Enhanced Resonance Energy Transfer and White-Light Emission from

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Enhanced Resonance Energy Transfer (RET) and White Light Emission from Organic Fluorophores and Lanthanides in Dendron based Hybrid Hydrogel Prashant Kumar, Sivalingam Soumya, and Edamana Prasad ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00018 • Publication Date (Web): 08 Mar 2016 Downloaded from http://pubs.acs.org on March 8, 2016

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ACS Applied Materials & Interfaces

Enhanced Resonance Energy Transfer (RET) and

White

Light

Emission

from

Organic

Fluorophores and Lanthanides in Dendron based Hybrid Hydrogel Prashant Kumar, Sivalingam Soumya and Edamana Prasad* Department of Chemistry, Indian Institute of Technology Madras (IIT M), Chennai 600 036, India KEYWORDS: White light emitting gel, lanthanides, resonance energy transfer, poly (aryl ether) dendron, hybrid hydrogel.

ABSTRACT

In this paper, we have investigated the use of poly (aryl ether) dendron based gel as a medium for resonance energy transfer (RET) from organic donors (phenanthrene, naphthalene, and pyrene) to lanthanides {Eu(III) and Tb(III)} ions. The gel has been prepared through the selfassembly of glucose cored poly(aryl ether) dendrons in DMSO-Water mixture (1:9 v/v). The efficiency of RET was calculated by metal centered emission quantum yield measurements in the gel medium. While there was no resonance energy transfer observed between the donor-

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acceptors pairs in solution, efficient RET has been observed in the gel medium. The metal centered quantum yield values for phenanthrene-Eu(III), naphthalene-Eu(III) and pyrene-Eu(III) systems were 11.9%, 3.9% and 3.6% respectively. Partial RET in the system has been utilized to generate white light emission from the gel by incorporating an additional lanthanide ion {Tb(III)} along with the organic donors and Eu(III). The CIE (Commission Internationale d’ Eclairage) coordinates obtained for gels formed by phenanthrene-Tb(III)-Eu(III) {PTE}, naphthalene-Tb(III)-Eu(III) {NTE} and pyrene-Tb(III)-Eu(III) {PyTE} were {(0.33,0.32) (0.35,0.37) and (0.35,0.33), respectively. The correlated colour temperature (CCT) for PTE, NTE and PyTE white light emitting gels were calculated and the values (5520 K, 4886 K and 4722 K, respectively) suggest that the above mentioned system generate cool white light. Introduction Luminescence from lanthanide ions through Resonance Energy Transfer (RET) has gained remarkable attention due to its potential applications in optoelectronics, cellular bio-imaging, and bioanalytical fields.1-3 In order to use lanthanide emission through RET, it is of paramount importance to stabilize the long lived excited state of lanthanides.4-7 While the excited state lifetime of lanthanides is in milliseconds, it can be easily quenched in solution phase due to the frequent solvent collisions and non-radiative decay processes through vibronic coupling with solvent molecules.8-13 One of the ways to overcome these hurdles is to place lanthanide ion in a gel medium. Gel medium enhances luminescence from lanthanide ions by, (i) rigidifying the donor-acceptor pairs with appropriate orientations of the dipoles which enhances RET, and (ii) significantly reducing the non-radiative decay due to vibronic coupling by replacing the solvent molecules by the gelator ligands.14,15


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While a large body of the initial work regarding lanthanide based RET has been carried out in inorganic gels, recent years have witnessed the use of hydrogels and organogels as a medium for lanthanide based RET.16 For example, RET from terpyridine derivatives to trivalent europium in a gel phase, where the gel is formed by the self-assembly of sodium salt of N-adamantyl-1,ωamino-11-undecanoic acid and N,N’-bis(O-methyl-S-tyrosine) oxalamide, has been reported.15 Lanthanide assisted energy transfer involving terpyridine europium complex in poly(N,Ndimethylacrylamide) based hydrogel has also been reported.17 In a different attempt, Zhang et al. have studied RET from quinolinate complex to various lanthanide ions in a xerogel.18 Recently, Maitra et al. have designed a europium cholate based hybrid hydrogel in which europium ion was sensitized by pyrene.19 We have initiated the use of poly (aryl ether) dendron derivative as a gel medium for lanthanide luminescence by RET. There are a number of reasons for using poly(aryl ether) dendron based hydrogel to study lanthanide luminescence by RET; (i) poly(aryl ether) dendron based low molecular weight gelators (LMWGs) have potential applications in optoelectronics;2022

(ii) the solubility of poly(aryl ether) dendron derivatives in polar solvents facilitates the

formation of hybrid hydrogels containing lanthanide ions; and (iii) poly(aryl ether) based gels are highly stable, both mechanically (modulus value ~105 Pa) as well as thermally (Tgel~80 0C) .23,24 Furthermore, poly(aryl ether) dendron derivative has functional groups which can complex with lanthanide ions, which provide more insight to the positioning of the metal ion in the gel matrix, which otherwise remains obscure. Initially, the resonance energy transfer between polycyclic aromatic hydrocarbons (pyrene, naphthalene and phenanthrene) as donors and Eu(III) as acceptor was investigated in the gel medium formed by the self-assembly of glucose cored poly(aryl ether) dendrons. The rate of


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energy transfer in the hybrid gel {poly (aryl ether) dendron derivative+ polycyclic aromatic hydrocarbon+ lanthanide ion} has been determined and the quantum yield of the metal centered luminescence has been determined. We hypothesized that the robust gelation properties of the poly(aryl ether) dendron derivative along with its ability to coordinate lanthanide ions would largely enhance the efficiency of RET. We have also generated white light emission through partial resonance energy transfer between organic donors and lanthanide ions {Eu(III) and Tb(III)}.The CIE coordinate values obtained for the white light emission were (0.33, 0.32), (0.35, 0.37) and (0.35, 0.33). Photophysical aspects of the various gel systems have been investigated systematically using steady state and time resolved experiments and the role of dendron based gel as a conducive medium for RET has been demonstrated. Experimental Section Materials: The glucose cored poly aryl ether dendron was synthesized using a reported procedure.24

Phenanthrene,

pyrene,

naphthalene,

europium{Eu(III)}

acetate

and

terbium{Tb(III)} acetate were purchased from Sigma Aldrich (USA), Central drug house, and Spectrochem Chemicals

Pvt. Ltd. All purchased materials were used without further

purification, unless otherwise stated. Instrumentation: 1H, 13C NMR data were taken by Bruker 400 MHz spectrometer (1H, 400MHz 13

C, 100 MHz). Mass spectra were recorded using a Micromass Q-TOF mass spectrometer. FT-

IR spectra were recorded using JascoV-660 spectrophotometer at room temperature. The UVVis absorption studies were carried out using a Jasco V-660 spectrophotometer. Fluorescence experiments were carried out on a Horiba Jobin Yvon Fluoromax-4 fluorescence spectrofluorometer. Lifetime measurements were carried out by the time correlated single photon counting technique (TCSPC) with a micro channel plate photomultiplier tube (MCP-PMT) as the


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detector and LED as the excitation source. Rheology experiments were carried out by Antorn Par Rheometer (MCR 102). Synthesis of Hydrogel: Europium based hybrid hydrogel was obtained by mixing donors with glucose cored poly(aryl ether) dendron in DMSO-water (1:9 v/v) mixture, followed by the addition of europium{Eu(III)} acetate. In a typical procedure, phenanthrene (0.2 mg, 0.0011 mmole) was taken in a vial along with glucose cored poly(aryl ether) dendron (1 mg, 0.0016 mmole) and europium {Eu(III)} acetate (5.4 mg, 0.0164 mmole). To this solid mixture, DMSOwater (1:9 v/v) was added and the mixture was sonicated for three minutes. After sonication, the mixture was kept at room temperature for an hour, by which the whole mass was turned to a gel. The hydrogel formation was confirmed by the inverted vial method. This gel is highly stable at room temperature without any solvent leakage, up to 6 months. Quantum yield calculation: Photoluminescence quantum yield of the gel samples were carried out by calibrated integrating sphere in a SPEX Fluorolog spectrofluorimeter. The sample placed in the sphere was excited using a Xe arc with 365 nm as the excitation wavelength. The absolute quantum was calculated by de Mello method

25,26

using the following equations.

φPL =  Ei ( λ ) − (1 − A) E o (λ )  / Le (λ ) A ......(1)

A = [ Lo (λ) − Li (λ)] / Lo (λ) .......(2) where Ei(λ) and Eo(λ) are the integrated luminescence intensity as a result of

direct

excitation and secondary excitation of the sample, respectively. A is the optical density of sample, measured by using eq (2). Lo(λ), Li(λ) and Le(λ) are the integrated excitation intensity for direct excitation, secondary excitation and excitation profile for an empty sphere, respectively.


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Calculation of RET efficiency (E): The efficiency of energy transfer was quantified by quantum yield measurement for metal centered {Eu(III)} emission.27-32 Calculation of rate of energy transfer (k) and distance measurements: The rate of energy transfer (k) and distance (r) between donor and acceptor were calculated by the following equations, 27-32

=

=

(3)

/

(4)

where τD is the donor lifetime in the absence of acceptor and Ro is the Förster distance (distance at which RET efficiency is 50%), r is the distance between donor and acceptor, E is the efficiency of energy transfer, and k is the rate of energy transfer. The Förster distance (R0) was calculated using equation 5, = 0.211(

! J(#))

(5)

where, κ2 is the orientation factor (the value of κ2 is taken as 2/3 assuming random orientation of the donor and acceptor molecules), n is the refractive index of the medium (1.34 for DMSO: water mixture)33 and J(λ) is the spectral overlap. QD is the quantum yield of the donor in absence of acceptor. The calculation of spectral overlap integral is carried out using the following equation, '

J(#) = % &! (#)() (#)# *(#) =

,

% + (-)./ (-)-0 1,

% + (-)1-

(6)

where, FD (λ) is the corrected fluorescence intensity of the donor in the wavelength range λ to λ+∆λ with the total intensity (area under the curve) normalized to unity, εA is molar extinction coefficient of the acceptor. The spectral overlap was found to be significant in the case of pyrene-Eu(III) system, compared to naphthalene-Eu(III) and phenanthrene-Eu(III) systems (Figure S1).


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Results and discussion Poly aryl ether based dendron systems have high propensity to aggregate in a variety of solvents.34,35 In the present study, a glucose attached poly (aryl ether) dendron has been utilized and the gelation propensity of the gel was mainly due to non-covalent forces such as H-bonding and π-π stacking. The structure of the gelator, along with the organic donor molecules used in the present study is depicted in Chart 1.

H

I

H HO O

N H

OH

OH H OH

H OH

N

II O

O O

L III Chart 1.Structure of the energy donors {(I) naphthalene (II) phenanthrene (III) pyrene} and the gelator (poly (aryl ether) dendron with glucose unit appended (L). The critical gel concentration of poly (aryl ether) dendron derivative was found to be 1mg/mL. The organic donor molecules, Eu(III) ion, and glucose cored poly (aryl ether) dendron are dissolved in DMSO-Water mixture (1:9 v/v). The poly aryl ether dendron derivative forms stable gel in presence of the donor-acceptor pairs. The morphology of the gel was examined through scanning electron microscopy (SEM). Figure 1 shows the SEM image of the gel in presence and absence of the metal ion {Eu(III)}, along with the donor molecules. Gelator shows fibrellar type morphology, with width in the range of 200-550 nm, in presence and absence of the metal ion.


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The morphology of the gel remains unchanged after the addition of the metal ion, which indicates that the interaction between the gel fibres and the metal ions does not perturb the selfassembly of the gelator molecules. Similarly, we have carried out the morphological investigation of phenanthrene and naphthalene incorporated glucose cored poly(aryl ether) dendron in presence and absence of the metal ion and no change in the morphology was observed after the addition of the metal ion (Figure S2).

Figure 1. SEM images of the xerogel with pyrene as the donor molecule; a) in absence and b) in presence of Eu(III). The images represent similar morphologies, indicates that the metal ion does not perturb the self-assembly. In order to understand the molecular level interaction between the metal ion and the gelator, FT-IR spectrum of the xerogel was recorded at room temperature. Figure 2a shows the FT-IR spectra of the xerogel in presence and absence of the metal ion{Eu(III)}. In the absence of the metal ion, the gelator shows characteristic peaks representing imine and amide stretching vibrations at 1587 cm-1 and 1646 cm-1, respectively.24,36 However, the peaks were shifted to 1573


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Transmittance(%)

a)

b)

90 -1

1587 cm

-1

1646cm

60 -1

1639 cm

30 -1

1573 cm

0

1200

1400

1600

-1

1800

Wavenumber(cm )

c)

Dendron Glucose Europium Acylhydrazone unit

29Å Figure 2. a) FT-IR spectra of L in the absence (red line) and presence (wine line) of the metal ion {Eu(III)}; b) Possible coordination of the metal ion with gelator; c) Proposed structure of self -assembly in the hybrid gel. cm-1 and 1639 cm-1, respectively, upon addition of the metal ion. This suggests that Eu(III) is coordinated through the imine and amide bond of the gelator. The propensity of gelation in the present system is regulated by non-covalent interactions like ππ stacking and H- bonding. The mechanism of gelation in glucose cored poly (aryl ether) dendron has been studied in detail and reported in our previous work.24 Results from powder XRD analysis of the xerogel substantiate the previous results that the self-assembled gelator molecules are ordered in a lamellar pattern, with an interdigitated fashion with respect to the sugar moiety. The structural features of the self-assembled system in the presence and absence of


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the metal ion has been analyzed using small-angle X-ray diffraction (XRD). The end to end distance of the self-assembled dimer molecule, before and after addition of the metal ion, was found to be 31Å (2θ=2.850) and 29Å (2θ=3.0420), respectively (Figure S3). The slight decrease in the length in presence of the metal ion suggests that the metal ion co-ordinates with the gelator, as indicated by the FT-IR studies. Geometry optimization of dendritic glucose with and without the metal ion was carried out via molecular mechanics simulations (MM+) using Hyperchem professional software package. The length of the optimized structure of metal incorporated dendritic glucose dimer was found to be 29.8 Å (Figure S4), which was closely matching from the experimentally observed d value (~29 Å). This indicates that metal ion incorporation does not alter the interdigitated structure of the lamellar arrangement. Based on the above results, a schematic representation of the metal coordination and self-assembly of the dendritic gel is shown in Figure 2 (b) and (c). The structural features of the self-assembled system in the presence and absence of the metal ion has been analyzed using small-angle X-ray diffraction (XRD). The end to end distance of the self-assembled dimer molecule, before and after addition of the metal ion, was found to be 31Å (2θ=2.850) and 29Å (2θ=3.0420), respectively (Figure S3). The slight decrease in the length in presence of the metal ion suggests that the metal ion co-ordinates with the gelator, as indicated by the FT-IR studies. Geometry optimization of dendritic glucose with and without the metal ion was carried out via molecular mechanics simulations (MM+) using Hyperchem professional software package. The length of the optimized structure of dendritic glucose dimer was found to be 33.08 Å, in the absence of the metal ion. The value is slightly higher than the experimentally obtained value (31 Å), which indicates that the dendritic glucose molecules self-assemble in an interdigitated fashion. The length of the optimized structure of metal incorporated dendritic


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glucose dimer was found to be 29.8 Å (Figure S4), which was closely matching from the experimentally observed d value (~29 Å). Based on the above results, a schematic representation of the metal coordination and self-assembly of the dendritic gel is shown in Figure 2 (b) and (c). Glucose cored poly (aryl ether) dendron shows thermo reversibility in presence and absence of metal ion{Eu(III). However, the gel melting temperature (Tgel) of the system increases from 60 0

C to 80 0C, upon incorporation of the metal ions. This suggests that the incorporation of metal

into the gelator results in the increased mechanical strength of the materials due to additional molecular interactions between the carbonyl and imine functionalities of glucose based dendron and the metal ion. Further, rheological experiments have been conducted with the gel in the absence and presence of the metal ions. The plot of modulus values vs frequency indicate that the storage modulus (G') of the system increases from 18570 Pa to73447 Pa, after incorporating the metal ions (Figure S5). This also substantiates the argument that the stability of the system increases due to the metal ion incorporation. Figure 3 shows the absorption spectra of pyrene incorporated glucose cored poly(aryl ether) dendron {III-L} (10-3 M) in the absence and presence of Eu(III) (10-3 M) in the gel state. Poly aryl ether dendon Pyrene+poly aryl ether dendron Pyrene +poly aryl ether dendron +Eu(III)

1.0

Absorbance

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


0.8 0.6 0.4 0.2 0.0

300

400

500

Wavelength(nm)

Figure 3. Normalized UV-vis absorption spectra of pyrene (10-7 M) incorporated glucose cored poly(aryl ether){III-L} dendron (10-3M) in the absence and presence of the metal ion {Eu(III)}{[Eu(III) =10-3 M]} in gel.


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The band at 260 nm is due to the absorption of benzene rings present in the ligand. After the incorporation of pyrene (10-7 M), an additional peak was observed in the 337 nm, which is attributed to the π-π* transition in pyrene moiety.37,38 After addition of the metal ion {Eu(III)} in the pyrene incorporated glucose cored poly(aryl ether) dendron, a new shoulder band was observed at 283 nm, which is presumably due to the complex formation between europium and poly (aryl ether) dendron (wine color line) (vide supra). Similarly, we have incorporated naphthalene {I-L} (Figure S6) or phenanthrene {II-L} (Figure S6) in the gel. In the case of I-L and II-L, additional bands were observed at 290 nm, and 347 nm, respectively.39,40 After addition of the metal ion {Eu(III)} ion in the system, there was an additional shoulder band at 297 nm in both the systems. Figure 4(a) shows the emission spectra of III-L in the gel state, upon excitation at 337 nm. Since the emission from the gel has four maxima at 377, 395, 418 and 445 nm, the structured bands were attributed to pyrene monomer emission.41 The concentrations of the donor (10-7 M) and gelator molecules (10-3 M) were carefully chosen to avoid any aggregate formation of the donor molecules, which would decrease the energy transfer efficiency. Figures 4(b) and 4(c) show the emission spectra of I-L and II-L systems in the gel medium. As it is evident from the spectra, both naphthalene and phenanthrene exhibit significant monomer emission peaks.42-44 Since the emission spectra from the gel state resembles that of the monomer emission, it can be concluded that the donor molecules are not aggregated . The donor molecules are likely to be distributed in the hydrophobic part (i.e. benzyl ether moiety) of the gel fibers, due to their hydrophobic nature. Upon addition of the metal ions, a significant reduction of the emission intensity of the donor molecules was observed. Subsequently, a concomitant increase in the luminescence intensity from Eu(III) was observed, which is attributed to the resonance energy transfer. Three transitions


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(580, 593 and 616 nm) from Eu(III) were observed and they are assigned to 5D0-7F0, 5D0-7F1 and 5

D0-7F2 transitions, based on literature reports.45,46

Furthermore, we have carried out time resolved luminescence study for the above mentioned donor acceptor pairs. The fluorescence lifetime of the donor was quenched in presence of the acceptor, substantiating the resonance energy transfer process (Figure S7).

b) 50

15 12

Pyrene+Dendron Pyrene+Dendron+Eu(III)(3mM) Pyrene+Dendron+Eu(III)(6mM)

Intensity(a.u.)

a) Intensity(a.u.)

9 6 3 0 350

400

450

500

550

600

c)

Naphthalene+Dendron Naphthalene+Dendron+Eu(III)(3mM) Naphthalene+Dendron+Eu(III)(6mM)

40 30 20 10 0 300

Wavelength(nm)

Intensity(a.u.)

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


400

500 600

650

700

Wavelength(nm)

10 8

Phenanthrene+Dendron Phenanthrene+Dendron+Eu(III)(3mM) Phenanthrene+Dendron+Eu(III)(6mM)

6 4 2 0

400 450 500 550 600 650

Wavelength(nm) Figure 4. Steady state emission spectra of a) pyrene (λexc= 337nm) (10-7 M); b) naphthalene (λexc= 290 nm) (10-4 M); and c) phenanthrene (λexc=347 nm) (10-7 M) in the gel phase in absence and presence of Eu(III). [Eu(III)]=3mM and 6mM. Among the three donor-acceptor systems, significant spectral overlap was observed only in the case of pyrene-Eu(III) system{III-L} (Figure S1). The overlap integral between donor (pyrene)


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emission and acceptor {Eu(III)} absorption was determined, and Förster distance (Ro) was calculated using equation 5. The value of Förster distance was found to be 13.34 Å in this case. The efficiency and rate of the energy transfer were calculated using the experimentally determined metal centered quantum yield values. Using the Förster distance, the distance between the pyrene-Eu(III) was calculated and the value was found to be 23 Å. This is somewhat surprising since the total distance between two self -assembled dendron gelators was 29 Å and if the pyrene molecule is placed near to the benzene rings of the dendron unit, one would expect~14.5Å as the distance between pyrene and the metal ion. The discrepancy can be explained as follows: (i) the self-assembled structure shown in Figure 2 c might be perturbed in presence of donor molecules or (ii) presumably the energy transfer occurs between the donor to a metal center which is not placed near to the donor, due to lack of suitable orientation dipoles. Since there was no spectral overlap between the other donors (phenanthrene and naphthalene) and Eu(III), the distance between the donor-acceptor pairs was not calculated. The highest value for energy transfer efficiency (11.9%), and rate (3.4x106 sec-1) was obtained for phenanthreneEu(III){II-L}system. The efficiency of energy transfer in terms of metal centered quantum yield, and rate of energy transfer systems are summarized in Table 1. Table 1. Rate and efficiency of energy transfer between organic fluorophores (I, II,III) and lanthanide{Eu(III)}. S.No.

System

keta(in sec-1)

ϕPLb(%)

1

L+I+Eu(III)

9.0 x 105

3.9

2

L+II+Eu(III)

3.4 x 106

11.9

3

L+III+Eu(III)

3.0 x 105

3.6

a- rate of energy transfer, b-Absolute quantum yield


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We have carried out RET (resonance energy transfer) experiments in solution phase for the above mentioned donor acceptor pairs. After exciting the donor molecules {(pyrene λexc= 337 nm), (naphthalene λexc= 290 nm), and (phenanthrene λexc= 347 nm)}, sensitized emission form europium {Eu(III)}was not observed (Figure S8). This suggests that gel medium is essential in keeping the donor acceptor moieties at fixed distance and orient them with suitable geometrical positions for efficient resonance energy transfer. The present system exhibits superior properties in terms of metal centered emission quantum yield, and rate of energy transfer compared to the reported cases.40,47-50 Generation of white light emission: White light is a combination of three primary colors i.e. blue, red and green.51,52 Recently, white light emitting organogels based on lanthanides were prepared via partial resonance energy transfer phenomenon.53,54 As a result of partial RET, the systems mentioned in the paper can emit in the blue region due to the presence of organic fluorophores (pyrene, naphthalene, and phenanthrene), and red region due to the presence of Eu(III). Therefore, it is reasonable to propose that after the addition of materials which emits green color with desired intensity, white light can be generated in the above mentioned systems. Herein, we have chosen terbium{Tb(III)} as a green emitter. The triplet state of pyrene, naphthalene, and phenanthrene were 16,890 cm-1, 21,270 cm-1, and 21,730 cm-1, respectively.55,56 The emissive state of terbium{Tb(III)} was 20,430 cm-1.8 In the three systems studied, namely, pyrene-Tb(III)-Eu(III){PyTE}, naphthalene-Tb(III)-Eu(III){NTE}, and phenanthrene-Tb(III)Eu(III){PTE}, emission from both Eu(III) and Tb(III) has been observed. Since the triplet states of naphthalene and phenanthrene are higher than that of both Eu(III) and Tb(III), partial energy transfer can occur from the triplet state of the donors (naphthalene, phenanthrene) to Eu(III) and Tb(III) emissive states. However, the pyrene triplet state is lower than that of the Tb(III)


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emissive level. This suggests that the resonance energy transfer takes from the singlet state of pyrene to Tb(III). In addition to this, cascading energy transfer, i.e., from the donors (naphthalene, phenanthrene) to Tb(III), followed by a second energy transfer from Tb(III) to Eu(III) is equally possible in such three component systems.53 To generate the white light emission, we have taken different concentrations of donors and metal ions. Initially, the metal ions concentration was varied keeping the donor concentration intact and the optimized values for Eu(III) and Tb(III) were 10-4 M and 10-2 M, respectively. Next, the concentration of the donor molecule has been varied to obtain the best result for white light generation. The optimized value for the donor molecules are as follows: [pyrene] =10-8 M; [naphthalene] =10-5 M; [phenanthrene] =10-7 M. Figure 5(a) shows the fluorescence spectra from (PyTE, NTE, PTE) in which the donor molecules show the characteristic emission peak from 370-450 nm which covers the blue part of the electromagnetic spectra. Similarly terbium {Tb(III)} and europium {Eu(III)} emit in the green, red part of the spectra, respectively. Tb(III) shows characteristic emission peaks at 490, 545, 584, 621 nm while Eu(III) shows the characteristic emission peaks at 593 and 616 nm.53 After mixing of these components at a desired amount, we obtained CIE value (0.35, 0.33) for PyTE, which is very close to the ideal white light CIE values(0.33, 0.33) (Figure 5b). Similarly, we have obtained CIE values {0.35, 0.37},{0.33, 0.32} for NTE and PTE respectively (Figures 5c and 5d) . The CIE values obtained for identical systems in solution phase were largely away from the white light region (Figure S9), corroborating our previous hypothesis that gel medium is essential for desired results through resonance energy transfer.


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a)

Intensity(a.u.)

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


30

PyTE NTE PTE

25 20 15 10 5 0

400

450

500

550

600

650

Wavelength(nm)

Figure 5. Fluorescence emission spectra of a) PyTE (λexc=337nm), NTE (λexc=330nm), PTE (λexc=347nm) and CIE (1931)coordinates diagram for b) PyTE, c) NTE, d) PTE, based white light emitting gel. (e) photograph of UV-illuminated white light emitting gel. Figure 5e shows the photograph of UV-illuminated white light emitting gel. In addition to this we have measured CCT (correlated color temperature ) for the white light emitting gel using the McCamy’s formula.57 The color temperature values were 4722 K, 5520 K, and 4886 K for PyTE, PTE, and NTE respectively. The CCT values suggest that the above systems have cool


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white light and such light (cool white light) have potential application to develop fluorescent light and electronic flash bulb. CCT values for white light emitting gel systems are summarized in Table 2. Table 2. CIE and CCT values for the white light emitting systems. S.No.

System

CIE

CCT(in K)

1

Pyrene+Tb(III)+Eu(III)

0.35, 0.33

4722

2

Phenanthrene+Tb(III)+Eu(III)

0.33, 0.32

5520

3

Naphthalene+Tb(III)+Eu(III)

0.35, 0.37

4886

There are limited number of works reported in the literature which describe the generation of white light from lanthanide incorporated gel system.53,54 Compared to the reported examples, the preparation of the gel has been carried out at lower temperature in the present case with a single step procedure. More importantly, the present work demonstrates the first time use of phenanthrene as a donor system for lanthanide ions, resulting in the white light emission with purity close to that of ideal white light. Conclusion We have demonstrated the utility of poly (aryl ether) dendron based gel systems as a medium for faster and efficient resonance energy transfer from different donors (naphthalene, phenanthrene, pyrene) to europium{Eu(III)}. Utilizing coordinating gelators such as poly (aryl ether) based dendrons, it is possible to get better insight into the location of the donor-acceptor systems in lanthanide based light emitting systems. The results indicate that gel medium essentially assists resonance energy transfer through 1) avoiding the non-radiative decay through vibronic coupling by replacing the solvent molecule by gelator. 2) rigidifying the donor-acceptor pairs with


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appropriate orientation of the dipole which enhance RET. Generation of white light emission from organo-lanthanide systems has been demonstrated and the results indicate that judicious choice of donor-acceptor systems and gelators play an important role in regulating the purity of the white light generated from such systems. ASSOCIATED CONTENT Spectral overlap spectrum, Absorption spectra of different donors, SEM images, time resolved luminescence study, CIE coordinates. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Fax: +91-44-2257-4202. Tel: +91-44-2252-4232 Notes The authors declare no competing financial interest ACKNOWLEDGMENT The authors thank Prof . A.K. Mishra, Department of Chemistry IIT Madras for TCSPC, Dr. M.L.P. Reddy, NIIST, TVM for integrating sphere facilities and Dr. R. L.Gardas, Department of Chemistry IIT Madras for rheology experiments. P.K. wants to thank University Grant Commission (UGC), New Delhi, India for fellowship. We thank Department of Chemistry, IIT Madras for the experimental facilities provided.


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Table of content

Efficient RET between organic fluorophores and lanthanides, along with white light emission is feasible with the support of poly(aryl ether) dendron based gel medium.


28

Enhanced Resonance Energy Transfer and White-Light Emission from

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