Bent versus Linear Benzocoumarins - American Chemical Society

Nov 5, 2014 - Hyunsoo Moon,. † ..... We have not observed, however, any noticeable blue ... the expected blue shift is that the absorption wavelengt...
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Molecular-Shape-Dependent Luminescent Behavior of Dye Aggregates: Bent versus Linear Benzocoumarins Hyunsoo Moon,† Qui Pham Xuan,† Dokyoung Kim,*,† Yonghwi Kim,‡ Jae Woo Park,‡ Chong Han Lee,§ Hyeong-Ju Kim,§ Ayano Kawamata,⊥ Soo Young Park,§ and Kyo Han Ahn*,† †

Department of Chemistry and Center for Electro-Photo Behaviors in Advanced Molecular Systems, POSTECH, 77 Cheongam-Ro, Nam-Gu, Pohang, Gyungbuk 790-784, Republic of Korea ‡ Department of Chemistry, POSTECH, 77 Cheongam-Ro, Nam-Gu, Pohang, Gyungbuk 790-784, Republic of Korea § Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Republic of Korea ⊥ Department of Organic Chemistry, Graduate School of Pharmaceutical Sciences, Tohoku University, 6-3 Aoba, Aramaki, Aoba-Ku, Sendai 980-8578, Japan S Supporting Information *

ABSTRACT: Aggregation patterns of dye molecules can govern their photophysical properties in the solid state. The linear and bent shaped dipolar benzocoumarins showed contrasting luminescence behavior in solution and in the solid state. Single crystal structures of both compounds showed πstacking patterns with eclipsing but opposite dipole moments when viewed orthogonal to the stacking plane. Although the bent molecules are stacked in parallel in the solid state, they behave as independent molecules owing to the unfavorable excited state resonance interaction and hence emit strong fluorescence because each dipolar molecule now is in a hydrophobic environment surrounded by other molecules. This is an unusual example where the shape-dependent stacking governs the solidstate luminescence of dyes, being suggested here as a nonresonant π-stacking system.

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INTRODUCTION Luminescent molecules continue to attract great attention for their potential applications to various research fields, including optoelectronic materials,1 dye-sensitized solar cells,2 and chemo- and biosensing and imaging materials.3 Consequently, aromatic molecules that show efficient light absorption and emission properties have been intensively sought after.4 In addition to the molecular structure of dyes, their aggregation patterns can also govern their absorption and emission properties. For example, luminescent properties of fused aromatic compounds such as anthracene and pyrene are heavily dependent on their aggregation state. They are highly fluorescent in solution, but poorly emissive in the concentrated or solid state.5 The flat nature of such dye molecules elicits efficient intermolecular stacking in the concentrated or solid state, leading to nonemissive H-aggregates or emissive Jaggregates depending on the aggregation patterns. Both H- and J-aggregates are composed of parallel dye molecules stacked plane-to-plane and end-to-end, respectively, in two-dimensional dye crystals. According to Kasha’s exciton theory, two dye molecules stacked on each other in the H-aggregation can generate two excitonic states, S1 and S2, through interaction of transition dipoles.6 The lower excitonic state S1 has zero transition dipole moment because the dipole moment of each molecule overlaps in the opposite direction. In other words, © 2014 American Chemical Society

orbital interactions between two H-aggregated dye molecules generate the nonemissive lowest transition state. This Haggregation model is simple and thus widely used to explain the aggregation-dependent nonfluorescent behavior of “flat” aromatic dyes. It should be noted that some “non-flat” molecules such as 2,3,4,5-tetraphenylsilole, tetraphenylethylene, and their analogues show the aggregation induced emission (AIE) behavior owing to the conformational restriction of the aryl substituents7 or planarization of the dye molecule7b,8 in the aggregated or solid state. In this contribution, we wish to disclose the shape-dependent luminescent behavior of benzocoumarin dyes. Specifically, a bent benzocoumarin emits strong luminescence in the solid state, in contrast to the corresponding linear benzocoumarin that is nonemissive, even though both compounds show highly ordered parallel stacking patterns in the solid state. This observation provides a rare example of the molecular shapedependent contrasting luminescent behavior. Recently we set out to explore benzocoumarin derivatives that can be excited at longer wavelength and with better photophysical properties, in particular, two-photon absorption properties, than the conventional coumarin dyes.9 The Received: October 22, 2014 Published: November 5, 2014 6613

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Fluorescence System (PTI, Photon Technology International) with a 1 cm standard quartz cell. A solution of P1 or P2 at 10 μM in the given solvent was used for the spectroscopic analysis. Quantum yields of both the compounds were determined in ethanol by using rhodamine 6G (ΦF = 0.6) as a reference dye.9b Quantum yield of compounds in solution were calculated according to the following equation:

benzocoumarin P2 (2H-benzo[g]chromen-2-one series) and its analogues showed highly desirable photophysical properties for bioimaging applications. These benzocoumarin dyes, being donor−acceptor (D−A) type dipolar dyes, can generate intramolecular charge transfer (ICT) excited states in polar media.10 Dipolar dyes generally exhibit luminescent properties sensitive to the polarity of media, owing to the ICT nature. In the course of our study on the synthesis of other types of benzocoumarins and evaluation of their photophysical properties, we have observed that benzocoumarin P1 (a 3Hbenzo[f ]chromen-3-one derivative) shows the opposite luminescent behavior from that of the linear analogue P2 (a 2H-benzo[g]chromen-2-one derivative), both in solid and in solution states (Scheme 1). The contrasting emission behavior

Φ = Φr

where Φ is the quantum yield, I is the measured integrated emission intensity, n is the refractive index, A is the optical density, and subscript r indicates reference. Quantum Mechanical Calculations. Quantum mechanical calculations were carried out at the DFT level using a commercial computation program (Spartan ’08, Wave function, Inc.). The conformations and geometries of P1 and P2 in the ground state were optimized in the absence of solvents by the density functional theory (DFT) method at the B3LYP level−1 implemented in the program. The excited state, HOMO, LUMO, transition dipole moment, and oscillator strength were calculated at the configuration interaction with a single excitation (CIS) level. To obtain transition dipole moment directions of P1 and P2 by theoretical means, we first optimized the geometries of P1 and P2 monomers at the level of SOSCIS(D0)13 with the 6-31+G* basis set on first excited state. The transition dipole moments were then calculated at the same level. The dimer structures were determined based on the X-ray crystal structures, and the dipole moments of the monomers were imposed on these structures. Crystal Structure Analysis. Single crystals were obtained from a two-phase solvent system. P1 or P2 was dissolved in dichloromethane and transferred into an NMR tube. Hexane was added slowly to the tube to form two solvent layers, which was kept for a few days without disturbing until single crystals appeared. The diffraction data of single crystals of P1 or P2 were collected on a Bruker KAPPA APEX II CCD diffractometer equipped with a graphite monochromated Mo Kα (λ = 0.71073 Å) radiation source. The data were corrected for Lorentz and polarization effects (SAINT), and multiscan absorption corrections based on equivalent reflections were applied (SADABS). The structure was solved by direct methods and refined by full-matrix least-squares on F2 (SHELXTL). All the non-hydrogen atoms were refined anisotropically, and hydrogen atoms were added to their geometrically ideal positions. The crystallographic data are provided in the Supporting Information. Optical and fluorescence images of the crystals were captured using a CCD camera (DFC 480 R2, Leica) installed on an inverted microscope (DMI 6000B, Leica) equipped with a 515−560 nm excitation filter cube. Measurement of the Solid State Quantum Yield. The solid state quantum yield of P1 was measured for its nanoaggregates by the integrating sphere method using a Hamamatsu absolute PL quantum yield spectrometer (Quantaurus-QY, C11347-11). Preparation of Solid State Samples. A nanoparticle suspension of each of P1 and P2 was prepared as follow. To a solution of P1 or P2 in THF (1.0 × 10−5 M) was added distilled water to make a suspension in water/THF (98:2, v/v) by vigorous mixing using a vortex mixer. The resulting mixture was further sonicated for 30 min to give a nanoparticle suspension, which was used for dynamic light scattering (DLS) analysis, solid-state fluorescence measurements, and time-resolved fluorescence spectroscopic analysis. To take the absorption spectrum, the nanoparticle suspension of P1 or P2 was attached onto a fluorescence-free adhesive that was placed on a quartz substrate (2.5 cm × 2.5 cm). Spectroscopic Characterization of the Solid State Samples. UV−visible absorption spectra were recorded on a Shimazu UV-1650 PC spectrometer. Photoluminescence emission and excitation spectra were obtained using a Photo Technology International, Felix32 QM-4 (all samples were excited at 375 nm with a monochromated beam), and the obtained spectra were corrected by taking into account of the sensitivity of the detection system and the characteristics of the lamp,

Scheme 1. Structures of Coumarin, Benzo[f ]coumarin (P1), and Benzo[g]coumarin (P2)

has prompted us to further investigate their photophysical properties and crystal stacking patterns. Specifically, we have found that a dye molecule such as P1 can emit in the solid state, even though the dye molecules stack exactly in the parallel way. In other words, the strong luminescence of P1 in the solid state provides an unusual example of a “nonresonant” π-stacking system. The results suggest that the solid state luminescence of dyes can be controlled by exploring a “shape-dependent” excited state resonance interaction.6e

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A r I n2 Ir A nr 2

EXPERIMENTAL SECTION

Materials. The chemical reagents were purchased from Aldrich or TCI. Commercially available reagents were used without further purification. Anhydrous solvents for organic synthesis were prepared by passing through a solvent purification tower. Thin-layer chromatography (TLC) was performed on precoated silica gel 60F254 glass plates. 1H and 13C NMR spectra were measured with a Bruker AVANCE III 300 MHz and AVANCE III 600 MHz FT-NMR spectrometer. Coupling constants (J value) are reported in hertz. Mass spectral analysis was recorded with a JEOL JMS-700 spectrometer and reported in units of mass to charge (m/z). HRMS was performed at the Korea Basic Science Center, Kyungpook National University. Synthesis. Both benzo[f ]coumarin (P1) and benzo[g]coumarin (P2) were synthesized starting from commercially available naphthalene-2,7-diol through the different synthetic routes established (Scheme S1, Supporting Information).9a,11 The dimethylamino group was introduced by the Bucherer reaction.12 The introduction of a formyl group at the C-1 position of naphthalene-2,7-diol was carried out under Lewis acidic conditions after protecting the hydroxyl group in the methyl ether form. Regeneration of the hydroxyl group followed by the Knoevenagel condensation with dimethyl malonate afforded P1 in overall 17% yield. The introduction of formyl group at the C-3 position of naphthalene-2,7-diol was carried out through directed lithiation followed by treatment with N,N-dimethylformamide after protection of one of the hydroxyl groups by methoxymethyl ether (MOM). Deprotection under acidic conditions followed by the Knoevenagel condensation with dimethyl malonate afforded P2 in overall 20% yield. Experimental procedures and characterization data are given in the Supporting Information. Spectroscopic Analysis. UV/vis absorption spectra were obtained using a HP 8453 UV/vis spectrophotometer. Fluorescence spectra were recorded on a Photon Technical International 6614

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Figure 1. Fluorescence emission spectra of bent P1 and linear P2 (a) in DMSO (10 μM) and (b) in the solid state. The fluorescence emission spectra were measured under excitation at the absorption maximum wavelength of each compound: 463 nm (in DMSO) and 475 nm (solid) for P1; 457 nm (in DMSO) and 400 nm (solid) for P2. Inset photos were taken under irradiation at 365 nm with a UV lamp. respectively. Time-resolved fluorescence lifetime experiments were performed by the time-correlated single photon counting (TCSPC) technique with a FluoTime200 spectrometer (PicoQuant) equipped with a PicoHarp300 TCSPC board (PicoQuant) and a PMA182 photomultiplier (PicoQuant). The excitation source was a 377 nm picosecond pulsed diode laser (PicoQuant, LDH375) driven by a PDL800-D driver (PicoQuant) with 70 ps fwhm. The multiexponential least-squares fitting procedure was carried out with the Fluofit software (PicoQuant), taking into possible double excitations of the IRF (Instrument Response Factor) within the deconvolution.

Table 1. Absorption and Emission Data of P1 and P2 λabs (nm)

state

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RESULTS AND DISCUSSION Photophysical Properties. The emission spectra of both benzocoumarins P1 and P2 were measured in solution and solid states (as nanoaggregates whose size were determined by dynamic light scattering, see Figure S1, Supporting Information). Strikingly, bent P1 showed weak fluorescence in polar media but strong fluorescence in the solid state and also in the crystalline state, whereas linear P2 showed opposite emission behavior and thus showed strong fluorescence in polar media (except water) but very weak fluorescence in the solid state (Figure 1; Figure S2, Supporting Information). For example, bent P1 emitted weak fluorescence in polar solvent such as DMSO; however, it emitted strong fluorescence in the red region (λmax = 594 nm) in the solid state with a large Stokes’ shift of 119 nm (see the absorption spectrum of P1 in Figure S3, Supporting Information). Also, P1 showed a high quantum yield of ΦF = 0.78 in the solid state, as determined for P1 aggregates by the integrating sphere method (Table 1). Vigorous mixing of a solution of each of the benzocoumarins in tetrahydrofuran (1 × 10−5 M) with increasing amount of water (the final composition, water/THF = 98:2 by volume) using a vortex mixer caused aggregation of P1, which was filtered and dried to obtain nanoaggregates of P1. Nanoaggregates of P2 were also similarly obtained. In contrast to bent P1, linear P2 emitted strong fluorescence in polar organic media of DMSO (λmax = 604 nm, ΦF = 0.61) but very weak fluorescence in the solid state. The observed emission behavior of both P1 and P2 in solution can be explained by evoking their dipolar nature. Both P1 and P2 emit polarity-dependent fluorescence as they belong to donor (D)−acceptor (A) type fluorophores, which form ICT excited states (Figure S4−S8, Supporting Information). Bent P1 showed moderately strong fluorescence in polar organic solvent such as toluene, dichloromethane, and

solution in toluene solution in DMSO nanoaggregates

451 463 475

solution in toluene solution in DMSO nanoaggregates

430 457 400

absorbance P1 0.14 0.12 0.027 P2 0.11 0.10 0.054

λem (nm)

fwhma

ΦF

580b 675c 594d

102b 108c 105d

0.04e 0.78f

542b 604c 642d

81b 102c 87d

0.61e 0.06f

a

Full width at half-maximum. bUnder excitation at 451 and 430 nm for 10 μM P1 and 10 μM P2, respectively. cUnder excitation at 463 and 457 nm for 10 μM P1 and 10 μM P2, respectively. dMeasured for the nanoaggregates of P1 or P2 under excitation at 475 and 400 nm, respectively. eQuantum yields were determined in EtOH using rhodamine 6G (ΦF = 0.6) as a standard.9b fMeasured for the nanoaggregates of P1 or P2 by the integrating sphere method.

tetrahydrofuran but weak fluorescence in highly polar solvents such as acetonitrile, dimethyl sulfoxide, and methanol; in contrast, linear P2 emitted strong fluorescence even in the highly polar solvents. Depending on the degree of the ICT from the amine donor to the acceptors (the ester and lactone), which is dependent on the polarity of media, P1 and P2 thus show solvent-dependent emission behavior. P2 is more emissive in solution than P1, because the former has a larger transition dipole moment than the latter (see below). Both P1 and P2 show significant red shifts in their absorption maxima as the polarity of media increases (430 → 467 nm; Tables S1 and S2, Supporting Information). Also, both P1 and P2 show poor emission in water, as in the case of other D−A type dyes that contain a dialkylamino substituent as the donor group.14 In water, such dipolar dyes can have a twisted ICT excited state (TICT), which is in general poorly emissive. Evaluation of the emission intensity of bent P1 in a mixed solvent composed of water and an organic solvent with varying water content shows that it starts to emit only in the medium of high water content (≥90%) where P1 exists in its aggregated form (Figure 2); this result can be readily explained by evoking the ICT excited state of P1 as well as its solid-state luminescence behavior that is obviously not due to the conformational restriction as in the case of AIE dyes.7 Further experiments, including fluorescent lifetime measurements, polarity-dependent fluorescence measurements, crystal 6615

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its packing pattern, seems to govern its emission behavior in the solid state. The crystal packing patterns of P1 and P2 (Figure 4) show that they aggregate by stacking with each other; the distances between the two stacked layers are 3.228 Å (pitch angle 0°) and 3.504 Å (pitch angle 76.97°), respectively (Figure 4e), which would allow π-stacking interactions if any. The crystal packing pattern of P1 molecules shows that in a stacked column they aggregate by stacking with each other in an exactly parallel way. The stacked columns of P1 both in the adjacent and in the diagonal positions confront each other with a tilt angle of 0°. The crystal packing pattern of P2 molecules shows that in a stacked column they also aggregate by stacking with each other in an exactly parallel way, but, in this case, two adjacent stacked columns are tilted a little from each other (tilt angle 28.09°) whereas the stacked columns in the diagonal position are on the same plane (tilt angle: 0°). If we look into two stacked molecules, they overlap in the plane-to-plane mode, plausibly owing to favorable dipole− dipole interaction in such dipolar dyes. The dipole moments of the two molecules exactly eclipse each other, but in the opposite direction when the two molecules are superimposed by moving along the line perpendicular to the stacking plane (Figure 5). The head-to-end stacking is likely due to the favorable dipole−dipole interaction. Given that both P1 and P2 show the parallel stacking pattern in the crystalline state, the Kasha’s exciton model of “parallel transition dipoles” may be evoked to explain their luminescence behavior in the solid state.6e According to the exciton energy diagram for a molecular dimer with parallel transition dipoles, the energy level of the out-of-phase dipole arrangement is lower than that of the in-phase dipole interaction. Because the transitions from the ground state to the lower energy excited state are forbidden (no change in the transition dipole moment or in the oscillator strength), usually we observe fluorescence quenching. Thus, the nonemissive property of linear P2 in the aggregated state can be explained by Kasha’s exciton model. This exciton model has been used to explain the nonemissive behavior observed for some flat aromatic dyes, known as the Haggregation model. In this model, normally a blue shift in the absorption spectra results because the transition from the ground state to the higher energy excited state is allowed (Figure 6). We have not observed, however, any noticeable blue shift in the absorption spectra of P2 depending on the water content in the medium of water−THF; rather small red shifts were observed as the medium polarity increased (Figure S11, Supporting Information). A plausible reason for the absence of the expected blue shift is that the absorption wavelength of such a dipolar dye is also dependent on the medium polarity; as the water content increases, that is, as the medium polarity increases, a red shift in the absorption wavelength occurs, which change seems to outweigh a blue shift expected from the increase of degree of aggregation as the water content increases. Then, how can we explain the strong luminescence of bent P1 in the solid state? A prerequisite of the Kasha’s exciton model is that there should be an “excited state resonance interaction” between two-stacked molecules. It seems that P1 lacks such an excited state resonance interaction because of geometrically unfavorable stacking. We have compared the stacking geometries of P1 and P2 molecules by focusing on two stacked molecules (see the stacked figures of P1 and P2 in Figure 4). In the case of P1, two stacked molecules show that the central benzene rings do not substantially overlap with each

Figure 2. Photos of fluorescence emission of P1 (10 μM) in (a) CH3CN/H2O and (b) THF/H2O at different ratios (the Arabic numbers indicate the volume percentages of H2O) taken under UV irradiation at 365 nm.

structure analyses, and quantum mechanical calculations were carried out. The fluorescence lifetime of P1 (1.607 ns) was found to be shorter than that of P2 (5.285 ns), as measured by time-resolved fluorescence spectroscopy carried out for a nanoparticle suspension of P1 or P2 in water/THF (98:2, v/ v) (Figure S9, Supporting Information). In addition, the fluorescence lifetime of P1 (1.502 ns) in DMSO was also found to be shorter than that of P2 (4.122 ns) in DMSO, measured at 10 μM concentration of each compound (Figure S10, Supporting Information). The large difference in the fluorescence lifetime between P1 and P2 supports their contrasting luminescence behavior in the solid states. Analysis of Crystal Packing Patterns. To understand why P1 emits in the solid state, we have analyzed crystal packing patterns of P1, along with that of P2. Single crystals of both P1 and P2 were grown from dichloromethane/hexane two-phase medium. Bright and fluorescence images of single crystals of P1 are given in Figure 3. Rod-like single crystals with a rectangular edge show strong red fluorescence on their surface. Single crystals of P2 are nonfluorescent, as in the case of its nanoaggregates. As noted above, the stacking pattern can cause special photophysical properties of aggregated dye molecules. Obviously, the molecular shape of a molecule, in addition to

Figure 3. Photos of bright field and fluorescence images of single crystals of P1. (a, b) Bright field images. (c, d) Fluorescence images collected in the wavelength range of 515−560 nm using a filter cube. FOV (field of view): 500 μm (a, c); 200 μm (b, d). 6616

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Figure 4. Crystal packing patterns of P1 and P2. (a) Crystal structures of P1 and P2 where oxygen and nitrogen atoms are shown in red and blue, respectively. (b) A top-view of two stacked molecules. (c) A top view (b-axis) of four nearby groups of two stacked molecules. (d) 3D views (a-axis). (e) Side views (c-axis).

molecule overlaps with the heterocyclic ring in the other. It seems that the parallel stacking between the benzene and heterocyclic rings seem to cause insufficient excited state resonance interaction, compared with the stacking of benzene rings. Indeed, stacking between the benzene and heterocyclic rings can lead to luminescence in the solid state, as observed in the boron complexes based on (benzothiazol-2-yl)phenol derivatives reported by us and others.15 Thus, the different folding geometries seem to cause the opposite luminescence behavior between P1 and P2 in the solid state. Although P1 molecules are stacked parallel with each other in the solid state, they seem to behave as independent molecules plausibly owing to the unfavorable excited state resonance interaction (Figure 6) and thus emit strong fluorescence because each dipolar molecule now is in a hydrophobic environment surrounded by other molecules. As mentioned previously, such dipolar dyes show environment-sensitive emission behavior, showing strong fluorescence in organic solvent and very weak fluorescence in aqueous media. This is an unusual case where the shapedependent stacking governs the solid-state luminescence of dyes, being suggested here as a nonresonant exciton system. Attempts to identify the excited state resonance interaction depending on the molecular shape-dependent stacking by quantum mechanical computations met with failure at this moment, leaving this subject open for further scrutinization. Quantum Mechanical Calculations. For a qualitative estimation of orbital interactions between two stacked molecules, we compared HOMO and LUMO orbitals of both stacked P1 and P2. The HOMO and LUMO interactions between two stacked molecules may be considered as a primitive model for estimating the excited state resonance interaction. HOMOs of both P1 and P2 show larger electron

Figure 5. Schematic illustration of the packing patterns for (a) P1 and (b) P2. The red ring represents the nonbenzene moiety. In both cases, the dipole moments of upper and lower layers exactly overlap but in the opposite direction. The two layers are represented slightly off from the top view. R1 = CO2Me; R2 = NMe2.

Figure 6. Exciton band energy diagram for the resonant (Haggregation) and nonresonant (in the case of P1) dimers with parallel transition dipoles.

other upon folding two molecules together, in contrast to the case of P2 where the central benzene rings well overlap each other. Also, in both cases, the terminal benzene ring in one 6617

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derivatives. The bent benzocoumarin shows strong fluorescence in the solid state, whereas the linear one does not. Analysis of the crystal structures and packing patterns reveals that both compounds show parallel stacking patterns that conform to Kasha’s exciton model of parallel transition dipoles. The bent benzocoumarin, however, seems to experience an unfavorable excited state resonance interaction, providing an example of a nonresonant stacked model. Frontier molecular orbital interactions between stacked molecules with opposite dipole moments suggest a different degree of orbital overlapping, offering a clue on the opposite optical behavior of the two molecules in the solid state. This study demonstrates the potential of molecular shape control for the development of solid-state luminescent materials.

distribution near the donor side, whereas their LUMOs show larger electron distribution near the acceptor side. When the HOMO and LUMO of P1, in the plane-to-plane stacked mode as in the crystal structure, are compared with respect to the orbital size and symmetry, the orbital overlap is poor; in contrast, significant orbital overlap is present in the case of P2 (Figure 7). Because of the poor orbital interactions between

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ASSOCIATED CONTENT

S Supporting Information *

Full experimental details, synthesis, photophysical properties, X-ray crystal structure analysis, quantum mechanical calculations, other supporting figures and tables, including packing structures and information, and crystallographic information in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org. X-ray crystallographic information files (CIFs) for P1 and P2 are also available from Cambridge Crystallographic Data Centre (CCDC 996298 and CCDC 996299, respectively).

Figure 7. Theoretical electron distribution of the HOMO−LUMO energy states for the stacked pair of (a) P1 and (b) P2.

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stacked molecules of P1, the excited state resonance interaction may become insignificant, which precludes application of Kasha’s resonant dimer model to this case. Bent P1 molecules in the solid state behave independently and thus emit strong fluorescence. In contrast, for the P2 dimer, which shows significant orbital overlap, its poor emission behavior in the solid state is explained by Kasha’s exciton model of parallel transition dipoles. Linear benzocoumarin P2 shows a larger transition dipole moment and oscillator strength compared with those of bent benzocoumarin P1, conforming to the fact that P2 emits stronger fluorescence than P1 in polar aprotic solvents (Table 2).

*K. H. Ahn: e-mail, [email protected]; tel, +82-54-279-2105; fax, +82-54-279-5877. *D. K. e-mail [email protected]. Author Contributions

Q. P. Xuan initially carried out synthesis of the compounds, characterization of their photophysical properties, and the crystal structure analysis. H. Moon carried out all the experiments again and carried out the quantum mechanical calculations with the help of J. W. Park. Y. Kim solved the crystal structures. C. H. Lee, H.-J. Kim, and S. Y. Park obtained the solid state luminescence data. A. Kawamata resynthesized compound P1. K. H. Ahn directed the experiments and prepared the manuscript together with D. Kim. All authors have given approval to the final version of the manuscript. H. Moon and Q.P. Xuan contributed to this work equally.

Table 2. Calculated Photophysical Properties of P1 and P2a

P1 P2

E (LUMO) [eV]

E (HOMO) [eV]

ΔE [eV]

transition dipole moment

oscillator strength

−2.17 −2.18

−5.48 −5.37

3.31 3.19

1.82 2.36

0.33 0.66

AUTHOR INFORMATION

Corresponding Authors

Notes

The authors declare no competing financial interest.

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Calculations were done for the ground state and for the first excited singlet state at the DFT level of theory using the hybrid exchangecorrelation functional of Becke’s three-parameter and the Lee−Yang− Parr approximation (B3LYP) implemented in Spartan 08′. a

ACKNOWLEDGMENTS This work was supported by grants from the EPB Center (No. R11-2008-052-01001) through the National Research Foundation and Ministry of Health & Welfare (No. HI13C1378), Korea.

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To obtain the orientation of the transition dipole moment of S0−S1 transition, theoretical calculations were carried out. The transition dipole of S0−S1 transition of both P1 and P2 was oriented with respect to the longitudinal axis of each molecule; hence, P1 and P2 can be considered as parallel transition dipoles (Figures S18 and S19, Supporting Information).

REFERENCES

(1) (a) Katz, H. E.; Bao, Z.; Gilat, S. L. Acc. Chem. Res. 2001, 34, 349. (b) Walzer, K.; Maennig, B.; Pfeiffer, M.; Leo, K. Chem. Rev. 2007, 107, 1233. (2) (a) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Chem. Rev. 2010, 110, 6595. (b) Imahori, H.; Umeyama, T.; Ito, S. Acc. Chem. Res. 2009, 42, 1809. (3) (a) Yang, Y.; Zhao, Q.; Feng, W.; Li, F. Chem. Rev. 2013, 113, 192. (b) Jun, M. E.; Roy, B.; Ahn, K. H. Chem. Commun. 2011, 47, 7583.

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CONCLUSIONS In summary, we have disclosed unusual, shape-dependent luminescence behavior of bent and linear benzocoumarin 6618

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dx.doi.org/10.1021/cg501567s | Cryst. Growth Des. 2014, 14, 6613−6619