Exploiting Direct and Cascade Energy Transfer for Color-Tunable and

Article pubs.acs.org/JPCC

Exploiting Direct and Cascade Energy Transfer for Color-Tunable and White-Light Emission in Three-Component Self-Assembled Nanofibers Carlo Giansante,†,‡,§ Christian Schaf̈ er,†,‡ Guillaume Raffy,†,‡ and André Del Guerzo*,†,‡ †

Univ Bordeaux, ISM, UMR 5255, 351 Cours de la Libération, 33400 Talence, France CNRS, ISM, UMR 5255, 351 cours de la Libération, 33400 Talence, France

‡

ABSTRACT: The co-self-assembly of a blue-light emitting organogelator and specifically designed green and red emitting hosts yields lightharvesting nanofibers with tunable emissive properties. In particular, under near-UV excitation white-light emission is achieved in organogels and their constituting nanofibers as demonstrated by confocal fluorescence microspectroscopy. Steady-state and time-resolved emission spectroscopies reveal that color tuning in three component nanofibers is achieved exploiting excitation energy transfers occurring between the blue-emitting anthracene derivative and the green- and red-emitting tetracenes, whereas the latter emission is further sensitized through a cascade blue-to-green-to-red transfer sequence. Moreover, excitation energy migrates through the blue-emitting components by exciton hopping before being transferred to the acceptors, thus contributing to the light-harvesting process.

1. INTRODUCTION Self-assembly of chemical species by weak noncovalent interactions is a widespread strategy exploited by Nature in its forms and functions1 and is attracting increasing interest in the realization of artificial systems2 at all size scales.3 Self-assembly has indeed brought considerable advantages in the bottom-up approach to the realization of multichromophoric nanostructures for light-harvesting and opto-electronic purposes.4−6 Ordered assemblies of different chromophores, indeed, promote fast and efficient energy (and electron) transfer processes, further sustained by the migration of the excitation energy. Great efforts are recently being devoted to the realization of highly organized quasi-one-dimensional self-assembled nanostructures7 and particularly to those originating from low-molecular-weight organic chromophores.8 Such self-assembled nanofibers might represent an alternative toward the realization of useful photoactive nanomaterials exploiting relatively simple solution processing techniques. Networks of self-assembled nanofibers can lead to solventtrapping and thus to the formation of organogels.9 Therefore supramolecular gels represent suitable systems for promoting photoinduced energy transfer processes.10 In this regard, the gelating ability of an anthracene derivative bearing two alkyloxy side chains (indicated as B in Scheme 1) has already been reported:11,12 the gelation process relies on the self-assembly of B into blue fluorescent nanofibers that can be exploited as lightharvesting matrices capable of hosting suitable dialkyloxy tetracene derivatives and sensitizing their emission via an excitation energy transfer process.13 The high degree of order at the molecular level in B nanofibers14,15 contributes to align the © 2012 American Chemical Society

emission dipole of the B energy-donor with absorption dipoles of suitably designed energy-acceptor guests facilitating energy transfer processes. Recently we have proposed the co-assembly of B with a green-emitting tetracene derivative (indicated as G in Scheme 1) and an almost-red-emitting tetracene derivative (indicated as R in Scheme 1) leading to white-light emitting nanofibers:16 G and R show appreciable fluorescence quantum yields and bear long alkyloxy side chains that allow their incorporation into or onto B nanofibers without significantly altering the molecular-level order.16 In this paper, we show that the range of colors that can be reached with these three components is controllable and wider than what could be achieved with a two-component system. Moreover, we exploit steady-state and time-resolved emission spectroscopy of B gel containing G and R at different doping ratios to evaluate the energy transfer processes underneath colortuning and white-light emission:17 (i) B−to−G and B−to−R excitation energy transfer, discriminating faster and slower contributions; (ii) a cascade B−to−G−to−R energy transfer; (iii) B−to−B excitation energy hopping, determined by means of a suitable model system, H2B (Scheme 1),18 constituted by an organogelator similar to B that is transparent at the wavelength used to excite B. Received: July 24, 2012 Revised: September 19, 2012 Published: September 19, 2012 21706

dx.doi.org/10.1021/jp3073188 | J. Phys. Chem. C 2012, 116, 21706−21716

The Journal of Physical Chemistry C

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Scheme 1. Molecular Structures of 2,3-Didecyloxy-anthracene (B), 2,3-Dihexadecyloxy-5,12-diphenyl-tetracene (G), 2,3Dihexadecyloxy-5,6,11,12-tetraphenyl-tetracene (R), and 2,3-Didecyloxy-9,10-dihydro-anthracene (H2B)

Figure 1. (a) Picture of W-gel (2.0 mM B in deoxygenated MeOH with 0.012 equivalents of G and R) under UV light, λex = 365 nm; (b) fluorescence intensity confocal microscopy image (30 × 30 μm, 80 × 80 nm/pixel, λex = 385 nm, λem > 405 nm) of the W-gel; and (c) corrected fluorescence spectra of W-gel (gray line, λex = 365 nm) in bulk and (black line, λex 385 nm, λem > 405 nm) of a 10 × 10 μm portion (scattered laser is visible around 385 nm).

2. RESULTS AND DISCUSSION 2.1. B-gel and B + G + R Mixtures. The anthracene derivative B displays a robust self-assembling ability in various organic solvents such as alcohols or DMSO, affording translucent gels that excited in the near-UV show a structured emission spectrum reminiscent of the constituting anthracene without pollution by lower-energy emissive defects (λmax = 365 nm, λem = 412 nm, Φem = 0.08).19 This organogel is constituted by nanofibers that can be exploited as light-harvesting matrices capable of hosting and sensitizing by excitation energy transfer nacene derivatives, suitably designed to be coassembled with B. Thus green-emitting G (2,3-dihexadecyloxy-5,12-diphenyltetracene, λmax = 489 nm, λem = 502 nm, Φem = 0.53;20 CIE coordinates x = 0.262, y = 0.631) and orange-emitting R (2,3dihexadecyloxy-5,6,11,12-tetraphenyl-tetracene, λmax = 523 nm, λem = 555 nm, Φem = 0.69;20 x = 0.470, y = 0.525) have been designed to be blended in B nanofibers leading to luminescent nanostructures.16 When both G and R are added in small quantities (up to 3.6 × 10−5 M) to a 2.0 × 10−3 M MeOH hot solution of B, only the latter emits upon near-UV excitation. In contrast, when the solution is cooled to room temperature, the three molecules coassemble, and upon selective excitation of B with UV-light, the blue emission of B decreases while those of G

and R are observed. The intensity of G and R emission depends on their concentration, as shown below. A particular case is reached when 0.012 equivalents of both G and R are added and form a white-light emitting gel (W-gel, Figure 1a). The corresponding CIE 1931 chromaticity coordinates for a twodegree field of view (x = 0.306; y = 0.331) nearly match those of the most common white light standard (daylight simulator D65, x = 0.31292; y = 0.32933). The fluorescence quantum yield of the W-gel reaches a moderate value of 11%,19 indicating also that the addition of highly emissive acceptors improves the overall efficiency of the gel. Confocal microspectroscopy reveals that G and R donors are well-dispersed into or onto B nanofibers since no large phase-separation and inhomogeneous acceptor dispersion is observed (Figure 1b) and that such threecomponent nanofibers emit white light (Figure 1c). Partial excitation energy transfer processes occur upon B selective excitation and give rise to tunable and white-light emission: herein we elucidate these mechanisms. 2.2. B-to-G Excitation Energy Transfer. In order to investigate excitation energy transfer from B to G, a 2 mM B-gel in deoxygenated MeOH has been blended with G up to 36 μM. B can be selectively excited due to the low absorbance (AG) of G (at λex = 365 nm, AB = 9.7, since for B, ε365nm = 4850 M−1 cm−1 and 21707



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Figure 2. (a) Fluorescence spectrum (λex = 365 nm) of 2 mM B-gel in deoxygenated MeOH (solid line) and absorption spectrum of 10 μM G in THF solution (dashed line), at RT; (b) energy transfer rate as function of the distance between B and G.

Figure 3. (a) Fluorescence spectra (λex = 365 nm) of 2 mM B/deoxygenated MeOH organogel at RT upon addition of G up to 36 μM; (b) fluorescence decays (λex = 371 nm, λem = 436 nm) of 2 mM B/deoxygenated MeOH organogel at RT (full circles) in absence and (empty circles) in presence of 36 μM of G; (c) fluorescence decays up to 30 ns time scale of 36 μM of G (λem = 502 nm) in a 2 mM B/deoxygenated MeOH organogel at RT upon (black line, λex = 371 nm) B selective excitation and (dark gray line, λex = 456 nm) G selective excitation (IRF shown in light gray for clarity).

[B] = 2 mM, whereas AG ≤ 0.058, since for G, ε365 nm = 1600 M−1 cm−1 and [G] ≤ 36 μM). The feasibility of a resonant energy transfer process between B donor molecules and G acceptors is linked to a good overlap integral (JBG) between B emission and G absorption (Figure 2a, eq 1). J is defined as

coaxial triads where the emission dipoles, along the small axis of the molecule, are at a relative angle of 60°)14,15 and the refractive index of anthracene for doped B-gel estimated at 1.59.22 Gels of B including G with concentrations increasing up to 0.018 equivalents show a decrease in B steady-state emission and the appearance of G emission (Figure 3a) when B is selectively excited (λex = 365 nm). Accordingly, the CIE coordinates vary linearly from (0.159, 0.039) to (0.223, 0.383, “greener-thanturquoise”, Figure 4). Furthermore, the fluorescence lifetime of B decreases upon addition of G (λex = 371 nm, λem = 436 nm, Figure 3b), whereas the fluorescence of G rises upon selective excitation of B (λex = 371 nm, λem = 502 nm, black line in Figure 3c) and decays with a longer decay time than B (see also selective excitation of G (λex = 456 nm, λem = 502 nm, dark gray line in Figure 3c, some scattering observed at short times). All of these findings straightforwardly account for a photoinduced energy transfer process from B to G occurring in the doped gels. It is worth noting the differences between Stern−Volmer plots of the average fluorescence lifetime of B (full circles in Figure 5a) and the steady-state fluorescence intensity of B (empty circles in Figure 5a). The variation of τ0/τ is linear up to 0.005 equivalents of G, but reaching a plateau around 0.015 equivalents of G, while I0/I increases linearly up to 0.020 equivalents of G. These trends

∞

J (λ ) =

∫0 FD(λ)εA (λ)λ 4 dλ ∞

∫0 FD(λ) dλ

(1)

where FD(λ) is the emission spectrum of the donor and is dimensionless and εA is the absorption spectrum of the acceptor expressed in M−1 cm−1.21 The value of the overlap integral is large enough (JBG = 1.4 × 1014 M−1 cm−1 nm4) to allow energy transfer on the nanometer scale. Indeed, at a distance of R0 = 1.84 nm between B and G, the energy transfer efficiency is 50% (eq 2). R0 is defined as R 0 = 0.0211(n−4κ 2 ΦDJ(λ))1/6

(2)

where ΦD is the donor fluorescence quantum yield, κ is the orientation factor, N is the Avogadro number, and n is the refractive index.21 The R0 value has been obtained assuming κ2 = 1/4 (fluorescence polarization microscopy suggests that G takes the place of a B molecule in B-gel,16 in which B is packed in 2

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kET is known to depend on the distance (d) between donor− acceptor (B−G) pairs according to kET(d) =

(4)

To be more precise, the distance between a donor in its excited state (B*) and an acceptor have to be considered, since G is always surrounded by B molecules when dispersed into a fiber. Hence, the distribution of B*−G distances induces a distribution of energy transfer rates kET. In this case, with below 0.005 equivalents of G (Figure 5a) the kET rates are mostly 1.02 nm. At higher G loading (above 0.005 equivalents, Figure 4a), a faster quenching appears in addition to the “dynamic” quenching. This “static” quenching reflects kET rates >5.0 × 109 s−1 between B and G and can be attributed to B*−G pairs that are at distances shorter than 1.02 nm. The probability that an excited state meets an acceptor G is thus increased with a higher concentration of G in the B-matrix. The quenching of B is accompanied by a sensitization of the emission of G. The ratiometric plot of G-to-B fluorescence (Figure 5b) increases linearly with the addition of G, suggesting that in this concentration range all added acceptor molecules are dispersed into the B-donor fiber and accessible for energy transfer processes. Time-resolved fluorescence also shows that upon selective excitation of B the excited states of G are populated with a rise time τrise = 2.9 ns (with 0.018 equivalents of G, Figure 3c) and decay-time τ = 21.4 ns, thus clearly slower than upon direct excitation (14.0 ns in the gel and τ = 13.8 ns in THF, kTHF = 3.8 × 107 s−1 = (26 ns)−1). This difference is discussed in r section 3. 2.3. B-to-R Excitation Energy Transfer. In analogy to the previous section, investigation of excitation energy transfer from B to R has been conducted on a 2 mM B-gel in deoxygenated MeOH titrated with R up to 36 μM. Also in this case, B can be selectively excited due to the low absorbance (AR) of R (at λex = 365 nm, AB = 9.7, since for B, ε365nm = 4850 M−1 cm−1 and [B] = 2 mM, while AR ≤ 0.086, since for R, ε365nm = 2400 M−1 cm−1 and [R] up to 36 μM). The overlap integral JBR is only 15% smaller than JBG and thus also sufficient for energy transfer to occur (Figure 6a, JBR = 1.2 × 1014 M−1 cm−1 nm4). The value at which

Figure 4. Plot of CIE coordinates of: pure B (gel), G and R (solutions; crosses), and the mixed gels B + G (0−0.018 equiv, diamonds), B + R (0−0.018 equiv, squares), B + G (0.018 equiv) + R (0 to 0.018 equiv, triangles), B + R (0.012 equiv) + G(0 to 0.018eq, circles).

are reminiscent of combined static and dynamic quenching mechanisms, despite such terminology is rather counterintuitive since no appreciable diffusion of G molecules occurs within B nanofibers. Time-integrated fluorescence (I0/I) is known to be comprehensive of all quenching mechanisms (ηtot Q ) occurring at any time scale, including those much faster than the instrumental response (“static quenching” efficiency ηstat Q ) and those occurring at a time scale comparable to that of fluorophore lifetime (“dynamic quenching” efficiency ηdyn Q , measurable directly by τ0/ τ).23 The quenching efficiencies thus correspond to ηQtot = ηQstat + ηQdyn

6 1 ⎛ R0 ⎞ ⎜ ⎟ τD ⎝ d ⎠

(3)

At low G loading (below 0.005 equivalents), τ0/τ and I0/I are equivalent, indicating a pure “dynamic” quenching of B fluorescence in the gel. The instrumental response being ∼200 ps (Figure 3b), an upper limit to the detectable rate of fluorescence resonance energy transfer kET is ∼5.0 × 109 s−1.

Figure 5. (a) Stern−Volmer plots of (empty circles, λex = 365 nm, λem = 436 nm) steady-state and (full circles, λex = 371 nm, λem = 436 nm) time-resolved fluorescence quenching of B upon G addition up to 36 μM to a 2 mM B-gel in deoxygenated MeOH (lines are guide for the eye); (b) ratiometric plot of G steady-state fluorescence sensitization upon B direct excitation (λex = 365 nm). 21709



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Figure 6. (a) Fluorescence (λex = 365 nm) spectrum of 2 mM B-gel in deoxygenated MeOH (solid line) and absorption spectrum of 10 μM R in THF solution (dashed line), at RT; (b) energy transfer rate as function of the distance between B and R.

Figure 7. (a) Fluorescence spectra (λex = 365 nm) of 2 mM B/deoxygenated MeOH organogel at RT upon addition of R up to 36 μM; (b) fluorescence decays (λex = 371 nm, λem = 436 nm) of 2 mM B/deoxygenated MeOH organogel at RT in the absence (full circles) and in presence (empty circles) of 36 μM of R; (c) fluorescence decays of 36 μM of R (λem = 580 nm) in a 2 mM B/deoxygenated MeOH organogel at RT upon (black line, λex = 456 nm) R selective excitation and (dark gray line, λex = 371 nm) B selective excitation.

Figure 8. (a) Stern−Volmer plots of (empty circles, λex = 365 nm, λem = 436 nm) steady-state and (full circles, λex = 371 nm, λem = 436 nm) time-resolved fluorescence quenching of B upon R addition up to 36 μM to a 2 mM B-gel in deoxygenated MeOH (lines are guide for the eye); (b) ratiometric plot of R steady-state fluorescence sensitization upon B direct excitation (λex = 365 nm).

the energy transfer efficiency from B to R is 50% is R0 = 2.11, assuming κ2 = 2/3 (previous fluorescence polarization microscopy studies suggested indeed that R assumes a random orientation in a W-gel).16

Addition of up to 0.018 equiv of R to B provides gels with decreasing steady-state emission of B and increasing emission of R (Figure 7a; λex = 365 nm). The CIE coordinates vary linearly from (0.159, 0.039) to (0.318, 0271, light purple, almost white, 21710



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Figure 4). Besides, the fluorescence decay-time of B decreases in presence of R (λex = 371 nm, λem = 436 nm, Figure 7b), whereas the fluorescence profile of R shows a rise-time upon selective excitation of B (λex = 371 nm, λem = 580 nm, dark gray line in Figure 7c) that is not observed upon selective excitation of R (λex = 456 nm, λem = 580 nm, black line in Figure 6c). All of these findings straightforwardly account for a photoinduced energy transfer process from B to R occurring in R doped gels. The blend B + R exhibits trends similar to B + G. Indeed, τ0/τ coincides with I0/I (Figure 8) below 0.005 equiv of R (within experimental error), but a “static” quenching appears at higher concentrations of R. In this case, the faster processes are relevant at average donor−acceptor distances (dBR) below 0.97 nm. The sensitized emission of R also increases linearly with the addition of R, as shown by the ratiometric plot in Figure 8b, suggesting a good blend of donor and acceptor molecules. The kinetics of the sensitization process are also comparable to the previous case, occurring with a rise time τrise = 3.1 ns (with 0.018 equivalents of R, Figure 7c) and yielding a decay-time of R (τ = 19.4 ns). This decay is slower than upon direct excitation of R in solution (λexc = 456 nm, τ = 13.8 ns in the gel and 13.6 ns in THF, kTHF = 5.1 × r 107 s−1 = (20 ns)−1). Similarly to the case of G in a mixed gel, the increased decay-time of R is discussed in section 3. Discrimination between time-integrated and time-resolved quenching (Figures 4a and 8a) allows evaluation of very fast (“static”) and slower (“dynamic”) quenching efficiencies as a function of quencher concentration

ηQstat ηQdyn

=

K stat Kdyn

2.4. G-to-R Excitation Energy Transfer. The resonant energy transfer from G to R should also be considered since the spectral overlap between G and R is even higher than those between the anthracene and the tetracenes (JGR = 3.8 × 1014 M−1 cm−1 nm4, Figure 9a). The G-R distance at which resonant energy transfer is 50% efficient (R0) is 3.53 nm (Figure 9b) assuming κ2 = 2/3 (G and R relative random dispersion in B-gel) and n = 1.59 (refractive index of anthracene),22 making the energy transfer possible also at low G and R loads. In order to investigate such an excitation energy transfer from B-sensitized G* to R in a mixed gel, a 2 mM B-gel containing 36 μM G in deoxygenated MeOH has been titrated with R up to 36 μM (Figure 10a). Even in this three-component system, the selective excitation of B is guaranteed by the low absorbance of the dopants (at λex = 365 nm AB = 9.7, since for B, ε365nm = 4850 M−1 cm−1 and [B] = 2 mM, while AG + AR ≤ 0.144, since for G, ε365nm = 1600 M−1 cm−1 and for R ε365nm = 2400 M−1 cm−1), and the emission of G at 502 nm does not overlap with the emission of the other components. Figure 10a shows that the sensitized emission of G is reduced with the addition of R (see also triangles in Figure 10c), whereas the emission of R gradually increases. In an analogous experiment, when G is added to a B + R mixed gel (Figure 10b), the emissions of both G and R are enhanced. The slopes of G ratiometric fluorescence variation are identical, although with opposite signs (Figure 10c), when adding R to a B + G gel or G to a B + R gel. However, the sensitization of G is reduced by half as compared to the case where G is added to B alone (compare empty circles to filled circles in Figure 10c). This suggests that the presence of R reduces the probability of populating the excited states of G. To analyze the emission variation of R upon addition of G, the contribution of G at 580 nm needs to be subtracted (both emissions overlap). Figure 10d (gray line) shows the neat fluorescence of R deduced after subtraction of the emission of G at 580 nm, estimated in the presence of B and R from the previously described experiment (triangles in Figure 10d). The resulting values indicate that the emission of R increases by 46% with the addition of G, in agreement with the 50% decrease of G emission upon addition of R (Figure 10c). These results not only indicate a competition between G and R for sensitization by B in a three-component gel but also reveal a further energy transfer process from G-to-R upon selective excitation of B. As in the B-to-G and B-to-R energy transfer processes, time-resolved fluorescence reveals that the G-to-R energy transfer occurs at different time scales. Due to a “dynamic” energy transfer, the lifetime of the sensitized fluorescence of G at 502 nm decreases from 21.4 to 17.3 ns when 36 μM of R are added to a B + G gel (excitation in the UV, Figure 11, Table 2). This quenching clearly demonstrates the occurrence of G-to-R energy transfer, with a “dynamic” kinetic constant of (90 ns)−1, accounting for the quenching of 19% of G* upon addition of R. The remaining quenching of G* (31% out of 50%) are due to two effects: (i) a “static” component of the G− to−R transfer estimated to occur at distances dGR < 3.53 nm and (ii) a lack of sensitization of G by B due to the addition of the competing R molecules. In contrast, the decay-time of fluorescence at 580 nm increases from 19.4 to 24.0 ns when 36 μM G is added to a B + R gel (Figure 11b, Table 2). Although in the B + R gel the emission at 580 nm is due only to R, in the three-component gel it is due to both G and R yielding a multiexponential decay profile. This decay is expected to contain only a smaller contribution of G emission decaying with a lifetime of 17.3 ns (see Figure 10d). Thus, the R emission lifetime

(5)

through a modified Stern−Volmer equation I0 = (1 + Kdyn[Q ])(1 + K stat[Q ]) (6) I tot where τ0/τ = 1 + Kdyn[Q]. When ηQ = 1 − (I/I0), combination with eqs 3 and 5 gives the quenching efficiencies values reported in Table 1 when [G] = 36 μM or [R] = 36 μM. It should be noted Table 1. Quenching Efficiencies of B in the Presence of 0.018 equiv of G or/and Ra B

I0/I

ηtot Q

τav(B)

ηdyn Q

ηstat Q

+ G 0.018 equiv + R 0.018 equiv + G, R 0.018 equiv

2.3 1.8 3.4

57% 45% 71%

6.7 4.6 5.7 3.8

32% 15% 44%

25% 30% 27%

a tot ηQ

stat tot dyn = 1 −(I/I0), whereas ηdyn Q = 1 − (τ/τ0) and ηQ = ηQ − ηQ .

a large difference in ηdyn Q values for B in presence of G or R, accounting for the difference in relative donor−acceptor orientation and spectral overlap, while only slight differences can be observed in ηstat Q values. Addition of both G and R to a 2 mM B-gel in deoxygenated MeOH results in further decrease of B fluorescence steady-state intensity and lifetime (see below). Interestingly, evaluating total and dynamic quenching at [G] = [R] = 36 μM reveals that (i) the efficiency of B “dynamic” quenching is almost the sum of that achieved by adding the acceptors individually and (ii) that “static” quenching is instead rather insensitive to the nature of the acceptor unit (G or R, see Table 1) and does also not change when both tetracenes are added although the total concentration of quencher is higher. 21711



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Figure 9. (a) Fluorescence (λex = 460 nm) spectrum of 10 μM G in THF solution (dashed line) and absorption spectrum of R in THF solution (solid line), at RT; (b) energy transfer efficiency as function of the distance between G and R.

Figure 10. Ratiometric fluorescence data of organogels, where B is always 2 mM, in B + G, G is 36 μM, and in B + R, R is 36 μM and titrations are performed from 0 to 36 μM (all are deoxygenated using MeOH at RT, λex = 365 nm). (a) spectra of B + G upon titration with R; (b) spectra of B +R upon titration with G; (c) emission of G: (triangles) upon titration with R of B + G, (circles) upon titration with G of (full circles) B or (empty circles) B + R; (d) emission at 580 nm of R and/or G: (full circles) upon titration with R of B, (empty circles) upon titration with G of B + R, (triangles) calculated contribution of G upon titration with G of B + R, (gray line) resulting calculated contribution of R upon titration with G of B + R.

2.5. B-to-B Excitation Energy Migration. A final process that has to be considered corresponds to the homotransfer B-toB, also named exciton hopping, possibly occurring in competition with the heterotransfers to G or R acceptors. Evidence for this mechanism can be obtained when comparing a B-gel to the model system H2B + B in which B−B interactions do

can be estimated to be slightly larger than 24.0 ns (see discussion below). Overall, the efficiencies of total, static, and dynamic quenching of G and sensitization of R are very similar (Table 2), indicating that no excited states are lost in the three-component system as compared to the two-component nanofibers. 21712



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Figure 11. (a) Fluorescence profiles (λex = 371 nm, λem = 502 nm) of 2 mM B + G 0.018 equivalents/deoxygenated MeOH organogel at RT (black line) without and upon addition of (gray line) R 0.018 equiv; (b) fluorescence profiles (λex = 371 nm, λem = 580 nm) of 2 mM B + R 0.018 equiv/ deoxygenated MeOH organogel at RT (black line) without and upon addition of (gray line) G 0.018 equiv.

H2B+B gel, with comparable bandwidth and wavelengths. B emission in a H2B-gel is only slightly blue-shifted, due to a slightly different microenvironment, providing thus a good model for the study of B-to-B exciton hopping in pristine B nanofibers.24 Furthermore, the homogeneity of the environment experienced by B molecules and the lack of interaction between them is also suggested by the monoexponential decay of B fluorescence (Figure 13a). Several differences can be observed between a B-gel and a H2B/B-gel. In a B-gel, the (S1 → S0) v0−0 transition is not observed, whereas it is clearly distinguished at 390 nm in a H2B/ B-gel. This can result from a reabsorption of the emission of B at these low wavelengths. Time-resolved fluorescence further confirms the B-to-B exciton hopping in a B-gel. Indeed, the B fluorescence anisotropy decays rapidly and reaches the steadystate value (r∞ = 0.06, Figure 13b) in ∼700 ps. Since the rotational diffusion of B molecules in B nanofibers can be neglected, any loss of B fluorescence polarization can be attributed to an energy transfer process. Besides, the faster decay of the fluorescence of B in a B-gel, as compared to B in H2B/B-gel, can be attributed to exciton hopping toward rare nonemissive traps and subsequent emission quenching.

Table 2. Quenching Efficiencies of G upon B Selective dyn Excitation Due to the Presence of R, ηtot Q = 1 − (I/I0), ηQ = 1 − stat tot dyn (τ/τ0), and ηQ = ηQ − ηQ , and Sensitization efficiencies of R upon B selective excitation due to energy transfer from G, ηtot S stat tot dyn =1 − (I0/I), ηdyn S = 1 − (τ0/τ), and ηS = ηS − ηS quenching of G B+G 0.018 equiv I0/I τ ηtot Q ηdyn Q ηstat Q

21.4

sensitization of R B+R 0.018 equiv

(B + G) + R both 0.018 equiv 2.0 17.3 50% 19% 31%

I0/I τ ηtot S ηdyn S ηstat S

19.4

(B + R) + G both 0.018 equiv 0.54 24.0 46% 19% 27%

not take place. In this case, B is dispersed in small amounts (0.01 equiv) in 2,3-didecyloxy-9,10-dihydroanthracene (H2B, Scheme 1), an organogelator that does not absorb the light used to excite 18 B (λabs max,H2B = 284 nm). H2B self-assembles into a network of nanofibers trapping the solvent and thus gelling the solution, so that B can be sufficiently dispersed into the H2B-gel to eliminate homoenergy transfers (Figure 12b). The (S1 → S0) v0−1, v0−2, and v0−3 emission bands of B can be observed both in the B-gel and

Figure 12. (a) Fluorescence time-resolved confocal microscopy image (30 × 30 μm, 80 × 80 nm/pixel, λex = 385 nm, λem > 405 nm) of a 2 mM H2B-gel containing B 0.01 equiv/deoxygenated MeOH organogel at RT; (b) (solid lines) absorption and (λex = 385 nm, dashed lines) fluorescence spectra of 2 mM (black lines) B-gel and (gray lines) H2B-gel containing B 0.01 equiv. 21713



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Figure 13. (a) Fluorescence decays (λex = 371 nm, λem = 436 nm) of (empty circles) a 2 mM B-gel and (full circles) a 2 mM H2B-gel containing B 0.01 equiv/deoxygenated MeOH organogel at RT; (b) (solid lines) steady-state and (empty circles) time-resolved anisotropy of a 2 mM B-gel.

yields color-tunable fluorescent nanofibers and soft materials, showing controllable blue, green and several tints of white-light emission. The CIE coordinates can be fine-tuned, by adapting the proportions of B, G, and R, within the range of values delimited by the extreme concentrations of G and R tested (see Figure 4). The emitted colors result from the sensitization of three components upon UV excitation, exploiting the partial excitation energy transfer from the directly excited blue-emitting anthracene matrix toward the green and red tetracene emitters. In a two-component system, the energy transfer kinetics occurs at various scales, ranging from kET > (200 ps)−1 for the “static” transfer from B to a first “shell” of the closest acceptors to ∼(3 ns)−1 for the “dynamic” processes with the most distant tetracenes (see risetimes of G and R in mixed gels). As expected for a Förster-type energy transfer mechanism, B sensitizes G more efficiently than R due to a better spectral overlap. Additionally, this three-component system is a rare case of nanofibres in which an energy transfer cascade has been clearly characterized. Indeed, a B-to-G-to-R sequential transfer contributes significantly to the sensitization of the red-emissive component, with kinetics of G-to-R transfer occurring on several scales, >(200 ps)−1 and ∼(90 ns)−1 (calculated from the decays of G, Table 2). The noteworthy efficiency of the intrananofiber energy transfer processes is a result of the high degree of molecular-level order that results from self-assembly and promotes favorable donor−donor and donor−acceptor dipole alignment. Thus, excitation energy can be directly transferred to the acceptors by Coulombic energy transfer. Moreover, exciton hopping over B molecules contributes further to the lightharvesting process. The possible photoinduced processes in three component B−G−R nanofibers are depicted in Scheme 2. The additional complexity of the three-component system is revealed by further analysis of the photophysical data. Indeed, although G and R compete to receive excitation energy from B, disregardful of the nature of the acceptor(s), the value of “static” energy transfer does not surpass a value of 25−30%. This could be due to a limitation of acceptor molecule concentration in the first ‘shell’ of excited B*. There is however no similar limitation for the “dynamically” sensitized acceptors. The current data do not clearly demonstrate whether these molecules are dispersed equally within the B matrix or whether some different microenvironments exist. Confocal fluorescence microscopy studies16 (CFM, spatial resolution ∼200 nm) and the dynamics

An important mechanism for exciton hopping is the Coulombic homotransfer between B donor molecules. Indeed, the Förster overlap integral JBB is 4.0 × 1013 M−1 cm−1 nm4 and only 3.5 times smaller than JBG, as estimated from the absorption spectrum of a B-gel (solid black line in Figure 12a) and the emission of B in the H2B/B model system (dashed gray line in Figure 12a). According to eq 2 and using the angles of 60° between emission dipoles deduced from the crystal structure of a series of B-analogue gelators bearing shorter alkyl chains,14,15 R0 is estimated at 1.49 nm for a pair of B’s in a triad in a B-gel. The assumption of a pure Coulombic isotropic energy homotransfer between the nearest B molecules in a triad with a lattice constant a = 0.52, estimated from the crystallographic structures mentioned previously, yields a theoretical value for the exciton diffusion length LD according to eq 725 LD =

3 a2 1 R0 = 3k f 3 a2

(7)

where kf is the rate constant for the homotransfer process. The resulting value is LD = 7.1 nm, in agreement with the exciton diffusion domain obtained for other organic crystals.25b The donor−donor energy transfer predominately occurs with the nearest neighbors, like the donor−acceptor process.26 In addition, transfers and migration toward more distant emissive or nonemissive energy acceptors are occurring as expected from the combination of donor−donor and donor−acceptor Förstertype processes. Considering the proposed organization of the molecules within the nanofibers, the energy hopping should be anisotropic, thus different along the c axis than along the a and b axes, though values given here are representative of an isotropic average.

3. DISCUSSION The nanofibers of the anthracene derivative B, composing an organogel, act as an efficient light-harvesting matrix capable of hosting G and R tetracene derivatives added in small amounts. Co-assembly of these three components occurs thanks to supramolecular interactions that are favored by the structural similarities of the molecules: all are linear acenes (anthracene and tetracenes) prolonged in their long axis with two alkoxy-chains (decyl or hexadecyl). The homogeneous dispersion of the acceptors, within the range tested of a maximum of 1.8% equiv each, is suggested by the linear variation of the spectral properties of the gels with regular addition of the tetracenes. This property 21714



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the sensitization processes, and a similar behavior is currently investigated in a new system (unpublished results).

Scheme 2. Energy Level Diagram Showing the Excited States Involved in the Main Photophysical Processesa of the B−G− R Three-Component Systemb

4. CONCLUSION The fluorescence studies and the microspectroscopy have revealed that the material is composed of nanofibers that individually emit light with color coincident with that of the bulk gel. The acceptor molecules are thus quite homogeneously dispersed into or onto the fibers, although it is not clearly established whether different species of acceptor or clusters are present on scales

Exploiting Direct and Cascade Energy Transfer for Color-Tunable and

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