White Light Emission in Butadiyne Bridged PyreneâPhenyl Hybrid

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White Light Emission in Butadiyne Bridged Pyrene-Phenyl Hybrid Fluorophore: Understanding the Photophysical Importance of Diyne Spacer and Utilizing the Excited State Photophysics for Vapor Detection Avik Kumar Pati, Santosh J. Gharpure, and Ashok Kumar Mishra J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b04956 • Publication Date (Web): 05 Jul 2016 Downloaded from http://pubs.acs.org on July 7, 2016

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The Journal of Physical Chemistry

White Light Emission in Butadiyne Bridged Pyrene-Phenyl Hybrid Fluorophore: Understanding the Photophysical Importance of Diyne Spacer and Utilizing the Excited State Photophysics for Vapor Detection

Avik Kumar Pati,† Santosh J. Gharpure*‡ and Ashok K. Mishra*† †

Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036,

Tamil Nadu, India. ‡Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India.

1

ACS Paragon Plus Environment


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ABSTRACT: Generation of white light emission (WLE) from a single organic fluorophore is challenging because of the need to get fluorescence covering the visible region (400–700 nm) upon excitation of the dye at near ultraviolet wavelength. Herein, we report white light emission from a butadiyne bridged pyrene-phenyl hybrid fluorophore in mixed-aqueous solvents as well as in polymer film matrices. The ability of the butadiynyl dye to emit from multiple excited states such as locally excited (LE) (400–500 nm), aggregate (excimer type) (475–600 nm) and charge transfer (CT) (500–750 nm) states spanning the emission almost throughout the visible range has made the generation of the white light to be possible. In highly polar solvent such as acetonitrile, the butadiynyl dye emits from the LE and CT states and the white light emission is achieved through a control of the dye concentration such that intermolecular CT (exciplex type) contributes along with the intramolecular CT and LE emissions. In mixed-aqueous systems such as water-acetonitrile and water-N,N-dimethylformamide, the CT emission is red-shifted (because of the high dielctric constant of water) and the contribution of the aggregate emission (originated because of the poor solvent water) is important in maintaining the relative distribution of the fluorescence intensities (LE, excimer and CT) in the entire visible region. The significance of the diyne spacer in achieving the WLE is delineated through a control study with a single acetylenic analogue. The LE, aggregate, and CT emissions are involved in generating bluish white light in a poly(vinyl alcohol) film matrix of the butadiynyl dye. Blue emission is noted in a poly(methyl methacrylate) (PMMA) film matrix of the dye with a major contribution from the LE and a minor contribution from the aggregate state. Exposure of the PMMA film of the dye to polar aprotic vapors assists in gaining the CT state emission such that the LE, aggregate, CT emissions cover the entire visible region to produce the WLE. This opens up a new strategy for selective vapor sensing. 2


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INTRODUCTION Search for novel molecular systems possessing the character of white light emission (WLE) has found growing interest because of their wide applications in light emitting devices, optical displays, and sensors.1–13 It still remains as a challenge to get emission from a material covering the visible region (400–700 nm), comprising of red (R), green (G), and blue (B) fluorescence or complementary emissions. The creation of white light from multi-fluorophores demands careful choice of all of the fluorophores such that intermolecular energy transfer processes are controlled.14–23 In this regard, single component white light emitters, usually certain groups of inorganic complexes/metal organic frameworks (some selected examples are cited here)24–33 and organic molecules34–44 have attracted special attention as the alternatives of the complex fluorophoric systems which emit white light. However, there are only limited reports on the WLE from single organic derivatives in the literature.34–44 Yang et al. described the WLE from a benzo[a]xanthene dye, where the fluorescence originated from neutral and anionic forms of the dye controlled through pH of the medium.34 Recently, Xie et al. reported aggregation induced WLE from a carbazolyl- and phenothiazinyl-substituted benzophenone.44 There has been increasing interest on various applications45–53 and photophysics54–59 of butadiynyl derivatives in recent years. In a program aimed at the understanding of photophysics of some small butadiynyl fluorophores,60–63 we observed recently that a donor (-NMe2)–acceptor (-CN) substituted diphenylbutadiynyl derivative (Me2NPBPCN) (Figure 1) shows the WLE in acetonitrile (CH3CN) through a control of locally excited (LE) and exciplex emissions. A few questions which remained unaddressed were that whether the excited state photophysics of butadiyne bridged hybrid fluorophores such as a pyrene-phenyl conjugate (PyBPNMe2) (Figure 1) could be utilized to generate the white light in CH3CN and the WLE can be achieved even in mixed3



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aqueous solvents. Thus, here, we aim to study (i) the WLE in the butadiyne bridged hybrid fluorophore PyBPNMe2 in CH3CN and (ii) CH3CN-water solvent mixtures; (iii) the photophysical role of the diacetylene (butadiyne) bridge over an acetylene spacer to the generation of the WLE; and (iv) an application of the WLE strategy towards detection of polar aprotic vapors in a polymer film matrix. EXPERIMENTAL METHODS Solution Preparation for Photophysical Studies. Spectroscopic grade solvents were used for all the photophysical studies. Stock solutions of 10-3 M concentration in CH2Cl2 were prepared for the derivatives. The solutions in other solvents were prepared by evaporating CH2Cl2 from the desired amount of the stock solution by purging nitrogen gas. The desired solvent was then added to it. Steady-State Absorption and Fluorescence Experiments. Absorption spectra were recorded with Shimadzu UV-2600 spectrometer. Fluorescence experiments were carried out with Horiba Jobin–Yvon FluoroMax-4 spectrofluorometer, with 450 W xenon lamp as light source. The emission spectra were collected with slit widths of 3/3 nm. Time-Resolved Fluorescence Decay Experiments. Fluorescence lifetime experiments were done using a Horiba Jobin Yvon TCSPC lifetime instrument in a time-correlated single-photon counting arrangement. 370 nm nano-LED was used as light source for experiments. The pulse repetition rate was set to 1 MHz and the instrumental full width at half maximum of the LED, including the detector response was around 800 ps. The instrument response function was collected using a scatterer (Ludox AS40 colloidal silica). The decay data were analyzed using IBH software. A value of χ2, 0.99 ≤ χ2 ≤ 1.3 was considered as a good fit, which was further verified by symmetrical distributions of residuals. 4


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RESULTS AND DISCUSSION The diacetylene bridged pyrene-phenyl hybrid fluorophore PyBPNMe2 and the acetylene bridged pyrene-phenyl hybrid dye DMAPEPy, which are under study, are shown in Figure 1. The synthesis and solution phase photophysical properties of the derivatives were described elsewhere.62,64

N

NC

Acceptor

Me2NPBPCN Spacer

Exhibits white light emission

Donor

Replacing the 'benzonitrile' acceptor by flat aromatic ring ('pyrene')

N

White light emission?

PyBPNMe2

Replacing the 'diacetylene' spacer by 'acetylene'

White light emission?

N DMAPEPy

Figure 1. Molecular structures of the diacetylene bridged phenyl-phenyl derivative Me2NPBPCN, the diacetylene bridged pyrene-phenyl hybrid fluorophore PyBPNMe2 and the acetylene bridged pyrene-phenyl hybrid dye DMAPEPy (P, B, Py, and DMA indicate ‘phenyl’, ‘butadiyne bridge’, ‘pyrene’, and ‘N,N-dimethylaniline’, respectively). White Light Emission in Neat Acetonitrile: Steady-State Fluorescence Studies. In pure CH3CN, PyBPNMe2 (concentration, C = 0.55 µM) shows LE and ICT emissions spanning over 400–500 and 500–740 nm, respectively (Figure 2a). The commission internationale de l'éclairage (CIE) chromaticity index of the overall emission spectrum of the dye at C = 0.55 µM is (0.27, 0.30). The CIE color coordinate provides the information of red (R), green (G), and blue (B) 5



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color contribution in the fluorescence. To achieve pure white light, it is required to have equal contribution (33.33%) of all the three colors (R, G, and B). Ideally the CIE index of pure white light is (0.33,0.33), in which the x and y co-ordinates indicate the contribution of red and green color, resectively. The contribution of blue color is [1 – (0.33 + 0.33)]. As concentration of PyBPNMe2 is increased, the fluorescence intensity of the longer wavelength CT emission increases (Figure 2a). This is also evident from Figure 2b, where the ratio of the fluorescence intensities at the exciplex and the LE emission wavelengths (I440/I580) decreases with increasing the fluorophore concentration. The enhancement of the fluorescence intensity at the longer wavelength suggests that the emission is of exciplex type. The fluorescence excitation profile at the exciplex emission wavelength (λem = 580 nm) (red color in Figure 2c) is being different from that of the LE state (λem = 440 nm) (black color in Figure 2c), it is understood that the exciplex is of static character. The WLE was achieved upon controlling the concentration of PyBPNMe2. The CIE value of PyBPNMe2 in 0.82 µM concentration was close to the pure white light (0.31, 0.34) (Figure 2d). The CIE index was tuned through changing the concentration of the dye (see Figure S1, Supporting Information). The white light emission from sample cuvette of PyBPNMe2 under xenon lamp excitation at 380 nm is shown in inset of Figure 2d.

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(c) 6

3.6x10

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λex = 380 nm

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0.1 0.6

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Figure 2. (a) Concentration dependent steady-state fluorescence spectra (λex = 380 nm), (b) fluorescence excitation spectra at two different emission wavelengths (λem = 440 and 580 nm), (c) plot of the ratio of fluorescence intensities at two emission wavelengths (I440/I580) versus fluorophore concentration and (d) CIE chromaticity diagram of PyBPNMe2 in CH3CN at C = 0.82 µM. White Light Emission in Mixed-Aqueous Media: Steady-State Fluorescence Studies. As an exciplex emission is generally sensitive to solvent polarity, it was thought that the CIE coordinate of PyBPNMe2 can be tuned in CH3CN upon addition of water. Not only the shift of the exciplex emission can be controlled but also certain level of aggregation can be added with gradual mixing of water (a poor solvent) in CH3CN such that the CIE value can be better adjusted for obtaining the WLE. At low (< ~0.82 µM) and high (> ~0.82 µM) fluorophore 7



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concentration, the relative intensities of the LE emission appearing in the blue region (400–500 nm) and the CT emission in the red region (500–750 nm) are such that the overall distribution of the fluorescence intensity is not sufficient for the generation of the WLE. With increase of water content in CH3CN, the exciplex emission is red-shifted and the aggregate emission starts appearing (the aggregate emission is closely overlapped with the LE and exciplex emissions, understood from time-resolved fluorescence decay experiments, discussed later), thus providing the necessary spectral components to bring the CIE towards the WLE. In 99% water in CH3CN, the dye PyBPNMe2 shows mainly aggregate emission appearing in the range 475–600 nm (Figure 3a).62 The progress of the CIE co-efficient with addition of water, however, is not regular (see Figure S2, Supporting Information). It was possible to obtain pure WLE in 40% water in CH3CN mixture with CIE co-ordinate (0.31, 0.33) (Figure 3b) at the fluorophore concentration of 1.3 µM. Thus, a fine balance of the three emissions (LE, excimer and exciplex) is essential for the generation of the pure WLE from PyBPNMe2 in mixed-aqueous media. Similar to the waterCH3CN solvent system, the steady-state emission spectra of PyBPNMe2 in water-DMF showed the red-shift of the exciplex emission on increasing water percentage and the aggregate emission at higher percentage of water (80–99%) (Figure 3c). The CIE index of the WLE was varied through the addition of different percentage of water content into DMF solution of the derivative (see Figure S3, Supporting Information) and the WLE was obtained in 20% DMF in water through the proper distribution of the LE, excimer and exciplex emissions with CIE co-ordinate (0.35, 0.36) (Figure 3d). It is to be mentioned that the WLE was not observed in mixed-aqueous solvents involving moderately polar solvents such as tetrahydrofuran (THF) (see Figure S4, Supporting Information). In the case of water-THF solvent system, the position of the CT

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emission band was such that the CT emission did not contribute much to the red/orange-red emission so as to achieve the required spectral criteria for the geneation of the WLE.

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(0.35, 0.36)

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Figure 3. (a) Steady-state fluorescence spectra of PyBPNMe2 in water-CH3CN, and (b) CIE chromaticity diagram of PyBPNMe2 in 40% water-CH3CN (λex = 380 nm, C = 1.3 µM). (c) Steady-state fluorescence spectra of PyBPNMe2 in water-DMF (λex = 380 nm), and (d) CIE chromaticity diagram of PyBPNMe2 in 20% water-DMF (λex = 380 nm, C = 1.2 µM). Insights into the Excited State Emitting Species Contributing Towards the White Light Emission in Mixed-Aqueous Media: Time-Resolved Fluorescence Decay Studies. Timeresolved fluorescence decay studies of PyBPNMe2 were carried out in mixed-aqueous solvents to better understand the excited states which contribute to the WLE. In pure CH3CN, the dye shows the LE and CT states with lifetimes 1.2 and 0.6 ns, respectively. When water was added in 9



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CH3CN (the mixed-aqueous system), the three emitting components (LE, aggregate and CT) were involved in the process of the generation of the white light. The aggregate state was characterized by a relatively longer lifetime ca. 5–6 ns. The aggregate state played a crucial role in generating the WLE in addition to the LE and CT state components. To illustrate, in 20% water-CH3CN, the excited state was dominated by the LE and CT states at λem = 425 (Figure 4a), 525 (Figure 4b), and 625 nm (Figure 4c). At λem = 425 nm, the relative excited state population (99%) as well as the relative emision (95%) of the LE state dominated; whereas at λem = 625 nm, the relative excited state population (81%) as well as the relative emission (64%) of the CT state was predominant. Although the aggregate state component (lifetime ~5–6 ns) was noted at λem = 425 and 525 nm in 20% water-CH3CN, the relative excited state population as well as the relative emission of the aggregate state was negligible. When the percentage of water was increased such as 40% water-CH3CN, the aggregate state contributed significantly along with the LE and CT states (Figures 4d–f). The dye PyBPNMe2 showed tri-exponential decay kinetics in all of the emission wavelengths λem = 425 (Figure 4d), 525 (Figure 4e) and 625 nm (Figure 4f). In 40% water-CH3CN, the LE state component which has a slightly longer lifetime (~2–3 ns) in comparison with 20% water-CH3CN system predominated at λem = 425 and 525 nm. The relative population and the relative emission of the aggreagte state (lifetime ~6 ns) was 13% and 60% respectively at λem = 425 nm and 17% and 41% respectively at λem = 525 nm. At λem = 625 nm, the relative population and the relative emission of the aggregate state was negligible and the CT state (lifetime ~0.5 ns) dominated (relative population and relative emission were 92% and 33%, respectively). Thus, in 40% water-CH3CN, the relative excited state population as well as the relative emission of the three emitting species throughout the visible region was such that the steady-state emission spectrum covered the entire visible region with proper distribution of 10


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fluorescence intensity producing the pure white light. On increase of further water content, the emission mainly comes from the excimer state showing bluish green emission with CIE value (0.22, 0.31) in 80% water in CH3CN. Prompt 20% water in CH3CN (λ em = 425 nm)

(b) 10000

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τ2 = 6.8 ns, α2 = 0.02, β 2 = 0.35

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Figure 4. Time-resolved fluorescence decay profiles and lifetime parameters of PyBPNMe2 in (a) 20% water-CH3CN at λem = 425 nm, (b) 20% water-CH3CN at λem = 525 nm, (c) 20% waterCH3CN at λem = 625 nm, (d) 40% water-CH3CN at λem = 425 nm, (e) 40% water-CH3CN at λem = 525 nm, and (f) 40% water-CH3CN at λem = 625 nm (λex = 370 nm for all of the cases; instrument limited short decay components are ignored). White Light Emission—‘Diacetylene’ Versus ‘Acetylene’ Spacer. In order to understand the importance of the diacetylene moiety in the generation of white light from PyBPNMe2, a control fluorescence experiment was carried out with an analogous derivative DMAPEPy64,65 having a single acetylene spacer. The dye DMAPEPy mainly showed the CT emission at ~541 nm. The CT emission got red-shifted on increasing the percentage of water in CH3CN (Figure 5a). The LE emission which was spread over 400–450 nm region, was of very low intensity in 11



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comparison with the CT emission. At higher water percentage (80–99%), the aggregate emission predominated and the emission maximum appeared at ~490 nm, which was ~35 nm blue-shifted compared to the dye PyBPNMe2. Thus, the position of the three emission bands as well as the distribution of the fluorescence throughout the visible region in DMAPEPy was such that the emission spectra in water-CH3CN mixtures did not satisfy the spectral requirement for the generation of the WLE. The chromaticity diagram shows that the CIE indices of DMAPEPy in different proportions of water-CH3CN are localized only on the blue and green regions (Figure 5b). The facts behind the incapability of the single acetylenic dye DMAPEPy in producing the WLE in contrast to the diacetylene analogue PyBPNMe2 can be comprehended as follows: (i) on decreasing the spacer length from diacetylene to single acetylene the energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) is increased (Figure 5c), which in turn makes the CT emission band of DMAPEPy to be less redshifted compared to the diacetylene derivative PyBPNMe2, thus contributing less towards the red emission; (ii) the single acetylene bridge in DMAPEPy being more rigid compared to the diacetylene spacer, the single acetylene derivative DMAPEPy shows efficient CT emission at the cost of the LE emission; (iii) the diacetylene derivative PyBPNMe2 because of having the possibility of twisting around the diacetylene spacer shows a balance between the LE and CT emissions through a compromise of the CT emission; and (iv) the aggregate emission in DMAPEPy is not strong enough to play a role in controlling the LE and CT emission intensities such that the CIE index is being attempted to shift towards the white light CIE co-ordinate (0.33, 0.33). Thus, the presence of the diacetylenic spacer in PyBPNMe2 is important to control the relative intensity of the emissions as well as the disposition of those emission bands in proper energy regions, which are crucial towards achieving the white light. 12


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(b)

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HOMO (-8.12 eV)

Figure 5. (a) Steady-state fluorescence spectra of DMAPEPy (C = 1 x 10-6 M) in water-CH3CN solvent mixtures (λex = 380 nm) and (b) CIE chromaticity diagram of DMAPEPy (C = 1 x 10-6 M) in water-CH3CN solvent mixtures. (c) Energies of frontier molecular orbitals of PyBPNMe2 (left panel) and DMAPEPy (right panel) in CH3CN calculated using CAM-B3LYP/6311+G(d,p) level of theory66 in Gaussian 09 computational suite67. White Light Emission in a Polymer Film Matrix through Detection of Polar Aprotic Solvent Vapors. The dye PyBPNMe2 in solid powder form exhibits bluish green emission63 with CIE co-ordinate (0.23, 0.33). Bryce and co-workers observed mainly the blue emission 13



from a butadiyne bridged carbazole-oxadiazole hybrid derivative in solution and in a film matrix, which showed white light upon fabrication of a light emitting device.7 Time-resolved fluorescence decay study for the solid powder (λex = 370 nm, λem = 520 nm) earlier63 showed triexponential decay behaviour with aggregate state lifetime ~2–16 ns and LE state lifetime ~1 ns. In an attempt to get the CT emission band in solid powder in addition to the aggregate and LE emissions so as to produce the WLE, the emission spectrum of the solid powder was recorded after exposure of the solid powder to CH3CN vapor for 1 minute. However, it rather increases the LE emission intensity (Figure 6a, ‘dark cyan’ color) with CIE co-ordinate (0.19, 0.26) (Figure 6b, ‘dark cyan’ color). The solid powder of PyBPNMe2 was also exposed to hexane and dichloromethane (DCM) vapors (Figure 6a, ‘red’ and ‘blue’ colors) and the CIE index was found to be changed (Figure 6b, ‘red’ and ‘blue’ colors). This indicated that the CIE index of PyBPNMe2 could be used as a possible tool for vapor detection in the solid state.

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Figure 6. (a) Steady-state emission spectra of PyBPNMe2 in solid powder form and after exposure to hexane, DCM and CH3CN vapors in the solids (λex = 380 nm). (b) CIE chromaticity diagram of PyBPNMe2 in solid powder form and after exposure to hexane, DCM and CH3CN vapors in the solids.

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λem = 520 nm τ1 = 1.4 ns, τ2 = 10.7 ns λem = 620 nm τ1 = 3.2 ns, τ2 = 12.8 ns,

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Figure 7. (a) Steady-state fluorescence spectrum of PyBPNMe2 in a PVA film matrix (λex = 380 nm), (b) CIE chromaticity diagram of PyBPNMe2 in the PVA film matrix, and (c) time-resolved fluorescence decay profiles of PyBPNMe2 in the PVA film matrix at different emission wavelengths (λex = 370 nm). In contrast to the solid state (powder) emission, the fluorescence spectrum of PyBPNMe2 in a poly(vinyl alcohol) (PVA) film matrix (Figure 7a) shows bluish white light with CIE co-ordinate (0.28, 0.31) (Figure 7b). The PVA film was kept into 10 µM solution of PyBPNMe2 in 20% water-methanol for 48 hrs and then the swollen film was dried for a day prior to fluorescence studies. Unlike the solution phase (CH3CN) WLE spectra which were composed of two well separated emission bands, the spectrum for the PVA film was quite broad. Time-resolved fluorescence decay experiment (λex = 370 nm) for the dye in the PVA film matrix showed bi/tri15



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exponential behaviour of the fluorescence decay at different emission wavelengths such as 425, 520 and 620 nm (Figure 7c). While 1.1 and 5.8 ns lifetime components were observed for the LE and aggregate states respectively at λem 425 nm, multiple aggregates with longer lifetimes ca. 10–13 ns (in comparison with the LE and CT lifetimes) were noted at λem 520 and 620 nm. The CT component with lifetime 0.6 ns was observed at λem 620 nm. An exposure of the PVA film of PyBPNMe2 to CH3CN vapor did not alter much the emission spectrum and thus, the CIE index remained almost unchanged. The CIE index also remained unaltered upon exposure of the PVA film of PyBPNMe2 to various types of solvent vapors. The emission behavior of PyBPNMe2 was next studied in a poly(methyl methacrylate) (PMMA) film matrix. The PMMA film was prepared by dissolving 1 gm of PMMA in 10 ml of 10 µM PyBPNMe2 in CH3CN at 60 °C and then pouring it on a petri dish uniformly. The film was taken out from the dish after drying it in oven at 60 °C for 2 hr. The PMMA film of the dye exhibited blue fluorescence (Figure 8a) with CIE index (0.19, 0.21) (see Figure S5, Supporting Information). The fluorescence of PyBPNMe2 in the PMMA film matrix originated from the LE (major contribution) and aggregate states, which were confirmed through time-resolved fluorescence decay experiment (Figure 8b). Excited state components with lifetimes ~1 and ~4 ns were noted for the LE and aggregate states, respectively (λex = 370 nm, λem = 450 nm). When the PMMA film was exposed to CH3CN vapor for 1 minute, the CT emission band appeared (Figure 8c) in addition to the LE and aggregate emissions exhibiting white light with CIE coordinates (0.29, 0.35) (Figure 8d). The non-smoothed spectrum (black line in Figure 8c) and the smoothed spectrum obtained by Gaussian curve fitting (red line in Figure 8c) gave the same CIE value. The smoothed spectrum has been depicted in Figure 8c to show that the original structured emission spectrum (black line in Figure 8c) (the structurings are because of 16


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inhomogeneity in the vapor exposed PMMA film and those are not vibronic emission bands) does not impact on the CIE index of the dye. It was latter prompted to check the selectivity of the PMMA film towards different types of solvent vapors to generate the WLE (see Figure S6, Supporting Information). Exposure of the film to non-polar vapors such as hexane, benzene produced blue emission, wherein the emission mainly originated from the LE state. On exposing the film to moderately polar vapors such as ethylacetate (EtOAc), DCM, tetrahydrofuran (THF) and polar protic vapors like methanol (MeOH), ethanol (EtOH) showed blue/bluish green emissions. However, the PMMA film of the derivative showed the WLE on exposing it to N,Ndimethylformamide (DMF) and acetone vapors, which furnished the CIE values (0.29, 0.29) and (0.30, 0.34), respectively (see Figure S7, Supporting Information). This suggested that the WLE could be generated in a PMMA film matrix of PyBPNMe2 through detection of polar aprotic vapors like CH3CN, DMF, acetone. This is the first report of its kind showing the possibility of vapor detection through the generation of white light in solid state. Vapor detection in the solid state using fluorescent molecules has attracted increased attention in the literature (a few selected examples are cited herein)68–76 because of the fact that fluorescence spectroscopy is a simple, efficient and sensitive detection technique. Detection of volatile organic compounds (VOCs) is important owing to the issues associated with human health and environmental concern. The fluorescence based systems which are mainly utilized in the detection of VOCs in the literature are (i) reaction based fluorogenic sensors,68,69 (ii) fluorescent organogel/fluorophore tagged supramolecular assembly,70,71 (iii) fluorophore labelled polymers/luminescent polymers,72–75 and (iv) fluorescent metal-organic frameworks76. In all of these systems, either the fluorescence is ‘switched on’ or ‘switched off” upon interaction/binding with vapor molecules. Here, the selective detection of polar aprotic vapors is carried out utilizing the excited state photophysical 17



properties of the butadiynyl dye. While the blue/bluish green emissions are noted from the dye in the PMMA film matrix on exposing non-polar and moderately polar organic solvent vapors (monitored through the variation of the CIE index of the dye), the white light is observed for the

(b) λex = 370 nm

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Figure 8. (a) Steady-state emission spectrum of PyBPNMe2 in a PMMA film (λex = 380 nm), (b) time-resolved fluorescence decay profile of PyBPNMe2 in a PMMA film (λex = 370 nm, λem = 450 nm), (c) steady-state emission spectra of PyBPNMe2 (λex = 380 nm) in a PMMA film exposed to CH3CN vapor, and (d) CIE chromaticity diagram of the film exposed to CH3CN vapor (white light emission from the PMMA film on exposure to CH3CN vapor is shown in inset).

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Conclusions In conclusion, the excited state photophysics of a butadiyne bridged pyrene-phenyl hybrid fluorophore were utilized to generate white light emission in highly polar solvent (acetonitrile), mixed-aqueous solvent (water-acetonitrile) and polymer film matrices. The key features of the present work include the following. (1) The locally excited (LE) and charge transfer (CT) emissions were involved in the generation of white light in the butadiyne bridged pyrene-phenyl conjugate in acetonitrile through a control of the fluorophore concentration such that intermolecular CT (exciplex type) appeared together with the intramolecular CT and LE emissions. The CIE index was found to be (0.31, 0.34) in CH3CN at the fluorophore concentration of 0.82 µM. (2) Multiple emissive pathways of the LE, aggregate (excimer type), and CT states were responsible for the generation of white light in mixed-aqueous solvents. The CIE index of the butadiynyl dye was noted to be (0.31, 0.33) in 40 % water in CH3CN at fluorophore concentration of 1.3 µM. The white light emission was also achieved in 20% water-DMF mixtures of the dye (concentration = 1.2 µM) with the CIE index (0.35, 0.36). This strategy of multiple emissions to generate white light is advantageous because the CIE index can be adjusted through the control of either of the emissions. (3) This work showed the importance of the diyne bridge over a single acetylene spacer in creating a suitable balance of the LE and CT emissions, which was an important requirement for the white light emission from such dyes. In addition to the balance of the LE and CT emissions, the importance of the control of aggregation as a crucial criterion for the white light emission in mixed-aqueous solvents was also established.

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(4) It was possible to change the CIE index in solid powder through exposure of organic solvent vapors, which opens up the possibility of vapor sensing using CIE as a detection tool. The LE and aggregate states were mainly involved in the vapor sensing process by the dye in the solid powder form. (5) Bluish white light emission was achieved in a poly(vinyl alcohol) (PVA) film matrix of the dye with CIE value of (0.28, 0.31). The LE, aggregate, and CT emissions contributed to the generation of the bluish white light in the PVA film matrix. (6) Finally, it was demonstrated that pure white light could be generated in a poly(methyl methacrylate) (PMMA) film matrix of the butadiynyl dye through detection of polar aprotic vapors. The dye in the PMMA film showed blue emission with a contribution from the LE and aggregate states. The exposure of the PMMA film to polar aprotic vapors such as CH3CN, DMF, and acetone helped in gaining the CT state emission in addition to the presence of the LE and aggregate emissions. The three emissions together covered the entire visible region producing the white light. The detection of non-polar and moderately polar organic sovent vapors was carried out through the variation of the CIE index of the dye. The CIE co-ordinates of the white light emission for CH3CN, DMF, and acetone were (0.29, 0.35), (0.29, 0.29), and (0.30, 0.34), respectively. AUTHOR INFORMATION Corresponding Author *†

Tel.: +91-44-22574207; Fax: +91-44-22574202; E-mail address: [email protected]

(Ashok K. Mishra). *‡

Tel.: +91-22-2576 7171; Fax: +91-22-2576 7152; E-mail address: [email protected]

(Santosh J. Gharpure). 20


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ACKNOWLEDGMENTS Authors thank DST, New Delhi for financial support. AKP thanks CSIR, New Delhi for a research fellowship. Supporting Information Supporting Information Available: CIE chromaticity diagram of PyBPNMe2 in CH3CN at different concentration of the fluorophore (λex = 380 nm); CIE chromaticity diagram of PyBPNMe2 in water-CH3CN solvent mixtures at fluorophore concentration of 1.3 µM (λex = 380 nm); CIE chromaticity diagram of PyBPNMe2 in water-DMF solvent mixtures at fluorophore concentration of C = 1.2 µM (λex = 380 nm); steady-state fluorescence spectra of PyBPNMe2 in water-THF solvent mixtures at fluorophore concentration of C = 1 µM (λex = 380 nm) and CIE chromaticity diagram of PyBPNMe2 in water-THF solvent mixtures at fluorophore concentration of C = 1 µM (λex = 380 nm); CIE chromaticity diagram of PyBPNMe2 in a PMMA film matrix (λex = 380 nm); steady-state emission spectra of PyBPNMe2 in a PMMA film matrix upon exposure to various types of vapors (λex = 380 nm); and CIE chromaticity diagram of PyBPNMe2 in a PMMA film matrix upon exposure to various types of vapors (λex = 380 nm). This information is available free of charge via the Internet at http://pubs.acs.org References: (1) Organic Light Emitting Devices: Synthesis Properties and Applications, ed. K. Mullen and U. Scherf, Wiley-VCH, Weinheim, Germany, 2006. (2) Sun, Y.; Giebink, N. C.; Kanno, H.; Ma, B.; Thompson, M. E.; Forrest, S. R. Management of Singlet and Triplet Excitons for Efficient White Organic Light-Emitting Devices. Nature 2006, 440, 908–912.

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White light emission LE emission N 5

9.0x10

5

6.0x10

Exciplex Excimer emission emission CIE = (0.31, 0.33)

5

3.0x10

0.0 400

450

500

550

600

Wavelength (nm)

32


650

700

White Light Emission in Butadiyne Bridged PyreneâPhenyl Hybrid

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White Light Emission in Butadiyne Bridged PyreneâPhenyl Hybrid