Photophysics of Diphenylbutadiynes in Water, AcetonitrileâWater, and

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Photophysics of Diphenylbutadiynes in Water, Acetonitrile-Water and Acetonitrile Solvent Systems: Application to Single Component White Light Emission Avik Kumar Pati, Rounak Jana, Santosh J. Gharpure, and Ashok Kumar Mishra J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b04954 • Publication Date (Web): 05 Jul 2016 Downloaded from http://pubs.acs.org on July 11, 2016

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The Journal of Physical Chemistry

Photophysics of Diphenylbutadiynes in Water, Acetonitrile-Water and Acetonitrile Solvent Systems: Application to Single Component White Light Emission

Avik Kumar Pati,† Rounak Jana,† 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. ABSTRACT: Diacetylenes have been the subject of current research because of their interesting optoelectronic properties. Herein, we report that substituted diphenylbutadiynes exhibit locally excited (LE) and excimer emissions in water and multiple emissions from the LE, excimer and intramolecular charge transfer (ICT) states in acetonitrile-water solvent systems. The LE, excimer and ICT emissions are clearly distinguishable for a diphenylbutadiynyl derivative with push (-NMe2)–pull (-CN) substituents and those are closely overlapped for non push–pull analogues. In neat acetonitrile, the excimer emission disappears and the LE and ICT emissions predominate. In case of the push (-NMe2)–pull (-CN) diphenylbutadiyne, the intensity of the ICT emission increases with increasing the fluorophore concentration. This suggests that the ICT emission accompanies with intermolecular CT emission which is of exciplex type. As the LE and exciplex emissions of the push–pull diphenylbutadiyne together cover the entire visible region (400–800 nm) in acetonitrile, a control of the fluorophore concentration makes the relative intensities of the LE and exciplex emissions such that pure white light emission is achieved. The white light emission is not observed in those diphenylbutadiynyl analogues in which the peripheral substituents of the phenyl rings do not possess strong push–pull character.

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INTRODUCTION Butadiynyl derivatives have found growing interest in recent years because of their widespread applications in supramolecular chemistry,1,2 molecular rotors,3–5 liquid crystals,6,7 suppressing polymer chain degradation,8 light emitting diodes,9,10 and solar cells11. The butadiyne scaffolds are prevalent in many biologically active molecules and natural products.12,13 The butadiynes have also found interest in vibrational and electronic spectroscopy.14–17 In this regard, the parent butadiynyl fluorophore ‘diphenylbutadiyne’ has been widely studied for opto-electronic properties.18–25 Thulstrup et al. investigated the changes of electronic states of the diphenylbutadiyne with variation of torsion angles and found that both planar and non-planar rotamers of the diyne contribute to the absorption spectra.15 A vibrational study of certain diacetylenes was carried out by Roman et al. and Fermi resonance was noted in the vibrational spectra of the diacetylenes.22 The excited states and the vibronic spectroscopy of the diphenylbutadiyne were studied by Sebree et al. and the electronic origin of excitation spectrum of the diyne was observed at 32,158 cm-1 with strong acetylenic progression.23 Although much of the recent studies on diphenylbutadiynes are focussed on understanding their vibrational properties, the electronic properties of the diphenylbutadiynes in solution phase is yet to be fully understood and this has been the focus of the present work. In a program aimed at the understanding of photophysics of diacetylenes, recently we found that (i) substituted diphenylbutadiynes show emissions from locally excited (LE) and intramolecular charge transfer (ICT) states in non-aqueous solvents;26,27 (ii) butadiyne bridged pyrene-phenyl hybrid fluorophores exhibit excimer emission in water and multiple emissions (LE, ICT and excimer) in mixed-aqueous solvents;28 and (iii) in solid powder form, diarylbutadiynes show aggregate emission.29 These studies make a case in general to investigate the aqueous and mixed-aqueous 2


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phase photophysics of substituted diphenylbutadiynes in detail. Thus, here we (i) study the photophysical properties of substituted diphenylbutadiynes in aqueous and mixed-aqueous media and, (ii) utilize the observed photophysics of the diynes towards generation of single component white light emission (WLE). Although there have been significant progresses on the generation of white light from complex/multi-fluorophoric systems (a few selected examples are cited herein),30–40 the number of organic molecules exhibiting the single component WLE are fairly less.41–55 The few reported examples include the organic molecules in which the WLE is achieved mainly through pH dependent emission,41–43 ESIPT fluorescence45–48 and aggregation induced emission49–52. Here, we show that a balance of the LE and exciplex emissions is important to achieve the WLE from a push–pull diphenylbutadiyne. This approach of achieving the WLE is attractive given the facts that (i) it does not require mixing of multiple fluorophores, (ii) does not need any adjustment of pH of the medium, and (ii) does not necessitate the control of energy transfer processes. 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 all of the butadiynyl derivatives. The solutions in other solvents of different polarities 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 either 3/3 or 5/5 nm. 3



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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. Either 340 or 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. RESULTS AND DISCUSSION The donor–acceptor diphenylbutadiyne Me2NPBPCN (P and B indicate ‘phenyl’ and ‘butadiyne bridge’, respectively) having a strong donor (–NMe2) and a strong acceptor (–CN) substituent, under study here, is shown in Figure 1a. In order to compare the photophysical properties of Me2NPBPCN in water, two analogous butadiynyl derivatives PBPCN containing a neutral donor (–Ph) and a strong acceptor (–CN) (Figure 1b) and PBPNMe2 possessing a strong donor (– NMe2) and a neutral acceptor (–Ph) (Figure 1c) are considered. The detailed synthetic procedures of all of the derivatives are reported elsewhere.26

Figure 1. Substituted diphenylbutadiynyl fluorophores (a) Me2NPBPCN, (b) PBPCN, and (c) PBPNMe2 under study (P and B imply ‘phenyl’ and ‘butadiyne bridge’, respectively). 4


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Steady-State Absorption and Fluorescence Studies in Aqueous Media. The absorption spectra of the derivatives are depicted in Figure 2. The absorption spectrum of the push-pull derivative Me2NPBPCN is structureless in aqueous media (Figure 2a, black line), which is in contrary to the spectrum of the dye in acetonitrile (CH3CN) in which vibronic bands are observed26 (Figure 2b, black line). The vibronic features of PBPCN (Figure 2a, red line) are less intense in water in comparison with the spectrum observed in CH3CN (Figure 2b, red line). The dye PBPNMe2 has some signatures of vibronic bands at the shorter wavelength region in water (Figure 2a, blue line), which resemble the vibronic features of the molecule in CH3CN (Figure 2b, blue line). The flattening of the absorption band of Me2NPBPCN and the less intense vibronic structures in the absorption spectra of PBPCN and PBPNMe2 could be attributed to ground state aggregates in water (the poor solvent for the organic derivatives). Such flat absorption bands owing to the aggregates were earlier observed in solid state absorption spectra of certain methoxyphenyl containing diacetylenes.29

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0.03 0.00 0.6

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Wavelength (nm) Figure 2. Steady-state absorption spectra of Me2NPBPCN (black color), PBPCN (red color), and PBPNMe2 (blue color) in (a) water (concentration, C = 1 x 10-6 M) and (b) in CH3CN (C = 1 x 10-5 M) (absorption spectra in CH3CN are modified from ref. 26 and are shown here for the comparison with the absorption spectra in water). The diphenylbutadiynes are found to be fluorescent in water. The push–pull diyne Me2NPBPCN exhibits the emission band centered at 520 nm (Figure 3a), which is different from the previously observed26 LE (400–450 nm) and ICT (600 nm) emission bands in nonaqueous solvents (for a comparison of the aqueous and non-aqueous phase emission spectra, see Figure S1, Supporting Information). The large Stokes shift (ca. 7085 cm-1, considering λem at 520 nm and λabs at 380 nm) as well as the unstructured emission spectrum rules out the possibility of 6


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J-type aggregate and hints at excimeric character.56,57 Unlike the emission spectrum of Me2NPBPCN, the emission spectrum of PBPCN is structured at shorter wavelength region (ca. 400–450 nm) and shows a shoulder peak at ca. 495 nm (Figure 3b). The structured emission is attributed to the LE emission and the shoulder band is assigned to the aggregate emission. The derivative PBPNMe2 shows mainly the LE emission (Figure 3c), which is in resemblance with the earlier observed emission spectrum26 in non-aqueous solvent. Notably, a tail is extended to the longer wavelength region of the emission spectrum of PBPNMe2, suggesting some contribution of the aggregate emission. In order to better understand the LE and aggregate emissions of the diphenylbutadiynes in water, the emission spectra were resolved into multiple Gaussian/Lorentzian curves. Although electronic emission spectra are generally Gaussian type, here, because of the presence of strong Raman scattering, it was difficult to fit all of the spectra in Gaussian function. Thus, the spectra were fitted into either multiple Gaussian or Lorentzian curves. The Raman scattering in Figures 3a–c was identified through standard spectroscopic technique of excitation of the aqueous sample of the diynes at different wavelengths. The Raman scattering was that signal which varied with different excitations, while the position of the fluorescence maximum remained unchanged (see Figure S2, Supporting Information). The Raman scattering observed in Figures 3a–c was also verified with the scattering observed in pure water at the same excitation wavelength (see Figure S3, Supporting Information). The shorter wavelength resolved emission spectra (blue color in Figures 3a–c) were assigned to the LE emission (multiple blue color spectra indicate vibronic bands in the LE transition) and the redshifted resolved band was assigned to the aggregate emission of the diynes. In order to shed light on to the resolved emission bands, time-resolved fluorescence decay experiments were carried out at two different emission wavelengths corresponding to the blue- and red-shifted resolved 7



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emission bands (Figures 3d–e). The excited state decays of the fluorophores fit into bi-/triexponential kinetics and the lifetime as well as the relative excited state population of the emissive states changes on moving from the shorter to the longer emission wavelengths. Me2NPBPCN (Figure 3d), PBPCN (Figure 3e), and PBPNMe2 (Figure 3f) show lifetimes 1.2, 0.8, and 1.4 ns at the emission wavelengths 470, 425, and 450 nm, respectively in water. The shorter lifetime component (~1 ns) in water corresponds to the LE state, which matches with previously observed lifetime in non-aqueous solvents at the shorter emission wavelengths. In contrast to the shorter lifetime components (~1 ns), relatively longer lifetime components are noted at the longer wavelength emission region. To illustrate, Me2NPBPCN show lifetimes 4.5 and 16 ns at 525 nm and PBPCN and PBPNMe2 show lifetimes 6.2 and 9.3 ns at 500 and 520 nm emission wavelengths, respectively in water. These relatively longer lifetime components (~5-16 ns) appearing in the longer wavelength region corresponding to the red-shifted resolved emission band are attributed to the aggregate emission. Not only the lifetimes of the LE and aggregate states get changed with the variation of the emission wavelengths, but also the relative excited state population of the two states varies with changing the emission wavelengths. For an example, in case of the push–pull diyne Me2NPBPCN, the relative population of the LE state is more (95%) at shorter wavelength (λem = 470 nm) in comparison with the population (5%) of the longer lifetime component. On the other hand, at the longer emission wavelength (λem = 525 nm), the population of the longer lifetime components is more (total 90%) in comparison with the shorter lifetime component. This suggests that the LE and aggregate emissions predominate at the shorter and longer wavelength regions, respectively. The aggregate emission maximum of Me2NPBPCN is ca. 25 and 55 nm red-shifted from the aggregate emissions of PBPCN and PBPNMe2 (considering the red-shifted resolved emission bands, olive green colors in Figures 8


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3a–c), respectively. The aggregate emission of Me2NPBPCN is also noted to be much redshifted in comparison with the common aromatic hydrocarbons58–60 such as naphthalene, anthracene, and pyrene. A single acetylene containing derivative 1,2-di-p-tolylethyne reported by Lewis et al.61 showed an aggregate emission maximum at ca. 447 nm, which is ca. 73 nm blueshifted than the push–pull diacetylene Me2NPBPCN. Fluorine substituted diphenylhexatriene studied by Sonoda et al.62 showed the aggregate emission centered at ca. 530 nm, which is only 10 nm red-shifted than the push–pull diyne. Thus, it is possible to achieve highly red-shifted excimer emission from simple donor–acceptor diynes.

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Figure 3. Steady-state emission spectra of (a) Me2NPBPCN, (b) PBPCN and (c) PBPNMe2 in water (C = 1 x 10-6 M) (dark red, blue and olive green colors indicate resolved Raman scattering, LE and aggregate emissions, respectively). Time-resolved emission spectra of (d) Me2NPBPCN (λex = 370 nm), (e) PBPCN (λex = 340 nm) and (f) PBPNMe2 (λex = 370 nm) in water (instrument limited short decay components are ignored). Understanding the Aggregate Emission in Water: Concentration and Temperature Dependent Steady-State Fluorescence Studies. To understand the aggregate emission band of 10


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the diynes, concentration and temperature dependent steady-state fluorescence studies were carried out. The diyne Me2NPBPCN was chosen for the studies as PBPCN and PBPNMe2 have poor solubility in water at higher concentration. At very low concentration of Me2NPBPCN (such as 10-7 M), only the LE emission is observed (Figure 4a). On decreasing the concentration, the aggregate emission band of Me2NPBPCN at ca. 520 nm disappears with an appearance of the blue-shifted LE emission (Figure 4b). As temperature is increased, the emission band at ca. 520 nm diminishes with a gradual appearance of the blue-shifted LE emission (Figures 4c,d).

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Figure 4. (a) Concentration dependent steady-state emission spectra and (b) fluorescence intensity vs concentration plot of Me2NPBPCN. (c) Temperature dependent steady-state emission spectra (C = 1 x 10-5 M) and (d) fluorescence intensity (C = 1 x 10-5 M) vs temperature plot of Me2NPBPCN. (Dark red color dotted ellipses indicate Raman scattering). 11



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Multiple Emissions from LE, Excimer and ICT States in Mixed-Aqueous Media: SteadyState and Time-Resolved Fluorescence Studies. When CH3CN is added to the diynes in water with increasing proportion, vibronic structures start appearing in the absorption spectra with an increase of the absorbance (see Figure S4, Supporting Information). This suggests that CH3CN helps in solubilizing the aggregates. In the steady-state emission spectra (Figures 5a–c), it is noted that the aggregate emission band of all of the fluorophores diminishes with an appearance of the LE emission on gradual addition of CH3CN in water. In 100% CH3CN, both the LE and ICT emissions are observed for the derivatives.26 Me2NPBPCN shows the highest excimer emission intensity at 10% CH3CN in water. The excimer starts breaking at 50% CH3CN in water and the LE emission intensity grows up from 50% to 70% CH3CN in water. The LE emission intensity becomes highest at 80% CH3CN in water and then starts decreasing from 90% to 100% CH3CN (Figure 5a). Similarly, PBPCN exhibits the highest excimer emission intensity at 10% CH3CN in water. The excimer emission starts disappearing at 20% CH3CN in water and then the LE emission starts appearing with a gradual increase of intensity from 20% to 80% CH3CN in water with the highest intensity at 90% CH3CN in water. The LE emission intensity then decreases from 90% to 100% CH3CN (Figure 5b). In case of PBPNMe2, the excimer emission is closely overlapped with the LE emission and the excimer emission is traceable only through time-resolved fluorescence decay experiment at different emission wavelengths, discussed in later part. The LE emission gets intensified from 40% to 80% CH3CN in water with the maximum intensity at 80% CH3CN in water (Figure 5c). The LE emission intensity decreases from 90% to 100% CH3CN with a broad emission spectrum at 100% CH3CN, composed of the LE and ICT emission bands (understood from time-resolved fluorescence decay experiments, discussed in later section). Thus, after disaggregation, the diphenylbutadiynyl fluorophores are 12


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better solubilized on gradual addition of CH3CN exhibiting the enhanced LE emission intensity. At higher percentage of CH3CN (90%–100% CH3CN), the charge transfer (CT) phenomenon probably plays a crucial role in the overall decrease of the emission intensity. The aggregate emission maximum of Me2NPBPCN is 43 nm red-shifted than the aggregate emission maximum of PBPCN in 10% CH3CN in water (see Figure S5, Supporting Information). The LE, excimer and ICT emissions are clearly observed and distinguishable for the push–pull diyne Me2NPBPCN (normalized spectra are shown in Figure 5d), whereas the ICT and excimer emissions are not apparently observed in the emission spectra of PBPCN (normalized LE and excimer emission spectra are shown in Figure 5e) and PBPNMe2 (normalized LE and ICT emission spectra are shown in Figure 5f), respectively. The LE, excimer, and ICT emissions of the diynes in mixed-aqueous media were compared with the emission spectra of the diynes in cyclohexane, solid powder, and N,N-dimethylformamide (DMF), respectively (Figure 5g for Me2NPBPCN, Figure 5h for PBPCN, Figure 5i for PBPNMe2) and both set of the spectra matched well with each other.

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650

Wavelength (nm)

Figure 5. Steady-state emission spectra of (a) Me2NPBPCN, (b) PBPCN, and (c) PBPNMe2 in mixed-aqueous media (C = 1 x 10-6 M, dark red color dotted ellipses indicate Raman scattering). Normalized steady-state emission spectra of (d) Me2NPBPCN in 80%, 10% and 100% CH3CNwater, (e) PBPCN in 90% and 10% CH3CN-water, and (f) PBPNMe2 in 80% and 100% CH3CN-water. Normalized steady-state emission spectra of (g) Me2NPBPCN in cyclohexane, solid powder, and DMF, (h) PBPCN in cyclohexane and solid powder, and (i) PBPNMe2 in cyclohexane and DMF. In order to better understand the LE, excimer and ICT emissions, time-resolved fluorescence decay experiments were carried out in mixed-aqueous solvents. It is noted that the decay curves (Figures 6a–c) as well as the relative contribution of the emissive states towards total emission 14


Page 15 of 34

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vary in different proportions of CH3CN-water mixtures (Figures 6d–f). In case of the donor– acceptor diyne Me2NPBPCN, in 80% CH3CN-water the relative contribution of the LE emission towards the total emission is dominant (90%) (Figure 6d), which is in accordance with the predominance steady-state fluorescence signal in 80% CH3CN-water (Figure 5d, pink color). However, a relatively small emission contribution (10%) is noted from the ICT state (Figure 6d, first panel). In 10% CH3CN-water solution of Me2NPBPCN, the relative emission contribution (91%) of the aggregate state predominates over that of the ICT state (9%) (Figure 6d, second panel). However, the relative emission contribution of the ICT state towards the total emission is more (87%) over the LE state (13%) in 100% CH3CN solution of Me2NPBPCN (Figure 6d, third panel). In cases of PBPCN and PBPNMe2, the relative emission contribution of the LE state predominates in 90% (Figure 6e, first panel) and 80% (Figure 6f, first panel) CH3CN-water mixtures, respectively and the aggregate state emission contributes more in 10% CH3CN-water (Figures 6e,f, second panel). However, the ICT emission contributes significantly along with the aggregate emission in 10% CH3CN-water mixture of PBPCN and PBPNMe2. In 100% CH3CN of PBPNMe2, the contribution of the ICT emission towards the total emission is although predominant over the LE emission (Figure 6f, third panel); both the LE and ICT emissions almost contribute equally for PBPCN (Figure 6e, third panel). This suggests that the ICT character of PBPNMe2 in 100% CH3CN is stronger than PBPCN. Thus, the time-resolved fluorescence experiments suggest that the observed fluorescence in the steady-state emission spectra of the diynes does not originate from a single emitting species; rather it comes from multiple emitting states and the emissions are mixed up together. The relative emission contribution of the individual emitting states is controlled through mixing different proportions of CH3CN in aqueous solution of the diynes. 15



60

80

120

140

10% CH3CN-water (λem = 525 nm)

100

80

10 20

40

60

80

100

Time (ns)

(d)

10 0 90

10% CH3CN-water

80 70 60 50

Aggregate state

40 30 20 10 0 90

ICT state

100% CH3CN

80 70 60 50

ICT state

40 30

30 10

10% CH3CN-water

80 70 60 50

Aggregate state

40 30 20

ICT state

10 0 90

100% CH3CN

80 70

Count

20

40

60

80

100

Time (ns)

80% CH3CN-water

90

ICT state

60 50 40

LE state

30

60 50 40 30

LE state

20 10 0 90

10% CH3CN-water

80 70 60 50

Aggregate state

40 30 20

ICT state

10 0 90

100% CH3CN

80 70 60 50

ICT state

40 30

20

20

20

10

10

10

LE state


70

LE state

20 0 90

120

80

ICT state

40

90

Time (ns)

(f)

60 50

100

80

Relative contribution towards total emission(%)

20


30

Time(ns)

60

70

LE state

60


10 40

80

ICT state


40

400

90% CH3CN-water

90

70 50

320

(e)

80 60

10 240


20

80% CH3CN-water

90

160

Time (ns) 90% CH3CN-water (λem = 425 nm)

100


Prompt

10

Prompt

10

100

Time (ns)

(λem = 600 nm)

100

Prompt

Prompt

1000

ICT state

40

100% CH3CN 1000

100

Prompt

10

Count

1000

100

(c)

100% CH3CN (λem = 520 nm)

1000

(λ em = 600 nm)

Count

Count

1000

Count

(b)

100% CH3CN 1000

Count

(a)

pt om Pr

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

Page 16 of 34

LE state

0 0.0 0.8 1.6 2.4 3.2 4.0 4.8 5.6 6.4 7.2 8.0

0 0.0 0.8 1.6 2.4 3.2 4.0 4.8 5.6 6.4 7.2 8.0

0 0.0 0.8 1.6 2.4 3.2 4.0 4.8 5.6 6.4 7.2 8.0

Excited state lifetime (ns)



Figure 6. Time-resolved fluorescence decay curves of (a) Me2NPBPCN (λex = 370 nm), (b) PBPCN (λex = 340 nm), and (c) PBPNMe2 (λex = 370 nm) in different CH3CN-water mixtures (the instrument limited short decay components are ignored and in case of 10% CH3CN-water, the scattering corrected decay curves are presented). Histograms of relative contribution of individual emitting states towards total emission versus excited state lifetime of those states for (d) Me2NPBPCN, (e) PBPCN, and (f) PBPNMe2 in CH3CN-water mixtures shown in upper panel. Understanding the ICT Emission through Curve Fitting of Steady-State Fluorescence Spectra in Mixed-Aqueous Media. From the time-resolved fluorescence decay experiments 16


Page 17 of 34

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discussed in Figures 6d–f, it is noted that the ICT emission appears in all of the proportions of CH3CN-water mixtures. However, when the percentage of CH3CN is more (for example, 90– 100% CH3CN-water mixture), the ICT emission predominates. In 50–80% CH3CN-water, the ICT emission is prevalent along with the dominant LE emission. On the other hand, when water percentage is high (for example, 10–40% CH3CN-water system), although the aggregate emission dominates, some contribution of the ICT emission is noted along with the aggregate emission. The hydrophobicity of the organic diyne molecules in water dominates over the dielectric constant of solvent water, resulting in a predominant aggregate emission over the ICT emission. Thus, to better understand the ICT emission in the mixed-aqueous solvents, particularly in 70–100% CH3CN in water (in which the ICT emission is mixed up with the LE emission), the steady-state fluorescence spectra of the diynes were fitted with multiple Gaussian/Lorentzian functions (Figures 7a–c). The longest wavelength resolved emission spectra of the derivatives were found to be red-shifted with increasing the percentage of CH3CN in water (Figures 7d–f). This suggests that the band is of ICT character, which is favored by the highly polar and ‘good’ solvent CH3CN molecules. The longest wavelength resolved emission spectrum of Me2NPBPCN containing a strong donor (-NMe2) and a strong acceptor (-CN) substituent experiences a red-shift of 130 nm from 70% to 100% CH3CN in water (Figure 7d). The diyne PBPCN which has a strong acceptor substituent (-CN) and a neutral donor (-Ph) shows only 18 nm red-shift of the resolved emission spectra from 70% to 100% CH3CN-water (Figure 7e). The derivative PBPNMe2, which possesses a strong donor (-NMe2) and a neutral acceptor (-Ph) exhibits a red shift of 68 nm on moving from 70% to 100% CH3CN-water (Figure 7f). Thus it is understood that the ICT character of the diynes follows the order as Me2NPBPCN> PBPNMe2> PBPCN. 17



5

2.4x10

5

8.0x10 6.0x10

5

1.8x10

80% CH3CN-water

5

5

4.0x10

5

1.2x10

5

2.0x10

4

6.0x10

0.0

0.0

5

90% CH3CN-water

5x10

5

4x10

4

100% CH3CN

8.0x10

4

6.0x10

5

3x10

4

4.0x10

5

2x10

4

2.0x10

5

1x10

0

0.0 400 450 500 550 600 650 700

(c)

5

2.5x10

70% CH3CN-water

5

2.0x10

5

5

1.0x10

5

1.2x10

4

4

5.0x10

6.0x10

0.0 5 5x10

90% CH3CN-water

5

4x10

4

9.0x10

5

6.0x10

5

3.0x10

3x10

4

2x10

4

1x10

0

0.0 350

400

0.6 0.4 0.2 0.0 450

500

550

600

Wavellength (nm)

450

500

550

600

350

650

700

(e) Normalized fluo. intensity

in water 70% 80% 90% 100%

400

100% CH3CN

5

1.2x10

5

400

450

500

550

600

1.28x10

4

5

1.5x10 4

6.40x10

5

1.0x10

4

3.20x10

4

5.0x10

0.00

0.0

5

4

70% 80% 90% 100%

0.6 0.4 0.2

400

450

100% CH3CN

1.2x10

4

9.0x10

9.0x10

4

6.0x10

4

3.0x10

4

6.0x10

4

3.0x10

0.0 350

400

450

500

550

600

650

0.0 350

400

450

(f)

in water

0.8

0.0 350

5

90% CH3CN-water

1.2x10

500

550

600

650

Wavelength (nm) % of CH3CN

1.0

80% CH3CN-water

5

2.0x10

9.60x10

Wavelength (nm) % of CH3CN

0.8

0.05 1.5x10

5

2.5x10

70% CH3CN-water

5

1.8x10

Wavelength (nm)

1.0

80% CH3CN-water

5

2.4x10

5

1.5x10

400 450 500 550 600 650 700

(d)

5

3.0x10

500

550

Wavelength (nm)

Normalized fluo. itensity


70% CH3CN-water

5

3.0x10


(b)

5

3.6x10


(a)


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

Page 18 of 34

% of CH3CN

1.0

in water 70% 80% 90% 100%

0.8 0.6 0.4 0.2 0.0 400

450

500

550

600

650

Wavelength (nm)

Figure 7. Gaussian/Lorentzian curve fitted steady-state fluorescence spectra of (a) Me2NPBPCN, (b) PBPCN, and (c) PBPNMe2 in 70–100% CH3CN-water mixtures (C = 1 x 106

M; violet and green colors indicate Raman scattering and LE emissions, respectively). The

normalized longest wavelength resolved emission spectra of (d) Me2NPBPCN, (e) PBPCN, and (f) PBPNMe2 in 70–100% CH3CN-water mixtures. Time-Resolved Fluorescence Decay Studies at the LE Emission Wavelengths. To shed light into the LE state emission in mixed-aqueous solvents, time-resolved fluorescence decay experiments were carried out at the LE emission region (ca. 420–470 nm) (Figure 8). As the content of CH3CN is increased, the longer lifetime component (ca. 5 ns) corresponding to the aggregate state disappeared and the lifetime of the LE component increases, in addition to the presence of the ICT component (ca. 0.5 ns). To illustrate, in case of Me2NPBPCN, at 20% CH3CN in water, the LE component with lifetime 1.3 ns and the aggregate component with lifetime 4.6 ns are observed at λem = 465 nm. When the percentage of CH3CN is increased, such as 60% CH3CN in water, the ICT state with 0.5 ns and the LE state with lifetime 1.7 ns are noted. The disappearance of the aggregate state lifetime suggests the solubilization of the diyne 18


Page 19 of 34

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in highly polar and good solvent CH3CN. In 100% CH3CN, the lifetime of the LE state gets decreased (1.1 ns) along with the presence of the ICT component with lifetime 0.6 ns (Figure 7a). Such fluorescence decay behaviour was also noted for PBPCN (Figure 8b) and PBPNMe2 (Figure 8c). Thus, it is observed that (i) the aggregate state lifetime component (ca. 5 ns) disappears on addition of around 10–30% CH3CN in water and (ii) the ICT state lifetime component (ca. 0.5 ns) is prevalent along with the LE state component in 40–100% CH3CN in water. Had the ICT state been formed directly from the LE state, the ICT state emission would have been predominant on increasing the CH3CN percentage (40–80% CH3CN) instead of the dominance of the LE state observed in Figures 5a–c. This hints at the possibility of the presence of planar and twisted ground state rotamers for the diphenylbutadiynes, where the planar form leads to the LE emission and the twisted form leads to the ICT emission. Earlier work by Thulstrup et al.15 indicated the presence of ground state rotamers in diphenylbutadiyne.

19



(a)

λex = 370 nm

(b)

Prompt 20 % CH3CN in water

λem = 465 nm

1000

60 % CH3CN in water

Prompt 20 % CH3CN in water (τ1 = 0.5 ns, τ2 = 2.4 ns)

60 % CH3CN in water

Count

(τ1 = 0.5 ns, τ2 = 1.7 ns)

100% CH3CN (τ1 = 1.1 ns, τ2 = 0.6 ns)

100

λex = 340 nm λem = 425 nm

(τ1 = 1.3 ns, τ2 = 4.6 ns)

1000

Count

(τ1 = 1 ns, τ2 = 3.1 ns)

100% CH3CN

100

10

(τ1 = 0.6 ns, τ2 = 2.2 ns)

10 15

30

45

60

75

15

20

Time (ns)

(c)

25

30

Time (ns)

10000

λex = 370 nm λem = 450 nm

Prompt 20 % CH3CN in water (τ1 = 1.4ns, τ2 = 5.5 ns)

1000

60 % CH3CN in water (τ1 = 0.9 ns, τ2 = 2.9 ns)

Count

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

Page 20 of 34

100 % CH3CN (τ1 = 0.5 ns, τ2 = 1.2 ns)

100

110

220

330

440

550

Time (ns)

Figure 8. Time-resolved fluorescence decay curves of (a) Me2NPBPCN, (b) PBPCN, and (c) PBPNMe2 at shorter emission wavelengths in mixed-aqueous solvents (instrument limited short decay components are ignored). Application to Single Component White Light Emission through Usage of the Multiple Emissive States. In earlier sections, it was noted that (i) the ICT emission of the donor–acceptor diyne Me2NPBPCN dominates at higher percentage of CH3CN (90–100%) with a small contribution from the LE state and (ii) there might be ground state twisted and planar forms contributing to the CT and LE emissions, respectively. Thus, it was anticipated that the push– pull diyne Me2NPBPCN could form an intermolecular CT complex on increasing its concentration such that the relative intensity of the shorter wavelength LE and the longer 20


Page 21 of 34

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wavelength CT emission bands could be controlled. The LE and ICT emissions of Me2NPBPCN span over ca. 400–500 and 500–740 nm, respectively in CH3CN covering almost the entire visible region. Thus, by balancing the LE and CT emission intensities, it is possible to adjust commission internationale de l'éclairage (CIE) index of the entire emission to generate white light (ideally, CIE 0.33, 0.33). As the concentration of Me2NPBPCN is increased, the fluorescence intensity of the longer wavelength CT emission band increases much higher than the shorter wavelength LE emission band (Figure 9a). This is clearly evident from Figure 9b, which shows that the fluorescence intensity at 600 nm (I600) and the ratio of the fluorescence intensities (I600/I450) keep on increasing at the higher concentrations. This suggests that the longer wavelength ICT emission accompanies with intermolecular CT emission. Such intermolecular CT emission, which is commonly termed as ‘exciplex’ emission,63 arises because of the strong push–pull nature of the butadiynyl derivative Me2NPBPCN. In order to shed light on to the exciplex formation, steady-state absorption (Figures 9c,d) and time-resolved fluorescence decay (see Figure S6, Supporting Information) experiments were carried out. The absorption spectra, depicted in Figure 9c, show that the S0→S1 absorption band intensity increases on increasing the fluorophore concentration, which is also reflected in the plot of the ratio of absorbances (A390/A290) versus fluorophore concentration (Figure 9d). The enhancement of the S0→S1 band absorbance on increasing the fluorophore concentration suggests that the intermolecular CT complex is formed in the ground state (static exciplex).63 Time-resolved fluorescence decay experiments at the longer emission wavelength (λem = 600 nm) show that on increasing the concentration of the fluorophore the lifetime of the CT state remains unchanged (~0.5 ns, see Figure S6, Supporting Information), which conforms to the formation of the static exciplex. The fluorescence excitation spectra at the shorter and longer emission wavelengths show two 21



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

Page 22 of 34

distinctively different spectra (Figure 9e) suggesting two different excitation pathways. This agrees with the fact that the exciplex, which is formed, is static in nature. The structured fluorescence excitation profile corresponds to the LE transition and the red-shifted fluorescence excitation spectrum belongs to the exciplex emission. Pleasingly, it was possible to obtain pure white light emission (WLE) in CH3CN solution of Me2NPBPCN (C ~4 µM) with the CIE value (0.33, 0.32) (Figure 9f). Thus, a fine balance of the LE and exciplex emissions is essential for the generation of pure WLE from the donor–acceptor diphenylbutadiyne Me2NPBPCN. A tuning of the color-coordinates is possible through a control of the fluorophore concentration (see Figure S7, Supporting Information). The derivative PBPNMe2, which does not possesses strong push– pull character, shows bluish white light emission in CH3CN with CIE (0.25, 0.31) (see Figure S8, Supporting Information). The LE and ICT emissions of PBPNMe2 are being closely overlapped with each other in CH3CN, the emission spectrum does not spread over the whole visible region and thus, the dye is incapable of showing the pure white light. Similarly, the PBPCN, which is having a strong acceptor (-CN) and a neutral donor (-Ph), does not satisfy the spectral requirements for the WLE. A pH dependent white light emission through controlled intermolecular CT was earlier described by Park et al. in an amino trimethylammonium oligophenylene vinylene derivative.43 The intermolecular CT phenomenon in a pyridinium derivative was used by Jin et al. to generate white light in solid state.55 The present study reports the usage of the LE and intermolecular CT (exciplex) emissions in the generation of the WLE from a simple donor–acceptor diyne.

22


(a)

5

5x10

5

4x10

5

3x10


5

6x10

λex = 370 nm

Probe concentration -6 1 x 10 M -6 2 x 10 M -6 4 x 10 M -5 1 x 10 M -5 2 x 10 M -5 4 x 10 M

5

2x10

5

1x10

0 400

450

500

550

600

650

5

6x10

(b)

5

5x10

8 Fluo. intensity at 600 nm (I600)

7

Ratio of fluo. intensities

6

5

4x10

(I600/I450)

5

5

3x10

4 3

5

2x10

2 5

1x10

1

0

700

Ratio of fluo. intensities

0 0

10

20

30

40 -6

Concentration of Me2NPBPCN (x 10 M)

Wavelength (nm) 0.5 Probe concentration -6 1 x 10 M -6 2 x 10 M -6 4 x 10 M -5 1 x 10 M -5 2 x 10 M -5 4 x 10 M

Absorbance

0.4 0.3 0.2 0.1 0.0

0.8

0.6

300

350

400

450

500

0

Wavelength (nm) 6

Ratio of absorbances (A390/A290)

1.0

0.4

250

3.0x10

(d)

1.2

Ratio of absorbances

(c)

0.9

(e)

λem = 440 nm

0.8

λem = 600 nm

6

2.5x10

10

20

30

40

Concentration of Me2NPBPCN (x 10-6 M)

(f)

0.7 0.6

6

2.0x10

0.5 6

Y


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



Page 23 of 34

1.5x10 1.0x10

0.2

5

5.0x10

0.0 280

(0.33, 0.32)

0.4 0.3

6

0.1 0.0 320

360

400

440

480

0.0

0.1

0.2

Wavelength (nm)

0.3

0.4

0.5

0.6

0.7

0.8

X

Figure 9. (a) Concentration dependent steady-state fluorescence spectra of Me2NPBPCN in CH3CN (λex = 370 nm), (b) plot of fluorescence intensity versus concentration of Me2NPBPCN, (c) concentration dependent steady-state absorption spectra of Me2NPBPCN in CH3CN, (d) plot of ratios of absorbances versus concentration of Me2NPBPCN, (e) steady-state fluorescence excitation spectra of Me2NPBPCN (C = 2 x 10-5 M) in CH3CN, and (f) CIE chromaticity diagram of Me2NPBPCN in CH3CN at C = 4 x 10-6 M. 23



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Page 24 of 34

Conclusions In summary, we have studied in detail, the emissive states of donor–acceptor diphenylbutadiynes in water, CH3CN-water and CH3CN solvent systems and applications of those emissive states in generation of single component white light emission (WLE) in CH3CN. The important findings of the present study are: (1) The donor–acceptor diphenylbutadiynyl fluorophore exhibits aggregate emission (ca. 520 nm) in water. The aggregate emission is of excimer type. The excimer emission is characterized by a relatively longer lifetime component (ca. 5 ns), in comparison with the ca. 1 ns LE and 0.5 ns ICT lifetime components observed in non-aqueous solvents. The observed aggregates in water are static in nature, as evident from the absorption spectral changes (structureless and flat absorption band, in contrast to the structured band in non-aqueous solvents) and the absence of rise time in time-resolved fluorescence experiments. (2) Addition of CH3CN into the aqueous solution of the push–pull diphenylbutadiyne breaks the aggregates, which is evident from the appearance of the shorter wavelength LE emission and the concomitant decrease of the excimer fluorescence intensity. In mixed-aqueous solvents, in addition to the LE and excimer emissions, a contribution of ICT emission is noted. (3) In pure CH3CN, the fluorescence intensity of the CT emission increases on increasing the fluorophore concentration, indicating an exciplex formation. Time-resolved fluorescence decay experiments at the exciplex emission wavelength (λem 600 nm) show that the lifetime of the exciplex does not change with increasing the fluorophore concentration. This suggests that the exciplex is static in nature. Fluorescence excitation profile at the exciplex emission wavelength (λem 600 nm) is completely different from that of the LE one, which agrees with the fact that the exciplex state follows different excitation pathway than the LE one. 24


Page 25 of 34

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(4) Finally, this work brings out the photophysical importance of the push-pull diyne in generating single component white light. The intensities of the LE and exciplex emissions of the push–pull diphenylbutadiyne are suitably balanced in CH3CN to generate the pure WLE with a good CIE value (0.33, 0.32) through the control of the fluorophore concentration (4µM). 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). ACKNOWLEDGMENTS Authors thank DST, New Delhi for financial support. AKP thanks CSIR, New Delhi for a research fellowship. Supporting Information Supporting Information Available: A comparison of steady-state emission spectra of Me2NPBPCN in aqueous media and non-aqueous solvents, steady-state fluorescence spectra of PBPNMe2 in water at different excitations, control experiments for Raman scattering, steadystate absorption spectra of Me2NPBPCN, PBPCN, and PBPNMe2 in mixed-aqueous media, normalized steady-state emission spectra of the diphenylbutadiynes in 10% CH3CN in water, time-resolved fluorescence decay curves of Me2NPBPCN in CH3CN with varying concentration of the fluorophore, chromaticity diagram of Me2NPBPCN in CH3CN with varying concentration of the fluorophore, emission spectra and chromaticity diagram of PBPNMe2 in CH3CN at C = 1.2 x 10-5 M. This information is available free of charge via the Internet at http://pubs.acs.org 25



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