From Dark TICT State to Emissive quasi-TICT State: The AIE

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From dark TICT state to Emissive quasi-TICT State: The AIE Mechanism of N-(3-(benzo[d]oxazol-2-yl)phenyl)-4-tert-butylbenzamide Wei Huang J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 29 Dec 2014 Downloaded from http://pubs.acs.org on December 29, 2014

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From dark TICT state to Emissive quasi-TICT State: The AIE Mechanism of N-(3-(benzo[d]oxazol-2-yl)phenyl)-4-tertbutylbenzamide

Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:

The Journal of Physical Chemistry jp-2014-089433.R2 Article 28-Dec-2014 Li, Jie-Wei; Nanjing University of Posts and Telecommunications (NJUPT), Institute of Advanced Materials (IAM) Qian, Yan; Nanjing University of Posts & Telecommunications (NUPT), Institute of Advanced Materials (IAM) Xie, Linghai; Nanjing University of Posts & Telecommunications (NUPT), Institute of Advanced Materials (IAM) Yi, Yuanping; Institute of Chemistry, Chinese Academy of Sciences, Li, Wenwen; Nanjing University of Posts & Telecommunications (NUPT), Institute of Advanced Materials (IAM) Huang, Wei; Nanjing University of Posts and Telecommunications (NJUPT), Institute of Advanced Materials (IAM)

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From Dark TICT State to Emissive quasi-TICT State: The AIE Mechanism of N-(3-(benzo[d] oxazol-2-yl)phenyl)-4-tert-butylbenzamide Jiewei Li,† Yan Qian,*,† Linghai Xie,† Yuanping Yi,*, ‡ Wenwen Li,† and Wei Huang*, †,§ †Key Laboratory for Organic Electronics & Information Displays and Institute of Advanced Materials (IAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing, 210023, China ‡Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China §

Jiangsu-Singapore Joint Research Center for Organic/Bio- Electronics & Information Displays

and Institute of Advanced Materials, Nanjing Tech University, Nanjing 211816,

AUTHOR INFORMATION Corresponding Author *Tel.: +86 25 85866008. Fax: +86 25 85866999. E-mail: [email protected], [email protected]. *Tel.: +86 10 62631259. Fax: +86 10 62525573. E-mail: [email protected]


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ABSTRACT: A phenylbenzoxazole-based organic compound, N-(3-(benzo[d]oxazol-2yl)phenyl)-4-tert-butylbenzamide (3OTB), has been synthesized and the mechanism of its condensed-state emission enhancement has been studied. Experimental and theoretical investigations indicate that prohibition of transition from the local excited state to the nonemissive twisted intramolecular charge transfer (TICT) excited state, but halfway to the intermediate emissive quasi-TICT excited state that results from partial restriction of free intramolecular rotations in condensed states, is responsible for the emission enhancement. Furthermore, it is easy to grow 3OTB nanosheets from THF/H2O mixed solvents. In addition, when molecular arrangement is more ordered, restriction of molecular rotation becomes severer, and consequently, stronger emission can be observed, so that the emission quantum efficiency is in order of crystalline > powder > nanosheet > amorphous film.

KEYWORDS: photophysics, aggregation-induced emission, twisted intramolecular charge transfer, condensed-state emission, theoretical investigation

INTRODUCTION Luminescent organic solids are of great interest from a scientific viewpoint owing to their great potential in various photofunctional applications such as nonlinear optics,1 organic light-emitting diodes,2 fluorescent biosensors3,4 and solid lasers.5 However, conventional organic fluorescent compounds bearing large delocalized π-conjugated moieties typically suffer from fluorescence quenching at high concentrations or in condensed states due to the facilitated non-radiative decay


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induced by strong intermolecular interactions such as exciton coupling and excimer formation, and thus hinders their application in optoelectronic devices. Therefore, many efforts have been devoted to suppressing non-radiative deactivation in the excited state of organic solids. In recent years, some novel materials with aggregation-induced emission (AIE) have been reported, such as siloes,6,7 tetraphenylethylene,8,9 triphenylethylene,10,11 cyanostilbene,12,13 triarylamine,14,15 and their combinations,16,17 which exhibit significant emission enhancement in aggregated/solid states, and have great potential in application of optoelectronic devices,18 photo memory,19 logic gates,20 and so on. The most popular AIE mechanism at the single molecular level involves restriction of intramolecular rotation (RIR),21 intramolecular charge transfer (ICT),22 twisted intramolecular charge transfer (TICT),23,24 or cis−trans isomerization.25 At the supra-molecular level, some types of specific molecular packing, such as J-aggregation,26,27 dimer/excimer stacking,28,29 herringbone stacking,30 or even the weakly coupled H-aggregation,

31

can also help to maintain

the emission in solid states, probably due to some specific emissive-favorable exciton coupling. However, these mechanisms are still limited, and further study of structure–property relationship is required for completely understanding the condensed-state emission mechanism and eventually for rational design of practically functional solid-state emitters.32,33 In our previous work,24,27,33-35 we have developed a series of AIE-active carboxamide derivatives of excited state intramolecular proton transfer (ESIPT) moieties, 2-(2′hydroxyphenyl)benzoxales (HBX, X = S, O), wherein the mechanisms of solid-state emission enhancement include restriction of the non-radiative TICT,33 specific molecular packing24 or combination of both,35 depending greatly on the subtle tuning of molecular structures. After removing the hydroxyl group of HBO, the carboxamide derivative of 2-phenylbenzoxole, N-(4-


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(benzo[d]oxazol-2-yl)phenyl)-4-tert-butylbenzamide (OTB) was found to be still AIE-active, mainly due to the reduced energy splitting and a facilitated inter-conversion between the lowest two excited states, as a result of the three-dimensional #-shaped cross-molecular packing in the solid state.35 In

this

present

work,

the

counterpart

N-(3-(benzo[d]oxazol-2-yl)phenyl)-4-tert-

butylbenzamide (3OTB), with a mere change of the substituted position of the tertbutylbenzamide group from the fourth to the third position of 2-phenylbenzoxole, exhibits a much different emission mechanism from OTB. The AIE of 3OTB is originated from an intermediate emissive quasi-TICT exited state, which is formed through incomplete transition of the local excited state to the “dark” TICT state due to partial restriction of intramolecular rotations in condensed states. EXPERIMENTAL AND THEORETICAL METHODS Materials. All the materials were received from Shanghai Chemical Reagents and used without further purification. The solvents were dried and distilled before use. Synthesis of 3OTB. 3-(benzo[d]oxazol-2-yl)aniline (3BOA) was synthesized using previously published methods.27 A sample of isophthalic acid (1.05, 5 mmol) and N, N’carbonyldiimidazole (CDI, 0.972 g, 6 mmol) were stirred in dry toluene under nitrogen and refluxed for 5h, The bath was cooled down, added by 1.21 g 3BOA (1.05 g, 5 mmol), and refluxed again. The reaction mixture was stirred overnight with plenty of pale yellow precipitate emerging. The solid was filtered subjected to silicagel column chromatography to produce 1.20 g of solid in 65% yield. 1H NMR (400MHz, DMSO, 298K): δ= 10.47 (s, 1 H, NH), 8.81 (s, 1H, Ar-H), 8.01-8.02 (d, 1H, Ar-H), 7.93-7.91(t, 3H, Ar-H), 7.82-7.85 (m, 2H, Ar-H), 7.51-7.62 (m, 3H, Ar-H), 7.41-7.48 (m, 2H, Ar-H), 1.34 (s, 9H, tBu-H). ESI-MS: m/z 371.42 [M+H] +.


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Anal.Calcd for C24H22N2O2 (370.17): C, 77.81; H, 5.99; N, 7.56. Found: C, 77.92; H, 5.85; N, 7.51. Characterization. The NMR spectrum was recorded on a Bruker AV 400 MHz NMR spectrometer. Accurate mass measurements were performed with an electrospray ionization (ESI) Mass Spectrometer. UV−vis absorption and photoluminescence spectra were obtained using Shimadzu UV-3600 and RF-5301PC spectrophotometers, respectively. Single crystal data were acquired with a Bruker Smart APEX CCD area detector using direct methods. Structure solutions were also obtained using direct methods and were refined through full-matrix leastsquares on F2 using SHELXL-97. A TEM image was obtained using HT7700 microscope at an accelerating voltage of 100 kV. Time-resolved fluorescence and lifetime measurements (error estimate is ± 40ps) were performed using a LifeSpec-ps fluorescence lifetime analytical spectrometer (Edinburgh Instruments). The fluorescence quantum yields of solids or nanoaggretates were measured with an integration sphere (error estimate is ±2%), and that of the local blue emission of the solutions were measured using 9,10-diphenyl anthracene as reference. Computational Methods. The geometries of the ground and the first singlet excited states were fully optimized by density functional theory (DFT) and time dependent DFT (TD-DFT) at the CAM-B3LYP/6-31G* level, respectively. The excitation energies at these optimized geometries and the configurations along the potential curve of the first singlet excited state were calculated by TD-DFT at the same level of theory. All the above quantum-chemical calculations were carried out using the Gaussian09 program.36 The universal force field was implemented to simulate the crystal growth using the morphology module in the Material Studio 7.0 with the fine quality.


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RESULTS AND DISCUSSION Synthesis and spectrum investigation. The synthesis of 3OTB was carried out following a procedure reported previously,37 by CDI-activated acylation of the amine 3BOA with 4-tertbutylbenzoic acid (Figure 1a). 3OTB is well soluble in common organic solvents, including chloroform, 1,2-dichloromethane, ethyl acetate, acetone and THF, acetonitrile but exhibits a poor solubility in water. 3OTB in THF solution exhibit almost no fluorescence with the fluorescence quantum yield being only 0.17%, but exhibits strong blue emission in either powder or nanoaggregates (Figure 1b). The fluorescence quantum yield of 3OTB powder is 8±2%, 47times higher than that of THF solution (0.17%), clearly exhibiting a characteristic of AIE.

Figure 1. Synthetic strategy of 3OTB (a) and photographs of 5×10−6 M 3OTB dispersed in THF, THF/water mixture (10/90 vol/vol) and powders (from left to right), respectively, under UV illumination at 365 nm (b). In solutions, the short-wavelength ultraviolet emission, exhibiting a small spectral shift from 343 to 351 nm by varying the solvent polarities (Figure 2b), is attributed to the local excited state (LE*) due to π-π* transition on the 2-phenylbenzoxole moiety as indicated by calculations (see Figure 3 and Scheme S1). However, the longer-wavelength emission, exhibiting large spectral shifts across a wide range from 470 nm in n-hexane to 525 nm in acetonitrile (Figure 2b), shows an obvious charge transfer (CT) characteristics whose transition dipole moment is strongly


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affected by solvent polarity.38 In nonprotic solutions (1×10-5 M), the TICT emission clearly exhibits a positive solvato kinetic effect (Figure 2b).39 That is, with increase of solvent polarity, the TICT emission red shifts together with a decrease of the relative intensity (see Table 1). However, in the protic solvents, the TICT emissions are not observed. This is possibly due to hydrogen bonding from a protic solvent which inhibits the TICT formation. In IPA solutions, a shoulder emission peaked around 413 nm is detected. This shoulder emission is also observed in dilute EtOH solutions (5×10-6 M and 10-6 M, see Figure S1). In addition, almost a complete charge transfer from the phenylamide linking to tert-butyl group to the N-(phenyl)aniline linking to bezoxazole is observed in the TD-DFT optimized excited-state geometry, with a large backbone twist of about 51° from the ground to excited-state geometry (Figure 3). 1.0

a) n-hexane THF DCM MeCN IPA EtOH

0.3

0.2

Normalized PL intensity

0.4

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.1

0.0 250

300

350

400

n-Hexane THF DCM MeCN IPA EtOH

b)

0.8 0.6 0.4 0.2 0.0 350

400

450

500

550

Wavelength (nm)

Wavelength (nm)

Figure 2. The absorption (a) and normalized fluorescence (b, λex=290 nm) spectra of 1×10-5 M 3OTB in different solvents. Table 1. The shift of maximum LE and TICT emission wavelengths of 1×10-5 M solutions with solvent polarity with dielectric constants included. Dielectric constants n-Hexane

1.88a

polaritya

LE emission peak (nm)

TICT emission peak (nm)

30.9

340

477

7.5

b

37.4

343

502

DCM

8.9

b

41.1

345

505

MeCN

36b

46

342

517

THF


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IPA

18.3a

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48.6

345

413

ETOH 24.3a 51.9 a b from reference 40. from reference 41;

340

—

Figure 3. The selected four atoms for the intramolecular twist angle (θ) (labeled in cyan in the upper inserted molecule). The two most stable ground/excited geometries of 3OTB, 1/1* and 1x/1x* (θ = 178.9°/178.5° and 151.8°/100.5°, respectively), and the natural transition orbitals for the excited states.

In contrast to significant change of luminescence spectra, the absorption profiles exhibit almost no shift in different solvents (Figure 2a). The maximum absorptions at 299 and 320 nm mainly corresponded to the π−π* transition of the benzoxazole ring, while the absorption at 289 nm mainly corresponded to the π−π* transition in the N-phenylbenzamide group (see Scheme S2). Thus, the longer-wavelength emission should be assigned to a TICT excited state (TICT*) emission. Consistently, the temperature-dependent (low temperature) emission spectra in meTHF solution indicated that, with decrease of temperature from 273 to 190 K, the relative intensity of


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the LE emission (peaked at 345 nm) increases while that of the TICT emission (peaked around

1.0

Normalized fluorescence intensity

540nm) decreases (Figure 4a). Normalized fluorescence intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a) 273K 250K 230K 210K 190K

0.8 0.6 0.4 0.2 0.0 350

400 450 500 Wavelength (nm)

550

1.0

b)

0.8 170K 150K 130K

0.6 0.4 0.2 0.0 350


550

Figure 4. The temperature-dependent emission spectra of 3OTB in meTHF solution (λex=290 nm).

The compound 3OTB is soluble in the THF/water mixed solvent with the water content of 0~80%. Consistent with the solvent effects, the main absorptions exhibit almost no spectral shift but the photoluminescence (PL) spectra exhibit significant changes (Figure 5). The TICT emission almost disappears by introducing the more polar water solvent, with the LE emission exhibiting a small shift (< 8 nm). However, when the water content is larger than 90%, the molecules begin to aggregate, as indicated by the sudden increase of emission, the typical tails in the visible region caused by Mie scattering (Figure 5a), nanoaggregates (nanosheets) observed from the TEM images (discussed below), and change of both the absorption and emission profiles. The relative intensity of the absorptions around 320 nm enhanced and a newly emerged emission around 450 nm suddenly increased (Figure 5). This new blue emission cannot be assigned to either LE* or TICT* emission, but to quasi-TICT* emission (to be discussed below). At longer excitation wavelength, the relative intensity of the LE*


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emission around 365 nm gradually decreases while that of the quasi-TICT* emission around 446 nm gradually increases (Figure S2).

400

a) THF:H2O 0:100 5:95 10:90 20:80 40:60 60:40 80:20 100:0

0.8 0.6 0.4 0.2 0.0 250

300


PL intensity (a.u.)

1.0

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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500

b)

THF:H2O 0:100 5:95 10:90 20:80 40:60 60:40 80:20 100:0

300 200 100 0

350


550

Figure 5. The absorption (a) and fluorescence (b, λex=290 nm) spectra of 1×10-5 M 3OTB in THF/water mixed solutions with different mixing ratios.

Nanoaggregate morphology and growth mechanism. The single crystals of the 3OTB were grown from ethyl acetate through slow evaporation. The X-ray crystallographic analysis reveals that the compound crystallizes in monoclinic space group P21/c with the following lattice parameters: a = 5.93 Å; b = 37.23 Å; c = 9.33 Å; and β = 104.87° and z = 4. There are four main weak interactions between one molecule and its surrounding molecules in the crystal state: (tertbutylbenzene)-H…(benzo[d]oxazol)-O,

(tertbutylbenzene)-H…(phenyl)-C,

(amide)-

O…(phenyl)-C and (amide)-O…H-N (amide) interactions, with the distance of 2.86, 2.89, 2.57 and 2.26 Å, respectively (Figure 6). Due to these weak interactions, the free configuration twist is partially restricted and a quasi-TICT* state can be formed as indicated by theoretical calculations (discussed below), leading to an enhanced emission in the crystal state.


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Figure 6. The main intermolecular interactions and distances (Å) in the 3OTB crystal. In the 10:90 (vol/vol) THF/water mixed solvent, 3OTB molecules gradually aggregate into two-dimensional (2D) flat rhombic nanosheets, as indicated by the TEM image (Figure 7a). To get further insight into the growth mechanism of its 2D anisotropic molecular self-assembly, the growth equilibrium and morphologies were simulated by Material Studio 7.0 Package (Accelrys Software Inc.),42,43 based on the single crystal data. The growth of the nanocrystals involves two steps: nucleation of an initial “seed” and the subsequent growth.44,45 In the nucleation step, the 3OTB molecules are supersaturated in solution (considering the poor solubility in the mixed solvent) and rapid nucleated to form an initial “seed” for further growth. Here, the nucleation can be regarded as an equilibrium process governed by, on one hand, the acquisition of a low aspect ratio to minimize surface area, and on the other hand, the attach energy of each facet. The morphology of the (nano)crystal depends on the growth ratio

R{rel = A* E hkl} s

where A is a proportional constant and E involves the attachment energy ( energies (

Esurf

E attach

of each facet,46 ) and the surface

) of a certain facet. In the former equilibrium growth of nucleation, the morphology

mainly depends on

E attach

of each facet. The subsequent growth based on the initial seed is a

nonequilibrium process, mainly governed by

Esurf

of various facets. The lower the

is, the slower the facet grows. As seen in Table 1, E

E attach

or

Esurf

attach {020}

is the lowest among all the facets; the


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growth of {020} facet will be the slowest in the nucleation process. The calculated

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E surf {020}

= 7.59

kcal/mol, is the lowest as well. Thus, the growth of {020} facet is most prohibited in the subsequent growth process. The

E attach

and

Esurf

of the low-surface-energy facets ({011}, {100},

{110} and {11-1} facets) of the crystal are 3~6 and 5~12 times higher than that of the {020} facet, respectively, and reveal relatively close values in both processes. Therefore, twodimensional morphologies of both the nanoaggreats (Figure 7a) and the crystals (Figure S3) are observed. The simulated morphology exhibits almost a flat hexagonal prism shape (Figure 7b), which is a little different from, but correlated well with the TEM images (the rhombic nanoplates, see Figure 7a). This is probably because the {11-1} facet of the nanosheets grows very fast due to its largest

E attach

surf and E .

Considering the flat 2-D samples lying on the plates, the peaks of {020} facet for various condensed states is highest in the XRD pattern (Figure 8). Compared with crystal and power, the XRD pattern of 3OTB nanosheets clearly exhibits a relatively higher {020} peak than other peaks, in accordance with their much flatter morphology.

Figure 7. The TEM image of 3OTB nanosheet (a) and simulated crystal growth morphology (b).


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Table 2. Attachment ( E

attach

surf ) and Surface Free ( E ) Energies of Various Crystal Facets {hkl}

simulated by universal force field. {hkl}

d{hkl}(Å) Surface area Eattach (kcal/mol) Esurf (kcal/mol)

{020}

18.61

53.46

-31.38

7.59

{011}

8.77

226.98

-97.27

37.91

{100}

5.73

347.42

-146.70

68.44

{110}

5.66

351.51

-150.06

69.95

{11-1}

5.46

364.53

-168.61

80.56

020

040

Film Power Nanosheet Crystal

060 012 022

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

10

20

30

40

2theta

Figure 8. The XRD patterns of 3OTB in film, power, nanosheet and crystal. Morphology dependent condensed-state emission. The fluorescence emission spectra of 3OTB in film, nanosheet, powder and crystal are shown in Figure 9, which are much different from that in the THF solution. The PL spectra in all these condensed states exhibit a blue emission peaked at around 440~446 nm. As expected, a narrower PL profiles was found for the crystal and powder than the film and nanoaggregate, and the film exhibits the broadest emission profile.


13


1.0 PL Intensity (Norm.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Solution -5 (10 M inTHF) Film Nanosheets Power Crystal

0.8 0.6 0.4 0.2 0.0

300

400 500 Wavelength (nm)

600

Figure 9. The fluorescence emission spectra (λex=290 nm) of 3OTB in THF solution and various condensed states (film, nanosheet, powder and crystal). From TD-DFT calculations, two stable molecular geometries — 1 and 1x are found in the ground state, with the twist angle (θ, labeled in cyan in Figure 1) of 178.9° and 151.8°, respectively. Correspondingly, in the lowest singlet excited state, there also exist two stable geometries — 1* and 1x*. The θ of 1* is 178.5°, indicating a very similar configuration to its ground state 1 (θ = 178.9°). However, 1x* exhibits a large geometrical twist compared with the ground state 1x. The change of θ is as large as 51°, from 151.8° in the ground state (1x) to 100.5° in the excited state (1x*). As seen from the natural transition orbitals, the electron transition in 1* (the calculated wavelength λcal = 315 nm, oscillator strength f = 0.877, see Scheme 1) is attributed to the π-π* transition localized in the benzo[d]oxazol group, while the electron transition in 1x* (λcal = 479 nm, f= 0.003) mainly occurs from the phenylformamide unit to the tert-butylbenzene group. Thus, the emissive LE* and the non-emissive TICT* is originated from 1* and 1x*, respectively. Our calculations show that configuration 1 is 0.22 eV more stable than configuration 1x while the energy of configuration 1x* is 0.43 eV lower than that of 1*. In addition, because of small energy barrier (~0.1 eV), the transition from the LE* (1*) to the TICT* (1x*) will be very easy (Figure 10). In dilute solutions, since 3OTB molecules are


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unrestricted and can freely rotate, the molecularly dispersed solutions are almost not emissive (Øf = 0.17%). The blue emission (peaked at 440~446 nm) in condensed states is far from the LE emissions (343~351 nm in solutions). In addition, the calculated absorption spectra and maximum wavelengths of the selected dimer from crystal structures show almost no spectral shift when compared with monomer (Figure S4). Thus, this condensed-state emission with such a large bathochromic shift (about 100 nm from the LE emission) should not be assigned to the LE emission. This is consistent with the theoretical result that the largest variation in λcal of the LE state 1* is only 25 nm even with fully free molecular rotation (θ from 60° to 180°). Although these blue emissions are much closer to the TICT emission (470 nm in the non-polar n-hexane solution), the much enhanced condense-state emission cannot be originated from the most stable TICT state 1x*, which is almost non-emissive (f = 0.003).

Wavelength (nm)

Energy (eV)

1.5

Oscillator Strength

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


*

1 * 1x

1.0 0.5 0.0 *

1 * 1x

450 400 350 300 10

-1

10

-2

10

-3

10

-4

*

60

1 * 1x

80

100

120

140

160

180

θ (degree)

Figure 10. The energy potential curves, and corresponding emission wavelengths and oscillator strengths with the twist angle (θ) for the LE and TICT excited states (namely, 1* and 1x*).


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At this point, we propose that, due to relatively large intermolecular distances (the shortest center-to-center distance is 4.684 Å) and highly flexible molecular backbone, intramolecular rotations of 3OTB is partially restricted in condensed states, leading to a quasi-TICT excited state (quasi-TICT*), that would be responsible for both the emission enhancement and large spectral shift from solution. Since the cross point of the PESs of the 1* and 1x* (θ = 155°) is close to the stable point of the LE state (178.5°), transformation from 1* to 1x* can occur readily in condensed states. However, full geometrical relaxation to the most stable TICT* requires very large molecular distortion (the change of θ = 66°) and thus is prohibited in condensed states. The intramolecular relaxation along the 1x* PES will be stopped at an intermediate point (the socalled quasi-TICT*) between the cross point and the most stable TICT* state. Correspondingly, the emission wavelength and intensity of this quasi-TICT* state is located between the strong LE* emission (λcal = 315 nm, f = 0.877) and weak TICT* emission (λcal = 479 nm, f = 0.003) (Figure 10). Therefore, an emission enhancement can be observed in condensed states with respect to dilute solutions. As the quasi-TICT* point approaches to the cross point, the emission will become stronger and less bathochromic (relative to the LE emission) in the condensed state. The emission in powder and crystal is peaked at 440 nm while that in nanoaggregates and films at 446 nm, indicating a greater restriction of molecular rotation in powder and crystal. Consistently, the emission intensity in powder (Øf = 9%) and crystal (Øf = 8%) is higher than that in spin-coated film (Øf = 2%). Moreover, compared with powder and crystal, the emission spectra of nanoaggregates and film are broader, probably due to less order of molecular packing as indicated by their XRD patterns (Figure 8). In powder and crystal, almost all the molecules are regularly oriented, exhibiting almost the same quasi-TICT* configuration, which is consistent to little changed fluorescence lifetimes detected at different emission wavelengths (Figure S5).


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Thus, relative narrow emission spectra are found in powder and crystal. However, in nanoaggregates or film, some molecules are highly oriented but some are disordered. The excited states can relax from the cross point to many intermediate positions, agreeing with the varied fluorescence lifetimes detected at different emission wavelengths. As a result, the emission

2.5

Solution 2.0 1.5

0.5ns 1ns 2ns 3ns 4ns 5ns 6ns

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spectra of nanoaggregates or film are broader. 1.0

Film

0.8 0.6

0.1ns 0.5ns 1.0ns 2.0ns 3.5ns 4.5ns

0.4 0.2 0.0 350

550

0.8

0.5ns 1.5ns 2.5ns 3.5ns 4.5ns 6.0ns 7.0ns 7.5ns

0.6 0.4 0.2 0.0 350


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Nanoaggregates

1.0

400

450

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Wavelength (nm)

Wavelength (nm) Normalized fluorescence intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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0.8 0.6 0.1ns 0.5ns 1.0ns 2.0ns 3.0ns 4.0ns

0.4 0.2 0.0 350

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Wavelength (nm)

Figure 11．．The time-resolved emission spectra of 3OTB in THF solution, film, nanoaggregates and powder (λex=290 nm). Consistently, the transition from LE* to full TICT* state in solution or to quasi-TICT* state in condensed states has been evidenced by the time-resolved emission spectra (Figure 11, deduced from series of fluorescence decays detected at different wavelengths). Although this fast transition has already occurred within 100~500 ps, the relative intensity of TICT or quasi-TICT emission does increase with time. In THF (HPLC grade) solution, with increase of time, the


17


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relative intensity of LE emission (peaked at 345 nm) gradually decreases, while that of the TICT emission (peaked at 510 nm) gradually increases. Similarly, in condensed states (film, nanoaggregates, and powder), the LE emission peaked around 345~365 nm decreases while the quasi-TICT emission peaked around 450 nm increases. In addition, this partial prohibition from the transition from LE to TICT state has also been proved by the low-temperature emission spectra. With decrease of temperature from 273 to 190 K, the wavelength TICT emission peak (around 540 nm) remains almost unchanged at 540 nm, with its relative intensity of the LE emission (around 345 nm) increases (Figure 4a). The longer wavelength emission blue shifted to 495 nm at 170 K, and further to 450 nm at 130~150K (Figure 4b). These are the quasi-TICT emissions due to more restriction of molecular twisting at lower temperatures. The 450 nm quasi-TICT emission at 130~150 K correlates well with those in condensed states (440~446 nm). Further decrease the temperature to 105~77 K, the quasiTICT emission can almost not be observed, and replaced by the structured phosphorescence (with detecting lifetime longer than µs magnitude, see Figure S6). Similarly, the quasi-TICT emissions have also been detected in concentrated solutions with various polarities. In each solution, with increase the concentration from 10-6 to 10-4 M, the relative intensity of the TICT emission (exhibiting prominent solvent effects) gradually increases with the maximum peak position almost unchanged, except that a new emission peaked at 483 nm is observed in 10-4 M MeCN solution (Figure S7). However, when the concentration is up to 10-3 M (lack of spectrum in n-hexane solution, because 3OTB cannot be fully diluted in n-hexane with 10-3 M concentration), all the TICT emission peaks shift to around 470 nm almost without exhibiting solvent effects (Figure 12). This is possibly because the closely packed molecules are much less sensitive to the solvent relaxation. Meanwhile, a new emission (peaked around 410 nm)


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between the LE and TICT emissions emerged, probably due to the quasi-TICT emission caused by the molecular aggregation. Nomalized fluorescence intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0

1x10-3 M 0.8 0.6 THF DCM MeCN IPA ETOH

0.4 0.2 0.0 350

400 450 Wavelength (nm)

500

550

Figure 12 . The fluorescence spectra of 10-3 M 3OTB in various solvents (λex=290 nm). CONCLUSIONS In summary, an AIE compound, 3OTB has been developed. The AIE mechanism of 3OTB is substantially different from our previous reported OTB molecule, even with a minor change of the molecular structure. Experimental and theoretical investigations indicate that the AIE of 3OTB arises from an emissive quasi-TICT excited state. With increasing ordering of molecular arrangements, intramolecular rotation is more restricted and correspondingly, the quasi-TICT* emission becomes stronger. In addition, 3OTB nanosheets can be easily grown from THF/water mixed solvents and a two-dimensional flat rhombic morphology is revealed by TEM and theoretical simulation. Our study shows that exploration of exact structure–property relationship is essential for understanding the AIE mechanism and rational design of practically functional solid-state highly emissive materials. ACKNOWLEDGMENTS


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We express our sincere gratitude to the National Natural Science Foundation of China (Grant Nos. 21373114, 21274064, 91333117), Project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, PAPD (YX03001), the Natural Science Foundation of Jiangsu Province of China (No. BM2012010), and Program for Postgraduates Research Innovation in University of Jiangsu Province (CXLX11_0422). SUPPORTING INFORMATION AVAILABLE Natural transition orbitals, oscillator strengths and wavelengths of the LE and TICT emissions, emission spectra of nanoaggregates at different excitation wavelengths, optical image of crystal, absorption spectra of dimers, fluorescence lifetimes of film, nanosheet, powder and crystal, and low temperature spectra. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES (1) Yuan, Z.; Collings, J. C.; Taylor, N. J.; Marder, T. B.; Jardin, C.; Halet, J.-F. Linear and Nonlinear Optical Properties of Three-Coordinate Organoboron Compounds. J. Solid State Chem. 2000, 154, 5-12. (2) Chiang, C. L.; Wu, M. F.; Dai, D. C.; Wen, Y. S.; Wang, J. K.; Chen, C. T. Red-Emitting Fluorenes as Efficient Emitting Hosts for Non-Doped, Organic Red-Light-Emitting Diodes. Adv. Funct. Mater. 2005, 15, 231-238. (3) Fujikawa, Y.; Urano, Y.; Komatsu, T.; Hanaoka, K.; Kojima, H.; Terai, T.; Inoue, H.; Nagano, T. Design and Synthesis of Highly Sensitive Fluorogenic Substrates for Glutathione STransferase and Application for Activity Imaging in Living Cells. J. Am. Chem. Soc. 2008, 130, 14533-14543.


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TOC GRAPHICS *

1

1.5

Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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*

1x

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solution Powder

LE*

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θ (degree)


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From Dark TICT State to Emissive quasi-TICT State: The AIE

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