Photon Upconversion from Chemically Bound Triplet Sensitizers and

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Photon Upconversion from Chemically Bound Triplet Sensitizers and Emitters on Mesoporous ZrO: Implications for Solar Energy Conversion 2

Jonas Sandby Lissau, Djawed Nauroozi, Marie-Pierre Santoni, Sascha Ott, James M. Gardner, and Ana Morandeira J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08907 • Publication Date (Web): 26 Oct 2015 Downloaded from http://pubs.acs.org on October 30, 2015

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The Journal of Physical Chemistry

Photon Upconversion from Chemically Bound Triplet Sensitizers and Emitters on Mesoporous ZrO2: Implications for Solar Energy Conversion Jonas Sandby Lissau,†,¶ Djawed Nauroozi,† Marie-Pierre Santoni,† Sascha Ott,† James M. Gardner,‡ and Ana Morandeira∗,† †Department of Chemistry - ˚ Angström Laboratory, Uppsala University, Box 523, SE-75120 Uppsala, Sweden ‡Department of Chemistry - Division of Applied Physical Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden ¶Currently at KTH Royal Institute of Technology E-mail: [email protected] Phone: +46 (0)18-471 3640

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Abstract Photon upconversion by sensitized triplet–triplet annihilation (UC-STTA) is studied in systems with triplet sensitizers and emitter molecules co-chemisorbed onto nanostructured ZrO2 films. UC-STTA is a promising strategy to overcome the ShockleyQueisser efficiency limit of single-threshold solar cells. The dye loaded mesoporous ZrO2 films studied herein allow high molecular densities and are good proxy systems for the study of photophysics relevant to dye-sensitized solar cells. Two sensitizer/emitter dye pairs are studied: platinum(II) deuteroporphyrin IX dicarboxylic acid / 4,4’-(10-(anthracene-9,10-diyl)dibenzoic acid and platinum(II) deuteroporphyrin IX dimethyl ester / methyl 4-(10-(p-tolyl)anthracen-9-yl)benzoate. Both dye pairs are closely related to the standard UC-STTA molecular pair platinum(II) octaethylporphyrin (PtOEP) / 9,10-diphenylanthracene (DPA). By chemically anchoring of the upconverting dye pairs onto ZrO2 films a significant improvement in UC-STTA efficiency is achieved with respect to previously studied co-physisorbed PtOEP/DPA. Controlled variation of the sensitizer/emitter dye ratios onto the surface shows that new energy loss mechanisms appear at high sensitizer surface coverage. Spectral signatures of porphyrin aggregates suggest separate sensitizer domains form, which limits the triplet sensitization of emitter molecules. The nanosecond timescale rise and decay of the observed UC emission are likely linked to the sample stability over time; UC emission is observed a year after sample preparation. These are promising properties for the application of this type of system for solar energy conversion.

Introduction Sunlight has the potential to meet humanity’s energy demands with minimal disruption to Earth’s climate. 1 One barrier to the widespread implementation of photovoltaics is the relatively low power conversion efficiency. The majority of commercially available solar cells are based on a single threshold structure, which inherently limits the efficiency of the solar energy conversion to approximately 30%, the Shockley-Queisser limit. 2 One possible 2


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approach to overcome the Shockley-Queisser limit is the use of spectral converters. 3 In this strategy, the solar spectrum is modified for a better match with the wavelength interval giving the highest solar energy conversion efficiency in a given solar cell. Photon upconversion is a sub-category of spectral conversion wherein sub-band gap photons are absorbed and their energies combined to produce photons above the band gap of the solar cell. Most strategies tested for photon upconversion rely on light intensities higher than those provided by the Sun for efficient photon upconversion to be observed. 4 However, photon upconversion by sensitized triplet–triplet annihilation (UC-STTA) allows for high upconversion efficiencies under non-coherent light excitation at intensities comparable to sunlight. 5 The UC-STTA process is initiated by the absorbance of a low energy photon by a sensitizer molecule that has a high singlet-to-triplet intersystem crossing yield and thus results in the efficient creation of a triplet excited state. The triplet excited state of the “sensitizer” is transferred by triplet energy transfer (TET) to an “emitter” molecule. When two emitters in the triplet excited state are in close proximity they can undergo triplet–triplet annihilation (TTA) to produce one singlet emitter excited state and one singlet emitter ground state. In the absence of quenching, the emitter singlet excited state can be observed as it decays radiatively by emission of a high-energy (upconverted) photon. In solar cells, this mechanism has the potential to convert low-energy photons, that are otherwise transmitted by the solar cell bandgap, into high-energy photons that can be absorbed by the solar cell and thereby enhance the photocurrent without sacrificing the voltage, leading to an overall gain in solar cell efficiency. The promising properties of the UC-STTA process have resulted in rapid development in this research field during the last decade, 6–15 and recently the first proof-of-principle experiments have shown increases in the solar energy conversion efficiency of established low-cost solar cell technologies, 16–18 among other applications. 19 A related approach to break the Shockley-Queisser limit aims at modifying the solar cell architecture to increase the cell’s theoretical maximum solar energy conversion efficiency. One approach in this category is the intermediate band (IB) solar cell, where low energy

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photons can be absorbed by an IB introduced in the solar cell band gap. 20,21 In practice, this approach has been typically limited by the short lifetime of excited states in the IB level. Due to the metastable molecular triplet states involved in the UC-STTA process, Ekins-Daukes and Schmidt proposed the use of this photophysical mechanism as a molecular approach to the IB solar cell, which would result in a theoretical maximum solar energy conversion efficiency of these solar cells above 40%. 22 An optimal solar cell technology for the implementation of this strategy is the dye-sensitized solar cell (DSSC), where the highenergy electron in the molecular excited state created in the UC-STTA process can be rapidly injected into the conduction band of the nanostructured TiO2 films used as photoanodes in DSSCs. 23 Implementation of the molecular approach to the IB solar cell by use of the UC-STTA mechanism in a DSSC structure has been studied experimentally in our group, applying model systems based on mesoporous ZrO2 films. These films have structural properties and refractive index very similar to the TiO2 material used in DSSCs, but ZrO2 has a higher conduction band energy, which inhibits electron injection from molecular excited states. 24–26 This property of ZrO2 has been used to prove the presence of the UC-STTA process on mesoporous metal oxides using a simple fluorimetry technique to observe the delayed anti-Stokes fluorescence of the singlet excited state produced by the UC-STTA process. 27 To date, the most efficient system studied for UC-STTA on mesoporous metal oxide films is a heterogeneous system, where emitter molecules are chemisorbed onto the nanostructured surface of the film and sensitizers diffuse freely in a solution surrounding the nanostructure. 23,28,29 It was found, however, that the efficiency of the UC-STTA process in these systems was greatly limited by the low concentration of sensitizers in the solution phase. It is known, that molecules adsorbed onto mesoporous films can reach very high local concentrations, 30 and the chemisorption of sensitizers onto the nanostructured ZrO2 surface along with emitters (co-adsorption) was therefore proposed as a simple solution to the encountered concentration limitation. 28

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In this article, co-adsorption based on chemisorption of sensitizer and emitter molecules onto mesoporous ZrO2 films is applied, and the UC-STTA emission properties of these systems are investigated. A schematic of the composition of this type of co-chemisorbed system is given in Chart 1. The nanostructured ZrO2 film with the co-chemisorbed sensitizer and emitter molecules is contained in a home-build sandwich-like sample cell, which is filled with pure Ar-saturated butyronitrile (BuN).

Sensitizer

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Chart 1: Structural composition of the studied systems. Sensitizer and emitter molecules are co-chemisorbed to the mesoporous ZrO2 surface. The concept of co-adsorption of different types of molecules onto mesoporous metal oxide films has been previously investigated in the field of DSSCs as a mean to increase the light harvesting efficiency. 31 It is important to note though, that even if the co-adsorption strategy applied in previous studies of DSSCs was efficient in improving the solar cell efficiency, it did not change fundamentally the energy level structure of the solar cell in a way that allows for efficiencies higher than the ∼30% limit first calculated by Shockley and Queisser. In the initial proof-of-principle experiments done on the UC-STTA process on nanostructured ZrO2 films, a well-known sensitizer/emitter pair for UC-STTA was co-adsorbed onto ZrO2 mesoporous films by physisorption. 27 The molecular pair consisted of the platinum(II) octaethylporphyrin (PtOEP) sensitizer and the 9,10-diphenylanthracene (DPA) emitter. This couple has previously been studied for UC-STTA applications in solutions, 32–36 in rigid solid matrixes, 37–39 in solid state organic films, 38–41 in organic glasses, 42 in single

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phase elastomers, 43 in polymer nanoparticles, 44 in Bragg structures 45 and even metal-organic frameworks (MOFs). 46 The UC-STTA efficiency of the molecular pair, when co-physisorbed onto mesoporous ZrO2 films, was greatly limited by low surface coverages of the molecules on the nanostructure. This condition led to poor orbital overlap between neighboring molecules on the surface, which limits TET by the Dexter mechanism, and limited the emitter triplet concentration due to the low concentration of sensitizers. The latter limitation is of great importance to the UC-STTA efficiency, since this quantity shows a square dependence on the triplet emitter concentration at the applied excitation light intensities. The co-chemisorption strategy applied in the studies of this paper is adopted in order to increase surface coverages of sensitizer and emitter molecules on the nanostructured surface of the ZrO2 films with respect to the co-physisorption method applied earlier. By increasing the surface coverages through chemisorption of the dyes we aim to address some of the problems encountered in the co-physisorbed system. A higher surface coverage decreases the distance between molecules, which can be expected to increase the energy migration and TTA rates. Increasing the sensitizer surface coverage is expected to cause a higher emitter triplet excited state concentration on the surface. Furthermore, when compared to the co-physisorbed system the chemisorbed system should have improved orbital overlap between adsorbed molecules due to the improved solvation by BuN of the sensitizer and emitter molecules on the ZrO2 surface. 27 Therefore, co-chemisorption is expected to lead to an overall improvement of the UC-STTA efficiency of the studied systems. In addition to the mentioned applications in DSSCs the nanostructured upconverters studied herein could also be implemented externally in other types of solar cells, where their high molecular density make them appealing alternatives to previously tested upconverters. 12,47 The use of chemical anchoring groups in this study has the further benefit that it allows for controlled variation of the emitter/sensitizer molecular ratio on the mesoporous metal oxide surface and an unprecedented study of the implications of this parameter to photon upconversion. Knowledge about the effects of varying surface coverage on the triplet

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sensitization efficiency of the upconversion process will also be useful in applications of mesoporous structures as high-density triplet sensitizer hosts to boost upconverting systems. 12,47 Improving the understanding of triplet exciton migration and TTA among organic molecules in the solid state is of great importance for a number of optoelectronic devices such as organic light-emitting diodes (OLEDs) 48 and bulk heterojunction (BHJ) organic photovoltaics (OPV), 49–51 as well as for potential third generation solar cell approaches such as singlet fission and triplet fusion (UC-STTA). 23,52–54 The molecules used in the studies of this article are shown in Chart 2. To allow chemisorption, the PtOEP sensitizer was substituted by the platinum(II) deuteroporphyrin IX dicarboxylic acid (Pt-PoDCA) sensitizer. For characterization in solution the corresponding ester, platinum(II) deuteroporphyrin IX dimethyl ester (Pt-PoDME), was used instead, due to its higher solubility. The DPA emitter was substituted for the 4,4 -(10-(anthracene-9,10diyl)dibenzoic acid (ADBA) molecule, which previously showed superior UC-STTA efficiencies in heterogenous ZrO2 film systems compared to a series of alternative DPA derivatives. 29 One of these derivates, the methyl 4-(10-(p-tolyl)anthracen-9-yl)benzoate (MTAB) molecule, has been subject to particularly thorough studies in heterogeneous systems. 28,29 Therefore, for the sake of comparison, a few results on the MTAB emitter co-chemisorbed onto ZrO2 with the Pt-PoDME sensitizer are also included in this paper. However, the main focus of the paper will be the ADBA/Pt-PoDCA co-chemisorbed pair, which shows higher UC-STTA efficiency.

Experimental Methods Synthesis and Characterization The ADBA and MTAB compounds were synthesized as described in references 28 and 29. PtOEP was used as received from Aldrich. Pt-PoDME was used as received from Frontier Scientific. 7



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Chart 2: Chemical structures of the compounds studied in this work.

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General Procedure for the Base Hydrolysis of Esters The ester was dissolved in a mixture of THF and MeOH (v/v = 1:1) and a solution of 2N NaOH was added. The resulting solution was stirred at room temperature over night. After the organic phase was removed under reduced pressure the aqueous phase was acidified with 2N HCl and the resulting precipitate was collected. The precipitate was washed with water and dried under vacuum to give the corresponding carboxylic acid. Platinum(II) Deuteroporphyrin IX Dicarboxylic Acid (Pt-PoDCA) The corresponding ester (50 mg) of the title compound was hydrolyzed with a 2N solution NaOH (15 ml) as described in the general procedure. After work-up the product was collected by filtration as an orange solid in 95% yield. Reaction scheme is given in Scheme S1 in the ESI. HRMS/NSI (+) m/z: 702.16881 [M]− , calculated for [C30 H27 N4 O4 Pt]− = 702.16809

Samples Mesoporous ZrO2 films were made by blade-casting of home-made ZrO2 paste onto MenzelGläser glass slides. The films were sintered on a hot plate at 450 ◦ C. The procedure is described in detail elsewhere. 27 ZrO2 film thicknesses of the samples are reported in the captions of the graphs showing data measured on the specific films. For the MTAB/PtPoDME|ZrO2 samples, the dyes were loaded onto the ZrO2 films by immersing the films in acentonitrile (AcN) solutions of MTAB (0.04-0.2 mM), Pt-PoDME (0.1-0.5 mM) and Ba(OH)2 · 8 H2 O (0.8-26 mM) for 3-4 days at 75 ◦ C. For the ADBA/Pt-PoDCA|ZrO2 samples, the ZrO2 films were sensitized with ADBA and Pt-PoDCA by immersing them in a 1:3 volume:volume ratio of ethanol:acetonitrile (EtOH/AcN, Solveco/Merck, Uvasol) solutions of ADBA (0.04 mM) and Pt-PoDCA (0.5-5 μM) for 3 days at room temperature. All films were rinsed after sensitization, by immersion in 20 mL AcN for 1 day, followed by immersion 9



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for 1 day in 20 mL butyronitrile (BuN, Fluka). The sensitized and rinsed ZrO2 films were incorporated into home-built glass cells, which were filled with pure BuN solvent and sealed using melted Surlyn (DuPont) inside of a glove-box. Reference 28 contains further details on the sealed cell construction.

Steady-State Spectroscopy Steady-state absorption measurements were done using a Cary 5000 spectrophotometer from Varian. Absorption spectra were corrected by subtraction of a constant value from the entire spectral range, resulting in a zero absorbance baseline around 700 nm. . Steady-state emission spectra were measured using a Fluorolog-3 spectrofluorometer from Horiba Jobin Yvon. This spectrofluorometer has double-grating excitation and emission monochromators and a 450-W Xe arc lamp as the excitation light source. Emission from the sealed sample cells was detected in front face detection mode with the sample surface oriented in 90◦ with respect to the excitation light. For the detection of upconverted emission, a longpass 380 nm optical filter was placed in front of the sample to cut off possible second order diffracted light from the monochromator gratings. Reference samples containing only the emitter molecule on the ZrO2 surface were measured under the same conditions as UC samples and showed no emission. The instrumental parameters used in the study of steady-state UC-STTA emission were set to match the parameters (slit widths, integration time, etc.) used in previous studies, 27 to facilitate easy comparison. All the reported spectra are corrected for the wavelength dependence of the monochromators and detectors and the fluctuation of the excitation light source. For UC-STTA emission measurements the excitation intensity was 5.5-7.5 mW/cm2 . More details on excitation light intensity dependence measurements can be found in reference 27.

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Transient Absorption and Time-resolved Emission Spectroscopies Time-resolved emission and transient absorption measurements were carried out using ∼10 ns laser pulses for excitation of the samples at 10 Hz. Details on the laser setup are described in reference 27. The 532 nm laser output was used for excitation of the sample. A 400-nm long-pass optical filter was used to remove any possible residual 355 nm light from the excitation beam. Unless otherwise stated, the laser intensity at the sample position was 29 mW/cm2 per pulse. In emission measurements the sample surface was oriented approximately 30◦ with respect to the excitation beam. The detectors, a CCD camera for spectral measurements and a PMT detector for kinetic traces, were placed in a 90◦ angle with respect to the excitation beam. The detected light was passed through a monochromator before arrival at the PMT detector. The time-resolved emission spectra were recorded using optical filters in front of the CCD camera to avoid spectral contamination by scattered laser light; an OG2 filter was used for phosphorescence measurements, and a BG12 filter was used for UC emission measurements. For kinetic trace measurements (PMT detector), the same filter was applied for UC emission, but a 610 nm long-pass filter was used in measurements of phosphorescence kinetics. In transient absorption measurements the sample was oriented in a 135◦ angle with respect to the excitation beam, and facing the probe light source (a pulsed Xe-arc lamp placed at 90 degrees with respect to the excitation beam). No optical filters were used in front of the detector in the transient absorption spectral measurements, but a fluorescence background spectrum was recorded and subtracted from the transient absorption spectral data. More details on the transient absorption setup and detection system can be found in reference 29.

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Results In the following section the sensitizer molecules are characterized by steady-state and timeresolved techniques in solution and chemisorbed onto mesoporous ZrO2 films. Characterization of the emitter molecules was presented in previous reports. 28,29 A section devoted to characterization of mesoporous ZrO2 films with co-adsorbed emitters and sensitizers is followed by a separate section for the study of the UC emission of these samples. Finally, a section is included for description of the long-term stability of the co-adsorbed UC systems.

Characterization of the Sensitizers In Solution The steady-state absorption and emission spectra of Pt-PoDME (∼ 5.5 × 10−5 M) in BuN solution are similar to those of the related reference compound platinum(II) octaethylporphyrin (PtOEP) in BuN solution (compare olive and blue spectra in Figure 1a). The only notable spectral difference between the two compounds is a small blue-shift of all the ob-

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Figure 1: (a) Steady-state absorption (solid lines) and phosphorescence (λexc = 534 nm, dotted lines) spectra of dilute BuN solutions of Pt-PoDME (olive) and PtOEP (blue). (b) Phosphorescence (λem = 646 nm) and transient absorption (ground state bleach, λTA = 379 nm, negative signal; and excited state absorption, λTA = 420 nm, small positive signal) kinetic traces of Pt-PoDME in BuN following a 532 nm laser pulse excitation. The solution was contained in a cell sealed in a glove box. The emission decay of Pt-PoDME in BuN solution following a 532 nm laser pulse ex12


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citation is shown in Figure 1b (high intensity, positive traces, right axis). Time-resolved spectral emission measured on Pt-PoDME in BuN at gate-delays of 500 ns and 20 μs displayed a rather similar shape (see Figure S1 in the ESI). The decay of excited state absorption at 420 nm and the recovery of ground state bleach at 379 nm of Pt-PoDME in BuN are displayed in Figure 1b together with the phosphorescence decay. The transient absorption kinetic data of Pt-PoDME in BuN could be globally fitted together with the phosphorescence decay data of the same sample using a sum of three exponential decay functions (see Figure S2 and Table S1 in the ESI). From this fit a weighted average phosphorescence lifetime of 4.9 μs can be calculated. The non-exponential decay behavior has also been observed for PtOEP in BuN solution and can be explained by homogeneous triplet–triplet annihilation (homo-TTA) among Pt-PoDME molecules. For comparison, the weighted average lifetime of PtOEP in BuN solution was measured to be 12.7 μs under similar laser excitation intensities. 29 It is noteworthy that, while the excited-state absorption and phosphorescence decays could be well fitted using a sum of two exponential functions, the ground state bleach recovery required an additional long time component with a recovery time constant of 9.5 · 101 μs accounting for 5% of the signal amplitude. This behavior is different from what was observed for PtOEP in BuN, where a perfect mirror image between phosphorescence decay and ground state recovery could be seen. 29 The transient absorption spectra of Pt-PoDME in BuN solution were, like the ground state absorption spectra, very similar to the same type of data measured on PtOEP in BuN solution (Figure S10 in the ESI). 29 The above measurements were also done in the presence of a ZrO2 film, but no significant changes in position, shape, intensity of the spectra and traces were observed (data not shown). This indicates that Pt-PoDME does not bind onto the film surface when in BuN solution.

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Chemisorbed to Nanostructured ZrO2 Films Absorption, emission and excitation spectra of a nanostructured ZrO2 film sensitized with Pt-PoDCA are shown in Figure 2. The absorption spectrum of Pt-PoDCA chemisorbed onto ZrO2 is markedly different from the absorption spectrum of Pt-PoDME in BuN solution (compare Figures 1a and 2). On the film, the Pt-PoDCA absorption spectrum shows a broadening (especially of the Soret band), a slight red shift, and a decrease in the ratio between the absorption of the Soret and Q-bands. These features in the absorption spectrum are generally ascribed to aggregation of the porphyrins, 27 which leads to a broader distribution of the oscillator strengths for the absorbing transitions. The phosphorescence spectrum features a strong emission band with maximum at 645 nm, similar to the one observed for Pt-PoDME in solution, and a new, small emission band around 768 nm, which is almost covered by the tail of the monomer phosphorescence. The excitation spectrum measured at the 768 nm emission band (dashed line in Figure 2a) shows a broadening of the bands compared to the excitation spectrum measured at the maximum of the monomer phosphorescence at 645 nm (dotted line). In addition, the 768 nm excitation spectrum features a small band around 590 nm (inset of Figure 2a), which is not observed in the 645 nm excitation spectrum. Exciting the sample at the 590 nm band results in an emission spectrum dominated by the 768 nm band (inset of Figure 2b). Far red emission bands have been repeatedly reported for PtOEP in solid state samples 42,55–58 and attributed to aggregate species. The weighted average phosphorescence lifetime in the Pt-PoDCA|ZrO2 system is 24 μs, which is significantly longer than the corresponding lifetime in solution (see Figure S3 and Table S2 in the Supporting Information).

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Figure 2: Steady-state absorption (solid) and excitation spectra (dashed and dotted) (a), and photoluminescence (b) of a mesoporous ZrO2 film sensitized with Pt-PoDCA and contained in a sealed cell filled with BuN. The excitation wavelength was 534 nm in the photoluminescence spectrum (590 nm in the inset) and the emission wavelengths were 645 nm (dotted line) and 768 nm (dashed line) in the excitation spectra. The inset in (a) shows a magnification of the data.

Emitter and Sensitizer Co-chemisorbed to Nanostructured ZrO2 Films Steady-state Absorption and Stokes Emission Absorption, fluorescence, and phosphorescence spectra for a selection of four samples of the emitter dye ADBA, co-adsorbed with sensitizer Pt-PoDCA, on nanostructured ZrO2 films are shown in Figure 3. Corresponding spectra of MTAB/Pt-PoDME co-sensitized ZrO2 are shown in Figure S13 and S15 of the ESI. The selection includes samples with different ZrO2 film thicknesses (these are reported in the figure captions) and a variation in the surface coverages of the sensitizer and emitter dyes, and the ratio between these, can be observed in the absorption spectra (Figure 3a). The porphyrin sensitizer has peak absorptions at 377 nm and 533 nm, while the absorption of ADBA has maxima at 360, 378, and 397 nm. The molar emitter/sensitizer ratios of the four samples of Figure 3 are given in the figure caption. The data series shown in Figure 3b illustrates how the shape and magnitude of the fluorescence spectra of the emitter dyes on the mesoporous ZrO2 film are complex functions of the dye loading, film thickness and possibly film structure. These are well-known observations in optical studies of sensitized mesoporous films, 28,29 and the resulting challenges in data 15



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Figure 3: Steady-state absorption (a), fluorescence (b), and phosphorescence (c) of mesoporous ZrO2 films (1.0 μm film, triangles; 1.7 μm film, squares; 2.0 μm, circles; 1.8 μm, no symbol) co-sensitized with ADBA and Pt-PoDCA in sealed cells filled with BuN. The excitation wavelength was 393 nm in the fluorescence spectra and 534 nm in the phosphorescence spectra. The emitter/sensitizer (mol/mol) ratios of the samples are estimated from the absorption spectra to be 97 (circles), 43 (squares), 10 (no symbol), and 8 (triangles), respectively.

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reproducibility are also well described. 27,29,59 In previous studies of mesoporous ZrO2 films sensitized with MTAB (alone) it was noted how an increasing MTAB absorbance would lead to a red-shift of the fluorescence spectrum and a decrease in the apparent fluorescence quantum yield, which was ascribed to inner-filter effect. 28 Energy transfer via exciton hopping among emitters on the surface could also contribute to the red shift. 60 In the co-adsorbed systems studied in this paper, the picture is further complicated by a sensitizer absorbance in the red-edge region of the emitter fluorescence spectra. Depending on the absolute and relative magnitudes of the emitter and sensitizer absorbances different types of inner-filter effects can be observed in fluorescence spectra, affecting the shape, magnitude and peak positions in various ways. A rather strong quenching of the porphyrin monomer phosphorescence in the MTAB co-sensitized systems makes clear the presence of far-red emission bands (768 nm) to various extent in different samples (see Figure S15 in the Supporting Information). There is no clear relation between the extent of monomer phosphorescence quenching and the intensity of this far-red emission. With the carboxylic acid anchor groups applied in the co-sensitized samples with ADBA it was possible to study a broader range of Pt-PoDCA surface coverages. It is notable in this series, that the sample with the highest Pt-PoDCA surface coverage is also the sample with the lowest intensity of phosphorescence per absorbed photons (see Figure 3a and c). A generally lower degree of monomer phosphorescence quenching in the ADBA systems compared to MTAB (selected spectra from each type of system are plotted on a common scale in Figure S16 in the ESI) accounts for less apparent 768 nm aggregation emission bands, since these are partly covered by the tail of strong monomer phosphorescence (Figure 3c). Time-resolved Stokes Emission and Transient Absorption Figure 4 shows the emission and excited-state absorption decays and ground state recovery on a broad range of time-scales of the ADBA/Pt-PoDCA|ZrO2 system. The short emission decay component is expected to partly originate in ADBA prompt fluorescence due to two-

17



photon excitation 61 (see Figure S9 in the ESI), but a fast triplet energy transfer component from sensitizer to emitter might also contribute to the fast decay. The samples also display a short (low magnitude) decay component in the transient absorption kinetic traces (see Figure 4). At 405 nm, the short decay component of the transient absorption kinetic trace

)DOmÆ( noitprosbA tneisnarT

is partly covered by the decay of emitter fluorescence on a very similar timescale. ).u .a( ecnecsenimulotohP

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150 100 50 0 -50 -100 -150 1

10

2

10

3

4

10 10 Time/ns

10

5

10

6

Figure 4: Photoluminescence (λem = 646 nm) and transient absorption (ground state bleach, λTA = 374 nm, negative signal; and excited state absorption, λTA = 405 nm, small positive signal) kinetic traces of co-sensitized Pt-PoDCA/ADBA|ZrO2 in BuN following a 532 nm laser pulse excitation. The sample was contained in a cell sealed in a glove box. The steadystate absorption, fluorescence, and phosphorescence properties of the sample are shown in Figure 3 (triangle markers). Excluding the initial short component discussed above, the kinetic traces of Figure 4 could be fitted globally to a sum of four exponential functions and a plateau (see Table S3 and Figure S4 in the ESI). It is notable that all the weighted average decay time constants are longer than the corresponding time constants for Pt-PoDME (alone) in solution (see Table 1 and Table S3 in the ESI); this is particularly true for the ground state bleach. Table 1 compares weighted average phosphorescence lifetime and steady-state phosphorescence intensity of co-sensitized ZrO2 samples with different sensitizer surface coverages to corresponding properties of sensitizers in the absence of emitters on ZrO2 film and in solution. Since the lifetime measurements are carried out at a much higher incident energy than the steady-state measurements, the ratio between the weighted average phosphorescence lifetime and the relative steady-state phosphorescence efficiency (τav /φ) is also included. This 18


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parameter reveals the relative importance of second order phosphorescence quenching processes in the different systems; the lower the value, the higher the importance of second order phosphorescence quenching processes. Table 1: Phosphorescence quenching of two ADBA/Pt-PoDCA|ZrO2 samples, compared to a Pt-PoDCA|ZrO2 reference sample and Pt-PoDME in BuN. System ADBA/Pt-PoDCA|ZrO2

LHE(λexc )a

LHE / τav c φd −1 b film thickness (μm ) (μs) (a. u.)

τav /φ (a. u.)

0.060

0.021

28

0.22

127

0.39

0.39

12

0.19

63

Pt-PoDCA|ZrO2

0.33

0.13

24

1.0

24

Pt-PoDME in BuN

0.023

n/a

4.9

0.47

10

a

Light-harvesting efficiency (LHE) is defined as 1 − 10−A , where A is the absorbance.

b

LHE divided by ZrO2 film thickness is applied here as a relative measure of sensitizer surface coverage.

c

Weighted average phosphorescence lifetime.

d

Normalized integrated steady-state phosphorescence intensity divided by LHE(λexc ).

The transient absorption spectra of Pt-PoDCA/ADBA|ZrO2 co-sensitized films feature a somewhat more positive signal in the 420-500 nm region as compared to Pt-PoDME in solution, which could be due to triplet-triplet absorption from ADBA (see Figure S11 in the ESI). The signal in this region appears to decay at a slightly different rate than the Q-band ground-state bleach of the porphyrin. The low transient absorption signal of the samples prevents further analysis of this spectral feature.

19



Photon Upconversion on Nanostructured ZrO2 Films Steady-state Photon Upconversion The steady-state photon upconversion spectra of the set of selected samples of ADBA/PtPoDCA|ZrO2 co-sensitized films are shown in Figure 5. Similar upconversion spectra for the MTAB/Pt-PoDME|ZrO2 series are shown in Figure S17 of the ESI. UC emission appears roughly in the same spectral range as the prompt fluorescence of the emitter. The exact shape and position of the UC spectrum is mainly dependent on the extent of inner-filter effects in the sample emission. In other words, it is dependent on the thickness of the film, emitter and sensitizer surface coverage, and penetration of the excitation wavelength, among others. See for instance Figures S18 and S19 in the ESI. ).u .a( noissimE detrevnocpU

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

400

420

440

460

480

500

Wavelength/nm

Figure 5: Steady-state upconverted emission of mesoporous ZrO2 films (1.0 μm film, triangles; 1.7 μm film, squares; 2.0 μm, circles; 1.8 μm, no symbol) co-sensitized with ADBA and Pt-PoDCA in sealed cells filled with BuN. The excitation wavelength was 534 nm. The corresponding absorption, fluorescence, and phosphorescence spectra of the different samples can be found in Figure 3. A general trend for the UC spectra shown in Figure 5 and Figure S17 in the ESI is that ADBA samples show higher UC emission intensities than the MTAB samples (one spectrum from each type of sample is compared on a common scale in Figure S20 in the ESI). Within each set of samples there are also some notable trends. For MTAB based samples an inverse correlation is observed between the intensities of UC emission and phosphorescence. The more phosphorescence quenching 62 in the MTAB systems, the higher the UC efficiency. 63 In the ADBA samples this correlation is not seen. Here, the sample with the highest degree of 20


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phosphorescence quenching shows the lowest UC-STTA emission efficiency, and increasing the Pt-PoDCA surface coverage leads to a decrease in the UC-STTA emission efficiency (see Table 2). It is also noteworthy in the ADBA sample set, that the sample showing the highest UC intensity is not the most UC efficient sample (measured by integrated UC emission intensity divided by the Q-band light harvesting efficiency, LHE, 64 at the excitation wavelength, see Table 2). Table 2: Q-band light harvesting efficiency LHE (absolute and per μm of ZrO2 filma ), emitter/sensitizer molar ratiosb , normalized (integrated) UC-STTA emission intensity, and normalized relative UC-STTA efficiencyc of four ADBA/Pt-PoDCA|ZrO2 samples. For comparison, previous results on co-physisorbedd and heterogeneouse systems are also included. System Co-chemisorbed

Co-physisorbed Heterogeneouse

d

LHE(λexc )

LHE / film thickness (μm−1 )

Emitter/sensitizer Normalized (mol/mol) UC intensity

Normalized (UC intensity/LHE)

Graph symbol

0.086

0.043

97

0.65

1.0

circle

0.13

0.076

43

0.55

0.56

square

0.38

0.21

10

1.0

0.34

solid line

0.40

0.40

8

0.20

0.066

triangle

0.055

0.010

n/a

0.11

0.27

(ref. 27)

0.034

n/a

n/a

4.2 · 101

1.6 · 102

(ref. 29)

a

LHE divided by ZrO2 film thickness is applied here as a relative measure of sensitizer surface coverage.

b

Estimated from their absorption spectra.

c

Measured here by the integrated UC-STTA emission intensity divided by LHE at the excitation wavelength (λexc ).

d

PtOEP and DPA were co-physisorbed onto a mesoporous ZrO2 film surrounded by de-aerated water. 27

e

ADBA was chemisorbed onto mesoporous ZrO2 , PtOEP was in BuN solution around the nanostructure. The system was contained in a cell, sealed in a glove-box. 29

To test the performance of Pt-PoDME as a sensitizer in a well known UC system, two similar sealed cells were constructed, both containing a MTAB sensitized nanostructured ZrO2 film, but differentiating in having either Pt-PoDME or PtOEP as a sensitizer in the BuN solution. The latter type of system, with PtOEP as sensitizer, has previously been thoroughly studied. 28,29 The two different sensitizers produced UC emission intensities on a similar order of magnitude in these heterogeneous systems (see Figure S21 in the ESI). 21



Excitation Intensity Dependence The UC emission intensity for a few of the ADBA/Pt-PoDCA|ZrO2 co-sensitized samples was high enough to allow for excitation intensity dependence studies throughout a reasonably large range of excitation intensities. The measured UC emission dependence on excitation intensity is shown for two ADBA based samples in Figure 6. The intensity dependence in the two samples is rather different. For the data series with circle markers in Figure 6 (measured on the low LHE/film thickness ratio sample giving rise to the UC spectrum with circle markers in Figure 5) a very clear quadratic dependence is observed. For the other sample (high LHE/film thickness ratio, no symbol solid line in Figure 5), a sub-quadratic dependence is observed.

).u .a( noissimE CU detargetnI

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10 10 10

2 7

Slope = 2

4 2 6

Slope = 2

4 2 5 9

1

2

3

4

5

6 7 8 9 -2

10

Power Density/(mW cm ) Figure 6: Integrated upconverted emission vs excitation power density for PtPoDCA/ADBA|ZrO2 co-sensitized films in sealed cells containing BuN (λexc = 534 nm). The solid lines are quadratic power dependence models. Steady-state absorption and fluorescence spectra of the samples can be found in Figure 3 (circles and no symbol lines, respectively). Phosphorescence and upconverted emission spectra of the samples are shown in Figures 3c and 5, respectively. As a reference, the steady-state phosphorescence of a sample with a Pt-PoDCA (only) sensitized ZrO2 film in BuN was also studied as a function of incident light intensity. The results revealed a shift from linear to sub-linear light intensity dependence at power densities below those applied in steady-state photon upconversion measurements (see Figure S12 in the ESI). 22


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Time-resolved Photon Upconversion In general, a higher UC signal in the ADBA/Pt-PoDCA|ZrO2 systems compared to systems with MTAB emitters allowed for more detailed time-resolved studies of the former systems. Time-resolved studies of the ADBA/Pt-PoDCA|ZrO2 system showed an emission signal in the ADBA fluorescence spectral region at short timescales (few ns), close to the instrumental resolution limit with a weak signal detectable up to 100 μs after excitation (see Figure 7a and b and Figures S7, S8, and S9 in the ESI). It should be noted that a fast decay of ADBA fluorescence as a result of two-photon absorption of 532 nm can be expected in the same spectral region and can therefore be difficult to distinguish from a fast decaying UCSTTA emission signal. 29 Comparison of the spectral shape at gate-delays of 3 and 40 ns reveals a red-shift very similar to the difference between the shapes of steady-state prompt fluorescence and UC-STTA emission (compare Figure 7c and d), which suggests that the majority of emission signal at short times (few ns) is prompt fluorescence due to emitter two-photon absorption, while the UC-STTA process is responsible for the main part of the emission signal after 30-40 nanoseconds. For comparison, the prompt fluorescence weighted average lifetime of ADBA on mesoporous ZrO2 films was previously reported to be 1.6 ns for excitation at 405 nm, based on TCSPC measurements. 29 It was noted, that this value depends on the specific film thickness and dye surface coverage. After 40 ns the shape of the time-resolved emission spectra of the co-sensitized sample (Figure 7b) in the 400-500 nm interval compares very well to the steady-state UC-STTA emission spectrum, and only an insignificant tail of the prompt fluorescence decay contributes to the emission signal at these long delay times. The long-lived signal attributed to photon upconversion could be fitted to a sum of three exponential decay functions with decay time constants of 4.7·101 ns, 2.8·102 ns, and 1.1 μs, and a weighted average decay time constant of 1.0·102 ns (see Table S5 and Figure S7 in the ESI).

23



40x10

3

1400

3 ns 9 ns 15 ns 21 ns 30 ns 40 ns

a)

30 20

40 ns 50 ns 60 ns 70 ns 90 ns 120 ns

b)

).u .a( noissimE devloser-emiT

).u .a( noissimE devloser-emiT

1200 1000 800 600 400

10

200

0 380 400 420 440 460 480 500

0 380 400 420 440 460 480 500

Wavelength/nm

Wavelength/nm

noissimE .vloser-emiT dezilamroN

noissimE etats-ydaetS dezilamroN

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 51

c)

lexc = lexc =

400

393 nm 534 nm

420 440 460 Wavelength/nm

480

500

d)

lexc =

532 nm

3 ns 40 ns 400

420 440 460 Wavelength/nm

480

500

Figure 7: (a and b) Time-resolved emission spectra (λexc = 532 nm) at various gate-delays of co-sensitized Pt-PoDCA/ADBA|ZrO2 in BuN contained in a cell sealed in a glove-box. The gate-width was 5 ns. The spectra were measured using an optical filter that distorts the red edge of the spectra. (c) Normalized steady-state prompt fluorescence (λexc = 393 nm, dashed line) and UC-STTA emission (λexc = 534 nm, solid line) spectra of the same sample. (d) Normalized time-resolved emission spectra (λexc = 532 nm) at gate-delays of 3 and 40 ns. The steady-state absorption spectrum of the sample is shown in Figure 3a (triangle markers) and the steady-state phosphorescence spectrum is shown in Figure 3c (triangle markers).

24


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Long-term Stability of the Samples The photophysics of three samples were studied over longer periods of time (freshly prepared, weeks, months, a year), with the samples stored in darkness in between measurements. Different types of optical measurements were done on these samples over time, so each sample was exposed to xenon arc lamp or laser pulse irradiation in between measurement of the presented data. Figure 8 shows the long-term stability of one of the studied samples. Remarkably, the UC emission of the sample increased one week after its preparation, although longer storage times lead to a decline of the signal. Nevertheless, the loss of UC emission with time is notably slow. Roughly half the initial UC signal could still be observed in a sample stored for one year, even though substantial amounts of oxygen had certainly leaked into the sealed cell sample (Figure S20 in the Supporting Information). All the studied samples show a clear increase in steady-state phosphorescence intensity for samples stored in darkness for up to a year (see Figure 8b and Figure S16 in the Supporting Information). However, no significant changes were observed in the phosphorescence decay of the aging samples (see Figure S5 in the Supporting Information). Figure 8c shows the consequences of sample aging on the fluorescence spectra. The fluorescence intensity in the ADBA/Pt-PoDCA co-sensitized samples follows the same trend as observed for UC emission; an initial increase after one week of aging followed by a decrease. A MTAB/Pt-PoDME co-sensitized sample was studied after a year, showing a blue-shift and a decrease in intensity of the fluorescence spectrum (see Figure S14 in the ESI).

25



a)

).u .a( ecnecserohpsohP

).u .a( noissimE detrevnocpU

1 2

400

420

440

460

480

500

b)

2 1

620

630

640

Wavelength/nm

650

660

670

680

Wavelength/nm 2

c)

).u .a( ecnecseroulF

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 51

1

400

420

440

460

480

500

Wavelength/nm

Figure 8: Steady-state upconverted emission (a), phosphorescence (b), and prompt fluorescence (c) of one of the mesoporous ZrO2 co-sensitized films shown in Figure 3 (similar symbols; ADBA and Pt-PoDCA, 1.0 μm film, triangles). The spectra were measured at different times after cell sealing; dotted red triangles, same day; long dashed red triangles, 1 week later; short dashed red triangles, 1 month later. In between these measurements, additional spectroscopic measurements were done, in which the samples were exposed to irradiation from a xenon arc lamp and/or laser pulses. The excitation wavelength was 534 nm for all UC emission and phosphorescence measurements and 393 nm for all fluorescence measurements.

26


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Discussion Sensitizer Dynamics vs Surface Coverage The selected set of four samples presented in this paper were all prepared by using the same concentration of ADBA in the sensitization solution, but different concentrations of Pt-PoDCA. Due to its carboxylic acid groups, Pt-PoDCA binds very well to the surface compared to its ester analogue Pt-PoDME, used for co-sensitization with MTAB. As the concentration of Pt-PoDCA in the sensitization solution was increased, the molar ratio between the two dyes in solution was changed, which was reflected in the surface coverage where the coverage of ADBA decreases as the coverage of Pt-PoDCA increases. Table 1 shows that the phosphorescence decay rate increases when the sensitizer surface coverage increases and the emitter one decreases. This relationship is the inverse of what could be expected if TET to emitters was the main sensitizer triplet decay pathway. In the co-sensitized ZrO2 films studied in this article, the sensitizers and emitters are present in the same phase, chemisorbed onto a nanostructured surface. The studied range of emitter/sensitizer (mol/mol) ratios is rather large (from 97 to 8, see Table 2), and it is clear from absorbance, emission, and excitation spectra of the samples, that porphyrin aggregates are present on the samples with the largest relative concentration of porphyrin, and therefore that at least a fraction of the porphyrins are chemisorbed closely together. The ZrO2 surface does not facilitate neat ordered organization of molecules on the surface. There is ample evidence that porphyrins are prone to segregating into islands, which may lead to aggregation. 31 The formation of Pt-PoDCA aggregates on the surface of ZrO2 , is confirmed in this study by the presence of a phosphorescence band at 750-800 nm (Figures 2-3c), which closely resembles the band previously assigned to PtOEP porphyrin dimers. 42,55–58 Aggregates can behave as energy traps and introduce an additional deactivation path at high surface coverage, therefore decreasing the phosphorescence lifetime. The long lived phosphorescence of Pt-PoDCA suggests that many Pt-PoDCA molecules in the triplet excited-state, do not

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

encounter ADBA molecules or do so inefficiently. As the surface coverage of porphyrins is increased, the probability of homogeneous TTA among sensitizers is also expected to increase. 65 The presence of a sub-linear dependence of phosphorescence intensity on incident light intensity in a Pt-PoDCA|ZrO2 reference sample further supports the idea that sensitizer homo-TTA is active in these systems (Figure S12 in the Supporting Information). Furthermore the τav /φ ratio, which is a measure of the relative importance of second order phosphorescence quenching processes in a given sample, was seen to decrease as the sensitizer surface coverage was increased, which is consistent with a sensitizer homo-TTA loss mechanism (see Table 1). In a reference system with no emitter (Pt-PoDCA|ZrO2 ) the τav /φ ratio decreased further due to the lack of first order TET to emitters, which increased the relative importance of the homo-TTA decay path. The lowest τav /φ ratio is found for Pt-PoDME in BuN solution, where the absence of quenching by emitters and the increased mobility of the sensitizers in the solution phase maximize the relative importance of excited state decay by homo-TTA (Table 1). Briefly, a decrease in phosphorescence lifetime is observed as the sensitizer surface coverage increases, which can be explained by sensitizer domain formation causing phosphorescence quenching by homogeneous sensitizer TTA and energy traps in sensitizer aggregates. Examination of the transient absorption traces of Pt-PoDCA/ADBA|ZrO2 (Figures 4 and S4 in the ESI), which are dominated by the porphyrin contribution, shows that there is a discrepancy between the transient signals at 405 nm and 374 nm. The signal at 405 nm is attributed to the excited-state decay of Pt-PoDCA and it matches well with the decay of the phosphorescence at 646 nm. The signal at 374 nm roughly follows the decay at 405 nm, but it has an additional long-lived component extending to the ms timescale (see Table S3 in the Supporting Information). From the extracted time constants, it is clear that there are more complicated kinetics observed at 374 nm than is observed for the excited-state. Firstly, there are two components that have substantial absorbance at 374 nm, Pt-PoDCA (Figure 2) and ADBA. 29 At early times, the signal is most likely attributable to the ground-

28


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state bleach of Pt-PoDCA, since it has a larger extinction coefficient than ADBA, despite the fact that absorbance from ADBA is near its maximum at 374 nm. As the system evolves, PtPoDCA triplet-energy-transfers to ADBA and a ground-state bleach in ADBA can develop. After Pt-PoDCA has decayed to the ground-state, the triplet from ADBA decays with slower time constants. Secondly, it is conceivable that some small fraction of Pt-PoDCA triplets may react directly with oxygen in solution leading to undesired photochemistry. This is a likely explanation for why Pt-PoDME was observed to have a substantially longer time component for the ground-state bleach in Figure S2. Due to the larger number of triplet excited-states for Pt-PoDCA in the co-adsorbed system, the reaction with oxygen may be more favorable, resulting in a larger fraction of triplet excited-states reacting with oxygen to produce undesired photochemistry. The need for a plateau in the modelling of the 374 nm signal of a Pt-PoDCA|ZrO2 reference sample is another indication that the slow decay of this signal in the co-sensitized samples cannot solely be ascribed to the presence of emitter triplet states (see Figure S3 and Table S2 in the Supporting Information). Lastly, it is possible that the singlet excited-state of ADBA could oxidize Pt-PoDCA. This could explain the rather high amplitude and exceptionally long lifetime of the 374 nm bleach seen in the Pt-PoDCA/ADBA|ZrO2 system compared to samples without ADBA present (compare Table S3 with Tables S1 and S2 in the Supporting Information). The singlet excited-state of ADBA is a potent oxidant that is thermodynamically capable of oxidizing Pt-PoDCA and the short distance between chemisorbed dyes on the surface compared to dyes in solution makes electron transfer reactions likely events during the lifetime of the singlet excited emitter molecule. The possibility of electron transfer reactions has implications for solar cell design. When designing co-chemisorbed systems for UC-STTA the ability of the sensitizer to undergo oxidation by the excited emitter should be taken into account.

29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

UC Efficiency Dependence on Sensitizer Surface Coverage Experimental data show that there is an inverse correlation between the Pt-PoDCA coverage and the UC-STTA efficiency; the higher the coverage the lower the efficiency 66 (see Table 2). This is an unexpected observation, since the quantum yield of UC-STTA is known to be linearly dependent on the concentration of triplet excited emitters and could therefore be expected to increase with the increasing absorbance of the triplet sensitizer. 33 One well known reason for a lack of linear increase in UC-STTA quantum yield with increasing triplet state concentration is that the UC-STTA process has entered the “strong annihilation limit”. But such a scenario should result in a constant quantum yield at high triplet state concentration, and not a decrease in quantum yield as it is observed here. A consequence of the above finding in the ADBA/Pt-PoDCA|ZrO2 samples is that the co-sensitized film that shows the highest UC-STTA efficiency is not the one that shows the highest UC-STTA intensity (see Table 2). This means that at a certain concentration of Pt-PoDCA the cost of getting higher UC-STTA emission intensities by increasing the PtPoDCA concentration is a loss in UC-STTA efficiency. Some additional decay path seems to be introduced as the Pt-PoDCA concentration increases. As discussed in the previous section, the islanding and aggregation of Pt-PoDCA would facilitate isolation and it might be an explanation for why increasing Pt-PoDCA surface coverage leads to a decrease in UC-STTA efficiency. In the excitation intensity dependence measurements presented in Figure 6, the UCSTTA emission magnitude was compared for the ADBA sample with the highest UC-STTA efficiency (circle markers) and the ADBA sample showing the highest UC-STTA emission intensity (line markers). The high UC-STTA efficiency sample shows the quadratic excitation intensity dependence which is the expected dependence for UC-STTA in a sample, where TTA is not the main deactivation channel for triplet excited emitter molecules. In contrast to this, when TTA becomes the dominant decay path for triplet excited emitter molecules (the “strong annihilation limit”), a sub-quadratic dependence can be expected, which is 30


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observed for the sample showing the highest UC-STTA emission intensity. The higher PtPoDCA surface coverage in the sample with the highest UC-STTA emission signal makes it conceivable that the “strong annihilation limit” can be reached at lower excitation intensity in this sample, since a relatively high triplet concentration can be obtained due to the high Pt-PoDCA surface coverage. However, the fact that the sample showing this subquadratic excitation intensity dependence shows a lower UC-STTA efficiency than the sample that follows a pure quadratic excitation intensity dependence makes room for alternative explanations. The excitation energy dependence equations commonly used to model sensitized photon upconversion are derived for homogenous solutions of sensitizer and emitters, and a sensitizer concentration significantly lower than emitter concentration is assumed. 33 In our previous work, we have seen that for other conditions, there can be alternative reasons for a subquadratic dependence of UC on the excitation energy. 29 More specifically, it was observed that separation of sensitizers and emitters into separate phases, introduced significant UC efficiency losses due to homogenous TTA among sensitizers. 29 Hence, in the present study, the sub-quadratic dependence of UC-STTA emission in the high porphyrin surface coverage sample observed at high excitation light intensities could be explained by homogeneous TTA among sensitizers, which is expected to be more efficient at higher excitation intensities. The existence of this alternative decay path is also consistent with the observed negative correlation between the sensitizer surface coverage and the UC-STTA efficiency seen in the ADBA based samples. The higher the Pt-PoDCA surface coverage, the more efficient becomes the homogeneous Pt-PoDCA/Pt-PoDCA TTA process and the less efficient becomes UC-STTA. Monguzzi and co-workers have done theoretical modelling of the measured excitation light dependence of UC-STTA in homogeneous solution systems with a fixed sensitizer concentration and various emitter concentrations. 8 In their data, they identified a saturation limit, where the available emitter population is saturated with triplet states, and where further increase in incident light intensity leads to a decrease in UC-STTA quantum yield. This

31



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saturation limit can be observed at gradually lower excitation intensities for increasing sensitizer/emitter molar ratios. A triplet saturation of the emitter population might therefore be an alternative explanation to the negative correlation between sensitizer/emitter (mol/mol) ratio and UC-STTA efficiency that is observed in the ADBA co-sensitized mesoporous ZrO2 systems of this study, as well as for the sub-quadratic excitation light intensity dependence of UC-STTA at high light intensity in the high sensitizer/emitter (mol/mol) ratio sample. It is conceivable that both triplet emitter population saturation and homogeneous sensitizer TTA are relevant processes in the ADBA/Pt-PoDCA|ZrO2 systems, working in concert to cause the observed UC-STTA behaviour.

Long-term Sample Stability The increase in steady-state phosphorescence signals observed in the long-term stability studies (see Figure 8b) can be partly ascribed to the presence of high concentrations of chemisorbed emitter molecules in the nanopores, where the chemisorbed sensitizers are also located. The small volumes in the nanopores and the high concentration of emitter molecules secure that the anti-oxidant effect of the emitters 29 will quickly deplete any oxygen that could have leaked into the nanoporous network when optical measurements are carried out. As emitter molecules react with singlet oxygen they loose their ability to accept triplet states from nearby sensitizers, which leads to an increase in phosphorescence intensity. Moreover, desorption of sensitizers that are strongly quenched by chemisorbed emitters could contribute to the observed long-term evolution of phosphorescence intensity. The prompt fluorescence from emitters shows a decrease in intensity and a hypsochromic shift due to reduced inner-filter effect, which is expected as a consequence of emitter degradation and desorption. The longer the period of time after the sample cell was sealed, the stronger is the effect on the emitter prompt fluorescence magnitude, which decreases as the emitter molecules react with singlet oxygen. The effect of emitter desorption is expected to be higher, the higher the emitter solubility is, and it should therefore be higher for MTAB 32


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compared to ADBA, which is supported by the steady-state prompt fluorescence evolution over time (compare Figure 8c and Figure S14 in the ESI). The initial increase of UC emission intensity after one week of sample storage seen in Figure 8a is a consequence of the light-induced oxygen removal described above, which reduces the degree of triplet state quenching by molecular oxygen and thereby gives a boost to the UC emission intensity. Generation of singlet oxygen has the highest probability of taking place close to chemisorbed sensitizers, where the concentration of triplet excited states can be assumed to be highest. Therefore the generated singlet oxygen is also most likely to react with the emitter molecules placed closest to the sensitizers. These emitter molecules have a special importance in the UC-STTA process either by producing the upconverted emission or by being the important first link in a triplet energy migration chain to the encounter of other triplet states for annihilation and UC emission production. Therefore, the photodegradation of the emitter molecules that initially leads to an increase of UC, due to its anti-oxidant effect, will in the long term cause a drop on the UC intensity. Another contributor to the decrease seen in UC-STTA emission efficiency over time is the desorption of emitter molecules from the surface, since it will decrease the efficiency of the TET from sensitizer to emitter and the triplet energy migration among emitter molecules on the film. Another factor that will affect the stability of the samples is the kinetics of UC. The faster the rise of the delayed fluorescence (UC emission), the shorter the triplet emitter lifetime and therefore, the smaller the losses due to triplet quenching with molecular oxygen, leading to photodegradation of the sample. The time-resolved emission data measured on the ADBA/Pt-PoDCA|ZrO2 system show a fast decay component (close to the instrumental resolution) in the spectral region where the UC-STTA signal is seen in the steady-state measurements (see Figure 7 and Figures S7 and S9 in the ESI). Due to the short lifetime of the component, it is possible that part of the observed emission is due to fluorescence from ADBA molecules excited directly by two-photon absorption, 29 which would be in agreement with the small red-shift observed in the emission spectrum at later delay times (Figure 7d). A

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similar shift is observed when comparing steady-state prompt and delayed (UC) fluorescence spectra (Figure 7c). Nevertheless, since only a very low signal was observed in the 400-500 nm interval at later times, it is reasonable that part of the short-lived signal around 440 nm is due to the UC-STTA emission, which has been observed in steady-state measurements that have no contribution from two photon absorption. The sensitization of this UC-STTA signal from Pt-PoDCA triplet excited states must therefore occur in the same or a faster time-scale and could be the reason for the short decay components seen in the transient absorption data at 374 and 405 nm (see Figures 4 and S4 in the Supporting Information). Consequently, the fast rise of UC emission suggested by the time-resolved data points at very fast TET and UC-STTA kinetics being active at the nanostructured surface. The fast kinetics as compared to solution systems would contribute to the observed long term stability of the co-adsorbed UC system by reducing the quenching effect of the increasing concentration of oxygen in the system. Briefly, in spite of the slow photodegradation of the emitter with time, it is important to stress that the co-chemisorbed studied systems are remarkably robust and that a large fraction of the initial UC emission can be observed even one year after preparation of the sample (Figure S20 in the Supporting Information). The most plausible, and apparently contradictory, reasons are the anti-oxidant effect of DPA chemisorbed onto the nanostructured network and the fast rise of UC emission.

Comparison with Co-physisorbed and Heterogeneous Systems Co-chemisorbed vs Co-physisorbed Systems The co-sensitized mesoporous ZrO2 film systems based on chemisorbed dyes presented in this paper show clear improvements in absolute UC-STTA signal intensity compared to similar systems previously studied based on physisorption (compare solid red and grey lines in Figure 9). 27 The highest UC-STTA emission intensity of the co-chemisorbed system, achieved with an emitter/sensitizer ratio of 10 (mol/mol), is an order of magnitude higher than the corresponding intensity in the co-physisorbed system. The absorbance of the porphyrin Q34


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band, where the samples are excited, is however quite different in the two different systems, and taking this difference into account gives a similar UC-STTA efficiency in the systems (see Table 2). Increasing the emitter/sensitizer ratio of the co-chemisorbed samples leads to a significant increase of the UC efficiency, although the absolute signal decreases. At an emitter/sensitizer ratio of 97 (mol/mol), the UC-STTA efficiency of the co-chemisorbed system becomes roughly four times higher than the corresponding efficiency of the co-physisorbed system (see Table 2). In this case, the surface coverage of Pt-PoDCA sensitizers is relatively low, which minimizes UC-STTA efficiency losses due to homogeneous sensitizer TTA and re-absorption of UC emission by the sensitizer (inner-filter effect). In the co-physisorbed system, UC-STTA efficiency losses due to inner-filter effect by the sensitizer are also thought to be minimal based on a very low sensitizer absorbance. 27 The higher UC-STTA efficiency in co-chemisorbed systems with relatively low sensitizer surface coverage, compared to cophysisorbed systems, is most likely a consequence of improved triplet energy migration among surface adsorbed emitter dyes, due to reduced inter-dye distances obtained by chemisorption, because of an improved dye-loading. In the co-physisorbed system, the dyes were thought to lie flat on the ZrO2 surface due to the polar water solvent surrounding the nanostructure. In the co-chemisorbed systems water was substituted for the less polar BuN solvent, which should allow chemisorbed dyes to “stick out” from the surface plane and possibly improve the orbital overlap with neighbouring molecules, which may lead to improved triplet energy migration and TTA. While the co-chemisorbed systems were contained in sample cells, sealed in a glove-box to minimize the oxygen concentration, the co-physisorbed systems were contained in a cuvette filled with water and purged with argon. The latter approach was taken since the heatsealing of sample cells results in desorption of the physisorbed molecules. The different approaches to oxygen removal can be expected to allow for very different amounts of oxygen in the two types of systems, and the co-physisorbed system, which was purged with argon,

35



showed a high degree of emitter molecule degradation after illumination, most likely due to endoperoxide formation. This emitter degradation was avoided in sealed cells freshly prepared in the glove-box. While the co-physisorbed systems degraded in a few hours, cochemisorbed systems in sealed samples showed UC-STTA emission after one year (see Figure S20 in the Supporting Information). After this long storage period the oxygen concentration of the sample interior is expected to have equilibrated with the ambient atmosphere, and the continued UC activity of the cell is a clear sign of the robustness of the the UC-STTA mechanism in these systems toward the presence of oxygen. ).u .a( noissimE detrevnocpU

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x 30

x 100

400

420

440

460

480

500

Wavelength/nm

Figure 9: A comparison of the relative intensities for steady-state upconverted emission of (red, solid line) a mesoporous ZrO2 film (film thickness is 1.8 μm) co-sensitized (chemisorption) with ADBA and Pt-PoDCA in a sealed cell filled with BuN, (red, dashed line) a previously studied ADBA|ZrO2 (1.8 μm film) sample in a sealed cell with a dilute (∼ 4.4 × 10−5 M) BuN solution of PtOEP, 29 and (grey line) a previously studied DPA/PtOEP|ZrO2 cosensitized (physisorption) system (film thickness is 5.3 μm). 27

Co-chemisorbed vs Heterogeneous Systems Comparison to heterogeneous systems for UC-STTA with chemisorbed emitters and sensitizers in solution is complicated by the rather different composition and function of these systems. 28,29 In particular, when comparing the relative UC-STTA emission efficiencies, it should be considered that in the heterogeneous systems, the emitters and sensitizers are separated into two phases. This means that the majority of absorbing sensitizers will have no contact to emitter molecules chemisorbed at the nanostructured ZrO2 film and therefore cannot contribute to the observed UC-STTA emission. Furthermore, the UC-STTA signal 36


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mainly originates from the front layer of the mesoporous film interfacing the bulk solution phase. In spite of this, the integrated intensity of the UC-STTA emission spectrum for the heterogeneous system shown in Figure 9 (red dashed line) is 42 times higher than the same quantity calculated for the co-chemisorbed system (red solid line). Taking into account the porphyrin Q-band light harvesting efficiency at the excitation wavelength in the two systems, the overall (global) UC-STTA efficiency is 160 times higher in the heterogeneous system than in the co-chemisorbed system showing the highest UC-STTA efficiency. Considering that in the heterogeneous system only sensitizers within the sensitizer excited state diffusion length from the mesoporous film interface in the bulk solution are thought to contribute significantly to the observed UC-STTA emission, the effective local UC-STTA efficiency in this confined volume is most likely another 160 times 67 higher compared to the UC-STTA efficiency of the co-chemisorbed system. The most likely reason for the higher efficiencies observed in the heterogeneous systems is the spatial distribution profile of the triplet excited emitters. In the heterogeneous systems, the majority of the observed UC-STTA emission is thought to originate from the interface between the bulk solution phase and the mesoporous film. Excited sensitizers from a relatively large volume can diffuse and transfer triplet state energy to chemisorbed emitters at the mesoporous film interface. Therefore, a high triplet emitter concentration can be reached at the interface with the bulk solution. An additional advantage of the localization of the UC process at the interface is that reabsorption of upconverted photons due to inner-filter effect is minimized. In contrast, in the co-sensitized systems the triplet emitter population is more diluted throughout the film phase together with the sensitizers. Since the UC-STTA emission intensity depends on the square of the triplet emitter concentration, the described effect of accumulation of triplet states in the interface region of the heterogeneous systems can be expected to have a large positive impact on the UC-STTA emission intensity. In principle, the issue of the low UC efficiency found in chemisorbed systems due to the spatial delocalization of triplet excited emitter states, could be addressed by increasing

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the concentration of sensitizers in the samples. In practice, this approach did not work as expected. For the MTAB/Pt-PoDME co-sensitized systems it was simply not possible to achieve high sensitizer concentrations. The ester groups applied as chemical anchors limited the yield of the chemisorption process and the resulting surface coverage of dyes, especially for the Pt-PoDME sensitizer dye. Using carboxylic acid groups instead of esters for molecular anchoring, allowed for a much better control of the chemisorption process and larger sensitizer concentrations in the samples. However, increasing the Pt-PoDCA sensitizer surface coverage did not lead to an increase in UC efficiency (see Table 2). As discussed above, this observation has several likely causes including the formation of Pt-PoDCA domains, where losses due to aggregate formation and homogeneous sensitizer TTA increases, 65 and a triplet saturation of the emitter population available for triplet energy transfer from sensitizers can occur. An additional problem is that increasing the Pt-PoDCA sensitizer surface coverage seems to decrease the ADBA concentration in the sample, even though total coverage of the surface is not thought to be achieved. This observation highlights the advantage in the heterogeneous systems of having the sensitizers in a separate solution phase, where their concentration, in principle, can be increased without losses in emitter surface coverage. Long-term stability experiments in co-chemisorbed systems seem to suggest that endoperoxide formation could have some degree of site preference, with emitter molecules chemisorbed close to sensitizer molecules being more vulnerable to this degradation reaction. This could be a serious drawback of the co-sensitized system architecture, because reactions with singlet oxygen could very efficiently cut off the initial energy transfer steps in the UC-STTA process. Moreover, desorption of emitter molecules from the surface can cut off energy migration pathways on the surface and thereby slow down the UC-STTA process. In this regard, the heterogeneous system architecture has the advantage that the initial energy transfer does not take place at one specific molecular site each time, but is instead randomized throughout the nanostructured surface, which should render the energy migration chains involved in the UC-STTA process more robust to degradation or desorption

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of chemisorbed emitter molecules. On the other hand the diffusion controlled kinetics of the UC-STTA mechanism in the heterogeneous systems 29 makes these systems much more vulnerable to the quenching by oxygen than the co-adsorbed systems where the fast UC-STTA kinetics (see Figure 7) makes these systems robust towards the presence of oxygen. In summary, the data suggest that further optimization of the co-chemisorbed systems will require modification of the dyes to achieve higher surface coverages and at the same time avoid the segregation of sensitizer and emitter dyes into separate domains. Even with an improved control of the distribution of sensitizers and emitters on the surface, it is likely that the inherent inner filter effect of the sensitizer will limit the potential for improvement in UC-STTA efficiency by increased dye surface coverage. It should be noted though, that for the use of the UC-STTA mechanism of these systems as part of a photoactive electrode, losses due to the inner filter effect are of minor importance, since in this case an injection of a high-energy electron is the wanted product of the UC-STTA process, rather than an emitted photon.

Summary Photon upconversion by sensitized triplet–triplet annihilation (UC-STTA) has been studied in systems with triplet sensitizer and emitter molecules co-chemisorbed onto nanostructured ZrO2 films. The studied sensitizer and emitter molecules were based on the well-known UCSTTA molecular pair PtOEP/DPA, functionalized with esters or carboxylic acids as anchoring groups. The Pt-PoDCA/ADBA system, which contains carboxylic acid anchoring groups, had the highest on film surface coverage and greatest reproducibility in the chemisorption process. In this case, a relative increase in UC-STTA efficiency of 400% was achieved with respect to the previously studied co-physisorbed PtOEP/DPA. 27 This improvement was attributed to more efficient triplet energy migration among the surface adsorbed molecules due to reduced intermolecular distances and more favorable dye orientation.

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Controlled variation of the Pt-PoDCA/ADBA molecular ratios on the film surface revealed the appearance of new energy loss mechanisms at higher sensitizer surface coverages. Steady-state absorption and emission spectra indicated the presence of porphyrin aggregates at high sensitizer surface coverage. Analysis of the dependence of the Pt-PoDCA phosphorescence decay and the relative UC-STTA efficiency on sensitizer surface coverage and excitation intensity, suggested the formation of separate sensitizer domains. In this case, excited sensitizer states could be deactivated by trapping in aggregate states, via homogenous triplet-triplet annihilation, and/or due to saturation of a limited emitter population in the vicinity, available for triplet sensitization from sensitizers. As well, long lived photobleaching of Pt-PoDCA in the presence of ADBA suggests holetransfer from ADBA to Pt-PoDCA. This unintended charge transfer chemistry will likely be a part of solid state solar energy conversion systems that utilize UC-STTA. In addition to energy transfer concerns in designing UC-STTA systems for solar energy conversion, electron transfer chemistry must also be taken into account. Upconversion dynamics on the nanosecond time scale were observed. The data indicate that molecular diffusion is not required for TET or TTA. The remarkably rapid energy transfer likely provides a certain degree of photoprotection to the studied samples over time. Overall, the system has a low sensitivity to O2 concentrations. Strikingly, UC emission was observed a year after sample preparation. The encountered limitations of the upconverting systems presented herein, can most likely be reduced or avoided by appropriate molecular design. In particular, the UC-STTA efficiency in co-adsorbed systems would benefit from improved emitter surface coverage, which might be feasible by the use of stronger anchoring groups, 68 energetic and/or structural modification of the dyes to maximize energy transfer yields while minimizing electron transfer, and a better mixing of sensitizer and emitter molecules on the surface. Aggregation is a well-known problem for porphyrin molecules, and a number of strategies have already been developed to limit this behavior. 31 If molecular engineering strategies can successfully

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prevent the aforementioned limitations in the co-chemisorbed nanostructured films, the resulting UC-STTA system is expected to present some great advantages due to fast energy migration along the nanostructured surface, which minimizes the sensitivity of the UC-STTA process to quenching by molecular oxygen, and a high molecular density allowing efficient light harvesting.

Acknowledgement This work was supported by the Swedish Research Council (VR). JMG and JSL thankfully acknowledge support from the Swedish Government through “STandUP for ENERGY”.

Supporting Information Synthesis procedure and reaction scheme. Time-resolved spectroscopic data of sensitizer molecules in solution and on mesoporous ZrO2 films; alone and co-adsorbed with ADBA. Phosphoresence excitation intensity dependence of a Pt-PoDCA sensitized ZrO2 film. Steadystate absorption and emission spectra of a series of MTAB/Pt-PoDME sensitized ZrO2 films, including long term stability studies. Comparison of steady-state spectroscopic data for heterogeneous systems with MTAB adsorbed onto mesoporous ZrO2 and either PtOEP or Pt-PoDME in BuN solution. This information is available free of charge via the Internet at http://pubs.acs.org

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(61) The short initial emission decay component is similar to what has been observed in heterogeneous systems with the sensitizer in solution. 29 It was argued that the short component was due to emitter fluorescence following a two-photon absorption of 532 nm laser photons by the emitter. It was shown that the tail of the fluorescence spectrum at the emission wavelength 646 nm has a significantly high intensity with respect to the phosphorescence at short time scales. This observation is reproduced in the co-sensitized systems of this study. (62) The relative degree of phosphorescence quenching can be compared between samples by integrating the phosphorescence intensity, and dividing the resulting value with the Q-band light harvesting efficiency (LHE = 1 − 10−A , where A is the absorbance) of the sample at the excitation wavelength. The lower this quantity is, the more phosphorescence quenching takes place. (63) As defined in Table 2. (64) LHE = 1 − 10−A , where A is the absorbance. (65) Jankus, V.; Snedden, E. W.; Bright, D. W.; Whittle, V. L.; Williams, J. A. G.; Monkman, A. Energy Upconversion Via Triplet Fusion in Super Yellow PPV Films Doped with Palladium Tetraphenyltetrabenzoporphyrin: A Comprehensive Investigation of Exciton Dynamics. Adv. Funct. Mater. 2013, 23, 384–393. (66) This trend could not be observed for MTAB/Pt-PoDME samples, due to a nearly constant sensitizer coverage in this series. (67) A factor of 49 μm/300 nm ≈ 160, where 49 μm is the effective light path length through PtOEP in solution and 300 nm is the excited state diffusion length of PtOEP in solution. 69 (68) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Dye-Sensitized Solar Cells. Chem. Rev. 2010, 110, 6595–6663. 49



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(69) Bansal, A. K.; Holzer, W.; Penzkofer, A.; Tsuboi, T. Absorption and Emission Spectroscopic Characterization of Platinum-Octaethyl-Porphyrin (PtOEP). Chem. Phys. 2006, 330, 118–129.

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Photon Upconversion from Chemically Bound Triplet Sensitizers and

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