Article pubs.acs.org/JPCA
Second-Order Photochemical Upconversion in Organic Systems Angelo Monguzzi* and Francesco Meinardi Dipartimento di Scienza dei Materiali, Università Milano Bicocca, via R. Cozzi 55, 20125 Milano, Italy S Supporting Information *
ABSTRACT: In order to extend the photon energy shift of sensitized upconversion processes based on triplet−triplet annihilation in multicomponent organic systems, we have demonstrated that it is possible to exploit a sequence of consecutive upconversion steps. We have therefore realized an all-optical device for double upconversion: a light blue-shift of more than 0.9 eV was obtained at an excitation irradiance of a few tens of milliwatts per square centimeter.
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quantum yield20) but also the energy shift between incident and emitted radiation, ΔEi−o.21,22 The photon energy is exactly doubled by nonlinear second harmonic generation (SHG), but this is not true for TTA-UC because of the dependence of ΔEi−o on the positions of the involved energy levels of both dyes. As a consequence, ΔEi−o in TTA-UC is always smaller than in SHG. In nonlinear optics, large upconversion gains are obtained through a sequence of frequency-doubling processes, and therefore, in this work we propose for the first time a prototypical optical device that, mimicking classical high-order harmonic generation, allows the production of multiple TTAUC to obtain large ΔEi−o at powers as low as a few tens of milliwatts per square centimeter. The best TTA-UC performance is usually obtained in liquid environments, where the dyes are free to meet each other, enhancing the energy transfer process as well as the annihilation of metastable excited states. However, in order to obtain double/multiple TTA-UC it is not possible to just mix two different donor−acceptor pairs in the same solution. In such a way, reabsorption of the (double) upconverted light, inner filter effects, and unwanted energy back-transfer processes prevent any efficient multiple TTA-UC emission. We have designed a simple device that allows the two dye pairs to be kept completely separated while maintaining the power density following the first TTA-UC step as large as possible. It consists of two concentric capillary tubes, each one containing the proper sensitizer/emitter pair in tetrahydrofuran (THF) solution (Figure 1). Excitation at low energy (1.95 eV, from a Roithner Lasertechnik RLTMRL-635-100 laser) is focused in the inner
he generation of high-energy photons upon excitation with low-energy ones is routinely obtained through nonlinear optical effects arising from the high-order terms of the transmission-medium macroscopic polarization. These processes are widely used in high-resolution optical microscopy (to enhance both resolution and contrast), for in vivo bioapplications (to increase the tissue penetration depth), and in laser technology (to reach emission wavelengths otherwise barely obtainable).1−4However, nonlinear optical processes require the simultaneous interaction of two or more coherent photons, which can be achieved only at excitation power densities of megawatts or gigawatts per square centimeter.5,6As a consequence, nonlinear materials are not suitable for photovoltaic applications, where it is necessary to develop photon-managing strategies with high conversion yield at the solar irradiance (0.1 W cm−2).7,8 In the past few years a new method to obtain photon upconversion with low-power noncoherent light sources has been successfully developed. It exploits the annihilation of metastable triplet states of an emitting chromophore that are indirectly populated via Dexter energy transfer from a second dye acting as a sensitizer. This annihilation process produces the excitation of high-energy singlet states of the emitter, whose radiative decay gives rise to the upconverted emission.8−12 A detailed description of the overall process, usually called sensitized upconversion or triplet−triplet annihilation-based upconversion (TTA-UC), is reported elsewhere.8 TTA-UC is actually the most promising technique to enhance photovoltaic device performances by recovery of solar sub-band-gap photons, as upconversion yields larger than >20% have been obtained under irradiances of a few milliwatts per square centimeter.7,13−19 TTA-UC figures of merit include not only its absolute efficiency and excitation power threshold (Ith, the excitation power density requested to get the maximum conversion © 2014 American Chemical Society
Received: January 25, 2014 Revised: February 4, 2014 Published: February 4, 2014 1439
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PtOEP/DPA. The capillaries were sealed in a glovebox under a nitrogen atmosphere to prevent oxygen quenching of the dye excited states. The kinetics of each TTA-UC process was previously studied, and the results showed that for excitation densities (Iexc) below the TTA-UC threshold Ith, the intensity of upconverted light (Iuc) is dependent on the square of Iexc, with the corresponding overall upconversion quantum yield continuously increasing. On the contrary, above Ith the relationship between Iexc and Iuc becomes linear, and the TTA-UC yield remains constant.8 Therefore, it is possible to write separate equations for Iuc below ([↓]) and above ([↑]) the threshold:20 1 [ ↓ ] ⇒ Iuc = ϕPLfγTT[(k T)−1ϕETα(E)Iexc]2 (1) 2
Figure 1. Outline of the proposed double UC optical device.
cylinder, which may eventually act also as a waveguide, producing the first TTA-UC. This becomes a homogeneous fluorescent green light source stimulating the second upconversion by the dyes in the external shell, giving rise to a blue emission of up to 2.9 eV. Figure 2A,B reports the
1 ϕ fϕ α(E)Iexc (2) 2 PL ET where ϕPL is the PL quantum yield of the emitter, f is the statistical probability to have a singlet upon triplet annihilation, γTT is the second-order rate constant for annihilation, and α(E) is the absorption coefficient of the sensitizer. The factor of 1/2 simply means that two low-energy photons are needed to produce a single high-energy one, implying a maximum theoretical TTA-UC yield of 0.5. As a consequence, the highest achievable efficiency for n-step TTA-UC is 0.5n (i.e., 0.25 for a two-step TTA-UC). For two coupled TTA-UC systems (TTA-UCA and TTAUCB) as in our study, the systems can be combined in four configurations that have different dependences of the doubly upconverted output (IBuc) on Iexc: [ ↑ ] ⇒ Iuc =
⎧[ ↓ , ↓ ] ⇒ I B ∝ I 4 uc exc ⎪ ⎪ ⎪ [ ↑ , ↓ ]⎤ B ⎥ ⇒ Iuc [TTA‐UCA , TTA‐UCB] = ⎨ ∝ Iexc 2 ⎪ [ ↓ , ↑ ]⎥⎦ ⎪ ⎪[ ↑ , ↑ ] ⇒ I B ∝ I ⎩ uc exc
Figure 2. (A, B) Molecular structures of the chromophore pairs employed for the first and second TTA-UC steps, respectively. (C) Absorption spectra (PdPh4TBP, red circles; PtOEP, green dashed line) and emission spectra (BPEA, greenish solid line; DPA, cyan crosses) of the electronic transitions relevant to the double TTA-UC.
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However, at least in our case, the third configuration, [↓,↑], can be neglected because it is unlikely that the intensity of the emission produced by TTA-UCA below its threshold is sufficient to bring TTA-UCB to its high-intensity excitation regime. The three different dependences of IBuc on the excitation power density Iexc are shown in eq 3: (i) if both TTA-UC processes are below Ith, the overall output is proportional to the fourth power of Iexc; (ii) when only one TTA-UC process, usually the first one, is above Ith, the overall output is proportional to the square of Iexc; and (iii) in the case that both TTA-UC processes are above Ith, the upconverted emission rises simply linearly with Iexc. Figure 3 presents on a log−log scale the experimental Iuc measured as a function of Iexc. At low excitation power, the data can be fitted with a straight line with angular coefficient m1 = 3.7, in good agreement with the expected value of 4 for the [↓,↓] case. As Iexc increases, the slope gradually changes, saturating at around m2 = 1.8, which indicates that the double TTA-UC is taking place in the [↑,↓] regime. The two straight lines used for the fitting cross at about 30 mW cm−2, and therefore, this is the expected threshold value for the first TTA-UC step. From the standard equation for the threshold, Ith = 2(kT)2[γTTα(E)ϕET]−1, and the well-known parameters for
molecular structures of the employed chromophores and the logical scheme used to obtain a two-step TTA-UC from the red light to the high-energy tail of the blue spectrum. Commercial dyes were used for this proof-of-concept experiment. The first conversion (TTA-UCA) was obtained by coupling palladium(II) meso-tetraphenyltetrabenzoporphyrin (PdPh4TBP) as the sensitizer and 9,10-bis(phenylethynyl)anthracene (BPEA) as the emitter. The greenish photoluminescence (PL) of BPEA between 2.6 and 2.2 eV is resonant with the Q-band absorption peak of the selected second-pair donor, namely, platinum(II) octaethylporphyrin (PtOEP), which quickly transfers its energy to 9,10-diphenylanthracene (DPA), leading to the final doubly upconverted emission (TTA-UCB). All of the absorption and emission spectra corresponding to the electronic transitions involved in these processes are reported in Figure 2C. The chromophores were purchased from Sigma-Aldrich and used as received (see Figures S1 and S2 in the Supporting Information). The dye concentrations were carefully optimized to maximize the conversion quantum yield (see Figure S2). The best efficiencies were accomplished with concentrations of 10−4 M/10−3 M for PdPh4TBP/BPEA and 10−4 M/10−2 M for 1440
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ACKNOWLEDGMENTS A.M. thanks the Fund “Dote Ricercatori”: FSE, Regione Lombardia. The authors are grateful for support of this work by Fondazione Cariplo (Project 2009.2657).
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Figure 3. TTA-UC luminescence intensity as a function of the excitation power density. The inset shows TTA-UC emission spectra at selected excitation irradiances (green ◆, 1083 mW cm−2; yellow ■, 900 mW cm−2; orange ▼, 460 mW cm−2; dark-red ●, 360 mW cm−2). The low-energy tails of these spectra are due to the residual green emission from the first TTA-UC step (see the Supporting Information for details).
the PdPh4TBP/BPEA pair, a theoretical Ith value of 25 mW cm−2 was obtained, in very good agreement with the experimental one. The following parameters were used: kT = 370 Hz and γTT = 7 × 10−12 cm3 s−1 are the BPEA triplet spontaneous decay rate and TTA rate, respectively; α(E) = 1.7 cm−1 is the absorption coefficient at the excitation wavelength for PdPh4TBP at a concentration of 10−4 M; and ϕET = 1 is the PdPh4TBP/BPEA energy transfer efficiency at the employed acceptor concentration.23 It should be noted that even at the highest employed excitation density, double TTA-UC did not reach the maximum efficiency regime [↑,↑] where Iuc and Iexc are linearly related. This is probably due to the broad emission of BPEA, which is not completely absorbed by the narrow and slightly mismatched S0 → S1 transition of PtOEP, as is required for efficient activation of the second TTA-UC step. In summary, an “all-optical” multiple photon upconversion with an energy gain larger than 0.9 eV has been obtained with excitation intensities of a few tens of milliwatts per square centimeter using commercially available dyes and a simple homemade configuration consisting of several concentric capillary tubes in a prototypical device. Kinetic analysis of the TTA-UC effect demonstrated the occurrence of a double TTAUC while also suggesting that there is room for further improvement. In particular, by the use of properly designed dye pairs with optimized spectral overlap it may be possible to increase the photon shift and to raise the overall double TTAUC efficiency.24−27 Finally, it should be noted that the proposed concentric capillary can be easily arranged in arrays to produce large-area double-TTA-UC panels.
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ASSOCIATED CONTENT
S Supporting Information *
Absorption, PL, and PL quantum yield measurements. This material is available free of charge via the Internet at http:// pubs.acs.org.
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[email protected]. Fax: (+39) 02 64485400. Notes
The authors declare no competing financial interest. 1441
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