Constructing Full-Color Highly Emissive Organic Solids Based on a X

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C: Plasmonics, Optical Materials, and Hard Matter

Constructing Full-Color Highly Emissive Organic Solids Based on a X-Shaped Tetrasubstituted Benzene Skeleton Rui Huang, Bin Liu, Chenguang Wang, Yue Wang, and Hongyu Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01251 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 25, 2018

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The Journal of Physical Chemistry

Constructing Full-Color Highly Emissive Organic Solids Based on a X-Shaped Tetrasubstituted Benzene Skeleton Rui Huang,‡ Bin Liu, ‡ Chenguang Wang,* Yue Wang and Hongyu Zhang* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China.

ABSTRACT

With the aim of highlighting the potential talent of the unique single-benzene skeleton versus the common large π-systems, we propose a general strategy to construct single-benzene fluorophores 1‒9 featuring fascinating emission properties in solid state, e.g. high fluorescence quantum yields and wide variety of emission colors. Our molecular design is X-shaped tetrasubstituted benzene of which two electron-donating groups and two electron-withdrawing groups are arranged in a X-shaped fashion. This molecular design enables the very small single-benzene skeleton to show intense fluorescence in solid state because the π‒π stacking and dipole-dipole interaction are inherently avoided in crystal. More importantly, by simply changing the electrondonating groups, the emission colors in solid state of these single-benzene fluorophores can be

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continuously tuned from deep blue to red. In addition, the potential application of these singlebenzene fluorophores as crystal lasing media have been demonstrated by amplified spontaneous emission measurement.

1. INTRODUCTION The design and synthesis of organic fluorophores which are highly emissive in solid state is an important and fundamental issue for their application in neat or aggregate state, e.g. thin-film based optoelectronic devices and nanoparticle based fluorescent bio-imaging.1‒15 However, organic fluorophores, which consist of extended π-systems, generally encounter the serious fluorescence quenching in solid state due to the strong intermolecular interactions, such as the π‒ π stacking of planar polycyclic skeleton and the dipole-dipole interaction of strong donoracceptor (D-A) framework.1‒4 Thus, in order to achieve efficient emissions in solid state, several strategies have been adopted for suppressing the undesired intermolecular interactions, such as the introduction of large steric bulky group and the construction of highly twisted skeleton (Figure 1).16‒28


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Figure 1. Representative fluorophores exhibiting strong emissions in solid state: full-color tuning by changing a) the electron-donating groups and b) the electron-accepting groups, respectively. The fluorescence maxima and quantum yields of these compounds in films are given. See references 17 and 22. Rather than the common fluorophores with large π-systems, our attention is focused on mini π-system based fluorophore consisting of even single benzene. We assume that if the fluorophore has a single-benzene π-system as well as a small dipole moment in excited state, the π‒π stacking and dipole-dipole interaction can be inherently avoided, and thus the fluorescence quenching in solid state should be significantly suppressed for such fluorophore. Is it a good strategy to construct highly emissive organic solids? This fascinating idea is apparently limited by the bottlenecks of: 1) the single-benzene based skeleton is too small, how can we get fluorescence which locates at visible region? 2) Such mini π-system means small radiative decay rate constant, how can we achieve intense fluorescence?


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In this content, several single-benzene based fluorophores showing strong emissions in solid state have been reported.29‒33 For example, Shimizu’s group developed 1,4-bis(alkenyl)2,5-dipiperidinobenzenes (Figure 2a) of which the emissions in solid state could be tuned in fullcolor range by changing the electron-accepting moieties.30 Katagiri’s group reported 2,5bis(methylsulfonyl)-1,4-phenylenediamine (Figure 2b) exhibiting intense sky-blue emission in solid state.31 Very recently, our group developed dimethyl 2,5-bis(methylamino)terephthalate 1 (Figure 2c) which shown strong red emission in crystal.32

Figure 2. Single-benzene based fluorophores exhibiting strong emissions in solid state: a) 1,4bis(alkenyl)-2,5-dipiperidinobenzenes developed by Shimizu’s group, reference 30; b) 2,5bis(methylsulfonyl)-1,4-phenylenediamine reported by Katagiri’s group, reference 31; c) redemissive compound 1 of our previous work, reference 32; d) the molecular design of singlebenzene based fluorophore of which two electron-donating groups (EDG) and two electronwithdrawing groups (EWG) are arranged in a X-shaped fashion; e) compounds 2‒9 of this work. The fluorescence maxima and quantum yields of these compounds in solid state are given.


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Figure 3. TD-DFT calculation result of 1, 2, 6 and 8: energy diagrams, Kohn-Sham HOMOs and LUMOs, vertical excitation wavelengths and oscillator strengths (f). For all of the compounds, the first excited states are mainly originated from the transitions of HOMOs→LUMOs. Inspired by the elegant works of Shimizu’s group and Katagiri’s group, herein we would like to propose a molecular design of single-benzene fluorophores. That is X-shaped tetrasubstituted benzene of which two electron-donating groups (EDG) and two electronwithdrawing groups (EWG) are arranged in a X-shaped fashion (Figure 2d). Such arrangement of substituents resulting in a characteristically separated HOMO and LUMO (Figure 3),30‒32 is the key factor to enable the very small single-benzene skeleton to get a large Stokes shift, and thus show fluorescence at bathochromic region while feature a small dipole moment in excited state. Associated with these characters, π‒π stacking and dipole-dipole interaction are inherently avoided in crystal. Thus the nonradiative decay is significantly suppressed and intense fluorescence is observed in solid state. More importantly, by simply changing the electron-


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donating groups, the emission colors in solid state of these single-benzene fluorophores 1‒9 (Figure 2e) can be continuously tuned from deep blue to red. In this work, besides of the basic properties including synthesis, photophysical properties, crystal structures, et. al, the potential application of these highly emissive solids as crystal lasing media has been preliminarily evaluated by amplified spontaneous emission measurement. Although the detailed properties of compound 1 have been reported very recently by our group,32 herein we emphasize a general strategy to construct full-color highly emissive organic solids rather than an individual case as our previous report. Moreover, we would like to highlight the potential talent of the unique single-benzene fluorophores versus the common fluorophores with large π-systems. Therefore, this work should give large contribution to the basic knowledge of organic fluorophores with very simple molecular structures.

2. EXPERIMENTAL SECTION 2.1. Synthetic Details. Melting points (Mp) were determined with DSC measurements. 1

H and 13C{1H} NMR spectra were recorded with a Bruker Avance 500 MHz spectrometer (500

MHz for 1H and 126 MHz for

13

C) in CDCl3 or DMSO-d6. The chemical shifts in 1H NMR

spectra are reported in δ ppm using tetramethylsilane as an internal standard, and those in

13

C

NMR spectra are reported using the solvent signals as an internal standard (CDCl3 δ 77.16 and DMSO-d6 δ 39.52). Thin layer chromatography (TLC) was performed on glass plates coated with 0.25 mm thickness of silica gel 60F254 (Merck). Column chromatography was performed using neutral 100–200 mesh silica gel (Qingdao Haiyang Chemical Co.). All reactions were performed under a N2 atmosphere. Commercially available solvents and reagents were used


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without further purification unless otherwise mentioned. Anhydrous toluene was distilled from sodium/benzophenone under nitrogen atmosphere. Anhydrous DMF was distilled from 4Å molecular sieves under reduced pressure. Dimethyl 2,5-bis(dimethylamino)terephthalate (2)32 and dimethyl 2,5-dibromoterephthalate34 were synthesized according to the literatures. Dimethyl 2,5-di(azetidin-1-yl)terephthalate (3). To the solids of dimethyl 2,5dibromoterephthalate (1.40 g, 3.98 mmol), RuPhos (350 mg, 0.750 mmol), Pd2(dba)3 (140 mg, 0.153 mmol) and K3PO4 (8.50 g, 40.0 mmol) were added toluene (20 mL) and azetidine (3.50 mL, 51.9 mmol) successively, and the resulting mixture was stirred at 120 °C for 10 h. After cooling to room temperature, EtOAc (50 mL) and water (20 mL) were added to the mixture. After separation, the organic layer was washed with water (20 mL), brine (20 mL), and then dried over anhydrous Na2SO4, and filtered. After concentration of the filtrate under reduced pressure, the resulting solid was purified by silica gel column chromatography (10/1 hexane/ EtOAc, Rf = 0.28) to afford 165 mg (0.542 mmol, 14 %) of 3 as yellow solids. Mp: 140 °C (DSC). 1H NMR (500 MHz, DMSO-d6): δ 6.65 (s, 2H), 3.79 (s, 6H), 3.69 (t, J = 7.5 Hz, 8H), 2.23–2.17 (m, 4H).

13

C{1H} NMR (126 MHz, DMSO-d6): δ 167.1, 142.7, 119.7, 115.3, 53.5,

52.1, 16.3. Dimethyl 2,5-di(pyrrolidin-1-yl)terephthalate (4). The compound was prepared in a similar manner as described for 3. The reaction of dimethyl 2,5-dibromoterephthalate (1.40 g, 3.98 mmol), pyrrolidin (10.0 mL, 122 mmol), RuPhos (350 mg, 0.750 mmol), Pd2(dba)3 (140 mg, 0.153 mmol) and K3PO4 (8.50 g, 40.0 mmol) in toluene (20 mL) provided 183 mg (0.551 mmol, 14%) of 4 as yellow solids. Mp: 174 °C (DSC). 1H NMR (500 MHz, DMSO-d6): δ 6.91 (s, 2H), 3.81 (s, 6H), 3.04 (br, 8H), 1.84 (br, 8H). 13C NMR (126 MHz, DMSO-d6): δ 168.6, 139.0,


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121.7, 115.6, 52.1, 51.5, 25.2. The structure was further determined by single-crystal X-ray diffraction analysis. Dimethyl 2,5-di(piperidin-1-yl)terephthalate (5). The compound was prepared in a similar manner as described for 3. The reaction of dimethyl 2,5-dibromoterephthalate (1.40 g, 3.98 mmol), piperidine (10.0 mL, 101 mmol), RuPhos (350 mg, 0.750 mmol), Pd2(dba)3 (140 mg, 0.153 mmol) and K3PO4 (8.50 g, 40.0 mmol) in toluene (20 mL) provided 203 mg (0.563 mmol, 14%) of 5 as yellow solids. Mp: 173 °C (DSC). 1H NMR (500 MHz, DMSO-d6): δ 7.22 (s, 2H), 3.81 (s, 6H), 2.85–2.81 (m, 8H), 1.59–1.57 (m, 8H), 1.50–1.47 (m, 4H). 13C NMR (126 MHz, DMSO-d6): δ 167.7, 146.2, 128.8, 121.0, 53.4, 52.1, 25.8, 23.6. The structure was further determined by single-crystal X-ray diffraction analysis. Dimethyl

2,5-dihydroxyterephthalate

(6).

To

a suspension

of dimethyl

2,5-

dioxocyclohexane-1,4-dicarboxylate (5.02 g, 21.9 mmol) in HOAc (20 mL) was added Nchlorosuccinimide (NCS) (3.01 g, 22.5 mmol) at 80 °C, and the resulting mixture was stirred for 1 h at the same temperature. After cooling to room temperature, the mixture was filtrated and the solids were washed with HOAc (5 mL), t-BuOMe (5 mL), and water (50 mL) to afford 4.43 g (19.6 mmol, 89 %) of 6 as pale yellow crystals. Mp: 174 °C (DSC). 1H NMR (500 MHz, CDCl3): δ 10.06 (s, 2H), 7.47 (s, 2H), 3.97 (s, 6H). 13C NMR (126 MHz, CDCl3): δ 169.6, 153.1, 118.5, 117.9, 52.9. The structure was further determined by single-crystal X-ray diffraction analysis. Diethyl 2,5-dihydroxyterephthalate (7). The compound was prepared in a similar manner as described for 6. The reaction of diethyl 2,5-dioxocyclohexane-1,4-dicarboxylate (1.41 g, 5.48 mmol) and NCS (0.750 g, 5.62 mmol) in HOAc (8 mL) provided 1.21 g (4.76 mmol, 87%) of 7 as pale yellow crystals. Mp: 130 °C (DSC). 1H NMR (500 MHz, CDCl3): δ 10.14 (s, 2H), 7.48


8

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(s, 2H), 4.42 (q, J = 7.0 Hz, 4H), 1.43 (t, J = 7.0 Hz, 6H). 13C NMR (126 MHz, CDCl3): δ 169.2, 153.1, 118.7, 117.9, 62.2, 14.2. The structure was further determined by single-crystal X-ray diffraction analysis. Dimethyl 2,5-dimethoxyterephthalate (8). To a solution of 6 (2.26 g,10.0 mmol) in anhydrous DMF (10 mL) was added sodium hydride (60% dispersion in mineral oil, 2.00 g, 50.0 mmol) and iodomethane (7.10 g, 50.0 mmol) successively, and the resulting mixture was stirred at 80°C for 2 h. After cooling to room temperature, the reaction mixture was slowly poured into water (50 mL). After filtration, the solids were washed with water (20 mL) to afford 2.34 g (9.20 mmol, 92 %) of 8 as white crystals. Mp: 140 °C (DSC). 1H NMR (500 MHz, CDCl3): δ 7.40 (s, 2H), 3.92 (s, 6H), 3.90 (s, 6H). 13C NMR (126 MHz, CDCl3): δ 166.1, 152.5, 124.1, 115.6, 56.9, 52.6. The structure was further determined by single-crystal X-ray diffraction analysis. Diethyl 2,5-dimethoxyterephthalate (9). The compound was prepared in a similar manner as described for 8. The reaction of 7 (1.13 g, 5.00 mmol), iodomethane (3.55 g, 25.0 mmol) and sodium hydride (60% dispersion in mineral oil, 1.00 g, 25.0 mmol) in DMF (30 mL) provided 1.10 g (3.90 mmol, 78%) of 9 as white crystals. Mp: 100 °C (DSC). 1H NMR (500 MHz, CDCl3): δ 7.37 (s, 2H), 4.39 (q, J = 7.0 Hz, 4H), 3.89 (s, 6H), 1.40 (t, J = 7.0 Hz, 6H). 13C NMR (126 MHz, CDCl3): δ 165.7, 152.6, 124.6, 115.6, 61.5, 57.0, 14.4. The structure was further determined by single-crystal X-ray diffraction analysis. 2.2. Theoretical Calculations. The calculation was performed using Gaussian 09 program at the CAM-B3LYP/6-31G* level of theory. The molecules were defined as symmetric ones using the keyword of “dsymm”. For the calculation of the radius of Onsager cavity, the keyword of “volume” was used.


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2.3. Thermal Properties. Differential scanning calorimetric (DSC) measurements were performed on a NETZSCH DSC204 instrument at a heating rate of 10 ºC min−1 under N2 atmosphere.

Thermogravimetric

analyses

(TGA)

were

performed

on

a

TAQ500

thermogravimeter at a heating rate of 10 ºC min−1 under N2 atmosphere. 2.4. Photophysical Properties. UV-vis absorption spectra were measured with a Shimadzu UV-2550 spectrometer with a resolution of 1 nm using dilute sample solutions in spectral grade solvents in a 1 cm square quartz cuvette. Emission spectra of solutions were measured with a PerkinElmer LS45 spectrometer with a resolution of 1 nm. Emission spectra of crystalline powders were measured with a C11347-11 Quantaurus-QY spectrometer with a calibrated integrating sphere system. Absolute fluorescence quantum yields were determined with a C11347-11 Quantaurus-QY spectrometer or an Edinburgh FLS980 spectrometer with a calibrated integrating sphere system. Fluorescence lifetimes were measured with an Edinburgh FLS980 spectrometer. All decay profiles were fitted reasonably well with a single exponential function unless otherwise mentioned. 2.5. X-ray Crystallographic Analysis. Single crystal X-ray diffraction data were collected on a Rigaku RAXIS-PRID diffractometer using the ω-scan mode with graphitemonochromator Mo•Kα radiation. The structures were solved with direct methods using the SHELXTL programs and refined with full-matrix least-squares on F2. Non-hydrogen atoms were refined anisotropically. The positions of hydrogen atoms were calculated and refined isotropically. 2.6. Amplified Spontaneous Emission. The crystal slice was irradiated by the third harmonic (355 nm) of a Nd:YAG (yttrium-aluminum-garnet) laser at a repetition rate of 10 Hz and a pulse duration of about 10 ns. The energy of the pumping laser was adjusted by using the


10

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calibrated neutral density filters. The beam was focused into a stripe whose shape was adjusted to 3.3 × 0.6 mm by using a cylindrical lens and a slit. The edge emission of the crystal was detected using a Maya2000 Pro CCD spectrometer.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization. The synthesis of these single-benzene based fluorophores 2‒9 is convenient and shown in Scheme 1. Dimethylamino substituted derivative 2 was prepared by methylation of 2,5-bis(methylamino)terephthalate,32 and cyclic amino derivatives 3‒5 were synthesized by Buchwald-Hartwig amination of 2,5-dibromoterephthalate. Simple oxidations of dimethyl/diethyl 2,5-dioxocyclohexane-1,4-dicarboxylate with Nchlorosuccinimide (NCS) provided hydroxyl derivatives 6‒7 which were further methylated to give methoxyl derivatives 8‒9. Although compounds 6‒9 have been synthesized in many years ago,35‒38 their solid-state emission properties have never been studied. The thermal stability of these compounds was also evaluated by DSC and TGA measurements (Table S1, Figure S1, S2).


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Scheme 1. Synthesis of compounds 2‒9. 3.2 X-Shaped Single-Benzene Fluorophore. We choose compound 2 as an example to illustrate the molecular design of X-shaped single-benzene fluorophore. The photophysical properties of 2 were initially investigated in nonpolar solvent cyclohexane (Figure 4, Table 1). The relative short absorption maximum (λabs = 380 nm) as well as small absorption efficiency (ε = 2570 M‒1 cm‒1) of 2 are reasonable in consideration of its mini π-system. Impressively, featuring a large Stokes shift of 151 nm (7480 cm‒1), 2 exhibits a green emission with the maximum (λem) of 531 nm even in nonpolar cyclohexane. This observation is quite different from the typical D-A type molecules of which the Stokes shifts are usually very small in nonpolar solvents.39‒41

Figure 4. Absorption (solid line) and fluorescence spectra (broken line) of 2 in various solvents.


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Table 1. Photophysical data for 2 in solutions and in crystals Absorption

Fluorescence

λabsb

ε

λem

(nm)

(M–1 cm–1)

(nm)

τ

kr

knr

(ns)

(108 s–1)

(108 s–1)

0.78

12.1

0.64

0.18

550

0.71

11.3

0.63

0.26

2730

566

0.66

14.2

0.46

0.24

387

2700

562

0.60

14.3

0.42

0.28

0.3046

378

2550

565

0.17

4.6d

0.37

1.8

‒

‒

‒

552

0.73

18.0

0.41

0.15

Solution or crystals

∆f

c-hexane

0

380

2570

531

toluene

0.0131

384

2700

CHCl3

0.1482

393

CH2Cl2

0.2171

CH3CN crystals

a

ΦFc

a

∆f is the solvent orientation polarizability. b The longest absorption maximum wavelengths. c Absolute fluorescence quantum yields determined by a calibrated integrating sphere system within ±3% error. d The average fluorescence lifetime of a double exponential function: 3.3 ns (0.262) and 9.4 ns (0.073). The double exponential decay should be attributed to the located excited transition as well as intramolecular charge transfer transition. The solvatochromism of fluorescence of 2 was then studied (Figure 4, Table 1). With increasing solvent polarity from cyclohexane to CH3CN, the fluorescence spectra of 2 are moved slightly in a bathochromic direction (the redshift is only 34 nm). The value of dipole moment in excited state of 2 is determined based on the Lippert-Mataga equation42 which expresses the Stokes shift ∆ν as a function of the solvent orientation polarizability ∆f:

∆ =

1 2( − ) ∆ + 4 ℎ

(9.05 × 10! )( − ) = ∆ [# $ ] +


13


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where ε0 represents dielectric constants of vacuum, h represents Planck constant, c represents light velocity, a represents radius of Onsager cavity, µg and µe represent the dipole moment in ground and excited state, respectively. Based on the linear fitting of Lippert-Mataga plots of 2, a slope of 3142 cm−1 is obtained (Figure 5). DFT-calculation at CAM-B3LYP/6-31G* level gives a value of 5.52 Å, and µg value of 0.0 D. Based on the Lippert-Mataga equation, the dipole moment in excited state µe of 2 is determined to be 7.2 D which is much smaller than that of typical D-A type molecules (around 20 D).39‒41

Figure 5. Lippert–Mataga plots of 2. In short, the X-shaped single-benzene fluorophore 2 is a unique skeleton which shows fluorescence in visible region with a large Stokes shift while has a small dipole moment in excited state. Such features of 2 should be mainly originated from the characteristically separated HOMO and LUMO, i.e. the HOMO and LUMO are arranged in a X-shaped fashion (Figure 3). More importantly, X-shaped fluorophore 2 shows high fluorescence quantum yields (ΦF) in solutions, e.g. 0.78 in cyclohexane and 0.60 in CH2Cl2 (Table 1). The high ΦF is an unusual feature on account of 1) the transition from ground state to excited state is less-allowed, as


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revealed by the small ε; and 2) the change of structure in excited state is dramatic, as demonstrated by the large Stokes shift. The high ΦF of 2 should largely rely on the dynamics of excited state. Thus, the radiative (kr) and nonradiative (knr) decay rate constants were determined based on the values of ΦF and the fluorescence lifetime (τ) (Table 1, Figure S3). The small kr of 2 (e.g. 0.64 × 108 s‒1 in cyclohexane and 0.42 × 108 s‒1 in CH2Cl2), which is one order of magnitude lower than the common fluorophores, is consistent with its small ε. According to the equation of ΦF = kr / (kr + knr), the ΦF of 2 should be dominated by the knr. In other word, the very small knr (e.g. 0.18 × 108 s‒1 in cyclohexane and 0.28 × 108 s‒1 in CH2Cl2) responses for the high ΦF of 2, and the suppression of nonradiative decay passway is the key factor to achieve intense fluorescence for X-shaped single-benzene fluorophore. The small knr should be related to the small dipole moment which minimizes the dipole-diploe interaction between fluorophore 2 and solvent molecules. Impressively, 2 shows intense fluorescence in solid state. A strong yellow emission with λem of 552 nm and ΦF of 0.73 is observed for crystals 2 (Table 1). The dynamics study of excited state of crystals 2 (kr = 0.41 × 108 s‒1, knr = 0.15 × 108 s‒1) also reveals that the very small knr renders the high ΦF. In order to get some insights regarding the suppression of the nonradiative decay passway in crystals 2, single-crystal X-ray diffraction analysis was performed. In crystal, molecule 2 adopts a highly twisted conformation due to the steric bulkiness (Figure 6a). Such intramolecular steric repulsion as well as the multiple intermolecular hydrogen bonds (Figure 6b) could constrain the free rotation/vibration of molecule. Along crystallographic c axis, the molecules are arranged in a rotated fashion to form columnar structures which are further parallel-aligned in crystal (Figure 6c). The long distance between neighboring molecules of 5.973 Å as well as the rotated arrangement of molecules clearly reveals that the π‒π stacking and


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dipole-dipole interaction do not exist in crystal 2. In other word, the structural characters of 2, in terms of the single-benzene based mini π-system as well as the small dipole moment in excited state, inherently avoid the π‒π stacking and dipole-dipole interaction. This should be the primary reason for the suppression of knr and the high ΦF of crystals 2. In addition, the following factors also give contributions to the suppression of knr: 1) intramolecular steric repulsion as well as the multiple intermolecular hydrogen bonds constrain the molecule; 2) the large Stokes shift avoids the self-absorption of fluorescence in solid state.

Figure 6. Crystal structure of 2 (CCDC 1581518): a) top view and b) side view of the twisted molecular conformation; c) the multiple intermolecular hydrogen bonds (marked by broken lines) between a molecule and neighboring ones; d) packing structure along crystallographic c axis. The linear distance (5.973 Å) of the adjacent molecules is given. 3.3. Full-Color Tuning of Emissions. The success of X-shaped fluorophore 2 largely inspired us. Thus, various electron-donating amine groups (azetidine, pyrrolidine and piperidine)


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were employed to construct single-benzene fluorophores 3‒5 in order to tune the emissions (Figure S4‒S7). As expected, all of them exhibit strong fluorescence in Table 2. Photophysical data for 1‒9 in solutions and in crystals

Cmpd 1

2

3

4

5

6

7

8

9

a or λabs

ε

λem

(nm)

(M–1 cm–1)

(nm)

toluene

488

6040

585

crystals

‒

‒

cyclohexane

380

crystals

τ

kr

 knr

(ns)

(108 s–1)

(108 s–1)

0.33

7.6

0.43

0.88

620

0.40

12.3

0.33

0.49

2570

531

0.78

12.1

0.64

0.18

‒

‒

552

0.73

18.0

0.41

0.15

cyclohexane

412

2490

539

0.52

11.5

0.45

0.42

crystals

‒

‒

563

0.76

16.5

0.46

0.15

cyclohexane

412

2480

525

0.47

11.5

0.41

0.46

crystals

‒

‒

537

0.72

17.4

0.41

0.16

cyclohexane

374

2390

535

0.58

11.6

0.50

0.36

crystals

‒

‒

536

0.88

20.6

0.43

0.06

cyclohexane

384

4790

446

0.81

12.1

0.67

0.16

crystals

‒

‒

493

0.23

30.4

0.076

0.25

cyclohexane

385

4240

446

0.78

11.2

0.70

0.20

crystals

‒

‒

466

0.37

6.9

0.54

0.91

cyclohexane

334

3060

386

0.55

4.6

1.2

0.98

crystals

‒

‒

412

0.43

8.4

0.51

0.68

cyclohexane

332

3470

386

0.52

4.4

1.2

1.1

crystals

‒

‒

415

0.70

10.4

0.67

0.29

Solution crystals

ΦFb

a

The longest absorption maximum wavelengths. b Absolute fluorescence quantum yields determined by a calibrated integrating sphere system within ±3% error.


17


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Figure 7. Full-color highly emissive organic solids of the X-shaped single-benzene fluorophores 1‒9: a) photographs of these crystals under UV light (365 nm); b) the emission spectra of these crystals. crystals: the λem and ΦF are 563 nm and 0.76 for 3, 537 nm and 0.72 for 4, 536 nm and 0.88 for 5, respectively (Table 2). The study of crystal structures of 4 and 5 also reveals that the π‒π stacking and dipole-dipole interaction are inherently avoided (Figure S14, S15), and thus the nonradiative decay passways (4: knr = 0.16 × 108 s‒1; 5: knr = 0.06 × 108 s‒1) are significantly suppressed and the intense emissions are observed in solid state. The molecular design of X-shaped fluorophore has been well illustrated by 1‒5 with various amino groups which exhibit strong red to green fluorescence in solid state (Figure 7). In order to further tune the emission color, the hydroxyl group and methoxyl group with decreased electron-donating ability were employed to construct single-benzene fluorophores 6 and 8. Their ethyl ester analogies 7 and 9 were also synthesized in order to perturb the emission properties


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(Table 2, Figure S8‒S13). We firstly compare methoxyl substituted 8 and dimethylamino substituted 2 to highlight that the energy gap and emission of X-shaped single-benzene fluorophore can be dramatically tuned by the electron-donating groups. In cyclohexane, 8 exhibits a short absorption band (λabs = 334 nm, ε = 3060 M‒1 cm‒1) and an intense bluish violet emission (λem = 386 nm, ΦF = 0.55) with a large Stokes shift of 4030 cm‒1 (Figure 8). The absorption and emission of 8 are much shorter than those of 2. DFT calculation revels that the HOMO level of 8 (‒7.96 eV) is much lower than that of 2 (‒6.31 eV) while the LUMO levels of them (‒0.48 eV for 8 and ‒0.29 eV for 2) are comparable to each other (Figure 3). Further TDDFT calculation demonstrates that the first excited states of 2 and 8 are mainly originated from the transitions of HOMOs→LUMOs. The calculated absorption maxima of 2 and 8 are 366 nm and 270 nm, respectively. Therefore, the weak electron-donating methoxyl groups render 8 a largely increased energy gap and a significantly blue-shifted emission. Similarly, the moderate electron-donating hydroxyl groups give 6 an emission of which the λem of 446 nm locates at the intermediate region between those of 8 and 2 (Table 2).

Figure 8. Comparison of absorption (solid line) and fluorescence spectra (broken line) of 2 (orange color) and 8 (blue color) in cyclohexane solutions.


19


Page 20 of 30

In crystal, 6 and 7 display strong sky blue (λem = 493 nm, ΦF = 0.23) and blue emission (λem = 466 nm, ΦF = 0.37), respectively, while 8 and 9 show intense deep blue emissions (8: λem = 412 nm, ΦF = 0.43; 9: λem = 415 nm, ΦF = 0.70) (Table 2). Again, the π‒π stacking and dipoledipole interaction are inherently avoided as revealed by the study of crystal structures (Figure S16‒S19), and thus strong deep blue to sky blue emissions of 6‒9 are obtained in solid state (Figure 7). In comparison to the others, the ΦF of crystal 6 is relative low because of the largely decreased kr (0.076 × 108 s‒1) which is one order of magnitude lower than its solution as well as crystals 7‒9. 3.4. Amplified Spontaneous Emission. The strong solid-state emissions of 2‒9 reveal their potential application as crystal lasing media of which high ΦF in crystal is the primary requirement. The lasing properties of organic crystals can be studied by means of amplified spontaneous emission (ASE).43‒46 Thus the ASE property of crystal 4 with suitable size and shape was studied. One isolated crystal was excited with a pulsed laser and its emission was subsequently recorded from the edge area. At low pump laser energy, the emission of crystal is featured as a broad band with a full-width half-maximum (FWHM) of about 70 nm (Figure 9a). Upon increasing the pump energy over a certain threshold, the emission is dramatically narrowed and the FWHM is largely decreased. Finally, a very narrow emission with the FWHM of 14 nm is observed. The threshold feature of FWHM versus pump energy as well as the nonlinear gain of emission peak intensity versus pump energy clearly reveal that crystal 4 is ASE active (Figure 9b). The threshold value of crystal 4, a crucial parameter for evaluating the ASE property, is determined to be 117 kW cm−2.


20

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Figure 9. ASE property of crystal 4: a) emission spectra as a function of the pump laser energy; b) dependences of emission peak intensity and FWHM on the pump laser energy.

4. CONCLUSIONS In summary, we proposed a molecular design of X-shaped fluorophore to construct full-color highly emissive organic solids based on a very small single-benzene skeleton. The fascinating emission properties of these single-benzene fluorophores, in terms of high fluorescence quantum yields in solid state and wide variation of emission colors ranging over deep blue to red, are by no means inferior to those of common fluorophores with large π-systems highlighting the potential talent of single-benzene skeletons. In addition, the amplified spontaneous emission


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Page 22 of 30

properties further increase the value of these single-benzene fluorophores. Therefore, this study not only provides a general strategy to construct highly emissive organic solids but also largely contributes to the basic knowledge of organic fluorophores with very simple molecular structures.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Additional thermal data, photophysical spectra, crystal packing structures, 1H and 13C NMR spectra. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (C.W.). *E-mail: [email protected] (H.Z.). Author Contributions ‡These authors contributed equally. Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT


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TOC Graphic


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Constructing Full-Color Highly Emissive Organic Solids Based on a X

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