Ultrafast SingletâSinglet Energy Transfer between an Acceptor

ARTICLE pubs.acs.org/JPCC

Ultrafast SingletSinglet Energy Transfer between an Acceptor Electrostatically Attached to the Walls of an Organic Capsule and the Enclosed Donor Shipra Gupta,† Aniruddha Adhikari,‡ Amit Kumar Mandal,‡ Kankan Bhattacharyya,*,‡ and V. Ramamurthy*,† † ‡

Department of Chemistry, University of Miami, Coral Gables, Miami, Florida 33146, United States Department of Physical Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India

bS Supporting Information ABSTRACT: Ultrafast Forster resonance energy transfer (FRET) between a donor (coumarin dye) and a cationic acceptor (rhodamine 6G) situated within and on the exterior walls, respectively, of a bimolecular capsule made up of anionic octa-acid (OA) is studied by femtosecond up-conversion. 1D and 2D 1H NMR spectral analyses reveal that donors coumarin-1, coumarin-153, and coumarin-480 form a 1:2 complex (guest to host) and are enclosed within the OA capsule. NMR and emission spectra suggest that the cationic acceptor rhodamine 6G is attached to the anionic exterior walls of the OA capsule. The acceptor emission displays a rise component of 13 ps which corresponds to a donoracceptor distance (RDA) of ∼13 Å and thus indicates the occurrence of ultrafast FRET between the donor and acceptor at close contact.

’ INTRODUCTION The possibility of significant alteration of the photophysical and photochemical behavior of organic molecules included within the restrictive space of a capsule comprised of two molecules of octaacid (OA) has been established in recent years.15 This observation led us to question if the excited state behavior of a guest molecule enclosed in the OA capsule could be influenced by a molecule present outside.611 In this context, our interest has been to explore the possibility of communication (exchange) between two such molecules, one encaged and the other free. Through steady-state and time-resolved electron paramagnetic resonance (EPR) studies, one of our groups recently established the interaction of two nitroxide molecules separated by the wall of the OA capsule.12,13 This was believed to be facilitated by orbital overlap between the two nitroxides through the walls of OA (superexchange process). These studies unequivocally proved that spin exchange is possible between two radicals, one present outside and the other inside the capsule. The same group also disclosed recently that the diffusion-limited rates of triplettriplet energy transfer between excited donor triplet and acceptor oxygen triplet in solution were altered to rates less than 108 M1 s1 when the donor was enclosed within the OA capsule.3,14 This energy transfer rate depended on the rate of capsule opening and closing that was estimated to be ∼107 s1. These results were interpreted to mean that the triplettriplet energy transfer occurs through partial overlap of the orbitals of the excited triplet donor and ground state oxygen facilitated by capsule opening. Unlike spin exchange described above, this process required capsule opening. r 2011 American Chemical Society

In contrast to the above collisional energy transfer (Dexter type), resonance energy transfer (Forster type singletsinglet energy transfer) does not require orbital overlap between the excited donor and ground state acceptor and could occur over longer distances.10 This criterion that obviates the need for capsule opening prompted us to investigate whether singletsinglet energy transfer between an encapsulated donor and a free acceptor could occur, similar to spinspin exchange, across the capsular wall. We hoped that results of this study would be helpful in our long-term goal of exploring the use of encapsulated dyes to capture and transfer solar radiation to an acceptor present at a distance. According to Forster theory, the rate of Forster resonance energy transfer (FRET) is inversely proportional to the sixth power of donoracceptor distance.10 Thus, FRET is expected to occur on an ultrafast (∼1 ps) time scale when the donor and the acceptor are at close proximity. Examples of such fast FRET include energy transfer in light harvesting systems,15,16 between organic dyes bound to nanoparticles1720 and between nanoparticles and bases in DNA.21 Ultrafast FRET between donors and acceptors in micelles and reverse micelles has been reported recently.2224 The present study focuses on ultrafast FRET between two molecules, one of which resides within a molecular capsule and the other just outside. In this context, we have used the well-investigated FRET pair, coumarin dyesrhodamine 6G. Received: January 6, 2011 Revised: March 9, 2011 Published: April 22, 2011 9593

dx.doi.org/10.1021/jp2001578 | J. Phys. Chem. C 2011, 115, 9593–9600

The Journal of Physical Chemistry C

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Scheme 1. Chemical Structure and Dimensions of OA with Proton Signals Assigned from AJ and Structure of Donors C480, C1, and C153 and Acceptor R6G

Figure 1. 1H NMR spectra of C1 alone in CD3CN (i), OA alone in buffered (sodium borate buffer) water (v), and C1 in the presence of OA at various guest to host ratios (ii), (iii), and (iv). Spectrum (ii) corresponds to a mixture in which the C1 to OA ratio is 1:8; (iii) to C1 to OA at a ratio of 1:4; and (iv) to C1 to OA to a ratio of 1:2. All spectra in water were taken at 10 mM sodium borate buffer solution. The host concentration was maintained at 1 mM, and the guest concentration varied. Signals marked AJ in (v) represent uncomplexed OA protons; signals marked a, a0 j0 in (iv) represent complexed OA protons; and signals marked * represent the guest alkyl proton signals.

Coumarin-1 (C1), coumarin-153 (C153), and coumarin-480 (C480) were chosen as donors (D) and rhodamine 6G (R6G) (Scheme 1) as an acceptor.25,26 These neutral and less watersoluble D’s are readily included within the negatively charged OA

capsule in water (pH ∼ 9).27 R6G, being water-soluble, positively charged, and larger in size than the capsular volume, remains close to the exterior wall of the capsule by being attracted through electrostatic interaction with the carboxylate anions of 9594

dx.doi.org/10.1021/jp2001578 |J. Phys. Chem. C 2011, 115, 9593–9600


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Steady-State Fluorescence Experiments. Fluorescence emission and excitation spectra were recorded on an Edinburgh FS920CDT steady-state fluorimeter, and nanosecond lifetime measurements were carried out on an Edinburgh FL900CDT fluorescence lifetime spectrometer. Stock solutions of 10 mM of OA in 100 mM of sodium tetraborate buffer and known concentration of coumarins in water were made and used for steady-state and time-resolved fluorescence studies. For experiments with C1 in the presence of OA, 20 μM of C1 was taken in a cuvette, and 6 equiv, i.e., 120 μM of OA, was added. Then the solution was made up to a volume of 3 mL, by addition of water. Similarly, for 16 μM of C153, 5 equiv (80 μM) of OA, and for 10 μM of C480, 8 equiv (80 μM) of OA were used to prepare 3 mL of donor@OA solutions. For FRET experiments, 5 mM Rhodamine 6G aqueous stock solution was added stepwise to the donor@OA solution, until no further change in the spectra was observed. Picosecond (ps) and Femtosecond (fs) Fluorescence Experiments. Femtosecond up-conversion setup (FOG 100, CDP) (IRF ∼ 350 fs) used in this study has been described previously.23 To fit the fs transient, we first determined the long ps components (4007000 ps) using a TCSPC setup (IRF ∼ 90 ps) described earlier.23 The long ps components were kept fixed to fit the fs data. The ultrafast components (13 ps) were determined from the fs up-conversion setup. The rate of FRET (kFRET) was calculated following F€orster theory29

kFRET ¼

Figure 2. Fluorescence spectra of (a) C480 (10 μM) in water (black line) and in the presence of OA (80 μM in 0.8 mM borate buffer) (red line), λex= 370 nm; (b) C1 (20 μM) in water (black line) and in the presence of OA (120 μM in 1.2 mM borate buffer) (red line), λex= 360 nm; (c) C153 (16 μM) in water (black line) and in the presence of OA (80 μM in 0.8 mM borate buffer) (red line), λex= 400 nm (a.u. = arbitrary units).

the capsule exterior. Location of the donor and the acceptor molecules and the nature of the complex formed are probed through 1H NMR technique. All three D’s formed 1:2 (guest:host) complexes with OA and are represented as D@OA2. The results of the singletsinglet energy transfer between encapsulated D’s and water-soluble R6G probed by steady-state and time-resolved (picoand femtosecond) fluorescence techniques are presented below.

1 τArise

1 R0 6 ¼ 0 τD R DA

where τ0D is the lifetime of the donor in the absence of acceptor, and τArise is the rise time of acceptor emission in the presence of donor. At a donoracceptor distance RDA = R0, the efficiency of energy transfer is 50% and kFRET = (1/τD0). The procedure to evaluate R0 is described in one of our groups’ previous publications.2224 General Protocol for Guest Binding Studies Probed by NMR. A D2O stock solution (600 μL) of host OA (1 mM) and sodium borate buffer (10 mM) taken in a NMR tube was titrated with the guest by sequential addition of 0.125 equiv of guest (1.25 μL of a 60 mM solution in DMSO-d6). The complexation was achieved by shaking the NMR tube for about five minutes. 1 H NMR spectra were recorded at room temperature under aerated conditions on a Bruker 500 MHz NMR. Addition of guest beyond 0.5 equiv led to a turbid solution at which stage the NMR spectrum demonstrated the presence of free guest in addition to the capsular complex. Protocols for Measurements by 1H NMR DOSY Studies. Diffusion NMR experiments were recorded on a Bruker 500 MHz NMR spectrometer. The experiments were performed at 25 °C, at a host concentration of 1 mM (in 10 mM sodium tetraborate). Assuming the complex to be spherical Rh can be calculated using the measured D according to the following equation R h ¼ kT=6πηD

’ EXPERIMENTAL SECTION Materials. Laser grade dyes coumarin 1, coumarin 153, and

coumarin 480 (and rhodamine 6G) were obtained from Exciton and used as received. The octa-acid was synthesized and purified according to the published procedure.28

ð1Þ

ð2Þ

where Rh is the hydrodynamic radius of the sphere in meters; k is the Boltzmann constant; T is the temperature in Kelvin; η is the solvent viscosity; and D is the diffusion constant in m2 s1.30 Since the molecular size of 1:1 and 1:2 complexes is different, the value of Rh is an indirect reflection of the nature of the complex. 9595



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Figure 3. 1H NMR spectra of (i) 0.5 mM R6G alone in D2O, (ii) 0.5 mM OA and 0.5 mM R6G in 5 mM sodium borate buffered D2O, and (iii) 0.5 mM OA alone in 5 mM sodium borate buffered D2O. Signals due to R6G protons are marked with a symbol *.

’ RESULTS AND DISCUSSION Encapsulation of Donor within OA. The host OA forms a 1:1, 1:2, or 2:2 complex depending on the guest molecule.27 Representative of the three donors used, the 1H NMR titration spectra for coumarin-1 (C1) shown in Figure 1 help determine the stoichiometry of the complex. The titration spectra for the other two coumarins are provided in the Supporting Information (SI) (Figures S1 and S2 in SI). In Figure 1(ii) and (iii), the proton signals of uncomplexed OA can be seen, but after addition of 0.5 equiv of C1, only complexed OA proton signals are visible. Further addition of guest resulted in signals due to uncomplexed guest also. At this stage, the appearance of distinct signals for the complexed and uncomplexed C1 suggests that there is no exchange between free and complexed guest molecules. The titration data suggested that C1 forms a 1:2 complex with OA. From the 1H NMR titration spectra shown in S1 and S2 (SI), we conclude that this is also true in the case of C480 and C153. The diffusion constant for free OA is measured to be 1.88 106 cm2/s by DOSY NMR experiments. The diffusion constant measured for the above complex (1.27 106 cm2/s) is consistent with that expected for a 1:2 complex.27 Consistent with the formation of 1:2 complexes, the diffusion constants for C480@OA2 and C153@OA2 were 1.27 106 and 1.24 106 cm2/s (Figure S3 in SI). Confirmation for inclusion of C1, C153, and C480 within the OA capsule is provided by the significant upfield shift of the guest proton signals relative to CD3CN (Figure 1 and Figures S1 and S2 in SI). The appearance of two independent signals for chemically identical protons of the two OA molecules that form the capsule suggests a lack of mobility (in the NMR time scale) of the donors within it.31 In the absence of rapid tumbling of the guest within the capsule, the chemically equivalent protons of the two OA would be magnetically nonequivalent due to the different parts of the guest occupying the two halves of the capsule. Thus, we visualize that the donor coumarin dye molecules are held tightly within the capsule. The above conclusion is also supported by the steady-state fluorescence data provided below. Coumarins C1, C153, and C480

Figure 4. (a) Absorption and (b) emission spectra of 50 μM of R6G in the absence (black line) and in the presence (red line) of 150 μM of OA in 1.5 mM sodium borate buffered water, λex = 525 nm. (The abbreviations O.D. = optical density and a.u. = arbitrary unit.)

belong to a family of laser dyes whose fluorescence maximum, quantum yield, and lifetime depend on the solvent polarity.32,33 The emission spectra of these in water in the presence and absence of OA are shown in Figure 2 (for detailed titration spectra, see Figures S4, S5, and S6 in SI). Upon excitation at 370 nm, coumarin480 (10 μM) in water showed a fluorescence maximum at 490 nm. 9596



Figure 5. Representations of spectral overlap between (a) the emission of C480@OA (black line) and the absorption of R6G in the presence of OA (red line); (b) the emission of C1@OA (black line) and the absorption of R6G in the presence of OA (red line); and (c) the emission of C153@OA (black line) and the absorption of R6G in the presence of OA (red line). Excitation wavelength (λex) in all cases is 375 nm.

On gradual addition of OA, the λemmax shifted gradually to 424 nm with an increase in intensity by a factor of 2. Similarly, upon exciting C1 at 360 nm, a blue shift of 46 nm from 458 nm in bulk water to 412 nm in aqueous-OA solution with a 3-fold increase in fluorescence intensity was noted. A similar trend was observed for C153 as well (λemmax = 547 nm in bulk water; 480 nm in aqueous-OA solution; and a 4-fold increase in fluorescence intensity). These marked blue shifts of the emission maxima and increase in emission intensity indicated that the donor molecules reside in a relatively nonpolar (dry) hydrophobic environment. Having previously established the hydrophobic nature of the OA capsule,27 we believe the above results support a model where the three donor molecules discussed in this presentation reside within the capsule protected from water. In all three guest systems, the shift reached a saturation point when the OA amount reached about 58 equiv of the guest amount (Figures S4, S5, and S6, SI). Further addition of OA did

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Figure 6. Steady-state fluorescence spectra demonstrating FRET between donor coumarins and acceptor R6G. (a) C480@OA and R6G; (b) C1@OA and R6G; and (c) C153@OA and R6G. The black line represents emission by the donor alone in the absence of the acceptor R6G, and the red line represents the emission in the presence of the acceptor. In this case, both donor and acceptor emissions are seen. Conditions for the spectra: (a) C480 at 10 μM, OA at 80 μM, R6G at 30 μM in 0.8 mM buffered water; λex= 370 nm; (b) C1 at 20 μM, OA at 120 μM, R6G at 50 μM in 1.2 mM buffered water; λex= 360 nm; and (c) C153 at 16 μM, OA at 80 μM, R6G at 48 μM in 0.8 mM in buffered water; λex= 400 nm.

not result in a significant shift, suggesting that under such conditions no guest molecules remain free in solution. Note that since the concentrations used for NMR and emission studies are different, different amounts of the host are needed to shift the equilibrium toward complete complexation. For energy transfer studies, an excess of OA was used to make sure that all donor molecules remain complexed. Association of Acceptor Rhodamine 6G to the Capsule’s Exterior. The diffusion constants for OA and R6G by themselves 9597



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Figure 7. Femtosecond transient decays of donor coumarins: (a) C480 (10 μM)@OA (80 μM in 0.8 mM buffered water) alone (black dots), and on addition of R6G (30 μM) (red dots), λex= 375 nm, λem = 440 nm; (b) C1 (20 μM)@OA (120 μM in 1.2 mM buffered water) alone (black dots) and on addition of R6G (50 μM) (red dots), λex= 375 nm, λem = 420 nm; (c) C153 (16 μM)@OA (80 μM in 0.8 mM buffered water) alone (black dots) and on addition of R6G (48 μM) (red dots), λex= 375, nm λem = 470 nm. Transient decays were recorded by monitoring the donor emission. The black line indicates the fit line to the recorded femtosecond transients.

Figure 8. Femtosecond transient decays of acceptor R6G: (a) R6G (30 μM) alone (black dots) and on addition of C480 (10 μM)@OA (80 μM in 0.8 mM buffered water) (red dots) and λex = 375 nm, λem = 560 nm; (b) R6G (50 μM) alone (black dots) and on addition of C1 (20 μM)@OA (120 μM in 1.2 mM buffered water) (red dots) and λex= 375 nm, λem = 570 nm; (c) R6G (48 μM) alone (black dots) and on addition of C153 (16 μM)@OA (80 μM in 0.8 mM buffered water) (red dots) and λex = 375 nm, λem = 570 nm. Transient decays of R6G were recorded by monitoring the acceptor emission and exciting the donor. The black line indicates the fit line to the recorded femtosecond transients.

as measured by DOSY experiments found to be 1.88 106 cm2/s and 2.6 106 cm2/s showed a decrease to 1.41 106 cm2/s and 0.9 106 cm2/s, respectively, when present together. This reduction in diffusion constants for both OA and R6G suggests the associated nature of these molecules when present together in solution. Noninclusion of R6G within the OA capsule is suggested by 1H NMR spectra provided in Figure 3. Upon addition of OA to an aqueous solution of R6G, there were no shifts in the 1H NMR signals of either of them, though some broadening was observed. Such broadening and absence of shift in the 1H NMR signals suggested their dynamic association and that R6G stay excluded from the OA capsule. Further support for association of R6G to OA comes from the red shift in the absorption and emission spectra of R6G in the presence of OA (Figure 4). As shown in Figure 4, unlike the blue shift observed in the case of donors@OA2 (Figure 2), a red shift in R6G absorption and emission maxima is noted (for titration spectra see Figure S7 in SI). Such shifts are known to result from

electrostatic interaction between R6G and perturbing molecules.34,35 On the basis of DOSY data, 1H NMR, absorption, and emission spectra, we believe that the acceptor R6G electrostatically interacts with the exterior of the OA cavitand. This type of interaction is consistent with R6G being positively charged and the OA exterior being negatively charged with eight carboxylate anion groups. In Figure S7 (SI) (titration spectra) one might note that initial addition of OA results in a decrease of emission (black lines), and further addition results in a shift in the λmax and enhancement of emission (red lines). Interpretation of this requires one to acknowledge that only the monomer of R6G emits, and dimers and higher aggregates do not. We believe the variation in the emission seen in Figure S7 (SI) is due to the fact that when a large excess of R6G is present in comparison to OA R6G molecules aggregate around the OA resulting in decrease of emission. When the amount of OA surpasses the amount of R6G, each OA is associated only with one molecule of R6G resulting in monomer emission. 9598



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Table 1. Femtosecond Decay Parameters of the Acceptor in the Presence and Absence of the Donors λex (nm)

λem (nm)

R6G þ OA

375

570

40 (0.09)

1100 (0.15)

R6G þ C153@OA

375

570

1.0 (0.24, 100%)

1800 (0.2, 16%)

6000 (1.04, 84%)

R6G þ OA

375

560

40 (0.09)

1100 (0.15)

5000 (0.76)

R6G þ C480@OA

375

560

1.5 (1.4, 100%)

1600 (0.4, 17%)

5100 (2.0, 83%)

R6G þ OA

375

570

40 (0.09)

1100 (0.15)

5000 (0.76)

R6G þ C1@OA

375

570

3.5 (1.56, 100%)

1000 (0.33, 13%)

5500 (2.23, 87%)

system

τ1a (a1b) (ps)

Table 2. Comparison of Parameters for Calculating RDA system

τArise

τ0D

R0

RDA

C480@OA2 þ R6G C153@OA2 þ R6G

1.5 ps 1.0 ps

4900 ps 7400 ps

48.8 Å 55.7 Å

13 ( 1 Å 13 ( 1 Å

C1@OA2 þ R6G

3.5 ps

4300 ps

42.5 Å

13 ( 1 Å

Steady-State Emission Studies. Before initiating the energy transfer studies, we wanted to confirm the existence of overlap between the donor emission and acceptor absorption, a requirement for FRET. The spectra for the three pairs are shown in Figure 5. It is obvious that the extent of overlap varies between the pairs, and it is the least for the C1R6G pair. Changes in the emission spectra of three coumarin dyes (C1, C153, and C480) included in the OA capsule upon gradual addition of R6G are provided in Figure S8 in the SI. For clarity reasons, only two traces for each dye are provided in Figure 6, one dye@OA2 in the absence and the other dye@OA2 in the presence of 50 μM R6G. Excitation of coumarin dye clearly results in the emission of R6G indicative of singletsinglet energy transfer. In the absence of the donor coumarins, at the excitation wavelength used in this study R6G absorbed light and emission resulted. However, the extent of absorption by R6G is expected to be negligible when the donor coumarins are present in solution. Absorption spectra of donor coumarins and acceptor R6G shown in Figure S9 (SI) confirms the above speculation. As can be seen from Figure S8 (SI), with a gradual increase in R6G concentration, the emission intensity of the donor steadily decreased, and that of the acceptor increased. From the steady-state spectra, the character of the quenching (static or dynamic) could not be deciphered due to the lack of changes in the lifetime (in the picosecond time scale; Figure S10 in SI) of the OA included donor dyes. To probe this further, femtosecond time-resolved experiments were carried out. Time-Resolved Emission Studies. In the picosecond studies, the excited singlet lifetime of the three donors included in OA (coumarin-480, coumarin-1, and coumarin-153) was found to be unaffected by the addition of acceptor R6G (Figure S10 in SI). This suggested that non-FRET donor molecules (i.e., donor alone in a cavity not being quenched by the acceptor) dominated the steadystate fluorescence. Under such conditions, occurrence of FRET could be detected by monitoring the rise time of the emission for the acceptor. In general, the donors that undergo FRET have a shorter decay. The acceptor molecules participating in FRET would exhibit a rise in their emission. This prompted us to probe the occurrence of ultrafast FRET between the donor and the acceptor through the walls of OA by the rise of the acceptor emission. Figure 7 shows the fs decay of the donors trapped within the OA capsule in the presence and absence of the acceptor R6G. Due to poor water solubility and low quantum yield of fluorescence

τ2a (a2b) (ps)

τ3a (a3b) (ps) 5000 (0.76)

emission, the lifetimes of C1 and C153 could not be measured in water in the absence of OA. The lifetime of C480 in water is reported to be 5.9 ns.32 In the presence of the acceptor, the donor displays a short initial decay component (∼1030 ps). We believe that the short decay of donors (close to the origin) in Figure 7 results from ultrafast FRET between the capsule included donors (C480, C1, C153) and the acceptor R6G at short distances. Conclusive evidence in favor of ultrafast FRET in the OA cavity is provided by the rise of the acceptor emission captured in a fs upconversion experiment. Figure 8 provides the rise of the acceptor R6G emission in the presence of the OA trapped donors, coumarin480, coumarin-1, and coumarin-153, respectively. The emission of the acceptor exhibits a rise time of 1.5, 3.5, and 1.0 ps (Table 1) for OA trapped donors, coumarin-480, coumarin-1, and coumarin-153, respectively. Using these rise times, the donoracceptor distance, RDA, was calculated using eq 1. Table 2 lists the values of experimentally obtained R0, τ0D, τArise, and calculated RDA for the three donoracceptor pairs: C480@OA2 and R6G; C1@OA2 and R6G; and C153@OA2 and R6G. It is interesting that similar values were obtained despite the varying parameters for calculating RDA in these systems. This observation leads to the conclusion that varying fundamental conditions for FRET like spectral overlap, donor lifetime (in the absence of acceptor), or acceptor rise time (in the presence of donor) among various donoracceptor pairs could still result in efficient ultrafast FRET provided the pair is in close proximity (13 Å in this case). One might wonder why the most pronounced difference in the decays occurs for the DA pair exhibiting the worst spectral overlap. We believe that due to low magnitude of overlap between the DA pair (C1-R6G) the rate of FRET becomes slower, and hence the rise is more distinct compared to the transient in the absence of donor. The difference is only captured in our up-conversion setup (IRF, resolution ∼350 fs). One might argue that the absence of change in lifetime of the donor in the presence of an acceptor (in the picosecond time scale) could be due to trivial energy transfer. In our study, the trivial energy transfer (donor emission being reabsorbed by the acceptor) is ruled out due to the delay noted for the emission of the acceptor R6G upon excitation of the donors. An additional point to note is the lack of any rise at the red end of the emission spectra of the coumarin dyes indicating the absence of solvation dynamics inside the OA cavity (Figure S11 in SI), consistent with our earlier conclusion of a nonpolar, water excluded interior of the capsule.27 This also supports that FRET is not due to donor molecules present in water. Demonstration of energy transfer in an environment lacking water molecules suggests the capsule to have remained closed.

’ CONCLUSION This work demonstrates the occurrence of ultrafast FRET between a donor (coumarin dyes) enclosed in an OA capsule 9599


The Journal of Physical Chemistry C and a cationic acceptor attached to the capsule’s anionic exterior. Though steady-state experiments gave us a glimpse of singlet singlet energy transfer, time-resolved femtosecond studies gave us a clear and concise picture of the events occurring at the ultrafast time scale. These studies also unambiguously proved our proposed supramolecular model for the donor@OA and acceptor assembly. As a next step, we would like to tune this system by introducing various physical changes such as pH or other host molecules to ultimately incorporate this system in higher-order supramolecular assemblies as a building block.

’ ASSOCIATED CONTENT

bS

1

H NMR, absorption and emission spectra, and transient decays. This material is available free of charge via the Internet at http://pubs.acs.org. Supporting Information.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; [email protected].

’ ACKNOWLEDGMENT V.R. is grateful to the National Science Foundation, USA, for generous financial support (CHE-0848017). K.B. thanks the Department of Science and Technology, India (Project Number: IR/I1/CF-01/2002 and J. C. Bose Fellowship), and Council for Scientific and Industrial Research (CSIR) for generous research support. A.A. and A.K.M. thank CSIR for research fellowships. V.R. and S.G. thank Mintu Porel and Steffen Jockusch for help with ns lifetime measurements. ’ REFERENCES (1) Baldridge, A.; Samanta, S. R.; Jayaraj, N.; Ramamurthy, V.; Tolbert, L. M. J. Am. Chem. Soc. 2010, 132, 1498. (2) Baldridge, A.; Samanta, S. R.; Jayaraj, N.; Ramamurthy, V.; Tolbert, L. M. J. Am. Chem. Soc. 2011, 133, 712. (3) Jayaraj, N.; Maddipatla, M. V. S. N.; Prabhakar, R.; Jockusch, S.; Turro, N. J.; Ramamurthy, V. J. Phys. Chem. B 2010, 114, 14320. (4) Jayaraj, N.; Zhao, Y.; Parthasarathy, A.; Porel, M.; Liu, R. S. H.; Ramamurthy, V. Langmuir 2009, 25, 10575. (5) Porel, M.; Jayaraj, N.; Raghothama, S.; Ramamurthy, V. Org. Lett. 2010, 12, 4544. (6) Farran, A.; Deshayes, K.; Matthews, C.; Balanescu, I. J. Am. Chem. Soc. 1995, 117, 9614. (7) Farran, A.; Deshayes, K. D. J. Phys. Chem. 1996, 100, 3305. (8) Parola, A. J.; Pina, F.; Ferreiera, E.; Maestri, M.; Balzani, V. J. Am. Chem. Soc. 1996, 118, 11610. (9) Pina, F.; Parola, A. J.; Ferreiera, E.; Maestri, M.; Armaroli, N.; Ballardini, R.; Balzani, V. J. Phys. Chem. 1995, 99, 12701. (10) Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Modern Molecular Photochemistry of Organic Molecules; University Science Books: Sausalito, CA, 2010. (11) Place, I.; Farran, A.; Deshayes, K.; Piotrowiak, P. J. Am. Chem. Soc. 1998, 120, 12626. (12) Jockusch, S.; Zieka, O.; Jayaraj, N.; Ramamurthy, V.; Turro, N. J. J. Phys. Chem. Lett. 2010, 1, 2628. (13) Chen, J. Y. C.; Jayaraj, N.; Jockusch, S.; Ottavianai, M. F.; Ramamurthy, V.; Turro, N. J. J. Am. Chem. Soc. 2008, 130, 7206. (14) Jayaraj, N.; Jockusch, S.; Kaanumalle, L. S.; Turro, N. J.; Ramamurthy, V. Can. J. Chem. 2011, 89, 203. (15) Jordanides, X. J.; Scholes, G. D.; Fleming, G. R. J. Phys. Chem. B 2001, 105, 1652.

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