.466 1.360 1.442 1.363 1.442 1.360 1.466 0.093
1.466 1.360 1.442 1.363 1.442 1.360 1.466 0.093
144.2
144.3
−149.6(7)
144.2
−144.3
140.1(6)
a
At the MP2/6-311G** level. bCrystallographically independent two molecules. cBond length alternation; BLA (r1 + r3 + r5 + r7)/4 − (r2 + r4 + r6)/3. For atom numbering, see Chart 1.
Table 6. Major Geometrical Parameters for the X-ray and the Ground-State Optimized Structures of 2 (Conformer A) calca X-ray Bond Length (Å) 1.460(3) C7C1 (r1) C1C2 (r2) 1.332(3) C2C3 (r3) 1.438(3) C3C4 (r4) 1.342(3) C4C5 (r5) 1.439(3) C5C6 (r6) 1.339(3) C6C17 (r7) 1.460(3) BLAb 0.112 Torsion Angle (deg) C8C7C1C2 (ϕ1) 165.3(2) C18C17C6C5 (ϕ2) −165.4(2)
Table 4. Crystal Data of 1 and 2 1
calca
X-ray b
A1 (C2)
A2 (Ci)
1.459 1.361 1.440 1.364 1.440 1.361 1.459 0.088
1.459 1.361 1.440 1.364 1.440 1.361 1.459 0.088
164.2 164.2
164.3 −164.3
a
At the MP2/6-311G** level. bBond length alternation; BLA (r1 + r3 + r5 + r7)/4 − (r2 + r4 + r6)/3. For atom numbering, see Chart 1.
The Ar−CH single bonds (C7−C1 and C6−C17, Chart 1) in 1, in molecule Y in particular, are longer than those in 2. The bond lengthening in 1 is to diminish the steric hindrance between H1 and H9, H2 and H16, H5 and H26, and H6 and H19. Because the averaged double bond lengths are similar for 1 and 2, the lengthening of the Ar−CH bonds in 1 results in larger bond length alternation (BLA) than in 2. The triene and naphthalene groups are basically planar in 1 and 2. The deviations from the least-squares planes for the triene chain and the naphthalene ring are less than 0.035 Å for all molecules. The Ar−CH single bond in 1 is somewhat twisted due to the steric hindrance around the Ar−CH bond, as seen in ϕ1 and ϕ2 (Table 5). Here, ϕ1 and ϕ2 are the torsion angles of C8−C7−C1−C2 and C18−C17−C6−C5, respectively, and are defined in the range from −180° to +180°. In 2, the steric hindrance is smaller than in 1 and the structure is more planar (Table 6). The molecular planarity in the ground state is thus somewhat but clearly different for 1 and 2 in the solid state.
structure analysis. The major geometrical parameters are summarized in Tables 5 and 6. In the crystal structure of 1, there exist two crystallographically independent molecules, X and Y. In the structure of 2, only one crystallographically independent molecule is included in a unit cell. Each molecule is noncentrosymmetric. 572
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Chart 2. Conformational Isomers of (a) 1 and (b) 2
neighboring molecule (x-1, y-1, z) is 2.808 Å (Figure 6a). In crystal 2, the distance of Cg of the half part of the naphthalene ring (C7−C8−C9−C14−C15−C16) of the molecule (x, y, z) and the naphthalene H25 of the neighboring molecule (1-x, y +0.5, 1-z) is 2.719 Å (Figure 6b). In 2, the triene carbon and triene hydrogen atoms of the neighboring molecules are also in close proximity. The distance between the triene C6 (x, y, z) and the triene H1 (1-x, y+0.5, 1-z) is, for example, 2.954 Å. The proximity of the molecules can at least partially be attributed to CH/π interactions between the H atoms of the naphthalene ring or the hexatriene chain, and the π-systems of the ring or the chain. The dihedral angles between the least-squares planes of the naphthalene rings of the adjacent molecules are 40−41° in 1 and 52° in 2, whereas, the dihedral angles between the triene planes of the adjacent molecules are 88° and 70° in 1 and 2, respectively. The larger difference between the dihedral angles of the triene and naphthalene planes in 1 than in 2 clearly results from the larger Ar−CH torsion angle for 1 due to the steric reason, as described above. Also in the parent naphthalene crystal, the molecules are arranged in a typical herringbone pattern.48 The dihedral angle between the two adjacent molecular planes is 47°.49 The value is similar to those between the two naphthalene planes in crystals 1 and 2. Further we can compare the structure of 1 with that of its Z,E,Z counterpart, which we reported previously.30 4. Optimized Structures and Ar−CH Torsional Potentials in the Ground and Excited States. 4.1. Ground States. Relative Ratio and Optimized Structures of Conformers A−C. For 1 and 2, the structures of the possible conformers A−C (Chart 2) are optimized at the MP2/6311G** level. Only the all s-trans isomer was taken into account in the present case to avoid the calculations being too complex. In both 1 and 2, the MP2 calculations gave two kinds of optimized structures for each conformer with respect to the mutual signs of the Ar−CH torsion angles ϕ1 and ϕ2. The values of ϕ1 and ϕ2 have the same signs in one of the structures and have different signs from each other in the other structure. These two structures are designated by adding 1 for the former structure or 2 for the later one to A−C, respectively, like A1 or A2. In conformations A and C, the optimized structures possess the molecular symmetry of C2 or Ci: A1 and C1 belong to the
Molecules 1 and 2 have three kinds of possible conformers A−C (Chart 2). Among them, only one kind of conformer, A, was found in the crystal structure.45 3.2. Molecular Arrangements. The crystal packing diagrams of 1 and 2 are shown in Figure 6. The molecules in the crystal structures are arranged in a herringbone fashion. In crystal 1, the distance between the gravity center (Cg) of the half of the naphthalene ring (C17−C18−C23−C24−C25C26) of the molecule (x, y, z) and the triene H6 of the
Figure 6. Crystal packing diagrams of (a) 1 and (b) 2. 573
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C2 symmetry with ϕ1 = ϕ2 and A2 and C2 belong to the Ci one with ϕ1 = −ϕ2. Meanwhile, B1 and B2 have no symmetry (C1). The relative energies of these optimized structures were calculated at the MP2/6-311G** level. In both 1 and 2, A1, B2, and C1 are slightly (0.01 kcal/mol) lower in energy than A2, B1, and C2, respectively. Hence, the two structures of each conformer are expected to exist at a ratio essentially equal at room temperature in solution. For 1, B2 and C1 are shown to be 1.00 and 1.94 kcal/mol higher in energy than A1, respectively. When any entropy terms, including that arising from the number of the symmetry operations are neglected, the Boltzmann distribution indicates that the populations of A (A1), B (B2), and C (C1) are 82, 15, and 3%, respectively, at 300 K. Thus, if we consider the relative energies only, A will predominantly exist at room temperature in solution. For 2, the calculation shows that A1 is the most stable, but B2 and C1 are only slightly higher in energy than A1 (0.32 kcal/mol for B2 and 0.64 kcal/mol for C1). Thus, A, B, and C of 2 will exist as an equilibrium mixture at room temperature in solution. On the basis of the Boltzmann distribution, the populations of A (A1), B (B2), and C (C1) are estimated to be 52, 30, and 18% at 300 K. Tables 5 and 6 show the calculated values of the major geometrical parameters for the main conformers A (A1 and A2) for 1 and 2. The results for B (B1 and B2) and C (C1 and C2) are summarized in Tables S4 and S5 in the Supporting Information. In the optimized structures of A1, the Ar−CH single bond length (C7−C1 and C6−C17) for 1 is slightly longer than the value for 2. Because the averaged double bond lengths are similar for 1 and 2, the BLA for 1 is slightly larger than the value for 2. The results are in qualitative agreement with those from the X-ray analysis. For 1, the absolute values of the Ar−CH torsion angles, |ϕ1| (=|ϕ2|) are 144° in A1 and A2 (Table 5), showing somewhat twisted conformation of the molecule. Meanwhile, for 2, |ϕ1| (=|ϕ2|) are calculated to be 164° in A1 and A2 (Table 6). Thus the optimized structures of A (A1 and A2) of 2 are all more planar than those of 1. The Ar−CH torsion angles are significantly different for 1 and 2 in the ground-state structures. Further, for 1 and 2, |ϕ1| (=|ϕ2|) in the optimized structures of A (A1 and A2) are very similar to those in the X-ray determined structures (Tables 5 and 6). Torsional Potentials around the Ar−CH Single Bond. Figure 7 shows the torsional potentials of (a) 1 and (b) 2 in the ground state that were calculated by rotating the torsional angle ϕ1 for conformation A. All the degrees of freedom except for ϕ1 were fully optimized at each point on the potential curves. In Figure 7 (and Figure 8), ϕ1 is varied in the range from 0 to 360° instead of that from −180 to +180°. For both molecules, the potential curves are essentially symmetrical with respect to ϕ1 = 180°. The barrier heights between A1 and B1 are ca. 2.7 kcal/mol for 1 and ca. 4.2 kcal/ mol for 2 at the MP2//MP2 level. The relatively low barrier heights suggest that the Ar−CH bond in A can rotate in a wide range of angles around the minima at room temperature in solution. We note here the potential curve for 2 is very flat in shape around ϕ1 = 180° with extremely a small barrier between A1 and A2. This suggests that the relative population of A over B and C may be larger than the value (52%) calculated only from the relative energies based on the Boltzmann distribution.
Figure 7. Ground-state torsional potentials for the C8−C7−C1−C2 bond of (a) 1 and (b) 2 calculated at the MP2/6-311G**//MP2/6311G** level. For atom numbering, see Chart 1. The solid lines are only guides for the eyes.
Figure 8. S1-excited-state torsional potentials for the C8−C7−C1−C2 bond of 1 and 2 calculated at the TDHF/6-311G**//TDHF/6311G** level. For atom numbering, see Chart 1. The solid lines are only guides for the eyes.
4.2. Excited States. Optimized Structures. We performed the geometry optimization of A for the S1 states of 1 and 2 at the TDHF/6-311G** and CIS/6-311G** levels. The major geometrical parameters are summarized in Table 7. The TDHF and CIS methods gave very similar results. In contrast to the somewhat twisted structures in the ground state, conformer A in the excited state is predicted to be completely planar with the C2h symmetry, in both 1 and 2. 574
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Experimental and Theoretical Methods). It should be noted, however, that the calculated excitation energies for both molecules are considerably higher than the experimental values determined from the absorption spectra. In 2, for example, the (0,0) band for the lowest energy transition (S0 → S1) was observed at 398 nm in MCH, whereas the corresponding bands obtained by the TDHF//MP2 and CIS//MP2 calculation were positioned at 340 and 320 nm, respectively. The overestimation of the excitation energies by CIS calculations has been reported for other short53 or diphenyl polyenes.9 The improvement by the TDHF method over the CIS one is probably due to the fact that the TDHF method partly includes electron correlation effects neglected in the CIS method which are important for polyene molecules.53 Consideration of solvent effects may further improve the calculation results to some extent. For 1 and 2, the main configurations of the S1 states52 are the HOMO−LUMO transitions (Tables S6 and S7). Figure S3 in the Supporting Information shows the profiles of the HOMO and LUMO of 1 and 2 in the MP2 optimized structures of A. They are delocalized over whole of the molecules, indicating that the lowest energy absorption bands observed around 380 nm are due to π−π* transition for both molecules. The observation of λa at almost the same positions in the experimental spectra shows that the π−π* transition energies of 1 and 2 are almost the same. The π-conjugation between naphthalenes and triene would therefore be similarly effective in 1 and 2 (Figure S3, Supporting Information), although the Ar−CH bond in the ground-state optimized structure of 1 is somewhat more twisted than that in the structure of 2. We can say that the difference in the Ar−CH torsion angle for 1 and 2 in the ground state did not lead to the difference in λa but led to the difference in ε in solution. Fluorescence Spectra. The calculation of the relative ratio for the ground-state conformers A−C of 1 suggests that A predominantly exists at room temperature in solution. The fluorescence of 1 should therefore originate mainly from only one kind of emitting species, the excited state of A (A*). This agrees with the experimental observation. For 2, the calculation of the relative energies for the ground states A−C suggests that they are equilibrated as a conformational mixture at room temperature in solution. In the excited state, however, A*, B*, and C* will deactivate respectively by fluorescence emission to A, B, and C before reaching the equilibrium within their τs (nonequilibrated excited-state rotamers (“NEER principle”)).21,23,54,55 This explains the weak λex dependence observed for the fluorescence spectrum of 2 in solution. Here, we should note that the absorption and fluorescence spectra and possibly τs may be very similar for A−C in 2 (see section 1.2). This is in contrast to the previous observation for the ground-state conformers of 2-DNE, whose spectroscopic properties were distinctly different each other.22 Fluorescence emission energies of 1 and 2 were calculated at the TDHF/6-311G**//TDHF/6-311G** level using the optimized geometries of A in the S1 excited state. The results are summarized in Tables S8 and S9 in the Supporting Information. The S1 → S0 transition energy was calculated to be 428 nm for 1, which was lower than the value of 403 nm for 2. Because the excitation energies were similar for 1 and 2, ΔEss became larger for 1 than for 2. Thus the calculation reproduces the experimental results in solution at least qualitatively. For both molecules, however, the calculated emission energies were rather higher, and the ΔEss values were rather smaller than those obtained by the experimental spectra in solution. As in
Table 7. Major Geometrical Parameters for the Excited-State Optimized Structures of 1 and 2 (Conformer A (C2h)) 1 TDHFa
2 CISa
Bond Length (Å) 1.424 1.425 1.383 1.381 1.397 1.400 1.388 1.384 0.026 0.031 Torsion Angle (deg) C8C7C1C2 (ϕ1) 180.0 180.0
C7C1 C1C2 C2C3 C3C4 BLAb
a
(r1) (r2) (r3) (r4)
TDHFa
CISa
1.422 1.385 1.394 1.395 0.020
1.423 1.381 1.397 1.389 0.026
180.0
180.0
b
The 6-311G** basis set was used. Bond length alternation; BLA (r1 + r3)/2 − (2r2 + r4)/3. For atom numbering, see Chart 1.
Thus, there exists only one optimized structure for this conformer in each molecule. Also in other π-conjugated molecules such as biphenyl, the optimized structures are shown to be very planar in the excited state.50 In the TDHF optimized structures of 1 and 2, the single bond lengths are significantly shorter than, and the double bond lengths are considerably longer than, those in the ground state. This results in the significantly small BLAs in the excited state. A similar trend was observed in other linear polyenes.51 Torsional Potentials around the Ar−CH Single Bond. Figure 8 shows the torsional potentials of 1 and 2 in the S1 excited state with respect to the torsional angle ϕ1 calculated at the TDHF/6-311G**//TDHF/6-311G** level. The potential curves for 1 and 2 are symmetrical with respect to ϕ1 = 180°, reflecting the C2h symmetry for the optimized structures. In the potential curve for 2, the conformer A in the excited state (A*) is slightly higher in energy than the conformer B (B*). The calculated barrier heights measured from A* in the potential curves are ca. 7.1 kcal/mol for 1 and 11.8 kcal/mol for 2. Thus, the energy barriers are larger in the excited state than in the ground state for both 1 and 2. This suggests that the Ar−CH internal rotation is more restricted and the molecules are more tend to be planar in the excited state than in the ground state. 5. Relationship between Structure and Spectroscopic Properties in Solution and in the Solid State. 5.1. Spectroscopic Properties in Solution. Absorption Spectra. To understand the electronic transitions responsible for the absorption bands of 1 and 2, the vertical excitation energies were calculated for conformers A by applying the CIS/6311G** and TDHF/6-311G** methods to the MP2/6311G** optimized structures. The results are summarized in Tables S6 and S7 in the Supporting Information. At the TDHF//MP2 level, the excitation energies from S0 with relatively large oscillator strength (f > 0.1) are calculated to be (a) 330 nm (S1), (b) 251 nm (S5), and (c) 195−197 nm (S8, S9, and S10) for 1. They are (a) 340 nm (S1), (b) 254 nm (S5), and (c) 203 nm (S7) for 2.52 The calculated excitation energies for the three lowest energy absorption bands are similar for 1 and 2, in agreement with the experimental observation in solution. The CIS//MP2 method gave similar results. On the other hand, the calculated oscillator strength for the lowest energy transition band (a) around 340 nm for 2 is 1.33 times larger than that for the (a) band around 330 nm for 1. This also agrees with the experimental results that molar extinction coefficient (ε) for the (a) band around 380 nm for 2 is 1.74 times larger than the value for 1 in DCM (see the 575
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red-shifted fluorescence in solution, should considerably be restricted. As a result of this, the solid-state fluorescence emission occurs from the excited state similar to the Franck− Condon state. The position of λf was thus not largely redshifted from λa, and ΔEss was reasonably small in the solid state. Unlike in 1, λf observed at 505 nm in the solid-state spectrum was red-shifted in 2 by 52 nm (2273 cm−1) from λf in MCH solution. The position of λa in the solid state also red-shifted from that in solution. As a consequence, ΔEss were similar in solution (4521 cm−1) and in the solid state (3029 cm−1), although the value was somewhat smaller in the solid state (Tables 2 and 3). Because the degree of geometrical relaxation should be small in the solid state as in solution, it is reasonable that λf is not largely red-shifted from λa and ΔEss is considerably small in the solid state. Considering the same λa and similarly small ΔEss, it is reasonable that λf of 1 and 2 were similar in the solid state. It is important in this case that the main origins of the emission of 1 and 2 are both monomeric (τs results), consistent with the herringbone molecular arrangements in the crystals. The values of ϕf for 1 were not largely different in solution and in the solid state, whereas ϕf for 2 in the solid state decreased to be about one-tenth of the values in solution (Tables 1−3). Thus, although ϕf of 1 and 2 were very different in solution, they became considerably similar in the solid state. The small ϕf for 1 in solution can be attributed to the efficient NRD during the geometrical relaxation in the excited state. In the solid state, such an NRD route would be inefficient due to the restriction of the geometrical change. Although the intermolecular interaction such as CH/π interaction in crystal should decrease ϕf to some extent, ϕf in the solid state was thus similar to or even somewhat larger than the values in solution.57 For 2, the NRD during the excited-state geometrical relaxation would not be highly efficient in solution, thus leading to the large ϕf. The relaxation would also be restricted in the solid state; however, the intermolecular interaction in crystal should lead to a significant decrease in ϕf. In relation to the decrease in fluorescence intensity due to CH/π interaction in crystal, as observed in 1 and 2, the electronic overlap between the πorbitals of the neighboring molecules is considered to be significantly large in the crystals of conjugated molecules such as oligothiophene.58 Finally, we will discuss the effects of intermolecular (excitonic) interactions on the solid-state λf. Using the molecular structures and arrangements in crystals 1 and 2 determined by the X-ray analysis, the excitation energies and f of the excited states for single molecules and some selected pairs of adjacent molecules were calculated at the TDHF/6311G** level. The results are summarized in Figures S4 and S5 and Tables S10 and S11 in the Supporting Information. In crystal 1, two crystallographically independent molecules, X and Y, are contained in a unit cell (Figure 6a and Table 5). The S0−S1 excitation energies of single molecules X (x1; 319 nm) and Y (y1; 295 nm) are significantly different from each other (Table S10, Supporting Information). The S1 state of the single molecule is split into the two states of S1 and S2 in the molecular pair. The examination of f for the S0−S1 and S0−S2 transitions and the S1−S2 energy separation for each pair reveals that the excitonic interactions in the x1−x2 and x1−x3 pairs (Figure S4, Supporting Information) are H-type in character and probably the most effective in the crystal. Interestingly, the H-character of the interactions in the x1−yi (i = 1−4) pairs is reduced to be only partial as a result of the
the excitation energy calculations,52 it is necessary to include further electron correlation effects in calculations to improve the present results, but this costs much computer resources. The main configurations of the S1 → S0 transitions for 1 and 2 are shown to be the HOMO−LUMO transitions (Tables S8 and S9, Supporting Information). The HOMOs and LUMOs obtained for the optimized structures of the S1 states by the TDHF method (not shown) are similar to those obtained for the ground-state optimized structures (Figure S3, Supporting Information) and delocalized over whole of the molecules. The optimized structure of 1 is somewhat twisted in the ground state and thus in the Franck−Condon state, whereas the structure at the energy minimum in the excited state (i.e., the emissive state) is predicted to be completely planar (Tables 5 and 7). Therefore, the geometrical relaxation in the excited state from the twisted Franck−Condon state to the planar emissive state would be relatively large. This should lead to the red-shifted fluorescence and large ΔEss observed in solution. The largely red-shifted emission of 1 in solution is thus not considered to be due to a charge transfer excited state (CT*). On the other hand, the geometrical optimization of 2 shows that the molecule is considerably planar in the ground state and thus in the Franck−Condon state (Table 6). The excited-state structure at the energy minimum is also shown to be completely planar (Table 7). Thus the degree of the geometrical relaxation in the excited state is expected to be relatively small. This results in the fact that λf is not largely redshifted from λa, and ΔEss is relatively small in solution. Although λa were similar for 1 and 2, λf was more red-shifted and ΔEss was larger for 1 than those for 2 in solution. These observations are explained by the larger degree of the excitedstate geometrical relaxation in 1. Considering the similarly planar structures of 1 and 2 at the energy minima in the excited state, the large difference in the fluorescence properties for 1 and 2 in solution can be attributed to the difference in the molecular planarity in the ground state.56 5.2. Spectroscopic Properties in the Solid State. Absorption Spectra. Similar to the observation in solution, λa of 1 and 2 were found at almost the same positions in the solid state. Consequently, the red shifts in λa for the main bands from 375−380 nm in solution to 437−438 nm in the solid state are the same for 1 and 2 (Tables 1−3). The red shifts should mainly be attributed to (i) the restriction of the intramolecular geometrical change (the rigidification) and (ii) the intermolecular interactions in the solid state. Because the Ar−CH torsion angles in the crystal structures of 1 and 2 are frozen at almost the same angles as those in the MP2 optimized structures (Tables 5 and 6), we can roughly say that the red shifts in λa due to the molecular rigidification are not very different for 1 and 2. The very similar λa for 1 and 2 in the solid state therefore suggests that the shifts due to the intermolecular interactions are also not greatly different for 1 and 2. This is consistent with the crystal structures determined by the X-ray analysis, where the molecules are similarly arranged in the herringbone fashion. Fluorescence Spectra. In the solid-state spectrum of 1, λf was observed at 496 nm, blue-shifted by 49 nm (1812 cm−1) from λf in MCH solution. In contrast, λa in the solid state redshifted from the position in solution. Correspondingly, ΔEss of 8389 cm−1 in solution largely decreased to 2722 cm−1 in the solid state (Tables 1 and 3). In the solid state, the large geometrical relaxation accompanied by the Ar−CH internal rotation in the excited state, probably the main reason for the 576
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significantly large difference in the single-molecule S0−S1 energies of X and Y. In crystal 2, only one crystallographically independent molecule is contained in a unit cell (Figure 6b and Table 6). The calculations indicate that the excitonic interactions are considerably strong and H-like for all molecular pairs examined, except for x1−x4 (Figure S5 and Table S11, Supporting Information). We note that the excitonic interactions are considered to be more H-like in 2 than in 1, although the molecules in the crystals are similarly arranged in a typical herringbone pattern. The interactions in the x1−x4 pairs are J-like in both 1 and 2 but rather weak. The H-type excitonic interactions in 1 and 2 would lead to the red-shift in λf for the molecular pairs relative to those for the single molecules. In fact, however, λf of 1 blue-shifted on going from solution to the solid state (Tables 1 and 3). The observation can be understood by assuming that the blue shift in λf due to the restricted geometrical relaxation in the solid state is larger than the red shift due to the H-type interactions in crystal. In 2, the geometrical relaxation in solution is less important than in 1, and we simply observed the red shift in λf (Tables 2 and 3) as a result of various kinds of intermolecular interactions in crystal including the H-type interactions as described above.
S11); full author list for ref 38. This material is available free of charge via the Internet at http://pubs.acs.org.
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*Tel: +81-29-861-6390. Fax: +81-29-861-3029. E-mail: y.
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr Y. Kawanishi (AIST) for the use of the fluorescence spectrometer, and Dr S. Tsuzuki (AIST) for valuable comments on the calculated results of torsional potentials in the ground state. We also acknowledge Dr S. Inoue (HORIBA) for his help in the τs measurements using a FluoroCube 3000-U. One of the authors (Y. Shimoi) acknowledges partial financial support from JSPS KAKENHI (No. 21540334).
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REFERENCES
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CONCLUSIONS The large difference in the fluorescence properties of 1 and 2 in solution can be attributed to the difference in molecular planarity in the ground state, arising from the different steric hindrance around the Ar−CH single bonds. Thus, the redshifted and weak emission of 1 relative to that of 2 would be due to the larger degree of the geometrical relaxation accompanied by the Ar−CH internal rotation, from the twisted Franck−Condon state to the planar state having small CT character at the energy minimum in the excited state. The weakly λex-dependent fluorescence spectrum of 2 in solution is explained by the Ar−CH ground-state isomerism (the NEER principle). The restriction of the Ar−CH internal rotation in the solid state leads to the inefficient excited-state geometrical relaxation in 1 and the inefficient ground-state rotational isomerism in 2, resulting in the similar fluorescence behavior of 1 and 2 in crystals. On the basis of the results of spectroscopic measurements, X-ray crystal structure analysis, and ab initio quantum chemical calculations, the structure−property relationship for the two structural isomers of dinaphthylhexatrienes, 1 and 2, can thus be understood fairly well in terms of the internal rotation around the Ar−CH bonds in the ground and excited states.
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AUTHOR INFORMATION
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ASSOCIATED CONTENT
S Supporting Information *
Crystallographic data of 1 and 2 in CIF format; crystal and structure refinement data; τs data of 1 and 2 at different λem in solution and in the solid state (Tables S1−S3); major geometrical parameters for the ground-state optimized structures of 1 and 2 (conformers B and C) (Tables S4 and S5); vertical excitation and emission energies of 1 and 2 (Tables S6−S9); absorption and fluorescence spectra of 1 and 2 in solvents with different polarity (Figures S1 and S2); HOMO and LUMO of 1 and 2 obtained by the ground-state optimized structures at the MP2/6-311G** level (Figure S3); calculation results of the excitonic interactions of molecular pairs in the solid state (Figures S4 and S5, Tables S10 and 577
dx.doi.org/10.1021/jp306103a | J. Phys. Chem. A 2013, 117, 566−578
The Journal of Physical Chemistry A
Article
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in particular, by the CIS calculation is reported for all-trans-1,3,5hexatriene.53 To describe the correct energy ordering between the 2Ag and 1Bu states for the present molecules, further electron correlation effects are necessary to be taken into account in calculation. If two excited-state potential surfaces are close in energy, a multireference method would be needed. (53) Hsu, C.-P.; Hirata, S.; Head-Gordon, M. J. Phys. Chem. A 2001, 105, 451−458. (54) Vroegop, P. J.; Lugtenburg, J.; Havinga, E. Tetrahedron 1973, 29, 1393−1398. (55) Jacobs, H. J. C.; Havinga, E. Adv. Photochem. 1979, 11, 305− 373. (56) Although the potential curve and the optimized structure for the 2Ag state should also be examined to discuss the fluorescence emission process of 1 more correctly,52 the present experimental results can be understood by the results of geometrical optimization for the 1Bu state. This suggests that the shape of the potential curve for the 2Ag state may considerably be similar to that of the 1Bu state at least for the Ar− CH torsional mode. (57) Shimizu, M.; Takeda, Y.; Higashi, M.; Hiyama, T. Angew. Chem., Int. Ed. 2009, 48, 3653−3656. (58) Garnier, F. Acc. Chem. Res. 1999, 32, 209−215.
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