Anomalous Effect of Intramolecular Charge Transfer on the Light

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Anomalous Effect of Intramolecular Charge Transfer on the Light Emitting Properties of BODIPY Hongcheng Gao,† Yu Gao,‡ Cong Wang,† Dehua Hu,† Zengqi Xie,*,† Linlin Liu,† Bing Yang,‡ and Yuguang Ma*,† †

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China ‡ State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, China S Supporting Information *

ABSTRACT: A new way to approaching highly emissive BODIPY dyes in both solution and solid state is achieved by introducing intramolecular charge transfer (ICT). A hybrid excited state that shows behaviors of both the ICT excited state and local excited state is discovered to be beneficial in avoiding the disturbance from the rotation vibration of the flexible phenyl substituents. Thus, the nonradiative transition process is suppressed, and the fluorescence efficiency goes up. By modification of the excited state, we realize emission enhancement of crystalline BODIPY derivatives with a PLQY from 0.08 to 0.27, and the compound expresses a good potential application prospect in organic semiconductors.

KEYWORDS: BODIPY, intramolecular charge transfer, solid emission, twisting vibration, electroluminescence

1. INTRODUCTION Boron dipyrromethene (BODIPY) is a class of versatile fluorophores widely studied in the field of biological labeling and ion probes, which typically shows very strong fluorescence with a narrow emission band in dilute solutions.1−3 However, the fluorescence from its solid state is usually seriously quenched due to the relatively strong intermolecular staking, which obviously limits the applications in optoelectronic devices.4−6 In addition, the very small Stokes shift of BODIPY molecules results in strong self-absorption behavior in the solid state, which is another penalty for the solid emission.7,8 In principle, introduction of bulky groups to the BODIPY core could increase the intermolecular stacking distance, which is beneficial in avoiding the aggregation caused quenching (ACQ). 9,10 Previously, bulky groups such as mesityl, trimethylsilylphenyl, tert-butylphenyl, and tetrephenylethene were successfully appended on the mesoposition of the BODIPY core, which for sure suppressed the aggregation effectively; however, the self-absorption of the fluorescence is still a problem.11,12 The strategy of introducing an electrondonating substituent to BODIPY cores was then intensively investigated to tune the emission color through the effect of intramolecular charge transfer (ICT), thus diminishing selfabsorption.13−15 Nevertheless, the low lying ICT excited state is sensitive to the ambient environment and usually leads to a poor fluorescence efficiency for the weak irradiation transition. Indeed, some twisted intramolecular charge transfer (TICT) fluorophores exhibit apparent aggregation-induced emission © XXXX American Chemical Society

(AIE) behavior: they are weakly emissive in the dissolved state but highly emissive in the aggregated state by restriction of the intramolecular twisting vibration, as well as with some BODIPY dyes.16−21 Recently, several twisted electron donor−acceptor molecules possessing hybrid local and charge transfer (HLCT) excited states were reported by our group.22,23 These molecules show not only properties of a local excited state but also properties of a charge transfer state; therefore, they reveal high fluorescence efficiency as an LE-state molecule and a relatively strong solvent effect in solutions as a CT-state material.24,25 Also, materials with an HLCT state usually emit brilliantly in the condensed state, which is the essential factor for the realization of highly efficient electroluminescence devices.26,27 In addition, the utilization of molecules with an HLCT state has been proven to be a promising method to achieve NIR emissive materials.28,29 In this work, we report a donor−acceptor−donor (D−A−D) type BODIPY-based fluorescence material (B4) that shows high efficiency in both the solution state and crystalline state. The ICT process plays an anomalous effect on the excited state of the molecule to avoid the coupling between Special Issue: AIE Materials Received: September 5, 2017 Accepted: March 2, 2018

A

DOI: 10.1021/acsami.7b13444 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ether/CH2Cl2 2:1). Recrystallization gave a dark red powder (321 mg, 78% yield). 1H NMR (600 MHz, CDCl3, Figure S9) δ 7.51−7.45 (m, 3H), 7.35 (d, J = 6.4 Hz, 2H), 7.26 (dd, J = 8.8, 7.0 Hz, 8H), 7.11 (d, J = 7.6 Hz, 8H), 7.06 (d, J = 8.6 Hz, 4H), 7.02 (dd, J = 14.3, 8.0 Hz, 8H), 2.57 (s, 6H), 1.33 (s, 6H). 13C NMR (151 MHz, CDCl3, Figure S10) δ 154.42 (s), 147.77 (s), 146.82 (s), 141.84 (s), 139.13 (s), 135.65 (s), 133.50 (s), 131.44 (s), 130.99 (s), 129.37 (d, J = 13.1 Hz), 129.10 (s), 128.23 (s), 127.49 (s), 124.66 (s), 123.13 (s), 13.65 (s), 13.01 (s). MALDI-TOF MS m/z (Figure S14): calcd 810.371; found 810.298 [M]+ (100%), 791.269 [M − F]+ (35%). 2.3.3. 2,6-Bis(4-diphenylaminylphenyl)-4,4,8-triphenyl-1,3,5,7tetramethyl-4-bora-3a,4a-diaza-s-indacene (B4). B4 was synthesized according to B3, and recrystallization gave a red powder (271 mg, 73% yield). 1H NMR (500 MHz, CDCl3, Figure S11) δ = 7.49 (m, 5H), 7.42 (d, J = 6.6 Hz, 2H), 7.28−7.24 (m, 6H), 7.21 (dd, J = 14.7, 7.2 Hz, 10H), 7.06 (d, J = 7.6 Hz, 8H), 7.00−6.97 (m, 8H), 6.91 (d, J = 8.6 Hz, 4H), 1.79 (s, 6H), 1.36 (s, 6H). 13C NMR (151 MHz, CDCl3, Figure S12) δ = 153.68, 147.82, 146.37, 142.43, 136.77, 136.19, 134.19, 133.72, 131.46, 131.29, 129.33, 129.05, 128.93− 128.58, 127.36, 125.80, 124.44, 123.22, 122.91, 77.37, 77.16, 76.95, 15.98, 13.36. MALDI-TOF MS m/z (Figure S15): calcd 926.4542; found 849.3495 [M − benzene]+ (100%), 926.3837 [M]+ (12%).

the rotation vibration of the phenyl substituents and molecular irradiation process.

2. EXPERIMENTAL SECTION 2.1. General Experiments. All chemicals and solvents were used as received from a commercial company without further purification. NMR spectra were recorded on a Bruker 500 MHz spectrometer. MALDI-TOF mass spectra were measured on a Bruker Autoflex speed TOF/TOF spectrometer. Single-crystal diffraction was performed on an Xcalibur Eos Gemini machine at 293 K. UV/vis spectra were recorded with a Shimazu UV-3600 plus under ambient conditions. Fluorescence emission spectra at 77 and 298 K were recorded on a Horiba Scientific fluoroMax-4 spectrofluorometer. Photoluminescence efficiency (ΦPL) was recorded by a Hamamatsu compact absolute photoluminescence quantum yield (PLQY) spectrometer. Fluorescence kinetics are measured by the method of time-correlated singlephoton counting (TCSPC) with a Spectra-Physics MAI TAI HP 1040S (femtosecond laser), a PicoQuant GmbH (detector), a Princeton Instruments Acton SP2150 (monochromator), and a PicoQuant HydraHarp 400 (multichannel picosecond event timer). For the fairly short lifetimes of B2, IRF was introduced to deconvolution and fitted automatically in the measurement software. Without special notes, all the data in the paper were obtained at room temperature (298 K). Cyclic voltammetry (CV) was performed on a CHI600D electrochemical workstation. Optimized geometry and frontier molecular orbitals were given by DFT calculation by the method of B3LYP/6-31G (d, p), and the natural transition orbitals were performed at the level of TD-ωB97X/6-31G (d, p). 2.2. Crystal Growth. Crystals of these dyes can be easily obtained from the mixed solvents of chloroform and methanol at room temperature. 2.3. Synthesis. Synthetic routes of the compounds are outlined in Scheme S1. B1 is synthesized according to the literature.30 2.3.1. 4,4-Diphenyl-1,3,5,7-tetramethyl-8-phenyl-4-bora-3a,4adiaza-s-indacene (B2). In a round-bottom flask, B1 (324 mg, 1 mmol) was dissolved in 50 mL of anhydrous diethyl ether, and 1 mL of phenylmagnesium bromide (3 M) was added dropwise under an argon atmosphere. The solution was stirred at room temperature for 3 h, and thin layer chromatography (TLC) was used to monitor the reaction process. The reaction was quenched by pouring the mixture into water, and the organic layer was extracted with CH2Cl2 and dried over anhydrous Na2SO4. The extract was concentrated by rotary evaporation for further purification by column chromatography (silica, petroleum ether/CH2Cl2 2:1). Recrystallization gave an orange-red solid (198 mg, 45% yield). 1H NMR (500 MHz, CDCl3, Figure S7) δ 7.50−7.45 (m, 3H), 7.39 (dd, J = 8.0, 1.3 Hz, 4H), 7.34−7.31 (m, 2H), 7.24 (dd, J = 6.1, 1.3 Hz, 4H), 7.21−7.17 (m, 2H), 5.91 (s, 2H), 1.78 (s, 6H), 1.39 (s, 6H). 13C NMR (126 MHz, CDCl3, Figure S8) δ 154.73 (s), 142.51 (s), 140.37 (s), 136.19 (s), 133.97 (s), 131.59 (s), 128.98 (s), 128.67 (s), 127.35 (s), 125.80 (s), 122.11 (s), 77.41 (s), 77.16 (s), 76.91 (s), 17.12 (s), 14.87 (s). MALDI-TOF MS m/z (Figure S13): calcd 440.2424; found 363.1587 [M − benzene]+ (100%), 439.1873 [M − H]− (8%). (The B−C bond constituted by the phenyls and the boron atom is easy to fracture.) 2.3.2. 2,6-Bis(4-diphenylaminylphenyl)-4,4-difluoro-8-phenyl1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (B3). B1−Br was synthesized by NBS (N-bromosuccinimide, 2.1 equiv) and B1 (1.0 equiv) in dichloromethane; after the mixture was stirred for 12 h, it was washed with water three times, and then, rotary evaporation and recrystallization gave the pure B1−Br, which was directly used without further purification (Scheme S1). Triphenylamine-4-boronic acid (433 mg, 1.5 mmol), B1-Br (241 mg, 0.5 mmol), and Pd(PPh3)4 (35 mg, 3% equiv) were dissolved in 5 mL of toluene and a 2 mL aqueous solution of Na2CO3 (2 M). The mixture was refluxed under argon protection for 24 h. After the reaction mixture was cooled down to room temperature, it was poured into water and washed with brine and extracted by CH2Cl2. Then, the organic layer was dried over Na2SO4. The dried solution was concentrated by rotary evaporation for further purification by column chromatography (silica, petroleum

3. RESULTS AND DISCUSSION 3.1. Molecular Design and Single-Crystal Structures. As depicted in Chart 1, fluorine atoms of B1 were replaced by Chart 1. Chemical Structures of B1−B4

phenyl groups to achieve B2, in order to increase the packing distance and avoid the F···H−C bond that is very common in classic BODIPYs.6 The basic idea for this substitution is to develop a highly solid-state emissive material that can be applied in optoelectronic devices. Then, two twisted triphenylamines (TPA) as electron-donating groups were added at the 2,6 positions of the BODIPY core, to render B3 and B4 D−A− D type molecules with ICT states. Both phenyl groups and TPA moieties are beneficial in preventing the tight intermolecular π−π stacking of BODIPY dyes, and the ICT process aroused by TPAs would also give a relatively large Stokes shift to minimize the self-absorption in condensed state. Molecular structures of B1−B4 given by single-crystal diffraction were placed in Figure 1, and more detailed crystal diffraction data were collected in Table S1. Crystal structures of B1 have been carefully studied previously.31,32 The phenyl group at the mesoposition and the BODIPY plane are nearly orthogonal with a large dihedral angle of 85.4° because of the B


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Figure 1. Molecular structures (top) and packing modes (bottom) of B1 (a), B2 (b), B3 (c), and B4 (d).

steric effect of the two methyl groups at the 1,7 positions. Molecules of B1 in the crystalline form like to pack densely as the mode of cofacial herringbone, and the packing distance is about 3.8 Å, exhibiting properties of strong π−π stacking. B2, as well, shows a relatively large dihedral angle at the mesoposition. However, as depicted in Figure 1b, the molecular packing model of B2 is different from B1. Due to the additional phenyl groups, B2 tends to form dimers through a head-to-tail mode with a relatively large intermolecular distance of 4.3 Å. Then, the dimers closely pack into crystals, and the distance between the dimers is 3.2 Å. When the 2,6 positions were substituted, torsion angles between the TPAs and the BODIPY plane of B3 are 40.3 and 43.6°, respectively, and for B4, they are 52.0 and 80.5°. A smaller torsion angle is usually beneficial for the delocalization of the electron cloud and the interaction between the donor and acceptor; thus, B4, with much larger torsion angles, might not so conducive to the occurrence of charge transfer. Meanwhile, with the fluorine atoms substituted by phenyl groups, the intermolecular distance and distance

between two BODIPY planes of B4 (17.5 and 6.3 Å, respectively, Figure 1d) are much larger than those of B3 (8.8 and 2.9 Å, respectively, Figure 1c). As a result, B4 is more likely to pack according to the TPA groups rather than the BODIPY plane, the intermolecular π−π stacking of BODIPY is suppressed to a great extent, and the ACQ effect should be minimized. Not only that, the phenyl groups at the boron atom also affect the planarity of the BODIPY core, and larger dihedral angles are found in B2 and B4 than in B1 and B3 (Figure S1). This kind of distortion is bad for the delocalization of the electrons and will influence the photophysical properties of the molecules to some extent. 3.2. Theoretical Calculation. Density functional theory (DFT) calculation gives the electron density distribution of the frontier molecular orbitals of these compounds, which are shown in Figure 2. B1 and B2 show almost the same electron density distributions, but the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of B2 are a little higher than those of B1. C


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Figure 2. Optimized geometry and frontier molecular orbitals for compounds B1−B4 in the ground state at the level of B3LYP/6-31G (d, p).

Figure 3. Natural transition orbitals for S0 → S1 and S0 → S2 excitations of B3 and B4 (μt: transition dipole moment (unit: Debye); f: oscillator strength).

Generally, a lower LUMO level means stronger electronwithdrawing ability; thus, with fluorine atoms substituted by phenyls, the electron-withdrawing ability of B2 is weakened in comparison with that of B1. When TPA moieties were attached, their distributions are still alike, while B3 exhibits some higher HOMO and LUMO energy levels. Several reasons lead to this result. One is that the electron-withdrawing ability of B1 is strong, so there are effective interaction between TPAs and the BODIPY core, with more electrons contributing to the energy levels. Another reason is that the twist angles of B3 are smaller

than those of B4, and smaller twist angles are definitely more conducive to the conjugation and electron delocalization between the donor and acceptor units. Surprisingly, the twist angles calculated from optimized configurations are coincident with the results of crystal structures. It should be noted that the HOMO levels of B3 and B4 are delocalized along the whole conjugated backbone; however, the HOMO−1 levels are mostly distributed on TPA groups. There are barely energy gaps between the HOMO−1 and HOMO (−4.96 and −4.86 eV for B3, −5.19 and −5.08 eV for B4); thus, in principle, both D


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Figure 4. Absorption and PL spectra of B1 (a) and B2 (b) in various solutions. Fluorescence spectra of B1 (c) and B2 (d) in THF at 298 and 77 K. Solution concentration: 1.0 × 105 M. Excitation wavelength for PL: 450 nm.

where α = x, y, z denotes the electronic coordinates. Ψf is the wave function of the excited state, and Ψ0 is the wave function of ground state. q characterizes the charge of the electron, and j characterizes the electronic state.33 If M = 0, it means either the electronic coordinates do not move at all, or there is no overlap between the wave functions of the two states; in that way, the transition is forbidden. Usually, the transition dipole moment of an LE transition is large, while it is quite small for a CT transition. Clearly, CT transition is the major process for S0 → S2 of B3 and B4 because of their small transition dipole moment (0.57 D for B3 and 0.48 D for B4) and low oscillator strength. With a large transition dipole moment (μt = 15.4 D) and a high oscillator strength, the LE transition plays a leading role in the S0 → S1 transition of B4. What is different is that the S0 → S1 transition of B3 shows a transition dipole moment of 4.08 D, which is much smaller than that of B4 and that means there is more of a CT transition process of B3. Obviously, these results can be ascribed to the stronger D−A interaction of B3 compared with that of B4, which was mentioned in the previous part of frontier molecular orbitals. As is well-known, the excited state is sensitive to its surroundings (such as solvent polarity), especially for the CT state. Therefore, we could foresee that B3 and B4 will perform different transition properties in solvents. 3.3. Photophysical Properties in Solution. The photophysical properties of B1 and B2 in various solvents are shown in Figure 4, and the relevant spectroscopic data are collected in Table 1. The absorption curves of B1 and B2 are similar in structure, but the maximum absorption wavelength of B1 is redshifted (2 or 3 nm). Also, in Figure 4a,b, we can see that the molar absorption intensity of B2 is weaker than that of B1 in solutions. Without a doubt, the red-shifted absorption spectra and larger absorbance of B1 are due to the less severe distortion

the transition from the above occupied orbitals to the LUMO may contribute to the lowest excited state of B3 and B4 in proper circumstances. This kind of frontier molecular orbital distribution is essential to the formation of an HLCT state. Energy levels given by cyclic voltammetry show relatively consistent results with the DFT calculation (Figure S2). The HOMO levels of B3 and B4 rise close to the HOMO of TPA (5.2 eV), proving the existence of ICT from TPA (donor) to BODIPY (acceptor). In order to further understand the influence of the subtle difference between B3 and B4, natural transition orbitals (NTOs), which were calculated based on the geometry of the ground state (S0) at the level of TD-ωB97X/631G (d, p) were also introduced (Figure 3). In all cases, the emission properties of a fluorophore are mainly affected by the nature of excited-states S1 and S2. As displayed in Figure 3, we can easily find that the holes and particles of B3 and B4 are mainly centralized on the BODIPY core, and only a tiny part of the holes are spread on the TPA moiety for S0 → S1 excitation. Though particles for S0 → S2 are similar to the distribution of S0 → S1, the holes are quite different and are principally dispersed on the TPA groups. For the sake of distinguishing the transition nature of S0 → S1 and S0 → S2, electronic transition dipole moments of these states were also investigated. The transition dipole moment M (μt) is given by the equation M2 = Mx2 + My2 + Mz2

with Mα = ⟨Ψf | ∑ qjαj|Ψ0⟩ j

E


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when considering the difference of the molecular structures of B1 and B2. Only a tiny difference is observed in the full-width at half-maximum (fwhm) of the absorption spectra of B1 and B2, which means the ground state of B1 and B2 are alike. However, the PL spectra of B2 reveal much broader fwhm values than those of B1, indicating that the excited states of B2 undergo more vibrational relaxation. The emission spectra in THF at 77 K is proof of our conclusion (Figure 4c,d). The fluorescence intensity of B1 at 77 K increases little, while the emission of B2 is dramatically enhanced, and the shoulder peak reappears. With further comparison of the excitation spectra in Figure S3, three vibration energy levels (21 000, 20 500, 19 500 cm−1) were found to contribute to the emission of B2, while only two for B1 (21 000, 20 000 cm−1) were found to contribute to the emission. Fluorescence kinetics obtained by TCSPC at 298 K shows that the lifetimes of B1 are relatively long (about 4 ns), while the lifetimes of B2 appear to be much shorter and are difficult to calculate precisely (less than 1 ns, Figure S4). The short lifetimes of B2 can be attributed to poor electron orbital overlap in the transition of LUMO → HOMO, since the BODIPY plane of B2 is distorted. The photophysical properties of B1 and B2 are almost insensitive to the solvent polarity displaying the nature of the LE transition. When it comes to B3 and B4, with TPA groups as donors attached to the BODIPY cores, abnormal emission properties emerge.

Table 1. Photophysical Properties of B1 and B2

B1

B2

solvent

λabs (nm)

fwhm (cm−1)

λem (nm)

fwhm (cm−1)

ΦPL

hexane i-PrEa ether CHCl3 THFb DCMc hexane i-PrEa ether CHCl3 THFb DCMc

504 500 499 500 501 499 501 498 497 497 498 498

721 738 744 771 778 761 760 771 777 795 796 795

516 510 511 510 513 513 514 510 510 511 512 511

921 941 949 974 968 963 1387 1265 1112 1156 1236 1205

0.71 0.82 0.95 0.92 0.87 0.91 0.013 0.015 0.016 0.018 0.019 0.021

a c

Abbreviation for isopropyl ether. bAbbreviation for tetrahydrofuran. Abbreviation for dichloromethane.

of the BODIPY plane (Figure S1). Contrary to B1, which emits intense fluorescence in solution and exhibits a shoulder peak at 550 nm, B2 is weakly emissive with a quantum efficiency as low as 0.02, and the shoulder peak becomes unapparent. Such weak fluorescence of B2 is proposed to be related to the rotation vibration of the phenyl groups attached on the boron atom,

Figure 5. Absorption and PL spectra of B3 (a) and B4 (b) in various solvents. Fluorescence kinetics of B3 (c) and B4 (d) detected at 600 nm (bottom) and 720 nm (top). Solution concentration: 1.0 × 10−5 M. Excitation wavelength: 500 nm for PL and 520 nm for fluorescence kinetics. F


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ACS Applied Materials & Interfaces Table 2. Photophysical Properties of B3 and B4 B3

B4

a

solvent

λabs (nm)

fwhm (cm−1)

λem (nm)

fwhm (cm−1)

ΦPL

τa (ns)

τb (ns)

kfc (108 s−1)

knrc (108 s−1)

hexane i-PrE ether CHCl3 THF DCM hexane i-PrE ether CHCl3 THF DCM

541 539 539 542 542 543 532 530 531 533 532 532

1951 2009 2002 2111 2085 2108 1640 1658 1663 1705 1785 1734

609 623 629 658 661 670 587 593 597 603 607 612

1531 2179 2275 2203 2350 2342 1559 1817 1924 2027 2363 2591

0.59 0.26 0.23 0.09 0.04 0.008 0.17 0.22 0.24 0.25 0.21 0.17

4.72 2.98 2.68 2.32 1.40 0.98 1.31 1.79 1.86 2.40 2.31 1.95

4.91 3.05 2.72 2.35 1.34 1.03 1.37 1.86 1.91 2.52 2.42 2.01

1.3 0.87 0.86 0.39 0.29 0.08 1.3 1.2 1.3 1.0 0.91 0.87

0.87 2.5 2.9 3.9 6.9 10.1 6.4 4.4 4.1 3.1 3.4 4.3

Detected at 600 nm. bDetected at 720 nm. cCalculated according to the data detected at 600 nm.

Figure 6. PL spectra of B1 (a), B2 (b), B3 (c), and B4 (d) in water/THF mixtures. Solution concentration: 1.0 × 10−5 M. Excitation wavelength: 450 nm for B1 and B2, 500 nm for B3 and B4.

As is illustrated in Figure 5 and Table 2, B3 shows conspicuous CT properties, such as an increasing Stokes shift, absorption, and emission fwhm values; emission efficiency and lifetimes of B3 are decreasing sharply in polar solvents. This is definitely due to the strong D−A interaction, which was mentioned several times in previous sections. However, compared with B3, the absorption and emission spectra of B4 are blue-shifted, which is mainly because of the weaker D−A interaction and partly because of the distorted BODIPY plane. In addition, the emission spectra of B4 red-shifts only a little in polar solvents, and the fluorescence and lifetimes go through a process of increasing first and then decreasing with increasing solvent polarity. It reveals the properties of neither traditional LE transition nor classic CT transition. Therefore, there must be different transition processes of B3 and B4 in the solutions.

Considering the NTO distribution of B3 and B4 in the previous section, it is comprehensible that the S0 → S1 transition is the leading process in low polarity solvents, and in polar solvents, the energy level of the S0 → S2 transition drops down close to the energy level of S0 → S1, thus the S0 → S2 transition becomes the major process.34 Then, we could attribute the emission of B3 to be mainly controlled by CT transition on the basis of the variation of fluorescence spectra, efficiency, and kinetics. Without a doubt, this is related to the strong electronwithdrawing ability of B1 and a large proportion of the CT transition in the S0 → S1 process of B3. Differently, the S0 → S1 process of B4 is closer to an LE transition rather than a CT transition, and B4 demonstrates a specific variation of fluorescence and lifetime; thus, the emission of B4 should be the result of the combined effect of the LE state and CT state. G


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Figure 7. AFM topographic images of thin films of B4 before and after annealing.

Because the lifetimes of B3 and B4 detected at the main emission peak (600 nm) and far red edge (720 nm) get almost uniform results, their fluorescence respectively should originate from only one kind of excited state. This kind of excited state in B4, which is the combined effect of LE and CT, is called HLCT state. The contribution of the CT transition (S0 → S2) in the HLCT state is closely related to the solvent polarity. The more polar the solvent is, the more CT transition will contribute to the excited state. Due to the large transition distance, the lifetime of CT state is relatively long; therefore, we can observe that the lifetimes of B4 are gradually increasing from hexane to chloroform. In Figure 5b, we can easily find that the proportion of the emission shoulder peak at 680 nm is increasing; meanwhile, the fwhm values of the emission spectra of B4 in THF and DCM are larger than those of B3, both demonstrating the existence of ICT in B4. In THF and DCM, the PLQY and lifetimes of B4 are slightly lowered, and the nonradiative transition constant (knr) is rising, providing direct evidence for the dominant role of the CT state, since the strong interaction between the CT state and solvent molecules will accelerate the nonradiative transition process. As for B3, the knr increases remarkably due to B3 having a stronger D−A interaction than B4, and this is also the reason for the deceasing lifetimes, PLQY, and kf of B3. Different from B2, which is facile to transition by a nonradiative process, B4 is much more radiative with a maximum PLQY up to 0.25, making it 10 times brighter than B2, though also B4 has more flexible phenyls. The fluorescence intensity and lifetimes of B4 at 77 K increase to some extent but not as notably as in B2, which means the adverse effect of rotation vibration for fluorescence efficiency is weakened (Figure S5). Comparing the fluorescence transition rate and nonradiation transition rate in Table 2, we find that kf of B4 does not change much, but knr is decreasing typically along with the solvent polarity. Integrating the previous discussion, we can affirm that the descending knr is aroused by the increasing contribution of the CT transition (S0 → S2). In other words, the combined effect of the LE and CT transitions (HLCT state) in B4 is conducive to suppressing the nonradiative transition process in low polar solvents, and the additional TPA groups play an essential role to change the transition process of B4. As a result, B4 is highly emissive and almost insensitive to solvent polarity. These phenomena reveal the properties and advantages of the HLCT state, and only when the donor and acceptor match with each other could we observe the transition process of the HLCT state. Successfully,

we realized emission enhancement of BODIPY derivatives through modification of the excited state of the molecule. 3.4. Solid-State Emission Properties. The aggregation properties of these compounds in THF/water mixtures were also investigated (Figure 6). As a classic BODIPY dye, B1 is highly emissive in the mixtures before aggregation. When aggregates of B1 are formed in a mixture with 90% water, a notable ACQ effect is observed, which is definitely assigned to the fierce intermolecular π−π stacking. Furthermore, the fluorescence efficiency of B1 is merely 0.08 as powder and 0.11 as crystal, far lower than its efficiency in solution. The emission properties of B2 in the mixtures are different. The fluorescence spectra red-shifts up to 20 nm when the water fraction increases to 80 and 90%, which can be attributed to the emission of the aggregates. The emission intensity does not vary much, because the fluorescence efficiency of B2 is originally low in solution. In fact, powders of B2 are weakly emissive with the PLQY being merely 0.04, and they are a bit brighter in crystalline state with an efficiency of 0.08. Both B1 and B2 exhibit severe ACQ effects, which badly restrict their utilization in organic semiconductors. However, the performance of the D−A−D compounds B3 and B4 in the mixtures is quite attractive. The fluorescence intensity of B3 and B4 is weakening with the water fraction increasing from 10 to 50%, and the maximum emission wavelength is red-shifting gradually, which exhibits the property of a TICT process. When the water fraction continues rising from 60 to 90%, remarkable AIE phenomena appear. The fluorescence intensity increases significantly, and the maximum emission wavelength is blueshifted. Because of the strong CT property, B3 demonstrates a more apparent TICT process and AIE phenomena, while the emission properties of B4 are more stable due to the HLCT state. The fluorescence efficiency of the crystalline state of B3 is only 0.10, implying that there is still strong stacking in the crystal. With fluorine atoms substituted by phenyls, the crystalline state of B4 shows a quantum yield up to 0.27 (Figure S6), much higher than that of B1 (0.11) and B2 (0.08). It is rather rare for a D−A−D molecule based on BODIPY to have such high efficiency. As a result, by modification of BODIPY cores, we realize the transformation from ACQ to AIE and obtain efficiently emissive BODIPY crystals. The substituents, at the same time, improve the solubility and film-forming properties of B4 considerably. Dissolved in chlorobenzene (15 mg/mL), neat thin films of B4 obtained by spin coating (2000 rpm/min) on the substrate of PEDOT:PSS(30 nm)/ITO get a roughness of less than 3 nm. After the films H



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are annealed at 120 °C for 10 min, the films still keep smooth (Figure 7). However, B3 can merely dissolve 5 mg in 1 mL of chlorobenzene, thus high quality and uniform films of B3 are difficult to prepare. With enhanced solution and solid emission, B4 expresses a good potential application prospect in organic semiconductors.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b13444. Detailed synthesis procedures, DFT calculation, cyclic voltammetry measurement, photoluminescence properties, electroluminescence properties, crystal structure and relevant data, characterization spectra (PDF) X-ray data: B2 (CIF) X-ray data: B3 (CIF) X-ray data: B4 (CIF)

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REFERENCES

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4. CONCLUSION In summary, we report a BODIPY-based D−A−D type molecule that shows relatively high efficiency in solution and the crystalline state. The ICT excited state coupled with a local excited state to form a hybrid excited state, which effectively avoids the disturbance from the rotation vibration of the flexible phenyl substituents. Meanwhile, by carefully and seriously analyzing PL spectra, PLQY, kf, knr, and the fluorescence kinetics, we find the interaction processes of the S0 → S1 transition and S0 → S2 transition, and further deepen our understanding of the excited state, especially the HLCT state. Thus, by modification of the BODIPY dyes, we realize improvement of fluorescence and film-forming properties. This work highlights that it is possible to manipulate the excited state through the ICT state to suppress the irradiative transition process in order to develop highly emissive molecules and introduce a prominent way to design BODIPY derivatives for the application of organic semiconductors.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; Fax: +86-20-87110606; Tel: +8620-22237036 (Y.M.) *E-mail: [email protected] (Z.X.) ORCID

Zengqi Xie: 0000-0002-9805-8176 Bing Yang: 0000-0003-4827-0926 Yuguang Ma: 0000-0003-0373-5873 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (51573055, 21334002, 21733005, 51761135101, 51473052, 51521002), the Ministry of Science and Technology of China (2015CB655003), the Fundamental Research Funds for the Central Universities, the Major Science and Technology Project of Guangdong Province (2015B090913002), and the Key Program of Guangzhou Scientific Research Special Project (201707020024) for their financial support. I


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