Implication toward a Simple Strategy To Generate Efficiency-Tunable

May 5, 2010 - The present contribution describes a unique strategy to produce tunable Fluorescence Resonance Energy. Transfer (FRET) emission only as ...
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J. Phys. Chem. A 2010, 114, 6097–6102

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Implication toward a Simple Strategy To Generate Efficiency-Tunable Fluorescence Resonance Energy Transfer Emission: Intertwining Medium-Polarity-Sensitive Intramolecular Charge Transfer Emission to Fluorescence Resonance Energy Transfer Bijan Kumar Paul, Anuva Samanta, and Nikhil Guchhait* Department of Chemistry, UniVersity of Calcutta, 92 A. P. C. Road, Calcutta-700009, India ReceiVed: March 2, 2010; ReVised Manuscript ReceiVed: April 7, 2010

The present contribution describes a unique strategy to produce tunable Fluorescence Resonance Energy Transfer (FRET) emission only as a function of medium properties through the implementation of Intramolecular Charge Transfer (ICT) reaction as the donor counterpart. Solvent sensitive emission (and quantum yield, ΦD) of N,N-dimethylaminonaphthylacrylonitrile (DMANAN) (donor) leads to modulation of spectral overlap (J(λ)) between donor emission and acceptor (Acridine orange (AO)) absorption spectra whereby yielding differential FRET efficiency in various solvents, i.e., intertwining of two photoprocesses viz. ICT and FRET results in realization of the claimed process in the title. 1. Introduction Research in the field of photochemistry has traditionally been a subject of elating fascination to the scientific world because of the profusely modulated photophysics of compounds sprouting through their photoexcitation, which has eventually led to the discovery of some interesting phenomena like excited state proton transfer,1 charge transfer,2 electron transfer,3 conformational transformations such as isomerization,4 differential modes and extents of solvation of molecules, clusters, ions, etc. in the excited state,5 resonance energy transfer,6–8 and so forth. With the progress of time, all these phenomena have enormously matured through the mammoth volume of works inclined along these directions and have eventually constructed some indispensable building blocks of photochemistry. Fluorescence Resonance Energy Transfer (FRET) is a widely prevalent photophysical process that involves the transfer of excitation energy of an electronically excited “donor” (D) molecule to an “acceptor” (A) molecule via nonradiative routes. About seven decades ago, an attempt to resolve the enigma surrounding fluorescence quenching experiments led Perrin7 to the observation of FRET and subsequent proposition of the operation of dipole-dipole interaction via which interaction between molecules separated by distances greater than their molecular diameter was possible without collision. Some two decades down the line, Fo¨rster8 shaped Perrin’s idea to an elegant theory to provide a quantitative interpretation for the nonradiative energy transfer in terms of his famous expression:

kET )

Kκ2ΦDJ(λ) r6

(1)

in which the rate of energy transfer, kET, is shown to be linearly proportional to donor quantum yield (ΦD), overlap between donor emission and acceptor absorption spectra (J(λ)), and orientation factor between the donor and acceptor molecules (κ2) and is inversely proportional to the sixth power of D-A * To whom correspondence should be addressed. Fax: +91 33 2351 9755. E-mail: [email protected].

SCHEME 1: Structures of DMANAN (donor) and AO (acceptor)

separation (r). Recent time has witnessed a splendid evolution of research focusing on the potential applicability of this technique. Successful exploitation of FRET has fathomed into areas such as DNA sensing,9 proteomics,10 probing various processes in macromolecular assemblies like living cells, proteins, etc.,11,12 detection of trace metal ions,13 and so on. However, most of the FRET-based sensing schemes are banked upon the explicit distance dependence of the technique, i.e., to generate the sensing response through modulation of D-A separation distance. Implementation of other factors in Fo¨rster’s equation, such as spectral overlap (J(λ)), relative dipole orientation of donor and acceptor (κ2), and donor quantum yield (ΦD) as an actuating mechanism in the development of sensing schemes have been underutilized. The present program concentrates on the investigation of the possibility of tuning FRET efficiency through modulation of J(λ) and ΦD only as a function of medium polarity. Execution of our purpose has been accomplished by using an ICT molecule (DMANAN) as the donor fluorohore and acridine orange (AO) as the acceptor (Scheme 1). The absorption spectra of AO is found insensitive to medium polarity within the range of solvents assayed for the present study, while the remarkable solvent polarity dependence of the emission profile of the donor, DMANAN, beautifully makes way for modifying the magnitude of J(λ) only as a function of solvent parameters. Simultaneously, the impact of solvent on donor quantum yield has automatically been

10.1021/jp101862w  2010 American Chemical Society Published on Web 05/05/2010

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incorporated. The motivation of the present work stems from two recent reports which describe the occurrence of FRET promoted by ESIPT14 and ESPT15 emission. However, our effort advocates the documentation of ICT induced FRET and claims to be the first example of this sort. Interestingly, the inherent sensitivity of ICT emission toward medium polarity has extended the study a step further to investigate the possibility of modulation of FRET efficiency merely by changing the solvent by virtue of the remarkable solvent polarity dependence of ICT emission (leading to modulate J(λ) and donor fluorescence yield, ΦD). Linear dependence of kET on J(λ) was experimentally verified long ago by Haugland et al.,16 but their technique required complicated synthetic skill. A recent report by Li et al.17 also focuses on the modulation of FRET with J(λ) and acceptor quantum yield, but their approach is also bound by the necessity of good synthetic skill. The present work is free from the shortcoming of frightening synthetic skills as the solvent polarity dependent emission of the ICT donor molecule simply leads to variation of J(λ). At the same time, another aspect, viz., the dependence of FRET on ΦD, is also examined. Although dependence of FRET on J(λ) is not so exclusively tuned in the study, the possibility is reported here. We are optimistic that the synthesis of the proper molecular system linking and thereby fixing the separation between the donor and the acceptor moieties (without altering the basic photophysical character of the molecules) will improve the results and establish the claim beyond doubts. 2. Experimental Section 2.1. Materials. N,N-Dimethylaminonaphthylacrylonitrile (DMANAN, Scheme 1) was synthesized and purified following the usual literature procedure and is described elsewhere.18 Acridine orange (AO, Scheme 1) was purchased from SRL, India. The compounds were used after vacuum sublimation followed by establishing the purity on TLC plate. All solvents used in the study (CH3CN, CH2Cl2, CHCl3, DOX (1,4-dioxan), CH3OH) are of spectroscopic grade (Spectrochem, India) and were used after proper distillation. Triply distilled water is used for preparing an aqueous solution. The purity of the solvents was also tested by running their fluorescence spectra in the studied wavelength region prior to use. 2.2. Instrumentation and Methods. The absorption and emission spectra were acquired on a Hitachi UV-vis U-3501 spectrophotometer and a Perkin-Elmer LS-55 fluorimeter, respectively. All experiments have been carried out at ambient temperature (300 K) with air-equilibrated solutions, using quartz cuvettes of 1 cm path length. Only freshly prepared solutions were used for spectroscopic measurements. During FRET measurements, addition of AO (from a stock solution) to a solution containing DMANAN was limited only within 20 µL so as to eliminate any possibility of significant volume change. Concentrations of the fluorophores (DMANAN and AO) were carefully maintained in ×10-6 M order so as to negate the possibilities of reabsorption or innerfilter effect. Fluorescence quantum yield (Φ) was determined with anthracene (λabs ≈ 350 nm and Φ ) 0.27 in CH3OH) as the secondary standard in the following equation:1b,2b,18,19

AS (Abs)R nS2 ΦS ) ΦR AR (Abs)S n 2 R

where Φ is quantum yield, “Abs” is absorbance, A is the area under the fluorescence curve, and n is the refractive index of

Paul et al. the medium. The subscripts S and R stand in recognition of the corresponding parameters for the sample and reference, respectively. 2.3. Computational Procedure. Elucidation of the molecular orbitals (HOMO and LUMO) of DMANAN has been accomplished by optimizing the geometry of the molecule on the GAUSSIAN 03W suit of program, using B3LYP hybrid functional and 6-31G (d,p) basis set.20 The action of the solvent reaction field is explored with calculation in the Polarizable Continuum Model (PCM) at the same level of theory with the same basis set. 3. Results and Discussion The operation of the ICT reaction in DMANAN (Scheme 1) has been previously reported by our group.18 The molecule DMANAN shows an almost solvent independent absorption maxima at ∼370 nm (except water; λabs ≈ 355 nm) due to ππ* transition. The meteoric influence of medium polarity on the excited state photophysics of DMANAN is manifested on the emission profile through the appearance of dual emission comprised of a solvent insensitive higher energy local emission (λem ≈ 430 nm) and a remarkably polarity sensitive lower energy charge transfer (CT) emission. The CT emission red shifts from ∼460 nm to ∼485 nm as the medium polarity is varied from ET(30) ) 36.0 (DOX) to 46.0 (CH3CN) through 39.1 (CHCl3; λem ≈ 473 nm) and 41.1 (CH2Cl2; λem ≈ 477 nm). While spectroscopic observations with DMANAN establish the operation of ICT phenomenon in the molecule, theoretical calculation has also been shown to correlate well to the experimental findings in the light of the Twisted Intramolecular Charge Transfer (TICT) model.18 In particular, a careful perusal of the molecular orbital picture (HOMO and LUMO) of DMANAN in the ground state optimized geometry and the donor (sNMe2)/ acceptor (sCHdCHsCtN) twisted geometrical state afford a clear view for the photoinduced TICT state formation in DMANAN.18 The HOMO-LUMO molecular orbital pictures for the global minimum structure and for different twisted geometries are illustrated in Figure 1. For the global minimum state, the nitrogen lone pair is found to be more or less uniformly distributed over the entire π network of the naphthalene ring for both HOMO (π) as well as LUMO (π*). This is a corroboration of the presence of delocalization in the ground state of the π cloud density via the agency of the π network of the molecular framework and predicts the π-π* nature of the transition, which is allowed with high calculated oscillator strength (0.4359). In the donor twisted state, a relatively large electronic density projection is obtained over the N-atom for the HOMO (n), i.e. twisting of the donor -NMe2 group onto a plane perpendicular to the naphthalene nucleus leads to mutual decoupling of orbitals thereby preventing the π electron delocalization in the ground state rendering the donor lone electron pair now be localized and available for charge transfer to the acceptor side. In the LUMO (π*) orbital, the electronic density projection over the donor group is found to be noticeably reduced while the acceptor group enjoys a reasonably cloud density. Thus the transition is of forbidden n-π* type. A decrease in the calculated oscillator strength value from 0.4359 in the global minimum state to 0.0063 in the twisted state further substantiates the interpretation. The HOMO and LUMO orbital pictures in the case of acceptor twisting are found to be, qualitatively, similar to that of the global minimum state. At the donor twisted geometry for both in vacuuo and a polar solvent (ACN), the nitrogen lone pair is localized (HOMO orbital) compared to a delocalized HOMO orbital for the global

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Figure 1. Molecular orbital pictures (HOMO and LUMO) of DMANAN for the (a) global minimum state, (b) donor twisted geometry, (c) acceptor twisted geometry in vacuuo, and (d) donor twisted geometry in a polar (ACN) solvent (as obtained from B3LYP/6-31G (d,p) level of calculation).

minimum and acceptor twisted states.2b,18,19,21,22 The localized lone pair at the donor twisted geometry favors the CT process in the twisted decoupled state. Though the n-π* transition is symmetry forbidden, it is energetically generated through Franck-Condon excitation and the CT state is generated through a small barrier crossing on the S1 surface.2b,18,19,21,22 On the other extreme, the ground and excited state spectral properties of AO (λabs ≈ 490 nm and λem ≈ 531 nm) come out to be indifferent to solvent polarity within the range of solvents studied. More precisely, AO displays two bands in the absorption profile;the main absorption band peaking at ∼492 nm and another small hump at ∼464 nm showing signatures of the well-known dimerization process of AO. The higher energy band corresponds to the dimeric species while the lower energy band corresponds to the monomer.23a–f (The estimated molar extinction coefficient of AO reveals a value ε ) 6.98 ( 0.1 × 104 M-1 at λabs ) 492 nm. An excellent congruity with the literature23d thus dictates that under the presently employed experimental conditions the normal spectroscopic properties of AO are not perturbed and the method used for evaluation of the constant is feasible.) On the emission profile AO exhibits the maxima at λem ≈ 531 nm due to the monomeric species.23a–f It is well documented in the literature that the dimeric species shows its characteristic emission band at ∼650 nm, but in order for the dimer emission to detect the concentration of AO should be of the order of 1 × 10-3 M or higher.23e–g Presently we have employed a very low concentration of AO (of 10-6 M order; section 2.2) because of reasons stated in section 2.2, whence it is justified to presume that it is the emission of the monomeric species of AO participating actively in the process under investigation. Indeed, we did not observe any characteristic emission band for the dimeric species (Figures 2c and 3). These two factors (i.e., such differential photophysics of DMANAN and AO) when coupled together formed an excellent combination to provide a scope for modulating J(λ) through medium polarity dependent donor emission and quantum yield, apart from constituting an efficient FRET pair. Figure 2a depicts the overlap of donor and acceptor absorption spectra in CH3CN

Figure 2. (a) Absorption spectral overlap between donor-DMANAN and acceptor-AO in CH3CN solvent. (b) Spectral overlap of donor emission and acceptor absorption spectra. (c) Emission spectra of DMANAN (3.51 µM) in CH3CN solvent (λex ) 370 nm) in the presence of increasing concentration of AO. Curves 1f12 correspond to [AO] (µM) ) 0.0, 0.66, 1.32, 1.98, 2.64, 3.30, 3.96, 4.62, 5.28, 5.94, 6.60, 7.26.

revealing only minimal absorption of AO (Scheme 1) at maximum absorption wavelength of the donor thereby ruling out the possibility of direct excitation of acceptor (AO) in the course of FRET study (similar is the observation in other media). Figure 2b stands in justification of our choice of DMANAN and AO as a suitable FRET pair through appreciable overlap between donor emission and acceptor absorption spectra, the subsequent execution of which is revealed in Figure 2c which illustrates the occurrence of FRET between DMANAN and AO through gradual quenching of donor fluorescence in the presence of increasing acceptor (AO) concentration. Figure 3 justifies the same in other media. Figure 2b also depicts the modulation of J(λ) along medium dependent emission of DMANAN. Confirmation for the operation of the claimed phenomenon has been evidenced from the following observations: (1) the absorption spectra of the DMANAN/AO mixture produced no new band in addition to those for the individual chemical entities thereby discarding the possibility of any ground state complex formation;15,19,24–27 (2) the emission profile too was silent toward generation of any new band even at the long wavelength region and thereby negates the possibility of excimer formation;15,19,24–27 (3) the fluorescence quantum yield of AO was considerably low in the absence of the donor compared to that in the presence of the same for λex ) λabsmax(donor);19,24–27 (4) observation of a characteristic isoemissive point at ∼507 nm in CH3CN (Figures

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Paul et al. donor (in the picosecond range) leads to neglecting the impact of diffusive motions.19,24 A direct comparison of our observations with earlier reports by Lakowicz19 and others13–17,19,24–27 also is in line with the occurrence of FRET in the present case. Now an exploration of the major objective of the present program, i.e., modulation of the FRET efficiency as a function of solvent properties through medium dependent emission of the donor (modulating J(λ) and quantum yield, ΦD), is achieved from the parameters summarized in Table 1. Energy transfer efficiency (E) has been evaluated from the following equation8,19

E)1-

I I0

(2)

where I and I0 are the fluorescence intensities of DMANAN, respectively in the presence and absence of the acceptor (AO). Table 1 highlights the tunability of E over a reasonable range of magnitude only through varying the medium polarity, from E ) 0.0362 in DOX to E ) 0.7534 in CH3CN (through E ) 0.0745 in CHCl3 and E ) 0.1188 in CH2Cl2), i.e., around 95.2% modulation in E brought about by a mere 21.74% change in the medium polarity (ET(30) scale) in a sequential track (Figure 5). However, the observed wide range of tunability of E as a function of solvent properties is an outcome of the conjugate effect of modulation in J(λ) and ΦD. Under the circumstances of the present investigation and given the complexity in Fo¨rster’s nonradiative energy transfer theory, it is not possible to deconvolute relative contributions from the two effects quantitatively as they are intimately associated through the following expressions:8,19

E)

Figure 3. (a) Emission spectra of DMANAN (3.51 µM) in CH2Cl2 solvent (λex ) 370 nm) in the presence of increasing concentration of AO. Curves 1f10 correspond to [AO] (µM) ) 0.0, 0.69, 1.38, 2.76, 4.14, 6.90, 9.66, 12.42, 15.18, 17.94. (b) Emission spectra of DMANAN (3.51 µM) in CHCl3 medium (λex ) 370 nm) in the presence of increasing concentration of AO. Curves 1f8 correspond to [AO] (µM) ) 0.0, 0.66, 1.32, 1.98, 3.30, 4.62, 5.94, 7.26, 8.58, 9.90, 11.22, 12.54, 13.86. (c) Emission spectra of DMANAN (3.51 µM) in DOX solvent (λex ) 370 nm) in the presence of increasing concentration of AO. Curves 1f9 correspond to [AO] (µM) ) 0.0, 0.69, 1.38, 2.76, 4.14, 5.52, 6.90, 8.28, 11.04.

2c and 3), such an isoemissive point would have been lacking had the quenching of donor emission been merely a case of reabsoprtion;15,19,24–27 (5) an attempt to follow the steady state fluorescence quenching of the donor on the Stern-Volmer (SV) equation (I0/I ) 1 + KSV[Q], here Q denotes the quencher, i.e., AO, and I and I0 are the fluorescence intensities of the fluorophore (here the donor DMANAN), respectively, in the presence and absence of the quencher)19 led to KSV (SternVolmer constant19) values (Table 1) falling within the normal range reported earlier for the FRET process14,15,19,25–27 (the linearity of Stern-Volmer plot (Figure 4a) indicates the operation of only one type of quenching mechanism) and 1 order of magnitude higher than the normal diffusion-limited quenching process (Table 1) suggesting the predominant role of resonance energy transfer through long-range dipole-dipole interaction for the observed fluorescence quenching rather than the simple diffusion-limited process.15,19,25–27 Also a fast lifetime of the

R06 R06 + r6

(3)

and the Fo¨rster’s distance, R0 (at which energy transfer is 50% efficient), depends on the spectral properties of both the donor and the acceptor molecules through8,19

R06 ) (8.8 × 10-25)[κ2η-4ΦDJ(λ)] (cm6)

(4)

(η is the refractive index of the medium and κ2 is the orientation factor between the donor and acceptor molecules). The overlap integral is expressed as8,19

J(λ) )

∫0∞ I(λ)ε(λ)λ4 dλ/ ∫0∞ I(λ) dλ

(5)

in which I(λ) is the normalized fluorescence intensity of the donor in the range λ and λ + ∆λ, and ε(λ) is the extinction coefficient of acceptor at λ. Table 1 also underlines the nice corroboration between variation of E and J(λ) with solvent, i.e., efficiency of energy transfer varies sequentially with J(λ) and solvent polarity parameter, ET(30). At the same time, incorporation of protic solvent (CH3OH and H2O) into the sequential track of solvent polarity is found to endorse a perturbation in the trend (Table 1 and Figure 5). Apparently such deviation seems quite natural since the mode and extent of solute-solvent interactions are modified considerably because of the protic character of the solvents, and we presume the solute-solvent intermolecular H-bonding (IerMHB) to play the key role.18,19,22 It is probable

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TABLE 1: Spectroscopic Parameters Obtained during FRET between DMANAN and AO in Different Media solvent ET(30) (kcal/mol) CH3CN CH2Cl2 CHCl3 DOX CH3OH Water

46.0 41.1 39.1 36.0 55.5 63.1

ΦDb

λem (nm)

Ec

0.0435 ( 0.001 0.0513 ( 0.002 0.0745 ( 0.002 0.1080 ( 0.002 0.0361 ( 0.002 0.0192 ( 0.002

485 477 473 460 485 498

0.7534 ( 0.02 0.1188 ( 0.02 0.0497 ( 0.02 0.0362 ( 0.02 0.6304 ( 0.02 0.3342 ( 0.02

J(λ) × 10-16 (L · mol-1 · cm3) KSV × 10-4 (M-1)d intercept of SV plota 5.1067 2.3027 1.7723 1.24061 2.5995 1.8436

19.81 ( 0.053 0.739 ( 0.051 0.234 ( 0.0153 0.211 ( 0.080 9.99 ( 0.048 4.916 ( 0.052

0.851 ( 0.1 1.020 ( 0.1 1.029 ( 0.1 1.007 ( 0.1 0.923 ( 0.1 1.006 ( 0.1

a Intercept of the Stern-Volmer plot justifies the potential applicability of the Stern-Volmer equation in the present case in terms of deviation from the ideal case where KSV is equal to unity and the excellence of linear fit to the data points (Figure 4a) is verified from the correlation coefficient r2 ) 0.99 ( 0.01. b ΦD: quantum yield of donor with anthracene as the secondary reference. c E: efficiency of FRET. d KSV: Stern-Volmer constant for donor fluorescence quenching in the presence of the acceptor.

Figure 4. (a) Stern-Volmer plot for fluorescence quenching of DMANAN in the presence of increasing AO concentration in different media indicated in the figure legend. (b) Variation of FRET efficiency (E) as a function of AO concentration in different environments (indicated in the figure legend). Error bars are within the marker symbols if not apparent.

Figure 5. Plot of variation of E and J(λ) (L · mol-1 · cm3) with ET(30) (kcal/mol) showing the sequential variation in polar aprotic solvents (1fDOX; 2fCHCl3; 3fCH2Cl2; 4fCH3CN) and its disruption upon introduction of protic solvents (5fCH3OH; 6fH2O).

that opening up the nonradiative deactivation channels operating via IerMHBs is the major contributing factor for such deviation. However, it appears relevant to figure out at this stage that the role of photoinduced electron transfer from protic solvents to the chromophore assisted by hydrogen bonding might put its

signature in the process of donor fluorescence quenching and hence the overall photophysics occurring within the system in protic solvents. Zhao et al.3c in a recent nice report have explored the phenomenon on oxazine 750 (OX 750) chromophore and have described it as a new fluorescence quenching mechanism for chromophores in protic solvents. Zhao et al.3c have demonstrated the occurrence of electron transfer (ET) reaction from alcoholic solvents to the chromophore (OX 750) operating via site-specific intermolecular hydrogen bonding (IerMHB) interactions and have explored the effect through femtosecond timeresolved stimulated emission pumping fluorescence depletion spectroscopy (FS TR SEP FD) with combined theoretical calculations. Their report depicts a drastic fluorescence quenching for OX 750 in protic solvents compared to its strong fluorescence in a polar aprotic medium (acetone) and henceforth a radiationless deactivation of the excited state is suggested. In the present case, however, we notice a relative decrement of fluorescence quantum yield of DMANAN on passing from aprotic to protic solvents (Table 1) and qualitatively assign the effect on the operation of solute-solvent intermolecular hydrogen bonding assisted radiationless deactivation channels.18,19,21,22 This is rendered even more prominent upon inspection of the fluorescence quantum yield values of the chromophore in protic solvents, e.g., 0.0388 ( 0.003 in C2H5OH, 0.0361 ( 0.002 in CH3OH, and 0.0192 ( 0.002 in water. A quantitative scrutiny of the fluorescence quantum yield values of DMANAN in protic solvents thus distinctly unravels the influence of protic solvents on fluorescence properties of the molecule and is found to juxtapose quite well with normal observations with ICT chromophores,18,19,21,22 but at the same time, a direct comparison of our results with the report of Zhao et al. reveals that the observed fluorescence quenching of DMANAN with increasing hydrogen bond forming ability of the protic solvents is not as drastic in magnitude as in the case of OX 750. Thus we presume that the action of protic solvents resulting in quenching of the fluorescence quantum yield of DMANAN in our case is primarily based on solute-solvent IerMHB-assisted activation of nonradiative decay routes,18,19,21,22 and specific interaction of the sort of protic solvent-to-chromophore ET reaction might not be playing an instrumental role here, as in that case a considerably more drastic quenching effect would have been observed.3c,19 Furthermore, as the molecular orbital picture (Figure 1) illustrates the presence of a significant amount of electronic density projection over the π network of the naphthalene chromophoric unit and with a view to the usual observation of high oxidation potential for naphthalene,28 the chromophoric unit in DMANAN does not seem to suffer from a substantial electron deficiency, whereas OX 750 contained a cationic chromophore.3c Furthermore, our observations of the impact of protic solvents on the fluorescence properties of DMANAN is strikingly corroborating to the usual observations

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with ICT systems,18,19,21,22 in particular to systems having similar molecular architectures.22b,c Insignificant perturbation of the local emission (λem ≈ 430 nm) of DMANAN in the course of FRET dictates the exclusive impact of the CT state as the active participant (donor) in the process,15 which advocates for the generation of long wavelength emission (λem of AO ≈ 531 nm) starting from an unpopulated ground state. The medium polarity dependence of the CT emission, in turn, leads to the sequential variation of E with solvent polarity (Table 1). 4. Conclusion In conclusion, the documentation of the first example of the ICT induced FRET process carries with it the promise of design, synthesis, and characterization of suitable molecular systems which will allow tuning of FRET efficiency over a wide range of magnitude utilizing the solvent-dependent emissive character of an ICT chromophore through modulated J(λ) and ΦD only as a function of medium parameters. We are optimistic that the present illustration will augment the impetus of the search for a simple strategy to furnish efficiency (E)-tunable FRET emission on the lexicon of FRET-based sensing schemes. Also the concept of generation of long wavelength emission via an unpopulated ground state enterprises its viable expansion in the future technology of development of tunable white-light emitting molecular dyads. Acknowledgment. Financial support from CSIR (Project no. 01(2161)07/EMR-II) and DST (Project no. SR/S1/PC/26/2008), Government of India is gratefully acknowledged. B.K.P. and A.S. thank C.S.I.R., India for research fellowships. References and Notes (1) (a) Catalan, J.; Perez, P.; del Valle, J. C.; de Paz, J. L. G.; Kasha, M. Proc. Natl. Acad. Sci., U.S.A. 2002, 99, 5793. (b) Sing, R. B.; Mahanta, S.; Kar, S.; Guchhait, N. Chem. Phys. 2007, 331, 373–384. (2) (a) Ishikawa, H.; Shimanuki, Y.; Sugiyama, M.; Tajima, Y.; Kira, M.; Mikami, N. J. Am. Chem. Soc. 2002, 124, 6220–6230. (b) Chakraborty, A.; Kar, S.; Nath, D. N.; Guchhait, N. J. Phys. Chem. A 2006, 110, 12089– 12095. (c) Zhao, G.-J.; Han, K.-L. Biophys. J. 2008, 94, 38–46. (d) Zhao, G.-J.; Han, K.-L. J. Phys. Chem. A 2007, 111, 2469–2474. (e) Zhao, G.-J.; Han, K.-L. J. Phys. Chem. A 2009, 113, 14329–14335. (3) (a) Beratan, D. N.; Skourtis, S. S. Curr. Opin. Chem. Biol. 1998, 2, 235–243. (b) Takai, A.; Gros, C. P.; Barbe, J.-M.; Guilard, R.; Fukuzumi, S. Chem.;Eur. J. 2009, 15, 3110–3122. (c) Zhao, G.-J.; Liu, J.-Y.; Zhou, L.-C.; Han, K.-L. J. Phys. Chem. B 2007, 111, 8940–8945. (4) (a) Hanekom, D.; Mckenzie, J. M.; Derix, N. M.; Koch, K. R. Chem. Commun. 2005, 767–769. (b) Banares, A.; Heikal, A. A.; Zewail, A. H. J. Phys. Chem. 1992, 96, 4127–4130. (5) (a) Felker, P. M. J. Chem. Phys. 2006, 125, 184313–184325. (b) Guchhait, N.; Banerjee, S.; Chakraborty, A.; Nath, D.; Naresh, G. P.; Chowdhury, M. J. Chem. Phys. 2004, 120, 9514–9523. (c) Adhikary, R.; Mukherjee, P.; Kee, T. W.; Petrich, J. W. J. Phys. Chem. B 2009, 113,

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