Halogenated (F, Cl, Br, or I) Diphenylhexatrienes: Crystal Structures

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Halogenated (F, Cl, Br, or I) Diphenylhexatrienes: Crystal Structures, Fluorescence Spectroscopic Properties, and Quantum Chemical Calculations Yoriko Sonoda,*,† Midori Goto,‡ Yuki Matsumoto,§ Yukihiro Shimoi,§ Fumio Sasaki,† and Akihiro Furube∥,# †

Electronics and Photonics Research Institute and ‡Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, Japan § Research Center for Computational Design of Advanced Functional Materials and ∥Research Institute of Instrumentation Frontier, National Institute of Advanced Industrial Science and Technology (AIST), Umezono 1-1-1, Tsukuba, Ibaraki 305-8568, Japan S Supporting Information *

ABSTRACT: A series of halogenated compounds, (E,E,E)-1,6-di(4-X-phenyl)hexa-1,3,5trienes (1: X = F, 2: X = Cl, 3: X = Br, 4: X = I), were synthesized and their crystal structures and fluorescence emission properties were systematically investigated. Single-crystal X-ray analysis reveals that molecules are arranged via X/X halogen bonds and/or CH/X-type hydrogen bonds to form a herringbone structure in 1 and π-stacked structures in 2−4. In the structures of 2 and 3, which are almost isomorphous, the distance and displacement for the nearest stacking molecules are smaller than those in 4. Although the structures of 2−4 are basically not greatly different from each other, the nearest-neighbor arrangements are πstacked in 2 and 3, but herringbone in 4. Steady-state and time-resolved measurements show that the solid-state fluorescence properties also strongly depend on the halogen size. The fluorescence spectra are red-shifted and the Stokes shifts are large in 2 and 3 relative to those in 1 and 4, resulting from the difference in molecular arrangement in the crystal structure. The experimentally observed clear correlation between crystal structure and optical transition energy is reproduced fairly well by quantum chemical calculations for the excited states of molecular pairs in the X-ray determined structures of 1−4.

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increase in the number of fluorine atom leads to the decrease in distance and displacement between the nearest stacking molecules in the crystal structures.20 The solid-state fluorescence spectra shift to longer wavelengths as the number of fluorine atom increases, while the spectra in solution (i.e., the properties of isolated molecules) are not greatly different. The observed large differences in the solid-state spectra should therefore be attributed mainly to the differences in molecular arrangement in the crystal structure. It is known here that not only fluorine but also other heavier halogen atoms play an important role in constructing the crystal structures of small organic molecules.25−30 The nature of halogen/halogen (X/X) and CH/X-type hydrogen bonds has been intensively studied recently from both crystallographic and theoretical points of view.31−33 Up to now, however, only a limited number of papers have reported the single crystal structures of all four kinds of halogenated (F, Cl, Br, or I) compounds having the same molecular backbones.34−37 From a photophysical point of view, on the other hand, it is well-known that heavy halogen atoms such as Br and I in organic molecules

INTRODUCTION α,ω-Diphenylpolyenes have long received considerable attention due to their interesting spectroscopic properties1,2 and are expected to be potential optical3 and photofunctional4−6 materials. Due to the highly emissive properties, they can be new fluorescent probes and imaging agents in medicinal chemistry.7−10 The electronic excited states of these polyenes have also been the subjects of many theoretical studies.11−14 Among diphenylpolyenes, (E,E,E)-1,6-diphenylhexa-1,3,5triene (DPH) is the shortest chromophore in which the 2Ag state is clearly lower in energy than the 1Bu state. It is wellknown that DPH exhibits dual fluorescence from the first (S1) (2Ag) and the second (S2) (1Bu) singlet excited states to the ground state (S0) (1Ag) in solution.2 As for the emission properties of DPHs in the solid state, we first showed that the unsubstituted parent compound exhibited measurable fluorescence in the microcrystalline state.15 The solid-state fluorescence from DPH16,17 and related linear polyenes18 has attracted much attention recently due to their efficient singlet fission processes.19 Our recent studies further revealed the strong correlation between crystal structure and fluorescence properties for a number of symmetrically20−23 and asymmetrically24 substituted DPHs. For a series of ringfluorinated DPHs having different substitution patterns, the © XXXX American Chemical Society

Received: April 18, 2016 Revised: June 3, 2016

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DOI: 10.1021/acs.cgd.6b00590 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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synthesized by the double Wittig reactions of 4-halobenzaldehydes and (E)-2-butene-1,4-bis(triphenylphosphonium chloride) (TCI). The elemental analysis of 4 was performed using a CE Instruments EA1110 for C and H, and an ion chromatography system Dionex ICS2000 with a pretreatment combustion unit Mitsubishi Chemical Analytech AQF-100 for I. HR-MS data were obtained using a Hitachi M80B spectrometer. 1H NMR spectra were measured using a Bruker Avance 400 (400.03 MHz) with tetramethylsilane (TMS) as internal reference. J values are given in Hz. All the solvents used in the absorption and fluorescence measurements were of spectroscopic grade (Dojin). For the solid-state spectroscopic measurements, the samples were in the microcrystalline state, not ground to a powder in all cases. The sample thickness was roughly estimated to be 0.05 mm. All spectroscopic measurements were performed at room temperature in air unless otherwise noted. (E,E,E)-1,6-Di(4-chlorophenyl)hexa-1,3,5-triene (2). To a solution of 4-chlorobenzaldehyde (WAKO) (1.41 g, 10.0 mmol) and the triphenylphosphonium salt (3.25 g, 5.0 mmol) in ethanol (25 mL) was added a solution of sodium ethoxide in ethanol (0.60 M, 17 mL) at room temperature under nitrogen atmosphere. After stirring for 22 h, aqueous ethanol (45% v/v, 130 mL) was added to the reaction mixture and the solution was vigorously stirred for 1 h. The resulting yellow precipitate was filtered off, washed with aqueous ethanol (60% v/v, 100 mL) and water (80 mL), and dried under vacuum at room temperature (crude yield 66%). The crude product (Z−E mixture) was irradiated in toluene (Tol) with Pyrex-filtered light at room temperature in air to induce Z → E isomerization. A high-pressure mercury lamp (500 W) was used as a light source. After the irradiation, the solvent was evaporated and the resulting yellow solid (predominantly E,E,E) was recrystallized twice from Tol to give single crystals of 2 suitable for X-ray analysis. The purity was checked by HPLC. Mp 496 K (223 °C) (lit.43 220−221 °C). 1H NMR (CDCl3) δ 7.36 (4H, d, J 8.6, arom.), 7.30 (4H, d, J 8.6, arom.), 6.86 (2H, ddd, J 15.6, 7.0, and 2.9, triene), 6.57 (2H, d, J 15.5, triene), 6.53 (2H, dd, J 7.0 and 2.9, triene). UV−vis λmax (MeCN) 360 nm (ε = 85 300 M−1 cm−1). (E,E,E)-1,6-Di(4-bromophenyl)hexa-1,3,5-triene (3). Triene 3 was prepared from 4-bromobenzaldehyde (Aldrich) and the phosphonium salt by a similar procedure that described for 2 (crude yield 63%). Double recrystallization of the irradiated crude product (predominantly E,E,E) from Tol gave single crystals of 3 suitable for X-ray analysis. The purity was checked by HPLC. Mp 519 K (246 °C) (lit.42 241−242 °C). 1H NMR (CDCl3) δ 7.44 (4H, d, J 8.5, arom.), 7.28 (4H, d, J 8.5, arom.), 6.86 (2H, ddd, J 15.5, 7.0, and 3.0, triene), 6.54 (2H, d, J 15.6, triene), 6.51 (2H, dd, J 7.1 and 3.0, triene). UV− vis λmax (MeCN) 362 nm (ε = 90 900 M−1 cm−1). (E,E,E)-1,6-Di(4-iodophenyl)hexa-1,3,5-triene (4). Triene 4 was prepared from 4-iodobenzaldehyde (WAKO) and the phosphonium salt by a similar procedure that described for 2 (crude yield 61%). The irradiated crude product (predominantly E,E,E) was recrystallized twice from Tol. Single crystals of 4 suitable for X-ray analysis were grown from a solution of the recrystallized product in Tol by very slow evaporation of the solvent at room temperature in dark. The purity was checked by HPLC. Mp 537 K (264 °C). Anal. Calcd for C18H14I2: C, 44.66; H, 2.91; I, 52.43. Found: C, 44.92; H, 2.72; I, 52.21. HR-MS Found: M+, 483.9162. Calcd for C18H14I2: M, 483.9185. 1H NMR (CDCl3) δ 7.64 (4H, d, J 8.5, arom.), 7.15 (4H, d, J 8.4, arom.), 6.87 (2H, ddd, J 15.6, 7.0, and 3.1, triene), 6.52 (2H, d, J 15.3, triene), 6.51 (2H, dd, J 6.9 and 3.1, triene). UV−vis λmax (MeCN) 367 nm (ε = 93 800 M−1 cm−1). Photochemical Reactivity. Irradiation of 1−4 in solution induced rapid Z,E-photoisomerization at room temperature in air as shown by UV−vis absorption spectra. Prolonged irradiation caused decomposition. In the solid state, 1 and 2 were photochemically stable, while 3 and 4 were somewhat photoreactive. All spectral measurements were therefore carried out with minimum exposure to light. Single Crystal X-ray Structure Analysis. The crystal structure of 1 was reported previously.20 The single crystal X-ray diffraction measurements of 2−4 were performed at 183 K using a Bruker SMART CCD area-detector diffractometer with graphite monochro-

in general enhance the singlet−triplet intersystem crossing (isc) probability, and this often results in the large decrease in fluorescence quantum yield (ϕf) in solution (the internal heavyatom effect (HAE)).38 Therefore, it is expected that the introduction of halogen atoms into organic molecules will lead to the large changes in solid-state photoproperties as a result of the change in molecular arrangement in the crystal structure and that in molecular photophysics due to the internal HAE. To the best of our knowledge, there have been very few examples of systematic and comprehensive studies on the effects of halogen substitution on the solid-state fluorescence properties of purely organic (metal-free) compounds.37 This kind of study is very important for our fundamental understanding and further development of emissive organic solids. In this study, we synthesized a series of 4,4′-dihalosubstituted DPHs (1−4, Chart 1) to investigate the crystal Chart 1. Chemical Structures of 1−4

structures and fluorescence spectroscopic properties systematically. Although the crystal structure of 120 and the fluorescence properties of 1 and 2 in solution39,40 have been reported by us and others,41 the details of their solid-state photoproperties are unknown at present. Triene 3 has been synthesized and identified previously;42 however, its photophysical properties are not reported. Triene 4 was newly prepared in this study. We also performed quantum chemical calculations for 1−4 to understand the excited states and electronic transitions involved in the photophysical processes in solution and in the solid state. Our results revealed that both the crystal structure and the solid-state fluorescence properties of 1−4 clearly depended on the size of halogen, and were strongly correlated with each other.

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EXPERIMENTAL SECTION

Materials. The preparation and purification procedures and spectroscopic identification data of (E,E,E)-1,6-di(4-fluorophenyl)hexa-1,3,5-triene (1) were described previously.20 Trienes 2−4 were B



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Table 1. Crystal Data of 1−4 formula formula weight crystal color, habit crystal size (mm3) crystal system space group a (Å) b (Å) c (Å) α (degree) β (degree) γ (degree) V (Å3) Z Dcalc (g/cm3) T (K) mp (K) R1 (I > 2σ(I)) a

1a

2

3

4

C18H14F2 268.29 pale yellow, needle 0.20 × 0.04 × 0.03 triclinic P1̅ 9.582(4) 11.523(5) 12.890(5) 85.425(8) 84.296(9) 75.810(8) 1370.8(10) 4 1.300 183(2) 463 0.0663

C18H14Cl2 301.19 pale yellow, rectangular 0.40 × 0.10 × 0.05 monoclinic P21/c 15.5614(18) 4.0163(5) 12.0474(14) 90 105.5750(10) 90 725.30(15) 2 1.379 183(2) 496 0.0355

C18H14Br2 390.11 pale yellow, rectangular 0.35 × 0.10 × 0.07 monoclinic P21/c 15.738(2) 4.0499(6) 12.1415(18) 90 104.5400(10) 90 749.08(19) 2 1.730 183(2) 519 0.0214

C18H14I2 484.09 yellow, plate 0.30 × 0.20 × 0.02 monoclinic P21/c 13.6707(10) 7.7422(6) 7.6376(6) 90 97.8330(10) 90 800.83(11) 2 2.008 183(2) 537 0.0186

Ref 20. For the measurements in the UV−vis region using CCD camera and those in the NIR region, a glass filter (Shott GG420) was used to remove the excitation light (Ex = 405 nm). In other cases, no filter was used on the emission side during fluorescence emission monitoring in the solid state. Measurements of Fluorescence Lifetimes. Fluorescence decay curve and lifetime (τs) measurements for Em < 800 nm were carried out by the time-correlated single-photon counting (TCSPC) method, using a Hamamatsu C11367 equipped with UV−vis LEDs (time resolution 800 nm were performed using the TCSPC method (Hamamatsu C7990) equipped with an excitation pulse LED (NanoLED-405L, Ex = 405 nm, 900 nm) were carried out at room temperature in air and under nitrogen atmosphere using a liquid nitrogen optical cryostat (Oxford, Optistat). The spectra were recorded using an InGaAs detector (Roper Scientific, model ADS-ST133 for OMA-V-1024/ LNTL) attached to a monochromator (Acton SpectraPro 2300i). In all cases, Ex was set at 405 nm by using an ultraviolet semiconductor laser. The samples were placed between quartz plates (10 × 10 mm2).

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RESULTS AND DISCUSSION Crystal Structures. Molecular Structures. Table 1 shows the single-crystal X-ray data of 1−4. The structure of 1 contains two molecules, A and B, in a unit cell. Both molecules are basically symmetric, although they have no symmetry centers C



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Figure 1. Crystal packing diagrams of (A) 1,20 (B) 2, (C) 3, and (D) 4.

crystallographically. In the structures of 2−4, only one molecule is contained in a unit cell, and each molecule has a center of symmetry. The ORTEP drawings are shown in Figure S1 (Supporting Information (SI)). The major intramolecular parameters of 1−4 are summarized in Table S1 (SI). The lengths of the CC single and CC double bonds, and the bond length alternation in the triene, are similar in molecules 1−4. As for the twist around the single bond connecting the benzene ring and the triene chain, the torsion angles of C1−C6−C7−C8 and C14−C13−C12−C11

in molecule A of 1 and those of C19−C24−C25−C26 and C32−C31−C30−C29 in B are smaller than the angles of C6− C1−C7−C8 in molecules 2−4 (Table S1). However, the mean deviations from the least-squares planes (LSPs) defined by all non-hydrogen atoms are clearly larger in 1 than in 2−4. Namely, although molecules 1−4 are all basically planar in the crystal structures, the planarity of 1 is different from those of 2−4. This is probably due to packing. Molecular Arrangements. The crystal packing diagrams of 1−4 are displayed in Figure 1. Molecules 1 are arranged in a D



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typical herringbone fashion (Figure 1A). The edge-to-face interactions are observed along the a-axis. The dihedral angle formed by the LSPs of A and B is 47.51(8)°. The crystal packing patterns of 2−4 are different from that of 1 (Figure 1B−D). In these crystal structures, the molecules are stacked along the b-axes. The lattice constants b are similarly small in 2 (4.02 Å) and 3 (4.05 Å), while it is relatively large in 4 (7.74 Å). The distances and displacements for the LSPs of the nearest stacking molecules (A1 and A2 in Figure 1B−D) in the structures of 2−4 are summarized in Table S2 (SI).54 As seen, the large stacking distance (d) in 4 relative to those in 2 and 3 comes not from the displacement along the long molecular axis (dp), but from the displacement along the short molecular axis (dr) between the molecules. Thus, the stacking would be very weak in 4. On the other hand, the dihedral angles formed by the LSPs of the two molecules in the neighboring stacks (A1 and A3 in Figure 1B−D) are 66.64(5)°, 68.43(6)°, and 68.99(8)° in 2, 3, and 4, respectively. Although the structures of 2−4 are basically not greatly different from each other, the nearest-neighbor arrangements are π-stack in 2 and 3, but herringbone in 4. As shown above, not only the packing patterns, but also the intermolecular distances and dihedral angles are very similar in 2 and 3. The crystals of 2 and 3 can therefore be regarded as isomorphous with the chlorine−bromine substitution. Also for other halogenated compounds, the crystal structure of the fluorinated derivative is clearly different from that of the chlorinated one, while the chlorinated and brominated derivatives are isostructural.34,55,56 Intermolecular Interactions. The nature of X/X bonds25−30 and its comparison with that of CH/X hydrogen bonds31−33 have attracted much attention recently from both crystallographic and theoretical points of view. In general, it is considered that, with increasing the halogen size, CH/X hydrogen bonds become less important and X/X halogen bonds become more important in constructing the crystal structures of halogenated compounds.33−37,55−58 This trend can clearly be seen in the structures of 1−4. Namely, CH/F hydrogen bonds are observed but “F/F halogen bonds” are not found in 1. In 2 and 3, both CH/X hydrogen bonds and X/X halogen bonds are observed. In 4, I/I halogen bonds are observed but “CH/I hydrogen bonds” are not found. Other weak interactions such as CH/π (in 1−4) and π/π (in 2 and 3) are also involved in the construction of these structures. Fluorescence Spectroscopic Properties. Properties in Solution. Figure S2 (SI) shows the absorption and fluorescence spectra of 1−4 in MCH at room temperature. The spectra of 1 and 2 are almost identical to those reported previously.39,40 The spectroscopic data are summarized in Table 2. The absorption maximum (λa) shifts to longer wavelengths with the increase in halogen size. Similar red-shifts in absorption with increasing the halogen size59 are observed for other symmetrically halogenated compounds such as 1,4dihalobenzenes60,61 and 4,4′-dihalostilbenes.62,63 The fluorescence emission maximum (λf) also red-shifts as the size of halogen increases; however, the shifts in λa are slightly larger than those in λf. As a result of this, the Stokes shift (ΔEss) calculated from λa and λf decreases with the increase in the halogen size. The absorption spectra of 1−4 closely resemble each other, and are also very similar to that of the unsubstituted DPH. In contrast, their fluorescence spectra are somewhat different in shape. Namely, the small shoulder observed as a blue-edge

Table 2. Absorption and Fluorescence Data of 1−4 in Methylcyclohexane at Room Temperature λaa (nm) λfb (nm) ΔEssc (cm−1) ϕfd τse (ns) kff (s−1) knrg (s−1)

1

2

3

4

351 425 4961 0.47 7.7 6.1 × 107 6.9 × 107

362 435 4636 0.48 7.2 6.6 × 107 7.2 × 107

364 435 4484 0.40 3.9 1.0 × 108 1.5 × 108

369 438 4269 0.12 0.74 1.6 × 108 1.2 × 109

a

Absorption maxima. bFluorescence emission maxima. Excitation wavelength: Ex = λa. cStokes shifts calculated from λa and λf. d Fluorescence quantum yields. ±5%. eFluorescence lifetimes. Excitation wavelength: Ex = 365 nm. Emission (monitor) wavelength: Em = λf. fRadiative rate constants: kf = ϕf/τs. gNonradiative rate constants: knr = (1−ϕf)/τs.

around 400 nm in each spectrum becomes less evident as the halogen size increases. For 1 and 2, these shoulders (the highenergy bands)39,40 are assigned to the emission from S2 (Bu) generated by the thermal repopulation from S1 (Ag), whereas the main bands in the spectra are assigned to the emission from S1. On the other hand, the absorption is assigned to the fully allowed S0 (Ag) → S2 (Bu) transition. If we assume that this is also the case with 3 and 4, then the observed larger red-shift in λa than that in λf with increasing the halogen size suggests that the S0 → S2 energy is more sensitive to the halogen size than the S1 → S0 energy. The fluorescence shoulder that has almost disappeared in the spectrum of 4 can thus be explained by the fact that the largely red-shifted S2 → S0 shoulder is hidden by the less shifted and strong S1 → S0 band. The fluorescence excitation spectra are fundamentally the same as the absorption spectra, and the maximum wavelength (λex) in the excitation spectrum agrees well with λa for all four trienes. In Table 2, ϕf decreases in solution as the halogen size increases. A similar trend is reported for halogenated 2-phenylbenzoxazoles.37 The fluorescence decay curve of each triene in solution is able to be fitted by a monoexponential function to give a single-component τs, indicating the monomeric origin of emission. The value of τs is nearly independent of Em. A large decrease in τs is observed on going from 3 to 4. The radiative and nonradiative rate constants (kf = ϕf/τs and knr = (1 − ϕf)/ τs, respectively) are summarized in Table 2. As shown, knr of 4 is calculated to be much larger than that for 3. This is probably due to the enhanced efficiency of quenching for the singlet excited state of 4 responsible for the fluorescence emission. More specifically, it can be attributed to the increased probability of singlet-to-triplet isc as a result of the internal HAE of I. We also note here that fluorescence emission process is in general not greatly affected by HAE.38 This is in accordance with our observation that both kf and knr increase with increasing halogen size, however, the change in knr is much greater than that in kf. The values of kf are much smaller than what would be expected for allowed transitions. This is in accordance with the forbidden nature of S1 (2Ag) → S0 (1Ag) fluorescence transition. Thus, in this case, the S1 → S0 fluorescence is observed despite the forbidden character of S1. This should be due to the Herzberg−Teller (HT) coupling, where the 2Ag state borrows intensity from the higher-lying 1Bu state.64 This also explains why kf increases systematically on E



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disappeared at 77 K. This results in the observation of very similar shapes in their spectra. At 77 K, the excitation spectrum red-shifts whereas the fluorescence spectrum blue-shifts from those at room temperature, leading to the large overlap of the excitation and fluorescence spectra for each triene. Correspondingly, ΔEss at 77 K calculated from λex and λf are smaller than the values at room temperature. We also see that the temperature effect on λa (λex) is slightly but clearly larger than that on λf. For 2, the spectrum at 77 K measured in this study (Figure S3(b)) is very similar to that at 166 K reported in the literature.40 In ref 40, the large red-shift in absorption is explained by the large decrease in the S0 →S2 transition energy with lowering the temperature, whereas the small (blue-)shift in fluorescence is attributed to the low temperature sensitivity of the S1 → S0 energy. Therefore, the disappearance of the fluorescence shoulders at 77 K in the spectra of 1−4 can be explained by assuming that the red-shifted S2 → S0 shoulders are obscured by the strong S1 → S0 bands. It is interesting that the red-shifts in λa (λex) and λf, and the decrease in ΔEss with the increase in the halogen size, are similarly observed at room temperature and 77 K, when the molecules are isolated. Properties in the Solid State. Figure 2 shows the solid-state fluorescence and excitation spectra of 1−4 measured at room temperature. The spectroscopic data are summarized in Table 4.

going from 1 to 4, since the energy difference between S1 and S2 becomes smaller, and thus HT coupling increases. Figure S3 (SI) shows the fluorescence and excitation spectra of 1−4 in MCH glass measured at 77 K. The spectroscopic data are summarized in Table 3. Table 3. Fluorescence Data of 1−4 in Methylcyclohexane Glass at 77 K λexa (nm) λfb (nm) ΔEssc (cm−1)

1

2

3

4

359 420 4046

369 431 3898

372 431 3680

377 433 3431

a

Fluorescence excitation maxima. Emission (monitor) wavelength: Em = 420 nm for 1, 430 nm for 2 and 3, and 435 nm for 4. bFluorescence emission maxima. Excitation wavelength: Ex = 351 nm for 1, 362 nm for 2, 364 nm for 3, and 369 nm for 4. cStokes shifts calculated from λex and λf.

For all trienes, the spectra are more distinctly structured at 77 K than those at room temperature in MCH solution. Similar temperature effects are often observed in the spectra of small organic molecules including other ring-substituted DPHs,15 and are usually attributed to the reduced molecular vibrations and other movements in low-temperature organic glasses. More importantly, the blue-edge shoulders observed differently in the room-temperature fluorescence spectra of 1−4 have all

Figure 2. Fluorescence excitation (blue) and emission (red) spectra of (a) 1, (b) 2, (c) 3, and (d) 4 at room temperature in the solid state. Excitation wavelength: Ex = 360 nm for 1 and 405 nm for 2−4. Emission (monitor) wavelength: Em = 461 nm for 1, 530 nm for 2 and 3, and 540 nm for 4. F



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dependences of λf and λex on the halogen size, ΔEss calculated from λf and λex are significantly larger for 2 and 3 than those for 1 and 4. These observations in the solid state are very different from those in solution (Table 2), in which both λa and λf redshift and ΔEss somewhat decreases with increasing in the halogen size. These results in the solid state can reasonably be explained by the difference in the crystal structure using the exciton theory.66,67 In the structures of 2 and 3, the molecules are arranged in a π-stacking fashion with relatively small distances and displacements (Figure 1B,C, and Table S2). In these cases, we can expect strong H-type excitonic interactions68,69 and large splitting energies (ΔE) between the S1 and S2 states of molecular pairs (and aggregates). As ΔE in the exciton theory corresponds to ΔEss in the experimental spectra, it is reasonable that we observe large ΔEss for 2 and 3 in the solid state. In the structure of 1, on the other hand, the molecules are arranged in a herringbone fashion (Figure 1A). Although the molecular arrangement in 4 is a π-stacked pattern, as those in 2 and 3, the distance and displacement for the stacking molecules in the structure are significantly large (Figure 1D and Table S2) as described above. These structural features of 1 and 4 will lead to weak excitonic interactions, resulting in the observed small ΔEss in the solid state. Thus, there is a very clear correlation between crystal structure and solid-state ΔEss for 1−4. In the solid state, ϕf of 1−4 are much smaller than the values in solution (Tables 2 and 4). Although ϕf of 3 is measured to be somewhat larger than those of the others, the values are

Table 4. Absorption and Fluorescence Data of 1−4 at Room Temperature in the Solid Statea λab (nm) λexc (nm) λfd (nm) ΔEsse (cm−1) ϕff ⟨τs⟩g (ns) kfh (s−1) knri (s−1)

1

2

3

4

406 406 461 2.9 × 103 0.013 0.86 1.5 × 107 1.2 × 109

423 419 525 4.8 × 103 0.018 1.5 1.2 × 107 6.7 × 108

435 430 527 4.3 × 103 0.034 0.98 3.5 × 107 9.8 × 108

427 428 500 3.4 × 103 0.016 0.11 1.4 × 108 8.9 × 109

a

In the microcrystalline state. bAbsorption maxima. cFluorescence excitation maxima. Emission (monitor) wavelength: Em = 461 nm for 1, 530 nm for 2 and 3, and 540 nm for 4. dFluorescence emission maxima. Excitation wavelength: Ex = 360 nm for 1 and 405 nm for 2− 4. eStokes shifts calculated from λex and λf. fFluorescence quantum yields. ±10%. gIntensity weighted mean lifetimes. Excitation wavelength: Ex = 405 nm. Emission (monitor) wavelength: Em = λf. h Radiative rate constants: kf = ϕf/⟨τs⟩. iNonradiative rate constants: knr = (1−ϕf)/⟨τs⟩.

The fluorescence spectrum of 1 shows two clear peaks around 440 and 460 nm, while the spectra of 2−4 exhibit broad bands with very weak shoulders. In 2 and 3, λf are located at longer wavelengths than those in 1 and 4. On the other hand, λex red-shifts with the increase in the halogen size. The position of λa shows a similar trend.65 As a result of the different

Figure 3. Fluorescence excitation (blue) and emission (red) spectra of (a) 1, (b) 2, (c) 3, and (d) 4 at 77 K in the solid state. Excitation wavelength: Ex = 360 nm for 1 and 405 nm for 2−4. Emission (monitor) wavelength: Em = 470 nm for 1, 540 nm for 2, and 530 nm for 3 and 4. G



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(Figure 3b,c), although the spectra of 3 shown in Figure 4b exhibit some shoulders at low temperatures. More interestingly, λf of 2 and 3 clearly red-shift as the temperature decreases. The spectral shift is larger in 2 than in 3. In the case of 4, the fluorescence spectrum exhibits three peaks around 465, 500, and 525 nm. The spectrum is more distinctly structured at 77 K than at room temperature; however, the positions of λf are almost the same at both temperatures. In the excitation spectra of 1 and 4, λex are located at fundamentally the same positions at room temperature and 77 K, whereas in 2 and 3, λex are significantly blue-shifted at 77 K from those at room temperature. Table 5 shows ΔEss calculated from λex and λf at 77 K. In the cases of 2 and 3, ΔEss at 77 K are considerably larger than the values at room temperature. This is a result of a combination of the blue-shift in λex and the red-shift in λf with temperature reduction. In 1 and 4, on the other hand, the temperature effects on ΔEss are small. The amounts of the shifts in λex and λf and the resulting change in ΔEss with the decrease in temperature are thus clearly different for 1−4 in the solid state. This is in sharp contrast to the observations in MCH solution and glass, in which λa (and λex) is red-shifted, λf is blueshifted, and ΔEss decreases with lowering temperature for all four trienes. At room temperature in the solid state, ΔEss strongly correlates with the molecular arrangement in the crystal structure, as described above. Therefore, the large increase in ΔEss observed for 2 and 3 with temperature reduction can at least partially be due to the decrease in intermolecular distance in the structure at low temperatures. On the other hand, the very small effect of temperature on ΔEss observed for 4 suggests the small change in crystal structure with temperature reduction. This is probably because the molecular movements in the crystal lattice are more restricted in 4 than in 2 and 3 due to the presence of the heavy atom of I. The small change in ΔEss observed for 1 with reduced temperature can be attributed to the herringbone molecular arrangement in its structure. As for the change in spectral shape, the fluorescence spectra of 2 and 3 are broad even at low temperatures. This suggests that the sharpening of the spectra due to the suppression of intramolecular vibration and other intramolecular structural deformation and the broadening of the spectra due to the increase in magnitude of the intermolecular interaction with reduced temperature are not greatly different. On the other hand, the large change in the spectral shape of 4 with temperature reduction suggests that the spectral sharpening is more effective than the broadening in this case. The small temperature effects on the spectral shape for 2 and 3 relative to that for 4 can thus be understood by the large increase in the magnitude of intermolecular interaction in 2 and 3 with temperature reduction. This is consistent with the different degrees of temperature effects on ΔEss observed for 2−4 described above. It has previously been shown that π-stacking (depending on its degree) leads to excimer-like features (i.e., loss of fine structure and red-shift), because of the charge transfer character of the lowest excited state.71 This gives rise to different intermolecular arrangements in the ground and excited states. The spectral broadening is even visible at low temperatures.72 Thus, in the present case, the broad spectra of 2 and 3 at 77 K are a direct signature of strong π-stacking. On the other hand, the structured spectrum of 4 at 77 K suggests only weak πstacking and can be the evidence for the herringbone

practically not greatly different. The intensity weighted mean lifetimes (⟨τs⟩) of 1−4 in the solid state are considerably shorter than τs in solution, and ⟨τs⟩ of 4 is much shorter than those of 1−3. The fluorescence decay curves show multiexponential behavior for all four trienes. Table S3(a) (SI) summarizes τs and ⟨τs⟩ at different Em ( 3 for the pairs of 1 and 3 in parallel arrangement (Table S6(b) and Figure S6(a), SI). For the pairs of 2, 4, and 5 with nonparallel molecular arrangement (Figure S6(a)), the S0 → S1 and S0 → S2 transitions have f < 0.01 and f = 0.3−1.0, respectively (Table S6(b)). For all the pairs we examined, the S0 → S1 transitions are forbidden and the lowest allowed transitions are S0 → S2. This indicates that the excitonic interactions are all H-type in the crystal structure of 1. The calculated splitting energies ΔE(calc), that is, the energy differences between S1 and S2, for the pairs 1−5, are summarized in Table 6. It is interesting that ΔE(calc), which should be strongly affected by the magnitude of excitonic interaction, are not very small for the nonparallel and parallel pairs of 1−5, even in the herringbone crystal structure of 1. For the nonparallel pairs 2, 4, and 5 in 1, ΔE(calc) are somewhat

arrangement as the primary packing motif as described above. The structured spectrum of 1 should also be a signature of herringbone arrangement. This is probably because the edge-toface arrangement of the adjacent molecules in the herringbone layers does not permit strong intermolecular vibronic coupling.71 Quantum Chemical Calculations. Isolated Molecules. The excitation energies of the isolated molecules of 1−4 are calculated at the levels of TD-HF/6-311G**//MP2/6-311G** and TD-CAM-B3LYP/6-311G**//CAM-B3LYP/6-311G**. The results are summarized in Table S4 (SI). At both levels of calculation, the transition energies to the lowest allowed singlet states successfully reproduce the experimentally observed red-shift in the absorption energy in solution with the increase in the halogen size. These transitions are S0 → S1 with main configurations of HOMO−LUMO for all four molecules. Table S5 (SI) shows the energies of HOMO and LUMO for each triene calculated at the levels of HF/6-311G**//MP2/6311G** and CAM-B3LYP/6-311G**//CAM-B3LYP/6311G**. Although both the HOMO and LUMO energies decrease with the increase in halogen size, the decrease in the LUMO is somewhat larger than that in the HOMO. This suggests that the experimentally observed red-shift in absorption can be ascribed to the further decrease in the LUMO energy. Figure S5 (SI) shows the HOMOs and LUMOs of 1−4 obtained by using the ground-state structures optimized at the CAM-B3LYP/6-311G** level. As seen, the lowest allowed transitions are π−π* in nature in all cases. Here, we comment on the ordering of the excited states. In polyenes, the optically forbidden 2Ag state is often lower than or close to the 1Bu state in energy due to electron correlation effects.73,74 In fact, for 1 and 2, it is suggested experimentally that the S0, S1, and S2 states correspond respectively to the 1Ag, 2Ag, and 1Bu states and thus that the lowest allowed transition is S0 → S2.39,40 In the present calculation, the 2Ag state for each molecule is higher in energy than the 1Bu state. In order to describe the correct location of the 2Ag state for the present molecules, it is necessary to take account of further electron correlation effects in calculation, in particular, double excitations.13,14,75 Excitonic Interaction in Crystals. The excited states for the selected pairs of neighboring molecules in the crystal structures

Table 6. Calculated Transition Energies of S0−S1 and S0−S2, and ΔE(calc)a for Selected Molecular Pairsb in the Crystal Structures of 1−4c

1

2 3 4

molecular pair

(arrangement)

S0−S1 (nm)

S0−S2 (nm)

ΔE(calc) (cm−1)

1 2 3 4 5 1 2 1 2 1 2

(parallel) (nonparallel) (parallel) (nonparallel) (nonparallel) (parallel) (nonparallel) (parallel) (nonparallel) (parallel) (nonparallel)

350 356 350 357 356 369 354 379 363 356 368

328 330 327 331 327 334 339 342 347 342 338

1940 2259 2005 2200 2433 2879 1261 2868 1286 1216 2444

a

Energy difference between S1 and S2. bFor the geometrical arrangements of molecular pairs in each crystal, see: Figures S6−S9 (SI). cCalculated at the TD-CAM-B3LYP/6-311G** level using the Xray determined crystal structures. I



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be larger or smaller than those in the experiment. Further, the values of ΔE(calc) reproduce the experimentally observed ordering of ΔEss for 1−4 at least qualitatively. Even as we pick up single molecular pairs in the crystal structures only, the calculation results agree fairly well with those from the experiments. Considering the molecular size and intermolecular distance, we comment that the quantum chemical calculations are adequate to evaluate the excitonic interaction beyond the limitation of the PDA.79 The approach of the nearest-neighbor molecular pairs presented in this paper is the first approximation in quantum chemical calculations. For more quantitative comparison with the experimental results in the microcrystalline solid state, it is necessary to include multiple exciton couplings under periodic boundary condition as well as couplings between further distant pairs and screening effects by other molecules.69 For example, the exciton has a bandwidth of 4J in a simple one-dimensional Frenkel exciton system with the nearest-neighbor coupling J, while the exciton-splitting is 2J in the dimer case. The results that ΔE(calc) are all smaller than ΔEss suggest such effects of multicouplings in the solid state. It should also be added that the assignment of the emitting state is furthermore complicated in the current solids due to the presence of the 2Ag state, which is not reproduced by the present calculations as shown above. For this reason, it remains unclear whether the 2Ag state could still be the lowest excited state also in the solid state.

larger than (but not greatly different from) those for the parallel pairs 1 and 3 at this level of calculation. Figure S6(b) (SI) shows the profiles of the HOMO and LUMO, which are shown to be involved in the lowest allowed S0 → S2 transition, for the nonparallel pair of 2 in 1. Upon excitation, the electrons move from one molecule to the other in the pair, showing the charge transfer (CT) nature of the transition. The HOMOs and LUMOs for pairs 4 and 5 are similar to those for 2. TD-DFT calculations with conventional functionals tend to underestimate the CT excitation energies and long-range corrected functionals like CAM-B3LYP improve this problem.76 However, we should note here that, in the TD-HF calculations, such CT states are not seen at least up to S4 for the nonparallel pairs 2, 4, and 5 (Table S6(a)). For the parallel pairs 1 and 3, on the other hand, the TD-CAM-B3LYP and TD-HF calculations afford similar results. In the cases of 2−4, the TD-CAM-B3LYP calculations show that the S0 → S1 transitions have f of (nearly) 0.000 and the S0 → S2 transitions have considerably large f > 3 for both parallel and nonparallel pairs of 1 and 2 in the crystal structures (Tables S7(b)-S9(b)). Namely, the S0 → S1 transitions are optically forbidden and S0 → S2 are the lowest allowed transitions for both pairs. This indicates that the intermolecular excitonic interactions in 2−4 are all H-type, as in 1. Calculations at the TD-HF level give similar results (Tables S7(a)-S9(a)). For 2 and 3, ΔE(calc) are calculated to be considerably larger for the parallel pairs 1 than those for the nonparallel pairs 2 (Table 6). The excitonic interactions are thus much greater for the molecular pairs with parallel arrangement. In the nonparallel pairs, intermolecular distance is larger than those in the parallel pair. In addition, each molecule in the nonparallel pairs is displaced along the molecular long axis with each other. These geometrical features coincide with the small ΔE(calc) in the nonparallel pairs. We note that the H-type energy-splitting is sustained for the nonparallel molecular pairs, although the degree of the displacement exceeds the criteria changing from H-type to J-type molecular configuration in the point dipole approximation (PDA). (The angle θ between the transition dipole moment and the direction connecting the centers of two molecules is ca. 48° in 2 and ca. 47° in 3, less than θc = 54.7° in the PDA.) This fact suggests the limitation of the PDA, as pointed out in refs 69, 77, and 78. This is probably due to the shorter intermolecular distance compared to the molecular size in the crystal structures. Meanwhile, in the case of 4, ΔE(calc) for the nonparallel pair 2 is significantly large relative to the value for the parallel pair 1 (Table 6). This again contradicts the PDA: In the approximation, the parallel pair is expected to have larger energy splitting that the nonparallel pair. Finally, we compare the calculated energies of the S0 → S2 and S0 → S1 transitions and ΔE(calc) at the TD-CAM-B3LYP level with the experimentally observed absorption (excitation) and fluorescence energies and ΔEss in the solid state at room temperature. The molecular pair with the largest ΔE(calc) in each crystal is shown to be the nonparallel pair of 5 in 1 and that of 2 in 4, and the parallel pairs of 1 in 2 and 3 (Table 6). The calculated S0 → S2 transition energies for these pairs successfully reproduce the experimentally observed red-shift in λa (λex) with the increase in the halogen size (Table 4). On the other hand, using the same molecular pairs as above, the S0 → S1 transition energies are calculated to be larger for 1 and 4 than those for 2 and 3. Thus, the results of calculation agree with the experimentally observed ordering of λf for 1−4, although the energy differences among them are predicted to

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CONCLUSIONS

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ASSOCIATED CONTENT

We synthesized a series of halogenated DPHs 1−4 to systematically investigate the crystal structures and fluorescence emission properties in solution and in the solid state. Both the crystal structure and the solid-state fluorescence property depend heavily on the size of halogen, and are strongly correlated with each other. The large increase in knr on going from 3 to 4 strongly suggests that the internal HAE on the probability of singlet−triplet isc is also effective in the solid state as in solution. The temperature effects on the solid-state fluorescence spectra are different depending on the halogen size. The increase in ΔEss with temperature reduction observed for 2 and 3 can be understood by assuming a decrease in intermolecular distance in the crystal structure at lower temperatures. We also performed quantum chemical calculations for 1−4 to understand the excited states and electronic transitions involved in the photophysical processes in solution and in the solid state. The experimentally observed clear correlation between crystal structure and transition energy can be reproduced fairly well by the calculations of the excited states for the selected molecular pairs in the X-ray determined structures. Our results also indicate that the quantum chemical calculations are adequate to evaluate the excitonic interaction beyond the limitation of the dipole approximation.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00590. Crystal structure data, fluorescence spectra in methylcyclohexane, NIR emission spectra and lifetimes in the solid state, the HOMO and LUMO results, electronic transitions for isolated molecules and molecular pairs in J



Article

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the crystal structures calculated at different levels of theory (PDF) Accession Codes

CCDC 1448491−1448493 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. TEL: +81-29-861-6390. FAX: +81-29-861-6252. Present Address #

Institute of Technology and Science, Tokushima University, 2−1, Minamijosanjima-cho, Tokushima, 770−8506, Japan Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge Electronics and Photonics Research Institute and National Institute of Advanced Industrial Science and Technology (AIST) for financial support. We thank Ms. K. Yaguchi (AIST) for her help in performing the fluorescence lifetime measurements for the solid samples in the NIR region. Thanks are also due to Ms. S. Shibasaki (AIST) and Ms. H. Kobashi (AIST) for the elemental analysis of triene 4. A part of this work was conducted at the Nano-Processing Facility, supported by IBEC Innovation Platform, AIST.

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Halogenated (F, Cl, Br, or I) Diphenylhexatrienes: Crystal Structures

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