Tuning for Visible Fluorescence and Near-Infrared Phosphorescence

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Tuning for Visible Fluorescence and Near-Infrared Phosphorescence on a Unimolecular Mechanically-Sensitive Platform via Adjustable CH-# Interaction Hongwei Wu, Pei Zhao, Xin Li, Wenbo Chen, Hans Ågren, Qing Zhang, and Liangliang Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15939 • Publication Date (Web): 11 Jan 2017 Downloaded from http://pubs.acs.org on January 11, 2017

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ACS Applied Materials & Interfaces

Tuning for Visible Fluorescence and Near-Infrared Phosphorescence on a Unimolecular Mechanically-Sensitive Platform via Adjustable CH-π Interaction Hongwei Wu,† Pei Zhao,‡ Xin Li,§ Wenbo Chen,ǁ Hans Ågren,§ Qing Zhang,*,† and Liangliang Zhu*,‡ † Shanghai Key Laboratory of Electrical Insulation and Thermal Ageing, School of Chemistry and Chemical Engineering, Shanghai Jiaotong University, 800 Dongchuan Road, Shanghai 200240, China. ‡

State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular

Science, Fudan University, Shanghai 200433, China. §

Division of Theoretical Chemistry and Biology School of Biotechnology, KTH Royal Institute of

Technology, SE-10691 Stockholm, Sweden. ǁ

Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power,

Shanghai University of Electric Power, Shanghai 200090, China.

Abstract: CH-π interaction assisted alignment of organic conjugated systems has played an important role to regulate molecular electronic and photophysical properties, whereas harnessing such a smart non-covalent interaction into the tuning of unimolecular complex emissive bands covering a wide spectral region remains a challenging research topic. Since the tuning for visible and near-infrared emissive properties in a single π-functional platform relates to its multi-color luminescent behaviors and potential superior application in analysis, bio-imaging and sensing, herein, we report a proportional control of the singlet and triplet emissions that cover visible and near-infrared spectral 1

ACS Paragon Plus Environment


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region, respectively, can be straightforwardly achieved by CH-π interaction assisted self-assembly at the unimolecular level. Employing an octathionaphthalene based single-luminophore as a prototype, we find that a strength-adjustable CH-π interaction assisted self-assembly can be established in mixed DMF/H2O and in the film state. The hybridization of planar local excited and intramolecular charge transfer transitions occurs on the basis, allowing a competitive inhibition to intersystem crossing process to generate a complex emission composed with visible fluorescence and near-infrared phosphorescence. Furthermore, reversible mechanochromic and mechanoluminescent conversions of the corresponding solid sample can both be observed to rely on a corresponding self-assembly alternation. These results can probably provide new visions for the development of future intelligent and multi-functional luminescent materials. Keyword: CH-π interaction, visible fluorescence and near-infrared phosphorescence, unimolecular complex emission, tunable intersystem crossing, mechanical stimuli

Introduction CH-π interaction, a special sort of hydrogen bonds, plays a significant role in various fields of chemical, material and biological science.1-7 In particular, CH-π interaction involved crystal engineering allows an extensive progress in organic electronics from the recent decades.8-10 In this way, numerous reports have demonstrated that CH-π interaction can effectively aid to establish unique self-assembly patterns so as to adjust photophysical and light-emitting properties at the crystal scale.11-14 To date, engineering photoluminescence from visible (Vis) to near-infrared (NIR) spectral region has been strongly concerned in π-functional system for versatile applications of analysis, 2


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sensing and imaging.15 However, facile tuning of emissive pathways between Vis and NIR emissions in a single π-functional platform has not been accessed. Inspired by those findings where excited-state properties of π-functional molecules can respond to a variety of smart noncovalent factors including CH-π interaction,13-17 we expect to present a rational strategy in which proportional control of Vis and NIR emission can be smartly regulated by CH-π interaction assisted unique self-assembly, so as to achieve potential advanced emissive applications at the unimolecular level. Since fluorescence and phosphorescence emitted from a single organic molecule normally cover distinct spectral wavelength regions, it is straightforward to employ the singlet-triplet radiative strategy to address the Vis-NIR complex emission simultaneously. Moreover, the involvement of phosphorescence can potentially broaden material usage in time-resolved emitting events with high-efficient internal quantum conversion.18-20 Although the singlet-triplet radiative strategy has been observed in a few materials related to a precise molecular self-assembly design,21-26 NIR phosphorescence at room temperature is relatively difficult to gain in metal-free organic systems, simply because excited electrons in a large π-conjugation are easy to decay either from singlet state or nonradiatively.27-29 Thereupon, we propose to utilize a multiple sulfur-atom based unimolecular strategy aiming to produce visible fluorescence and near-infrared phosphorescence that can be tuned by adjustable CH-π interaction. The key point is to develop an organic skeleton capable of appropriate π-electron properties and to impose a well-selected molecular self-assembly fashion into the regulation of intersystem crossing (ISC)30,31 to realize the above-mentioned complex emission. Recently, persulfurated

benzene-cored

compounds have been

found to show strong

aggregation-induced phosphorescence with the emission wavelength ranging from 400 to 600 nm.32,33 3



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These compounds can be easily synthesized from cheap and commercially available starting materials with particular modifications by different functional groups for post-processing.34-36 In this work, we try to employ a naphthalene core to anticipate that the extended π-conjugation will help lower the transition and decay energy for matching within the NIR spectral region. Meanwhile, the multiple sulfur-atom modification will also be beneficial to the generation of room-temperature phosphorescence.32-33 Thus, two relatively simple compounds containing a persulfurated naphthalene core were synthesized (see chemical structures of 1 and 2 in Figure 1 and their preparation routes and details in Supporting Information (SI)). Two endgroups, phenyl and methylphenyl, were introduced respectively for the mutual control study from the perspective of employing CH-π interactions with different strengths. The adoption of such a molecular design also takes advantage of avoiding unfavorable π–π stacking so as to intensify the aggregation-induced emission effect. As mechanoresponsive materials have been of great interest in optoelectronics and such a responsive mode can be applied onto solid samples for advanced luminescent applications,37-39 in our current design, mechanical stimuli was also considered as an in-situ tuning way to manipulate the control of the Vis and NIR emission in the solid state, accompanied with an analogous CH-π assisted self-assembly formation and dissociation. Owing to such an efficient excited-state tuning fashion, the leading materials can undergo remarkable and reversible conversions in apparent color and in luminescent tone, revealing promising usage in mechanochromic and mechanoluminescent convertible events.

4


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(a) S

S

S S S

S S

S

S

S

S

S S S

S

1

S

2 adjustable C-H--π π interaction

strong

(b)

C-H--π interaction

S

S T

T

G

NIR

Complex Band

Vis

G

Single Band

Figure 1. (a) Molecular structures of compound 1 and 2. (b) Schematic representation of a proposed formation of an adjustable self-assembly of 1 directed by CH-π interaction, as well as a stable aggregation of 2 with strong CH-π interaction: 1 can give rise to a complex emission composed with visible fluorescence and near-infrared phosphorescence in the aggregation state through a competitive inhibition to intersystem crossing process, whereas the aggregated 2 only produces single phosphorescence because of lacking such a tuning process.

Results and Discussion Photophysical properties in mixed DMF/water solution. The photophysical property of the self-assembly was firstly explored in DMF/H2O mixed solutions. From Figure 2a, we can find that two of the absorption bands of 1 (~370 nm and ~470 nm) changed along with the variation of the water fraction. The band around 470 nm can be regarded as an intramolecular charge transfer (ICT) band according to the assignment of the similar structures in the literatures,32,33 which was probably 5



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due to the combination of the electron delocalization properties of the aromatic rings and the stabilization role of the negative charge in twist sp2 hybridized sulfur atoms.40 The higher energy band (~370 nm) linked to a planar local excited (LE) band that normally originated from the competitive suppression of the ICT. 41,42 The ratio (A470/A370) changed nonlinearly as the water fraction is up to 60%. As a result, the emission behavior of 1 underwent an interesting trend. The emission of 1 was nearly quenched in pure DMF (Figure 2b). However, complex emission bands were found with the NIR peak around 650 nm remarkable when increasing the water fraction up to 40% (see also the corresponding red luminescent color in the insert in Figure 2b). When the water fraction reached 60%, the band in Vis spectral region (ca. 525 nm) became much stronger so as to make the emission color become pale yellow (Figure 2b). These results featured a specific aggregation-induced emission process

43,44

whereby the enhancement of different emission bands was observed with the band

proportion adjustable. In contrast, the band around 370 nm of 2 deeply fell when increasing the water fraction (up to 40%-60%) in DMF (Figure 2c), indicating the LE band was strongly suppressed under those conditions.41,42 In that case, it showed the aggregation-induced emission effect with only the maximum emission wavelength remaining around 660 nm in the NIR spectral region (Figure 2d). Compared to the complex emission bands of compound 1, namely, the compound 2 only showed single emission band upon self-assembly. In this way, the conventional H-aggregation and J-aggregation could be ruled out during this process. Since the compound has a open assemblable space with eight endgroup sites, the aggregated oligomers will not be dominant in the system. Hence we can conclude that for this single-luminophore that a peculiar excited-state regulation process 6


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occurs upon a unique self-assembly pattern of 1 with the change of the solvent environment, resulting in a complex emission composed with a Vis fluorescence (525 nm) and NIR phosphorescence (650 nm). These results were further confirmed by time-resolved emission study as below.

Figure 2. Tunable complex emission in DMF/water: (a) Absorption and (b) emission spectra of 1 in DMF/H2O excitated at 420 nm. The inset in (b) shows the photographs of 1 in corresponding states by a UV light (Ex: 365 nm). (c) Absorption and (d) emission spectra (Ex: 420 nm) of 2 in DMF/H2O. The inset in (d) shows the photographs of 2 in corresponding states by a UV light (Ex: 365 nm). LE and ICT states can be clearly assigned in these absorption and emission spectra. All experiments were carried out with the concentration of 60 µM at room temperature.

Solid-state emission. To further exploit the practical application potential of the well-defined emissive materials, the solid-state emission property prepared from selected solvent conditions were

7



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also investigated. Three different solid films of 1 from ethanol (namely, Eth film), dichloromethane (namely, DCM film) and ethyl acetate (namely, EA film) could be straightforwardly obtained from the corresponding solutions. Due to the fine tuning of the band-ratio (A470/A370) of 1 among the solvents, different solution colors as well as the corresponding film and film luminescent colors can be clearly distinguished (see Figure 3a-3c). All these films displayed a remarkable Vis-NIR complex emission phenomena (quantum yields: 0.5% ~ 1.3%) with the band proportion adjustable (Figure 3d). By time-resolved emission measurements with 100 µs delay (Figure 3e), we can find the Vis band disappeared but the NIR band remained, indicating a distinct luminescent lifetime in between. To accurately monitor the unique time-resolved emission behavior, we tested the emission lifetime of the two bands of 1. To our delight, it showed a fluorescence lifetime (0.38 ns) at 525 nm which is opposed to a phosphorescence lifetime (150 µs) at 650 nm emitted from the films (see Figure 3f and 3g). These results clearly confirmed a fluorescence band in Vis spectral region and a phosphorescence band in NIR spectral region, respectively, originating from the complex emission of 1. On the contrary, there are still no complex emission phenomena in the luminescent spectra of the films of 2 (Figure S2a), and only a phosphorescent lifetime as high as 1.5 ms was observed (Figure S2b). From literatures,32-34 the persulfurated aromatic structure has been demonstrated as a room-temperature phosphorescence emitter. In comparison of the different emission behaviors between

1

and

2,

we

can

conclude

that

a

unique

aggregation

factor

infers

the

phosphorescence-to-fluorescence conversion to give rise to a Vis fluorescence and a NIR phosphorescence in 1. This excited-state regulation process can be both achieved in solution and in solid state. 8


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Figure 3. Solvent dependent photophysics and time-resolved emission study: (a) Solution color, (b) Film color and (c) Film luminescent color of 1 prepared from ethanol (Eth), dichloromethane (DCM) and ethyl acetate (EA). (d) Emission spectra of three different films of 1. (e) Time-resolved emission spectra of the Eth film of 1 with time delay. Photoluminescent lifetime of the Eth film of 1 measured at (f) 525 nm and (g) 650 nm emission excited at 420 nm.

Next, Transmission (TEM) and scanning electronic microscopy (SEM) were employed to explore the self-assembly morphologies. Obviously, we can find a nanodisc morphology with an average length of 2 µm when the sample 1 was prepared from EA solution (Figure 4c). However, crosslinked nanowires were seen from the TEM image of 1 prepared from DCM solution (Figure 4b), whereas nanosheets (see also the magnified SEM image in Figure S3) were observed upon the sample prepared from ethanol solution (Figure 4c, Figure 3). Such a huge difference in self-assembly pathways went together with the generation of the complex emission properties of 1. In contrast, only spherical aggregates were found from TEM images of compound 2 under the same conditions (Figure 4d, 4e 9



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and 4f). As compared with compound 1, a minimal difference in morphology among these solvent conditions in compound 2 can be seen, only leading to the variation of the emission intensity in the solid samples of compound 2 (Figure S2). The Powder X-ray diffraction (PXRD) spectra of the films were also investigated. There were no diffraction peaks from the DCM film of 1 since it was in an amorphous state under that condition. However, the other two spectra exhibited different diffraction peak patterns, indicating two different crystalline states (Figure S4). On the contrary, the EA and ETH film of 2 showed the same diffraction peak pattern (Figure S5). From these studies, we can figure out that the self-assembly of 1 is adjustable whereas that of 2 is relatively stable.

Figure 4. Morphological study: TEM images of (a, b, c) 1 and (d, e, f) 2 prepared from three kinds of solvent conditions. The

concentration of the original solutions was 1mg/5 ml. Scale bar: 5 µm.

Proposed mechanism. The Vis fluorescence and the NIR phosphorescence in the single molecule 1 feature a large singlet–triplet energy gap,

27-29,45

which originates from the octathionaphthalene

structural design. The hybridization of LE and ICT state (Figure 2a) plays a key role for the construction of the complex emission induced by a variable CH-π interaction. According to the 10


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remade single-crystal structural images from literatures35,

36

by MacNicol et al， two molecular

conformations of 1 can be obtained with being subjected to different strengths of CH-π interaction. Each planar molecule comprises a large number of CH-π interactions (Figure 5a), whereas the relatively twist one only suffered from less CH-π interactions (Figure 5b). However, compound 2 only reveals a even twist molecular conformation in the single-crystal structure and it goes through more CH-π interactions peripherally due to the stabilization effect of multiple hydrogen atoms in methyl group (Figure 5c). Consequently, face-to-face self-assembly patterns occur through further extension of the non-covalent force in both of the conformations of 1 (Figure S6 and S7), indicative of the coexistence of a LE state and an ICT state can be achieved in the aggregation forms upon excitation. In contrast, only edge-to-face self-assembly pattern were found driven by twist conformation of 2 (Figure S8), featuring the LE state has been largely suppressed with only leaving the ICT level. These findings are in good agreement with the results of molecular dynamics simulation (Figure S9) and can be well illustrated in Figure 1b. The coexistence of the two conformations of 1 further reflects the tunable ability in self-assembly morphologies as shown in Figure 4. As compared with the single ICT band (~420 nm) in excitation spectra of compound 2 (Figure 5f), two separated bands maximum at 380 nm and 420 nm in excitation spectra, which can be assigned to LE and ICT band of 1, respectively, were observed in their solid films (Figure 5d). These spectral phenomena deeply proved a smart and adjustable CH-π interaction can work in 1 rather than 2. In terms of the above photophysical studies, a mechanism for the complex emission composed with visible fluorescence and near-infrared phosphorescence can be proposed (Figure 5e). Since the aggregation of 2 is only favourable for ICT state (see Figure 2c), the excited electron can easily 11



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transfer to triplet state by ISC to produce a single phosphorescence (Figure 5e). In contrast, hybrid LE and ICT state will be coexist in a system upon an adjustable CH-π interaction assisted self-assembly (see Figure 2a and Figure 5d). In this way, the excited electron from LE state will meet a competitive inhibition to undergo the ISC process, simply resulting in the coexistence of phosphorescence and fluorescence (Figure 5e).

Figure 5. Monomer display of (a) a yellow single crystal of 1 in a planar conformation grown from DMF, (b) a red crystal of 1 with twist conformation grown from 1,4-dioxane and (c) single crystal of 2 in a twist conformation grown from 1,4-dioxane. The CH-π interactions are marked with red lines. CH-π interactions of methyl group peripherally in compound 2 are marked with green lines (d) Excitation spectra of EA film and DCM film of 1 from emission of 650 nm; (e) Schematic representation to describe the complex emission composed with visible fluorescence and near-infrared phosphorescence. (F = fluorescence, P = phosphorescence, ISC = intersystem

12


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crossing, LE= planar local excited state, ICT = intramolecular charge transfer state); (f) Excitation spectra of EA film and DCM film of 2 from emission of 650 nm.

Mechanically induced luminescent. Solid-state photoluminescent materials are more popular for various of practical device applications.39, 46-49 The solid-state emission property of compound 1 based on self-assembly is in easy response to mechanical force. Mechanochromic and mechanoluminescent conversation can occur simultaneously (Figure 6a), which can be attributed to the absorption and emission changes respectively upon an analogous self-assembly alternation during grinding. The Vis fluorescent band in the complex emission pattern underwent a more remarkable decrease than the NIR phosphorescence during grinding, accompanied with a luminescent color conversion of 1 from orange to red with prolonging the grinding time (see Figure 6b and 6c). Hence we can deduce that such a mechanistic conversion of the luminescent color originates from the disintegration of self-assembly with respect to the adjustable CH-π interaction upon grinding. Interestingly, such a mechanochromic and mechanoluminescent behavior can be recovered by fuming. The PXRD spectra of the pristine, grinded and fumed forms of the solid sample of 1 (Figure 6d) were investigated accordingly. The strong peaks of the pristine sample in XRD trace was a representative property of the crystalline self-assembled samples. The signals completely vanished after grinding, indicating an amorphous nature. These XRD peaks restored after the sample was fumed in ethanol vapor, suggesting regeneration of the original self-assembly in solid state. The reversible mechanoluminescent property is accompanied with recovery of the emission spectra, revealing a successful reset performacne of the solid-state emissive material. The mechanochromic

13



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and mechanoluminescent conversion process cannot be observed from the solid-state sample of 2 since only intensity changed in its emission spectra during grinding (Figure S10 and S11).

Figure 6. Reversible mechanochromism and mechanoluminescence: (a) The photographs of the pristine and fumed Eth film of 1 in (a) daylight and (b) a UV light (365 nm). (c) Emission spectra upon 390 nm excitation of the pristine, ground and fumed Eth film of 1 under 390 nm excitation. (d) PXRD traces of pristine, ground and fumed Eth film of 1.

Conclusion In summary, we have employed CH-π interaction assisted self-assemblies to design organic functional molecules with a regulation of Vis singlet and NIR triplet emissive properties based on such an adjustable molecular alignment. Control of complex emission composed with visible fluorescence and near-infrared phosphorescence at the unimolecular level was achieved, whereby the specific self-assembly pattern, which can hybridize the LE and ICT state so as to bring a competitive inhibition into ISC process, plays a key role. Our system can also be delivered into the solid state and 14


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thereby exhibit reversible mechanochromic and mechanoluminescent conversion relying on an analogous molecular self-assembly alternation. We believe that this strategy can be valuable for addressing the challenges of next-generation optoelectronics for potential usage in future versatile light-emitting events.

Methods General. Sodium benzenethiolate, sodium 4-methylbenzenethiolate, perfluoronaphthalene, 1,3-dimethyl-2-imidazolidinone, were commercially available from Adamas-beta® and used as received. NMR spectra were tested on a Bruker AVANCE III HD 400MHz spectrometer. MS was measured by Flight Mass Spectrometer (5800). UV-Vis spectra were obtained upon a Shimadzu (1800) spectrophotometer;

while

emission

spectra

were

recorded

with

a

Edinburgh

FLS920

spectrofluorometer. The emission quantum yields of solution and solid powders were recored on QM40 with an integrating sphere (φ 150 mm) from Photo Technology International, Inc. (PTI, USA). Transmission electron microscopy was conducted on a Jeol JEM 2100 with drop-casting samples onto 300 mesh carbon grids on a copper support and drying at rt for 2 h. Scanning electron microscopy were performed at a Zeiss FE-SEM Ultra 55 (3 kV). Samples were dropped from the corresponding solutions onto silicon wafers with the same concentration as those for TEM samples. X-ray diffraction measurements was performed by a PANalytical X'Pert PRO. Nikon COOLPIX S8000 camera was used to take the photo images. The ground films were prepared by grinding under similar forces using a spoon.

15



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Synthesis of compound 1: A dry condenser was installed to an oven-dried, 100 mL two-necked round-bottom flask under nitrogen, then sodium benzenethiolate (1.32 g, 10 mmol), compound 1-2 (272 mg, 1 mmol) and DMI (10.0 mL) were introduced. The mixture was stirred with a magnetic bar at 40 °C for 72 h. when A precipitate was generated with addintion of water (100 mL). The yellow solid was obtained by filter, and then it was washed by water, ethanol for purification (500 mg, 51%). 1

H NMR (400 MHz, CDCl3): δ = 6.64 (m, 8H), 7.13 (m, 32H).

13

C NMR (100 MHz, CDCl3): δ =

142.79, 140.94, 139.03, 138.69, 137.36, 129.65, 128.82, 128.57, 127.25, 126.57, 125.76. MS: MALDI-TOF MS, m/z: [M + H]+ 993.7.

Synthesis of compound 2: The procedure is similar to compound 1. 1H NMR (400 MHz, CDCl3): δ =2.27 (S, 12H), 2.35 (S, 12H), 6.54 (d, J = 8.0 Hz, 8H), 6.86 (dd, J = 8.0 Hz, 16H), 6.98 (d, J = 4.0 Hz, 8H).

13

C NMR (100 MHz, CDCl3): δ = 135.86, 135.54, 135.31, 134.03, 129.38, 129.30, 129.23,

127.70, 21.29, 21.09.

Self-assembly in DMF/H2O: Compound 1 or 2 (6 mg) were dissolved in DMF (10 mL) and divided into 10 parts (each 1ml), followed by addition into DMF and water to obtain a variety of different proportions. UV/Vis and PL were used for the analysis of the solutions. Preparation of solid-state films: Compound 1 or 2 (1 mg) was respectively dissolved in ethanol, dichloromethane or ethyl acetate (1 mL), and then the above solutions were dropped onto a clean glass substrate. The solvent was removed by heating at 40 oC. 16


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Computational conditions: Density functional theory (DFT) calculations

50,51

were employed to

optimize the geometric configurations of 1 and 2. Molecular dynamics (MD) simulations of 1 and 2 were carried out in aqueous solutions by GROMACS program package52. For each compound, the starting structures were generated in a cubic box (V=10 × 10 × 10 nm3) by randomly inserting ten molecules. At room temperature and pressure, each starting structures was solvated by around 32500 water molecules and subjected to 100-ns MD simulations.

Acknowledgement. This work was supported by the NSFC/China (21644005) and National Program for Thousand Young Talents of China. X. L. and H. Å. thank the Swedish National Infrastructure for Computing (SNIC) for providing computational resources for project SNIC 2014-11/31. W. C. acknowledges the Shanghai Pujiang Program (15PJ1402600) and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.

Corresponding Author: [email protected] (L. Z.); [email protected] (Q. Z.). Supporting Information Available: Supporting experimental data (including PL, SEM, UV/Vis, photographs, MD simulation, etc.). This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Takahashi, O. CH…π Interaction in Organic Molecules. Springer International Publishing. 2015, 19, 47-68. (2) Kobayashi, Y.; Saigo. K. Periodic ab Initio Approach for the Cooperative Effect of CH/π Interaction in Crystals: Relative Energy of CH/π and Hydrogen-Bonding Interactions. J. Am. Chem. Soc. 2005, 127, 15054-15060. 17



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Tuning for Visible Fluorescence and Near-Infrared Phosphorescence

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