Triplet Sensitized Red-to-Blue Photon Upconversion - The Journal of

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Triplet Sensitized Red-to-Blue Photon Upconversion Tanya N. Singh-Rachford and Felix N. Castellano* Department of Chemistry and Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio, 43403

ABSTRACT Sensitized red-to-blue upconversion with a record 0.8 eV anti-Stokes shift has been achieved utilizing platinum(II) tetraphenyltetrabenzoporphyrin (PtTPBP) as the triplet sensitizer and perylene as the energy acceptor/annihilator in deaerated benzene. Selective 635 nm excitation of PtTPBP results in the observation of perylene fluorescence centered at 451 nm. Stern-Volmer analysis of dynamic phosphorescence quenching of PtTPBP by perylene yields a triplettriplet energy transfer quenching constant of 4.08 109 M-1s-1. Clear evidence for the subsequent triplet-triplet annihilation of 3perylene* was afforded by the quadratic dependence of the integrated perylene fluorescence spectra with respect to incident 635 nm light power. The maximum upconversion quantum yield of perylene fluorescence under our sensitized excitation conditions is 0.0065 ( 0.0001, as ascertained by relative actinometry. The present chromophore combination was successfully translated into the solid state using a low glass transition temperature polyurethane host polymer, which produced upconverted photons for months when stored under ambient conditions. SECTION Kinetics, Spectroscopy

U

excited with focused high peak power laser pulses at 860 nm, rendering upconverted 9,10-diphenylanthracene (DPA) emission in the blue region of the spectrum exhibiting a quartic (x4) incident light power dependence.21 To truly harness the full capabilities of upconversion technology, new chromophore combinations must be evaluated in the hopes of further extending the anti-Stokes energy shift. This goal inspired the present contribution wherein we successfully demonstrate red-to-blue photon upconversion achieving a record 0.8 eV anti-Stokes shift in sensitized TTA. Table 1 summarizes the various upconverting systems that have been investigated within our research group to date. Table 1 illustrates that most of the currently functioning chromophore combinations yield anti-Stokes energy shifts on the order of 0.5 eV. Hence, one of our current research goals seeks to identify other molecular compositions that will result in a significant increase in the net anti-Stokes energy gap. The chemical structures of the chromophores investigated in this study are shown in Chart 1. The ground state absorption and emission spectra (in benzene) of the red absorbing sensitizer (platinum(II) tetraphenyltetrabenzoporphyrin (PtTPBP)) and the perylene acceptor/annihilator are presented in Figure 1. PtTPBP displays strong absorption transitions for the Soret band at 430 nm and Q-band at 611 nm which tails to ∼650 nm whereas perylene possesses structured absorption transitions with a small shoulder at 368 nm and very strong peaks with wavelength maxima at 389 nm, 411 nm, and 439 nm. The emission spectrum of a

pconversion based on sensitized triplet-triplet annihilation (TTA) exhibits strong potential to facilitate photon management in promising device technologies. In this sensitized TTA process, short wavelength photons are effectively produced from the absorption of lower energy light by highly quenched triplet sensitizers. The triplet energy stored within the two long-lived quencher species can merge, producing a higher energy excited singlet state and the corresponding ground-state species. The excited single state dispenses some of its energy through the emission of a photon, which is significantly higher in energy in relation to the exciting light. This particular strategy was first introduced by Parker and Hatchard in 1962 using various combinations of organic chromophores to serve the role of both triplet sensitizer and acceptor/annihilator.1,2 Our group3-11 and others12-19 have demonstrated that this phenomenon is exceedingly effective when transition metal-based sensitizers are applied in these schemes in concert with energetically appropriate acceptor/annihilator species. Interestingly, most of the efficiently functioning upconversion systems based on sensitized TTA generally possess energy gaps on the order of ∼0.5 eV between the excitation light and the maximum singlet fluorescence emission band exhibited by the acceptor/annihilator. We recently reported an upconverting composition that offers one of the largest energy differences experimentally attained to date (0.58 eV),10 along with a completely organic-based combination that facilitated visible-to-UV conversion (0.64 eV).20 Most recently, we successfully applied two-photon light activation to bimolecular triplet energy transfer, producing an artificially large antiStokes shift of 1.38 eV for sensitized TTA.21 In this example, [Ru(dmb)3]2þ (dmb = 4,40 -dimethyl-2,20 -bipyridine) was

r 2009 American Chemical Society

Received Date: October 21, 2009 Accepted Date: November 6, 2009 Published on Web Date: November 13, 2009

195

DOI: 10.1021/jz900170m |J. Phys. Chem. Lett. 2010, 1, 195–200

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Table 1. Anti-Stokes Energy Gaps Measured in Various Triplet Sensitizer and Acceptor/Annihilator Chromophore Mixtures from Our Laboratorya donor [Ru(dmb)3]

acceptor 2þ

An

λexc/nm

Eexc /eV

λobs/nm

Eobs /eV

Eobs - Eexc/eV

reference

514.5

2.41

400

3.10

0.69

4

biacetyl

PPO

442

2.80

360

3.44

0.64

20

PtTPBP

2CBPEA

635

1.95

490

2.53

0.58

10

PdOEP

DPA

544

2.28

445

2.79

0.51

6

PdPc(OBu)8

rubrene

725

1.71

560

2.21

0.50

7

[Ru(dmb)3]2þ

DMA

514.5

2.41

431

2.88

0.47

9

Ir(ppy)3 PtTPBP

pyrene BD-1

450 635

2.76 1.95

390 527

3.18 2.35

0.42 0.40

28 8

[Ru(dmb)3]2þ

DPA

514.5

2.41

445

2.79

0.38

4

PtTPBP

BD-2

635

1.95

556

2.30

0.28

8

a

exc is excitation, obs is observed, PdOEP is palladium(II) octaethylporphyrin, PdPc(OBu)8 is palladium(II) octabutoxyphthalocyanine, ppy is 2phenylpyridine, An is anthracene, PPO is 2,5-diphenyloxazole, 2CBPEA is 2-chloro-bis-phenylethynylanthracene, DMA is 9,10-dimethylanthracene, BD1 is 4,4-difluoro-8-(-4-iodophenyl)-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene, and BD-2 is 2,6-diethyl-4,4-difluoro-8-(-4-iodophenyl)-1,3,5,7tetramethyl-4-bora-3a,4a-diaza-s-indacene.

Chart 1. Chemical Structures of (a) PtTPBP and (b) Perylene

Figure 1. Absorbance and emission spectra of PtTPBP and perylene measured in deaerated and aerated benzene, respectively, at RT. All are normalized to an arbitrary maximum of 1.0.

Ksv is the Stern-Volmer quenching constant. The bimolecular quenching constant (kq) is obtained from the slope of the Stern-Volmer plot according to the relation, Ksv = kqτ0. The Stern-Volmer plot of τ0/τ -1 versus [Q] is shown in Figure 2 and clearly demonstrates dynamic quenching. A rather large Ksv of 169385 M-1 and corresponding kq of 4.08 109 M-1 s-1 (τ0 = 41.5 μs) was obtained, the latter approaching the diffusion limit in benzene, 1.1 1010 M-1 s-1 at 25 οC.22 The primary quenching pathway is presumed to proceed through triplet-triplet energy transfer from PtTPBP to perylene, and the upconverted fluorescence that is observed and discussed below must result from TTA, implying relatively efficient triplet state production in the porphyrin quenching step. Selective excitation of PtTPBP in a freeze-pump-thaw degassed benzene solution containing perylene, upon 635 nm excitation resulted in clearly observable upconverted fluorescence from perylene, anti-Stokes shifted relative to the excitation wavelength, with an onset near 440 nm. Figure 3a presents a typical emission intensity dependence profile of this solution measured as a function of incident excitation power ranging from 0.0019 to 0.043 W/cm2, illustrating both the upconverted perylene emission as well as the residual unquenched phosphorescence of PtTPBP. Analysis of the sensitized upconverted integrated emission intensity as a

degassed solution of PtTPBP exhibits strong phosphorescence centered at 770 nm at room temperature (RT; τ = 41.5 μs) while that of a dilute solution of perylene shows intense structured fluorescence at 445 nm, 471 nm, 502 nm with a shoulder at 545 nm (Figure 1). The fluorescence spectrum of perylene is nearly a perfect mirror image with respect to its absorption spectrum. Perylene was chosen as the acceptor/ annihilator because of its high fluorescence quantum yield (Φ = 0.75)22 in the blue and its strategically positioned triplet state energy. At the concentrations of perylene that achieve the highest upconversion quantum efficiency, the perylene fluorescence quantum yield is somewhat attenuated (Φ = 0.58) with respect to the literature value quoted above. Notably, perylene exhibits moderate thermal and photochemical stability, which are also important considerations for implementation in an upconversion scheme. The bimolecular quenching of the triplet excited state phosphorescence in PtTPBP by energy transfer is readily quantified by the Stern-Volmer relation (eq 1), ð1Þ τ0 =τ ¼ 1 þ K sv ½Q where τ0 is the photoluminescence lifetime in the absence of the quencher while τ is the lifetime in the presence of the quencher; [Q] is the molar concentration of the quencher, and


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Figure 3. (a) Emission profile of the upconverted perylene emission in addition to the unquenched phosphorescence of PtTPBP in a mixture of PtTPBP/perylene upon 635 nm excitation in deaerated benzene measured as a function of the incident light. The arrow shows the position of the perylene excimer. (b) Normalized integrated emission intensity of the upconverted perylene emission (black squares) and the unquenched PtTPBP emission (red circles) measured as a function of the normalized incident optical power density. The black line shows the best quadratic fit (x2) to the data, and the red line is the best linear fit. Inset: Double logarithm plot of the normalized data; the slope of the black line is 1.8, and the slope of the red line is 1.0.

Figure 2. (a) Single wavelength phosphorescence intensity decays of PtTPBP at 770 nm upon 635 nm excitation measured as a function of increasing perylene in deaerated benzene. (b) Stern-Volmer plot generated from excited-state lifetime quenching of PtTPBP by perylene at 770 nm using first-order fits to the data from panel a.

function of the incident power density is displayed in Figure 3b. The solid black line passing through the data points represents the best quadratic fit (x2) to the data illustrating the nonlinear photochemistry driving these processes. For comparison, the integrated intensity of the unquenched phosphorescence of the porphyrin was also measured as a function of incident laser power and resulted in the expected linear excitation power dependence (solid red line). The data were more closely evaluated using the double logarithm plot shown in the inset of Figure 3b, which better differentiates linear and quadratic incident power dependencies. The slope associated with the solid red circles is 1.0 indicative of the one-photon excitation processes associated with the generation of the phosphorescence of the porphyrin. The slope of the solid black squares is 1.8, demonstrating that the observed fluorescence intensity is approximately proportional to the square of the incident power at 635 nm, i.e., [3perylene*]2. The combined data support a mechanism wherein selective excitation of the PtTPBP sensitizer at 635 nm leads to triplettriplet energy transfer to the acceptor in which TTA ultimately results in the generation of singlet excited perylene, which radiatively decays to the ground state by singlet fluorescence. A Jablonski diagram depicting the relevant chromophore energy levels and intermolecular processes necessary to produce the upconverted fluorescence of perylene in the current experiments is summarized in Scheme 1. Figure S1 compares the upconverted fluorescence of perylene following long wavelength triplet sensitization of PtTPBP at 635 nm with that of the conventional fluorescence of perylene obtained with 360 nm excitation. Both spectra


contain contributions from the singlet fluorescence of perylene, with the onset of the prompt fluorescence at 425 nm while that of the sensitized TTA-based fluorescence was redshifted by 13 nm with an onset at 438 nm. The slight red shift observed in the latter results exclusively from an inner filter effect due to the high concentration of perylene and cannot be circumvented in the upconversion experiments. It should also be mentioned that the small feature observed in Figure S1 at longer wavelengths (565 nm) is attributed to perylene excimer emission23 and is also a result of using perylene concentrations sufficient to facilitate observable frequency conversion. Also worth mentioning is that this inner filter effect contributes a slightly attenuated anti-Stokes energy shift (0.80 eV) relative to that which would be nominally observed in its absence, 0.84 eV. The percent relative quantum yield (QY) of upconverted fluorescence was measured as a function of perylene concentration, determined relative to a methylene blue quantum counter (Φf = 0.030)24 using excitation at 635 nm at 22 mW24 (Figure S2). Although the emission profile of methylene blue does not effectively overlap that of perylene, highly reproducible QY data were obtained over many independent measurements in our conventional single photon counting fluorimeter (see experimental section for details). Figure S2 exhibits a steady increase in the upconverted fluorescence of perylene with increasing concentration. A plateau is observed beyond

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Scheme 1. Qualitative Jablonski Diagram Illustrating the Sensitized TTA Upconversion Process between PtTPBP and Perylenea

a

ISC is intersystem crossing, TTET is triplet-triplet energy transfer, and TTA is triplet-triplet annihilation. Solid colored lines represents radiative transitions.

1.67 10-4 M perylene, resulting in a maximum percent QY of 0.65 ( 0.01. At present, we can only speculate about why the upconversion yield is relatively low. One possibility lies in the initial triplet sensitizer quenching step. While we clearly observe the net results of triplet energy transfer in our experiments, we do not account for the possibility of electron transfer quenching, which would not be productive in terms of upconversion. The Stern-Volmer constant is rather large, and this may indeed represent contributions from parallel quenching pathways. The fluorescence QY of perylene under our experimental conditions is 0.58, which is smaller than its optically dilute value (0.75), but nonetheless this slight attenuation cannot be solely responsible for the small upconversion QY. The production of excimeric perylene23 subsequent to sensitized TTA is clearly observed in our experiments (Figure 3a and Figures S1a and S2a). This pathway obviously degrades the energy stored in the singlet perylene excited state, conspiring to decrease the overall upconversion QY. Although not experimentally tractable here, the QY of TTA between the two excited perylene triplets is also a likely culprit to the overall low upconversion QY observed. Even in the face of all of these issues, the interactions between these two molecules following light excitation managed to produce a record anti-Stokes shift of 0.8 eV resulting from a sensitized TTA pathway. Importantly, the blue perylene fluorescence is readily discernible to the naked eye in a well-illuminated room. Figure 4a displays a digital photograph of the blue perylene emission observed as a result of 635 nm excitation in a solution mixture of PtTPBP/perylene. This upconverting mixture was combined using higher chromophore concentrations into a low glass transition temperature polyurethane polymer host material. The rubbery material facilitates chromophore diffusion at RT,11 as evidenced by the fact that the perylene emission is still readily visible by the naked eye (Figure 4b); however, to effectively demonstrate this to the readership, the same experiment using a bandpass filter is also presented (Figure 4c). This particular upconverting material is air stable, and the upconversion phenomenon can be visualized for months with selective excitation of the long wavelength sensitizer. It should be noted that this macroscopic polymer bar was not maintained under any special conditions and was


Figure 4. (a) Digital photograph of a mixture of 3.3 μM PtTPBP and 125 μM perylene in deaerated benzene under 635 nm excitation. Digital photograph of PtTPBP/perylene absorbed in a Tecoflex EG-80A polymer bar under 635 nm excitation (b) unfiltered and (c) when filtered through a bandpass filter.

stored in the ambient throughout the experimentation period. These latter results suggest the stability of the two chromophores with respect to air in a solid host material and shows that the upconverting phenomenon appears poised for realworld applications. The current experiments demonstrate that selective excitation of PtTPBP at 635 nm in a PtTPBP/perylene mixture in deaerated benzene results in upconverted blue perylene fluorescence centered at 451 nm, producing a record antiStokes energy shift of 0.80 eV for a sensitized TTA process. While this chromophore mixture produces readily discernible upconverted photons in both benzene solution and in the solid state (low Tg polyurethane host), the overall process is rather inefficient in the former as ascertained from direct QY measurements. The low upconversion OY observed likely stems from proportionate contributions occurring during each step of the conversion process following light absorption, illustrating the complexity of finding appropriate chromophore combinations to produce efficient wavelength conversion. Pt(II) meso-tetraphenyltetrabenzoporphyrin (PtTPBP) was purchased from Frontier Scientific and used without further purification. Perylene, spectroscopic grade benzene, and

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AUTHOR INFORMATION

methanol were purchased Aldrich. The polyurethane, Tecoflex EG-80A, was purchased from Lubrizol. Static absorption spectra were measured with a Cary 50 Bio UV-vis spectrophotometer from Varian. Steady-state luminescence spectra were obtained with a Photon Technology International (PTI) SPC spectrofluorimeter. Excitation was achieved by a 635 diode laser (LHR635-100EC) purchased from Lasermate. Incident laser power was varied using a series of neutral density filters. Photoluminescence lifetimes were measured on a nitrogen-pumped broadband dye laser (2-3 nm full width at half-maximum (fwhm)) from PTI (model GL-3300 N2 laser and model GL-301 dye laser) using an apparatus that has previously been described.25 Rhodamine 101 was used to tune the unfocused pulsed excitation beam. All luminescence samples were prepared either in a 1 cm2 quartz cell purchased from Starna cells or in a specially designed 1 cm2 optical cell bearing a side arm roundbotton flask and were degassed for 30 min with high purity argon or subjected to a minimum of three freezepump-thaw degas cycles prior to all measurements. The laser power was measured using a Molectron Power Max 5200 power meter. The fluorescence quantum yield of the concentrated solution of perylene and the upconverted fluorescence quantum yield measurements of perylene in benzene were obtained relative to DPA and methylene blue in benzene and in methanol upon 330 and 635 nm excitation, respectively, in the optically dilute technique utilizing eq 2.26 Here, Φunk, Aunk, Iunk, and ηunk represents the quantum yield, absorbance, integrated photoluminescence intensity, and refractive index of the sample at the excitation wavelength. The corresponding terms for the subscript “std” are for the reference quantum counters, DPA, and methylene blue, at the corresponding excitation wavelength, and “unk” is for the sample to be determined. Astd Iunk ηunk 2 ð2Þ Φunk ¼ Φstd Aunk Istd ηstd

Corresponding Author: *To whom correspondence should be addressed. Phone: (419) 3727513. Fax: (419) 372-9809. E-mail address: [email protected].

ACKNOWLEDGMENT This research was supported by the Air

Force Office of Scientific Research (FA9550-05-1-0276) and the National Science Foundation (CHE-0719050).

REFERENCES (1) (2) (3)

(4)

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The refractive index of benzene and methanol at this excitation wavelength was extracted from the literature, η(benzene) = 1.498 and η(MeOH) = 1.329.27 The integrated intensity of the upconverted fluorescence was analyzed in the region 400-650 nm, while that of methylene blue was analyzed in the region 600-850 nm resulting in an overall integrated emission range of 250 nm for both the sample and the standard. The photoluminescence quantum yield for methylene blue under these conditions is Φstd = 0.03,24 while the fluorescence quantum yield of DPA in benzene is 0.84.22 Quantum yield values presented are an average of at least two sets of independent measurements. Even though the emission profile of the standard methylene blue sample does not overlap the photoluminescence at short wavelengths, the experimentally determined quantum yields were reproducible using the relative actinometry.

(11)

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SUPPORTING INFORMATION AVAILABLE Prompt and delayed fluorescence spectra of perylene and the relative quantum yield of upconverted perylene emission. This material is available free of charge via the Internet at http://pubs.acs.org.


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