Highly Efficient Room-Temperature Phosphorescence from Halogen

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Highly Efficient Room-Temperature Phosphorescence from Halogen-Bonding Assisted Doped Organic Crystals Lu Xiao, Yishi Wu, Jianwei Chen, Zhenyi Yu, Yanping Liu, Jiannian Yao, and Hongbing Fu J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b10160 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 29, 2017

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The Journal of Physical Chemistry

Highly Efficient Room-Temperature Phosphorescence from Halogen-Bonding Assisted Doped Organic Crystals Lu Xiao,†,‡ Yishi Wu,† Jianwei Chen,†,‡ Zhenyi Yu,†,‡ Yanping Liu,†,‡ Jiannian Yao,†,‡ and Hongbing Fu*,†,§,‖ †

Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of chemistry, Chinese

Academy of Sciences, Beijing 100190, People's Republic of China. ‡

University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China.

§

Beijing Key Laboratory for Optical Materials and Photonic Devices (BKLOMPD), Department

of Chemistry, Capital Normal University, Beijing 100048, People’s Republic of China ‖

Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry,

Tianjin University, and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, People's Republic of China Corresponding Author *E-mail: [email protected]; [email protected].

ACS Paragon Plus Environment

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ABSTRACT. The development of metal-free organic room temperature phosphorescence (RTP) materials has attracted increasing attention because of their applications in sensors, bio-labeling (imaging) agents and anti-counterfeiting technology, but remains extremely challenging owning to the restricted spin-flip intersystem crossing (ISC) followed by low-yield phosphorescence that cannot compete with nonradiative relaxation processes. Here, we report a facile strategy to realize highly efficient RTP by doping iodo difluoroboron dibenzoylmethane (I-BF2dbm-R) derivatives into a rigid crystalline 4-iodobenzonitrile (Iph-C≡N) matrix. We found that halogen bonding between cyano group of Iph-C≡N matrix and iodine atom of I-BF2dbm-R dopant is formed in doped crystals, i.e., Iph-C≡N∙∙∙I-BF2dbm-R, which not only suppresses nonradiative relaxation of triplets but also promotes the spin–orbit coupling (SOC). As a result, the doped crystals show intense RTP with an efficiency up to 62.3%. By varying the substituent group R in I-BF2dbm-R from electron donating -OCH3 to electron accepting -F, -CN groups, the ratio between phosphorescence and fluorescence intensities has been systematically increased from 3.8, 15, to 50.

INTRODUCTION. Room-temperature phosphorescence (RTP) materials have attracted a great deal of interest, owing to their applications in various fields, such as luminescent sensors,1-2 biological imaging,3 and anti-counterfeiting technology.4 However, most of the phosphors typically contain expensive heavy-metal atoms, such as platinum and iridium coordinated complexes.5-6 As attractive alternatives, purely organic compounds have appeared at the front of phosphorescent materials, owing to their low cost, low toxicity, earth-abundant constituent elements, and flexibility in molecular design.7-8 However, metal-free organic RTP materials are relatively rare and generally exhibit low efficiency. This is because without heavy-metal atoms


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to enhance the spin-orbital coupling (SOC), the spin-forbidden S1 →T1 intersystem crossing (ISC) is typically too slow to compete with the spin-allowed S1 →S0 fluorescence transition, similarly, the T1 →S0 phosphorescence radiative decay cannot compete with the nonradiative deactivation of triplet.9 Recently several strategies have been advocated to develop purely organic RTP materials by either (i) enhancement of the singlet-to-triplet ISC process or (ii) suppression of nonradiative decay of the excited triplet state. The singlet-to-triplet ISC process can be promoted by mixing the singlet-triplet of different molecular orbital configurations (El-Sayed rule; for example, aromatic carbonyls),10-11 by decreasing the singlet-triplet splitting energy (∆EST) through intramolecular charge-transfer (ICT) interactions,12-13 and by introducing intermolecular heavy atom effect.14 To suppress nonradiative decay of the excited triplet state, restriction of phosphors in a solid medium was widely investigated by crystallization or aggregation,15-17 and by embedment of organic phosphors into a carefully selected matrix such as poly(methyl methacrylate) (PMMA),18 cyclodextrin,19-20 , β-estradiol21 and metal-organic frameworks (MOFs).22 Nevertheless, it remains difficult to realize highly efficient RTP because it’s needed to make the S1→T1 ISC and T1→S0 phosphorescence radiative decay dominant simultaneously in the competitive photophysical processes.


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Figure 1. (a)Chemical structures of I-BF2dbm-R dyes and Iph-C≡N. (b) The highest occupied (down) and lowest unoccupied (up) natural transition orbitals (NTOs) diagrams of I-BF2dbm-R dyes. Iodo difluoroboron dibenzoylmethane (I-BF2dbm-R) materials are the extensively studied phosphorescent chromophores because they display tunable dual emissions and oxygensensitive phosphorescence for ratiometric sensing in solid state.3, 23 However, they are generally fluorescent in solution and possess low photoluminescence (PL) efficiency in solid state.24-25 In this work, we designed and synthesized three iodo difluoroboron dibenzoylmethane (I-BF2dbmR) derivatives (left part of Figure 1a) via Claisen condensation of R-substituted acetophenone with methyl 4-iodobenzoate, followed by boronation in CH2Cl2 (Scheme S1). These compounds are fluorescent in solution and exhibit weak phosphorescence emission in PMMA film. However, when doping these dyes into a rigid crystalline 4-iodobenzonitrile (Iph-C≡N) matrix (right part of Figure 1a), highly efficient RTP was realized. We found that halogen bonding between cyano group of Iph-C≡N matrix and iodine atom of I-BF2dbm-R dopant is formed in doped crystals, i.e., Iph-C≡N∙∙∙I-BF2dbm-R, which not only suppresses nonradiative relaxation


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of triplets but also promotes the spin-orbit coupling (SOC). As a result, the doped crystals show intense RTP with an efficiency up to 62.3%. By varying the substituent group R in I-BF2dbm-R from electron donating -OCH3 to electron accepting -F, -CN groups, the ratio between phosphorescence and fluorescence intensities has been systematically increased from 3.8, 15, to 50. RESULTS AND DISCUSSION. To understand the electronic structures of these compounds, we performed density functional theory (DFT) calculations for the geometry optimization of the S0 state, and subsequently time-dependent DFT calculations for the geometry optimization of the S1 state at the B3LYP/DEF2-SVP level. The highest occupied (down) and lowest unoccupied (up) natural transition orbitals (NTOs) for the S1 state are depicted in Figure 1b. It can be seen that the lowest unoccupied NTOs are mainly delocalized over the BF2 diketone moieties, whereas the highest occupied NTOs are more likely to concentrate on the iodine atom by varying the substituent group R from electron donating -OCH3 to electron accepting -F, -CN groups, suggesting that the S1 state of I-BF2dbm-F and I-BF2dbm-CN has stronger ICT character than IBF2dbm-OCH3. It has been reported that strong ICT interactions of the S1 state may serve to decrease ∆EST for ISC and thus enhance the ISC process.12-13, 26 To confirm this speculation, we performed nanosecond transient absorption and singlet oxygen generation experiments by using hypocrellin A (HA) as a standard (Φ∆= 0.84 in CH2Cl2) and 9,10-diphenylanthracene (DPA) as a chemical trap to estimate the ISC efficiency of these dyes in CH2Cl2 solution (see the Supporting Information for details). As expected, gradually increased values of ΦISC= 0.44, 0.79 and 0.83 were obtained for I-BF2dbm-OCH3, I-BF2dbm-F and I-BF2dbm-CN, respectively.


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Figure 2. (a) Absorption and PL spectra of I-BF2dbm-OCH3 in dilute CH2Cl2 solution (top)， PMMA film (1wt% mixture of I-BF2dbm-OCH3 to PMMA) (middle) and I-BF2dbm-OCH3/IphC≡N doped crystals (1wt% mixture of I-BF2dbm-OCH3 to Iph-C≡N) (bottom). Insets show the corresponding luminescence photographs under 365 nm UV light. The corresponding (b) fluorescence and (c) phosphorescence decay curves of I-BF2dbm-OCH3 in different media. Figure 2a depicts the steady-state absorption (dash) and PL (solid) spectra of I-BF2dbm-OCH3 in dilute CH2Cl2 solution (top), PMMA film (1wt% mixture of I-BF2dbm-OCH3 to PMMA) (middle) and I-BF2dbm-OCH3/Iph-C≡N doped crystals (1wt% mixture of I-BF2dbm-OCH3 to Iph-C≡N) (bottom). Table 1 summarizes the related photophysical parameters. The solution of I-BF2dbm-OCH3 exhibits a maximum absorption peak at 407 nm (blue dash in Figure 2a), while its PL spectrum shows a maximum at 439 nm (blue solid in Figure 2a). We measured the PL quantum yield through an absolute method by using an integration sphere27 and found that IBF2dbm-OCH3 is highly emissive with a ΦI-BF2dbm-OCH3,solution=50.6% in the dilute solution (Table


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1). To further clarify the nature of the excited state, we performed time-resolved PL measurements (Figure 2b). The monomer emission of I-BF2dbm-OCH3 at 439 nm decays monoexponentially (see black square in Figure 2b) with a lifetime of 1.78 ns (Table 1). This short lifetime suggests that the PL of I-BF2dbm-OCH3 solution originates from the fluorescence emission. When I-BF2dbm-OCH3 was dispersed into spin-coated PMMA film (1wt% mixture of I-BF2dbm-OCH3 to PMMA)，a new emission at 517 nm was recorded in addition to the PL at 438 nm (cyan solid in Figure 2a). Figure 2c presents the PL decay curve of I-BF2dbm-OCH3 in PMMA film at 517 nm (red circle), which was fitted with a lifetime of τI-BF2dbm-OCH3/PMMA,517nm = 0.34 ms (Table 1). The PL lifetime at 517 nm is almost five orders longer than that at 438nm (red circle in Figure 2b) τI-BF2dbm-OCH3/PMMA,438nm = 1.56 ns. Therefore, we ascribed the PL origin at 517 nm to the phosphorescence emission. The PL quantum yields of I-BF2dbm-OCH3 in PMMA film were presented in Table 1, ΦF,I-BF2dbm-OCH3/PMMA=45.7% for fluorescence and ΦPh,I-BF2dbmOCH3/PMMA =

4.5% for phosphorescence.

Then the I-BF2dbm-OCH3/Iph-C≡N doped crystals were prepared by simple drop casting from a CHCl3 solution of I-BF2dbm-OCH3 and Iph-C≡N (1wt% mixture of I-BF2dbm-OCH3 to Iph-C ≡N) onto a quartz plate (see the Supporting Information for details). After a few minutes, the solvent was evaporated and doped crystals were formed. From the inset luminescence photograph in Figure 2a bottom, we can observe that the whole I-BF2dbm-OCH3/Iph-C≡N doped crystal exhibits bright green emission under 365 nm UV light. Iph-C≡N is emissive neither in solution nor in crystal with an absorption peak at 256 nm in dilute CH2Cl2 solution (Figure S1), thus indicating the emission originates from I-BF2dbm-OCH3. The inset luminescence photograph in Figure 2a bottom also indicates I-BF2dbm-OCH3 is uniformly


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distributed into the Iph-C≡N crystal. The absorption spectrum of I-BF2dbm-OCH3/Iph-C≡N doped crystals (see the dash green line in Figure 2a) shows broader profile and was red-shifted (the maximum peak was shifted from 407 nm to 419 nm) in comparison with the solution sample of I-BF2dbm-OCH3. The PL spectrum of I-BF2dbm-OCH3/Iph-C≡N doped crystals (solid green line in Figure 2a) exhibits obvious dual emissions, with one peak at 464 nm and another more intensive peak at 525 nm. We further performed time-resolved PL measurements and found that these two PL peaks present totally different decay dynamics. The 464 nm PL of I-BF2dbmOCH3/Iph-C≡N doped crystals decays with a short lifetime of 0.38 ns (blue triangle in Figure 2b, Table 1), thus is assigned to fluorescence emission. In sharp contrast, the 525 nm PL of IBF2dbm-OCH3/Iph-C≡N doped crystals decays with a much longer lifetime of 1.21 ms (blue triangle in Figure 2c, Table 1). Therefore, we ascribed the 525 nm PL of I-BF2dbm-OCH3/Iph-C ≡ N doped crystals to phosphorescence emission. It should be noted that the 464 nm fluorescence emission of I-BF2dbm-OCH3/Iph-C≡N doped crystals was red-shifted by a value of 25 nm in comparison with the 439 nm fluorescence emission of I-BF2dbm-OCH3 solution (Table 1). As the doping amount of I-BF2dbm-OCH3 is too small to cause aggregation effects, these significant shifts in absorption and fluorescence emission indicate strong intermolecular interactions between I-BF2dbm-OCH3 and surrounding Iph-C ≡ N molecules in the doped crystals. The PL quantum yield of I-BF2dbm-OCH3/Iph-C≡N doped crystals was measured absolutely and the ambient phosphorescence quantum yield is up to 39.7% (Table 1). However, the fluorescence quantum yield of I-BF2dbm-OCH3/Iph-C≡N doped crystals was 10.6% (Table 1), much decreased in comparison with the 50.6% fluorescence quantum yield of BF2dbm-OCH3 solution. The drastically decline in fluorescence indicates a more likely singlet decay through S1


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→T1 ISC process, which is evidenced by the much shorter fluorescence lifetime of I-BF2dbmOCH3/Iph-C≡N doped crystals (τF, I-BF2dbm-OCH3/Iph-C≡N = 0.38 ns).

Figure 3. (a) Only fluorescence is allowed for I-BF2dbm-OCH3 in CH2Cl2 solution. (b) In PMMA film, vibrational freedom of the dye molecule is partially limited, allowing triplets to decay emissively. Fluorescence and weak phosphorescence emissions are allowed. (c) The dye molecule is distributed into the Iph-C≡N crystal by substituting for two of the host molecules and is restricted in the rigid environment. Accompanied by an promoted spin–orbit coupling (SOC) attributed to Iph-C ≡ N···I-BF2dbm-OCH3 halogen bonding, strong phosphorescent emission is achieved. (d) A general Jablonski diagram for I-BF2dbm-OCH3/Iph-C≡N doped crystals is presented. The PL mechanisms for I-BF2dbm-OCH3 in solution, PMMA film and I-BF2dbm-OCH3/Iph-C≡ N doped crystals are illustrated in Figure 3a, b and c, respectively. Figure 3a shows that I-


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BF2dbm-OCH3 molecule has more freedoms in solution, and the vibrational loss of triplets is too much to make triplet emission efficient even under oxygen-free conditions, thus only blue fluorescence is observed. In Figure 3b, I-BF2dbm-OCH3 is dispersed in PMMA film, vibrational freedom of the dye molecule is partially limited, allowing triplets to decay emissively, thus fluorescence and weak phosphorescence emissions are allowed. As shown in Figure 3c, IBF2dbm-OCH3 is strongly restricted in the rigid Iph-C≡N matrix through strong Iph-C≡N···IBF2dbm-OCH3 halogen bonding and other intermolecular interactions, thus significantly suppressing the vibrational relaxation of triplets. Furthermore, it’s previously reported that halogen bonding promotes SOC and thus enhance RTP emission.14,

28-30

To confirm the

promoted heavy atom effect in this doping system, time-dependent DFT calculations for bimolecules of Iph-C ≡ N···I-BF2dbm-OCH3 were performed. As shown in Figure S14, more amplitude concentrated on the iodine heavy atom of I-BF2dbm-OCH3 in the highest occupied NTOs of the bi-molecules when compared to the isolated I-BF2dbm-OCH3. This means that IBF2dbm-OCH3 in bi-molecules of Iph-C ≡ N···I-BF2dbm-OCH3 should experience a more pronounced heavy atom effect. Therefore, the S1→T1 ISC process is promoted and fluorescence emission is decreased in I-BF2dbm-OCH3/Iph-C≡N doped crystals. And oxygen quenching is also minimized in I-BF2dbm-OCH3/Iph-C≡N doped crystals as oxygen is sufficiently hindered by Iph-C≡N crystals. As a result, these cooperative effects promoted strong RTP emission in IBF2dbm-OCH3/Iph-C≡N doped crystals.

To analyse the phosphorescence process of I-BF2dbm-OCH3/Iph-C≡N doped crystals in more detail, we assume ΦIC is constant for simplification, which is reasonable as it is only a small portion in S1 decay with a ΦIC = 0.054 in solution calculated according to ΦIC = 1 - ΦF - ΦISC.


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Then the ΦISC of I-BF2dbm-OCH3 in PMMA film and Iph-C≡N crystals and phosphorescence quantum yield ΦPh can be expressed by following equations (1) and (2), ΦISC=1-ΦF -ΦIC

ΦPh =ΦISC ⋅

kPh kPh +kPh,nr

(1)

(2)

where kPh is the rate constant for T1→S0 radiative decay, kPh,nr is the rate constant for T1→S0 nonradiative decay. As evaluated and listed in Table 1, the values of ΦISC for I-BF2dbm-OCH3 in solution and PMMA film are equivalent (44% and 48.9% for solution and PMMA film, respectively). However, the ΦISC in I-BF2dbm-OCH3/Iph-C≡N doped crystals is increased to 84% (Table 1), suggesting a promoted S1→T1 ISC process in I-BF2dbm-OCH3/Iph-C≡N doped crystals. We further calculated the values of kPh and kPh,nr for I-BF2dbm-OCH3 in PMMA film and doped crystals according to the equations listed in the footnote in Table 1. The kPh,nr is greatly reduced in I-BF2dbm-OCH3/Iph-C≡N doped crystals (kPh,nr = 0.43 ms-1, Table 1), being 6.2 times smaller than that in PMMA film (kPh,nr = 2.67 ms-1, Table 1), thus indicating strong suppression of nonradiative decay in doped crystals. Notably, the kPh of I-BF2dbm-OCH3/Iph-C ≡N doped crystals is also increased by 1.4 times than that of PMMA film (0.39 ms-1 and 0.27 ms-1 for doped crystals and PMMA film, respectively. Table 1), which is attributed by the promoted SOC. As oxygen quenching probably influence the RTP emission of I-BF2dbm-OCH3 in PMMA film, we also measured the PL spectra, the quantum yield and the lifetime of RTP of IBF2dbm-OCH3 in PMMA film under N2. The RTP under N2 slightly increases and the quantum yield increases from 4.5% to 9.8% as compared with that under air (see Figure S15 and the data listed in the brackets in Table 1). However, this increase value of 4.3% is very small as compared with the high ISC efficiency ΦISC,I-BF2dbm-OCH3/PMMA = 48.9%. All these results indicate oxygen quenching is only a small portion in nonradiative relaxation for I-BF2dbm-OCH3 in PMMA film under air. Although SOC theoretically increases both kPh and kPh,nr, it should be noted that the Iph-C≡N crystals could provide a much stronger rigid environment for I-BF2dbm-OCH3 with the assistance of strong Iph-C≡N···I-BF2dbm-OCH3 halogen bonding and other intermolecular


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interactions than PMMA film, thus strongly reduce kPh,nr.14, 31-32 Meanwhile, the strong hindrance to oxygen in Iph-C≡N crystals also reduces kPh,nr. As a result, kPh,nr is decreased while kPh is increased in I-BF2dbm-OCH3/Iph-C≡N doped crystals. The influence of Iph-C≡N crystals on IBF2dbm-OCH3 phosphorescence emission can be summarized using a general Jablonski diagram in Figure 3d. To further confirm the PL mechanism for I-BF2dbm-OCH3/Iph-C ≡ N doped crystals, we performed theoretical calculations by using the ONIOM (B3LYP/GENECP: UFF) approach to optimize the geometry.33 In the model, a I-BF2dbm-OCH3 molecule is doped in by substitution of two Iph-C≡N molecules without affecting the original crystal structure of Iph-C≡N (see the Supporting Information for details), which is reasonable as no position shift of characteristic peaks was observed in X-ray diffraction (XRD) patterns (Figure S2). The simulated result indicates that, in addition to the original C≡N···I and C-H···I contacts in pure Iph-C≡N (Figure S3), there are abundant intermolecular interactions between Iph-C≡N and I-BF2dbm-OCH3 in the doped crystals, for example, C-H···X (X= H, I), B-F···I, O-B···I, C=O···I and a significant short Iph-C≡N···I-BF2dbm-OCH3 contact of 2.552 Å, which is much shorter than the 3.182 Å C ≡N···I contact of Iph-C≡N itself (Figure 3c and S6). These interactions, especially the strong Iph-C ≡ N···I-BF2dbm-OCH3 halogen bonding confine I-BF2dbm-OCH3 in a strongly rigid environment, and the Iph-C≡N···I-BF2dbm-OCH3 halogen bonding promotes SOC as well. For comparison, 1,4-diiodotetrafluorobenzene (I2F4) was used as a host for I-BF2dbm-OCH3 to fabricate doped crystals. However, the I-BF2dbm-OCH3/I2F4 doped crystals (1wt% mixture of IBF2dbm-OCH3 to I2F4) are only weakly emissive (Φtotal = 4.1%) and the PL spectrum exhibits few shifts (Figure S4), thus also indicating that cyano group of Iph-C≡N plays an critical role in effecting the luminescence behaviors of I-BF2dbm-OCH3/Iph-C≡N doped crystals.


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Table 1. Photophysical properties of I-BF2dbm-R dyes in dilute CH2Cl2 solution, PMMA film (1wt% mixture of I-BF2dbm-R to PMMA) and I-BF2dbm-R/Iph-C≡N doped crystals (1wt% mixture of I-BF2dbm-R to Iph-C≡N ).

I-BF2dbm-OCH3

I-BF2dbm-F

I-BF2dbm-CN

solution

PMMA film

Iph-C≡N crystal

solution

PMMA film

Iph-C≡N crystal

solution

PMMA film

Iph-C≡N crystal

407

407

419

391

392

407

397

399

411

λF(nm)/ΦF(%)

439/50.6

438/45.7

464/10.6

427/10.3

424/9.3

453/4.1

437/2.5

433/3.7

456/1.0

λPh(nm)/ΦPh(%)a

/

517/4.5(9.8)e

525/39.7

/

499/4.3(8.7)

522/62.3

/

512/2.3(5.6)

522/50.0

τF(ns)

1.78

1.56

0.38

0.30

0.31

0.29

0.28

0.28

/

0.34(0.79) e

1.21

/

0.27(0.63) e

0.50

/

0.24(0.78) e

1.19

e

1.46

/

0.11(0.09)

e

0.50

λAbs(nm) a

τPh(ms) -1 b

e

e

0.39

/

0.20(0.17)

e

0.26

kPh(ms )

/

0.27(0.25)

kPh,nr(ms-1)c

/

2.67(1.01) e

0.43

/

3.50(1.41) e

0.54

/

4.05(1.19) e

0.34

ΦPh/ΦF

/

0.09

3.8

/

0.46

15

/

0.62

50

ΦISC (%)d

44

48.9

84

79

80

85.2

83

81.8

a

b

84.5 c

Luminescence quantum yields were measured absolutely at an excitation wavelength of 380 nm. kPh = ΦPh/(ΦISC∙τPh). kPh,nr = 1/τPh - kPh. The ISC efficiency in PMMA film and Iph-C≡N crystal were calculated according to ΦISC=1-ΦF-ΦIC. eData in brackets were for PMMA film under N2.

d


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Figure 4. Absorption and emission spectra of I-BF2dbm-R in dilute CH2Cl2 solution, PMMA film (1wt% mixture of I-BF2dbm-R to PMMA) and and I-BF2dbm-R/Iph-C≡N doped crystals (1wt% mixture of I-BF2dbm-R to Iph-C≡ N). Insets show the corresponding luminescence photographs of I-BF2dbm-R/Iph-C≡N doped crystals under 365 nm UV light.

We further studied the photophysical properties of I-BF2dbm-F and I-BF2dbm-CN. Figure 4 presents the steady-state absorption (dash) and PL (solid) spectra of I-BF2dbm-F (up) and IBF2dbm-CN (down) in dilute CH2Cl2 solution, PMMA film (1wt% mixture of I-BF2dbm-R to PMMA) and I-BF2dbm-R/Iph-C≡N doped crystals (1wt% mixture of I-BF2dbm-R to Iph-C≡ N). Similar to I-BF2dbm-OCH3, both I-BF2dbm-F and I-BF2dbm-CN are fluorescent in solution (see the solid blue lines in Figure 4). However, the solution fluorescence yields are greatly reduced to 10.3% and 2.5% for I-BF2dbm-F and I-BF2dbm-CN, respectively (Table 1).


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Meanwhile, the solution fluorescence lifetimes are also decreased, with τI-BF2dbm-F,solution = 0.30 ns and τI-BF2dbm-CN,solution = 0.28 ns (Table 1 and Figure S16). All these data suggest that the S1→T1 ISC processes are facilitated by varying the substituent group R in I-BF2dbm-R from electron donating -OCH3 to electron accepting -F, -CN groups, further confirmed ICT induced ISC. Phosphorescence emissions were recorded both for BF2dbm-F and BF2dbm-CN in PMMA film, and the ratio between phosphorescence and fluorescence intensities has been systematically increased according to the order of I-BF2dbm-OCH3, I-BF2dbm-F and I-BF2dbm-CN (cyan solid lines in Figure 2 and 4, Table 1). By doping I-BF2dbm-F and I-BF2dbm-CN into Iph-C≡N crystals, much stronger RTP emissions were observed. The I-BF2dbm-F/Iph-C ≡ N doped crystals achieved a RTP efficiency up to 62.3% (Table 1), which is ca. 90 times higher than its pure crystals (ΦI-BF2dbm-F,crystal = 0.72%, Figure S8) and comparable to the highest values reported up to now.34 And the RTP efficiency of I-BF2dbm-CN/Iph-C≡N doped crystals is also very high with a value of 50.0% (Table 1). The ratio between phosphorescence and fluorescence intensities has been systematically tuned from 3.8, 15, to 50 for I-BF2dbm-OCH3, I-BF2dbm-F and IBF2dbm-CN in doped crystals (green solid lines in Figure 2 and 4, Table 1). The absorption and fluorescence emission of the I-BF2dbm-F/Iph-C≡N and I-BF2dbm-CN/Iph-C≡N doped crystals also present obvious red shifts in comparison to their solution samples (see black dash and blue solid lines for solution, and orange dash and green solid lines for doped crystals in Figure 4). Furthermore, in addition to the reduced kPh,nr, the kPh in I-BF2dbm-F/Iph-C≡N and I-BF2dbmCN/Iph-C≡N doped crystals are both greatly increased as compared with their PMMA film samples, from 0.20 ms-1 to 1.46 ms-1 for I-BF2dbm-F and from 0.11 ms-1 to 0.50 ms-1 for IBF2dbm-CN (Table 1), suggesting the promoted SOC in these doped crystals. All of these


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features indicate the same Iph-C≡N···I-BF2dbm-R halogen bonding assisted mechanism exists in these three doped crystals. CONCLUSIONS. In summary, we report a facile strategy to realize highly efficient RTP by doping iodo difluoroboron dibenzoylmethane (I-BF2dbm-R) derivatives into a rigid crystalline 4iodobenzonitrile (Iph-C≡N) matrix. We found that halogen bonding between cyano group of IphC≡N matrix and iodine atom of I-BF2dbm-R dopant is formed in doped crystals, i.e., IphC≡N···I-BF2dbm-R, which not only suppresses nonradiative relaxation of triplets but also promotes the spin–orbit coupling (SOC). As a result, the doped crystals show intense RTP with an efficiency up to 62.3%. By varying the substituent group R in I-BF2dbm-R from electron donating -OCH3 to electron accepting -F, -CN groups, the ratio between phosphorescence and fluorescence intensities has been systematically increased from 3.8, 15, to 50. This comprehensive strategy opens the way to new high efficiency RTP and tunable fluorescencephosphorescence dual-emissive materials, providing potential applications in biological imaging and anti-counterfeiting technology. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Details on the synthesis and characterization of all new compounds; computational and spectral details; XRD (PDF). AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected]; [email protected]. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China (973) 2013CB933500, the National Natural Science Foundation of China (Grant Nos. 21190034, 91222203, 21273251, 21221002, 91333111, 21503139, and 21673144), the Beijing Natural Science Foundation of China (Grant Nos. 2162011), Project of State Key Laboratory on Integrated Optoelectronics of Jilin University (IOSKL2014KF16), Youth Innovative Research Team of Capital Normal University. REFERENCES (1) Lehner, P.; Staudinger, C.; Borisov, S. M.; Klimant, I. Ultra-Sensitive Optical Oxygen Sensors for Characterization of Nearly Anoxic Systems. Nat. Commun. 2014, 5, 4460. (2) Zhang, G.; Chen, J.; Payne, S. J.; Kooi, S. E.; Demas, J. N.; Fraser, C. L. Multi-Emissive Difluoroboron Dibenzoylmethane Polylactide Exhibiting Intense Fluorescence and OxygenSensitive Room-Temperature Phosphorescence. J. Am. Chem. Soc. 2007, 129, 8942-8943. (3) Zhang, G.; Palmer, G. M.; Dewhirst, M. W.; Fraser, C. L. A Dual-Emissive-Materials Design Concept Enables Tumour Hypoxia Imaging. Nat. Mater. 2009, 8, 747-751. (4) Deng, Y.; Zhao, D.; Chen, X.; Wang, F.; Song, H.; Shen, D. Long Lifetime Pure Organic Phosphorescence Based on Water Soluble Carbon Dots. Chem. Commun. 2013, 49, 5751-5753. (5) Wong, W.-Y.; Ho, C.-L. Functional Metallophosphors for Effective Charge Carrier Injection/Transport: New Robust OLED Materials with Emerging Applications. J. Mater. Chem. 2009, 19, 4457-4482. (6) Liu, Z. W.; Guan, M.; Bian, Z. Q.; Nie, D. B.; Gong, Z. L.; Li, Z. B.; Huang, C. H. Red Phosphorescent Iridium Complex Containing Carbazole-Functionalized β-Diketonate for Highly Efficient Nondoped Organic Light-Emitting Diodes. Adv. Funct. Mater. 2006, 16, 1441-1448. (7) Mukherjee, S.; Thilagar, P. Recent Advances in Purely Organic Phosphorescent Materials. Chem. Commun. 2015, 51, 10988-11003. (8) Yoshii, R.; Hirose, A.; Tanaka, K.; Chujo, Y. Functionalization of Boron Diiminates with Unique Optical Properties: Multicolor Tuning of Crystallization-Induced Emission and Introduction into the Main Chain of Conjugated Polymers. J. Am. Chem. Soc. 2014, 136, 1813118139.


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TOC Graphic


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Highly Efficient Room-Temperature Phosphorescence from Halogen

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