Triplet–Triplet Annihilation-Induced Up-Converted Delayed

Jun 11, 2014 - Triplet–Triplet Annihilation-Induced Up-Converted Delayed Luminescence in Solid-State Organic Composites: Monitoring Low-Energy Photo...
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Triplet−Triplet Annihilation-Induced Up-Converted Delayed Luminescence in Solid-State Organic Composites: Monitoring LowEnergy Photon Up-Conversion at Low Temperatures Hossein Goudarzi and Panagiotis E. Keivanidis* Centre for Nano Science and Technology @PoliMi, Fondazione Istituto Italiano di Tecnologia, Via Pascoli 70/3, 20133 Milano, Italy S Supporting Information *

ABSTRACT: Hitherto, the role of the enhanced intermolecular interactions and the effect of lowering the temperature on the process of triplet−triplet annihilation-induced upconverted delayed luminescence in solid-state composites systems have remained controversial. Here we address these issues by performing temperature-dependent time-integrated and time-gated luminescence spectroscopic studies on the model photon up-converting solid composite comprising the (2,3,7,8,12,13,17,18-octaethyl-porphyrinato) PtII (PtOEP) sensitizer, mixed with the blue-light emitting 9,10 diphenyl anthracene (DPA) activator. Atomic force microscopy imaging and photoluminescence (PL) spectra confirm that the strength of intermolecular interactions in the DPA:PtOEP system can be tuned by keeping the composite either in its binary or in its ternary form with the use of the optically inert matrix of polystyrene (PS). By diluting DPA:PtOEP in PS, the concentration of the DPA excimeric and the PtOEP triplet dimer quenching sites is reduced and the lifetime of the DPA up-converted PL signal is prolonged to the microsecond time scale. By lowering the temperature to 100 K, the DPA up-converted luminescence intensity increases by a factor of 3, and this is attributed to the increased energetic disorder of the DPA excited states in the PS:DPA:PtOEP ternary system. These findings provide useful guidelines for the fabrication of efficient solid-state photon up-converting organic layers. triplet annihilation (TTA) between two A molecules giving rise to high-energy singlet excited state of A, from which the upconverted emission takes place. It was further demonstrated that ET-UC occurs even when concentrated sunlight is utilized as the photoexcitation source.14−16 The class of the ET-UC composites has been studied in detail, and the photophysical properties of these systems were addressed either with the use of coherent and incoherent light sources17−22 or by theoretical modeling of the spin-statistics and photokinetics.23−25 A model up-converting system for the study of ET-UC mechanism comprises the organometallic complex of (2,3,7,8,12,13,17,18-octaethyl-porphyrinato) PtII (PtOEP) mixed with the blue-light emitting 9,10-diphenyl anthracene (DPA).11 In this composite, PtOEP and DPA serve as the S and A components, respectively. Selective photoexcitation of the DPA:PtOEP in the green, in the absorption Q-band of the PtOEP component where DPA is not absorbing light, results in the generation of blue singlet DPA luminescence. Composition-dependent studies26 have indicated that in the solid state the optimum PtOEP doping concentration is 2 wt %.

1. INTRODUCTION During the last few years, the process of low-energy photon upconversion via triplet−triplet annihilation (TTA-UC)1−3 has received attention as an attractive method for enabling the management of light in a broad range of applications.4−7 Photon up-converting systems can be easily fabricated by solution-processable small molecular and polymeric materials that can be utilized either as solid-state or liquid mixtures.8−11 In the framework of the TTA-UC photophysics, photon upconversion originates from a sequence of photophysical events that take place in binary composite systems of chromophores and that eventually result in the generation of delayed luminescence at photon energies higher than the photon energy used for photoexcitation of the composites. Hitherto, two major photophysical excited-state pathways have been identified for the explanation of the TTA-UC process in organic composite materials; TTA-UC occurs on the basis of either an energy transfer (ET-UC)9−11 or a charge transfer (CT-UC)8,12,13 intermediated photophysical reaction. According to all reports to date, ET-UC is the most efficient of the two mechanisms, and it involves the cascade of: (i) light absorption by a molecular sensitizer (S) producing singlet excited states in S, (ii) intersystem crossing (ISC) in the excited state manifolds of S switching the spin state multiplicity of the neutral photexcitation from singlet to triplet, (iii) triplet energy transfer (TET) from S to a molecular activator (A), and (iv) triplet− © XXXX American Chemical Society

Received: May 29, 2014 Revised: June 7, 2014

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Despite the broad attention that TTA-UC has attracted by many research laboratories,4,27−31 some of the previously reported experimental observations in the TTA-UC systems are not yet well understood. One of these open questions concerns the role of PtOEP aggregation on the process of TTA-UC in the solid state. It is well known that PtOEP in films forms emissive triplet dimers that exhibit luminescence centered at 780 nm, red-shifted in respect to the monomeric phosphorescence peak of PtOEP at ∼650 nm.32−36 The role of these PtOEP aggregates in the process of TTA-UC remains unclear.37−40 The second unresolved question is in regard to the effect of lowering the temperature on the TTA-UC process. Early studies in vitrified DPA:PtOEP solutions demonstrated that in rigid environments and at low temperatures the ET-UC mechanism can be effective.11 However, conflicting reports have appeared in the literature either describing an increase or a reduction of the TTA-UC luminescence intensity at low temperatures.8,41 These two questions are inherently related because for both mechanisms of TTA-UC (ET-UC and CTUC) in up-converting composites, energy migration40 within the omnipresent aggregates in the S and A phases is expected to have a temperature-dependent component. Recently, a temperature-dependent spectroscopic study addressed the effect of temperature on the process of TTA-induced delayed luminescence in single -component polymeric films.42 Although a down-conversion luminescence process was studied, useful conclusions were provided by that report concerning the effect of temperature on the electronic energy migration of triplet excited states in an energetic landscape of disordered density of states. There is a tremendous potential for using solutionprocessable solid-state photon up-converters in light-management applications. The research fields of solar cell devices,43 biological imaging,44 and photodetecting technologies45 for surveillance applications will be greatly benefited by the realization of efficient photon up-converting composite layers that operate on TTA-UC. Efficient solid-state photon upconverters will be valuable in the sensitization of optoelectronic devices that cannot produce photocurrent when illuminated with low-energy photons that are not absorbed by the photoactive layers of the device.46,47 For this reason, it is imperative to fully clarify the processes that dictate the TTAUC in the solid state. In this work, we set out to investigate the influence of low temperature on the process of TTA-UC in the solid-state binary DPA:PtOEP 2 wt % and ternary polystyrene (PS):DPA:PtOEP 2 wt % solid-state composites. By modifying the environment of the PtOEP molecules with the use of the photophysically inert matrix of PS, we tune the strength of the intermolecular interactions of the PtOEP aggregates. Surprisingly, we identify the role of DPA aggregates that to our knowledge they have not been taken into account in the TTAUC process. Our results interrogate the influence of the PtOEP and DPA aggregates in the excited-state pathways that lead to TTA-UC and provide rational guidelines for the fabrication of efficient solid-state photon up-converting layers.

Figure 1. (a) Chemical structure of PtOEP and DPA molecules used in this work; atomic force microscopy height images for films of (b) PS:PtOEP 2 wt %, (c) DPA:PtOEP 2 wt %, and (d) PS:DPA:PtOEP 2 wt %. In all images the AFM window has a length of 10 μm.

Binary composites of PS:PtOEP 2 wt % and DPA:PtOEP 2 wt % were prepared after degassing the chloroform solvent by ultrasonication and N2 purging. Ternary composites of PS:DPA:PtOEP 2 wt % were prepared in an identical fashion like the DPA:PtOEP 2 wt % composites by the addition of PS in the solution. To ensure the efficient dispersion of the DPA and PtOEP components in the ternary composite, the mass of PS was kept three times higher than the mass of DPA. Control films of DPA and PS:DPA were also prepared in the same way by keeping the PS:DPA ratio fixed to 3:1. The surface topography of the PS:DPA 2 wt %, DPA:PtOEP 2 wt %, and PS:DPA:PtOEP 2 wt % composite films was studied by atomic force microscopy using an Agilent 5500 in tapping mode under ambient conditions. Topography and phase images were recorded simultaneously. UV−vis absorption and photoluminescence spectra of the produced films were recorded with a UV-2700 Shimadzu spectrophotometer and a Horiba Jobin Yvon NanoLog spectrofluorimeter, respectively. Timeintegrated and time-gated spectra of the studied systems were recorded in a range of temperatures between 100 and 290 K. The samples were photoexcited, with the output of an optical parametric oscillator (Spectra-Physics VersaScan Midband 120) pumped by the third harmonic of a Nd:YAG Laser (SpectraPhysics INDI-40-10-HG), at 532 nm by a train of 10 ns pulses at a repetition rate of 10 Hz. No focal lens was used for the photoexcitation of the sample, and a laser spot size of 0.385 cm2 was kept for all samples measured. The emitted light was dispersed in a spectrograph (Andor Shamrock Spectrograph, SR303i) with a 150 lines per millimeter grating and detected with a gated intensified charge-coupled device camera (Andor iCCD, iStar DH320T-25U-73). A notch filter was placed in front of the spectrograph for minimizing the intensity of the 532 nm photoexcitation line and for allowing the simultaneous PL detection at the high and the low photon energies of the PL

2. MATERIALS AND METHODS The chemical structures of PtOEP and DPA molecules are shown in Figure 1a. Thin films of DPA, poly(styrene) (PS):DPA, PS:PtOEP, DPA:PtOEP, and PS:DPA:PtOEP were deposited via spin coating of degassed solutions in chloroform onto quartz substrates, in a nitrogen-filled glovebox. B

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Figure 2. Time-integrated photoluminescence spectra of (a) PS:PtOEP 2 wt %, (b) DPA:PtOEP 2 wt %, and (c) PS:DPA:PtOEP 2 wt % films registered at 100 (black lines) and 290 K (red lines). For clarity, the PL intensity in the spectral region of 535−900 nm of panels b and c has been reduced by a factor of 25 and 50, respectively. Temperature-dependent spectrally integrated PL intensity of (d) PtOEP phosphorescence in PS:PtOEP 2 wt % (open circles), (e) PtOEP phosphorescence (filled squares) and DPA up-converted luminescence in DPA:PtOEP 2 wt % (open squares), and (f) PtOEP phosphorescence (filled triangles) and DPA up-converted luminescence (open triangles) in PS:DPA:PtOEP 2 wt %. In all cases, the photoexcitation of the samples was at 532 nm.

Rrms[PS:DPA:PtOEP] = 0.67 nm. The surface texture of the binary DPA:PtOEP 2 wt % composite (Figure 1b) is found to be dominated by islands of well-ordered clusters with a typical cluster size of 0.5 μm × 1 μm. In contrast, the surface texture of the ternary PS:DPA:PtOEP 2 wt % system (Figure 1c) is found to be very similar to that of the reference sample PS:PtOEP 2 wt % (Figure 1a). Nonetheless, in comparison with the PS:PtOEP 2 wt %, the ternary PS:DPA:PtOEP 2 wt % system exhibits a higher areal density of small-sized spherical aggregates with a typical diameter of 0.250 μm. 3.2. Time-Integrated Photoluminescence. The emission spectra of the PS:PtOEP 2 wt %, DPA:PtOEP 2 wt %, and PS:DPA:PtOEP 2 wt % blend films, at 100 and 290 K, are presented in Figure 2a−c. Following the photoexcitation of the samples at 532 nm, where only the Q-band of the PtOEP component absorbs,40 the typical PtOEP phosphorescence emission is detected in all samples, in the spectral region of 625−665 nm. Additional spectral features can be clearly seen in the PL spectra of the DPA:PtOEP and PS:DPA:PtOEP films. In particular, PL emission is detected in the spectral regions of 415−475 (PL band I), 475−520 (PL band II), and 740−820 nm (PL band III). These spectral features are annotated in the PL spectra of DPA:PtOEP (Figure 2b), and they can also be seen in the PL spectra of the PS:DPA:PtOEP film (Figure 2c). The PL bands I and II are detected at photon energies higher than the photon energy used for the photoexcitation of the samples, and they are assigned to the TTA-induced DPA upconverted PL emission. From previous literature, it is well

spectrum. During the spectroscopic characterization, the samples were kept in liquid-nitrogen variable-temperature cryostat (Janis VPF-100) that was evacuated by a turbomolecular pump (Pfeiffer TSH 071E Economy Dry Vacuum Pumping Station) so that a dynamic vacuum of a typical pressure of 10−7 mbar was maintained during the measurements. Temperature-dependent time-integrated and time-gated spectra were recorded in the range of 100−290 K by connecting the cryostat to a Lake Shore temperature controller (model 325) unit. To accurately monitor the sample temperature, we used a second temperature sensor (670-HTSTD) in contact with the surface of the measured sample. Photoexcitation intensity-dependent measurements were performed by using the combination of a set of neutral density filters of known transmittance values at 532 nm.

3. RESULTS 3.1. Atomic Force Microscopy. The surface topography of the DPA:PtOEP 2 wt % and PS:DPA:PtOEP 2 wt % films was studied by atomic force microscopy (AFM) imaging. For reference purposes, a control sample of PS:PtOEP 2 wt % was also studied. Figure 1b−d presents the AFM images recorded for the PS:PtOEP 2 wt %, DPA:PtOEP 2 wt %, and PS:DPA:PtOEP 2 wt %, respectively. With respect to the PS:PtOEP 2 wt % and PS:DPA:PtOEP 2 wt % samples, the root-mean-square surface roughness (Rrms) of the DPA:PtOEP 2 wt % is found to be significantly increased. In particular, Rrms[PS:PtOEP] = 0.86 nm, Rrms[DPA:PtOEP] = 54.2 nm, and C

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the DPA molecules serve as templates that favor the electronic association of adjacent PtOEP molecules. Both DPA and PtOEP systems are planar molecules that have a strong tendency to undergo interfacial π−π stacking. Consequently, in both cases of an effective electronic coupling between adjacent DPA monomers or PtOEP monomers, a longer-range order is anticipated for the PtOEP and DPA domains. The PL spectra in Figure 2b suggest that DPA is very prone to the formation of DPA dimers that exhibit the PL signature DPA intermolecular states. The PL emission of DPA dimers is red-shifted with respect to the PL of the DPA monomers, and this can be clearly seen in the PL bands I and II in Figure 2b,c. The relative contribution of PL band I to the total DPA up-converted PL increases when going from DPA:PtOEP 2 wt % to PS:DPA:PtOEP 2 wt %. Control PL measurements of films made of DPA and PS:DPA after photoexcitaton at 390 nm showed a similar effect. (See Figure S3 in the Supporting Information.) The DPA emission of the pure DPA films is a superposition of the monomer-like DPA PL centered at 430 nm and a PL band centered at 500 nm. The two different emitting species are clearly resolved, and particularly for the case of the DPA intermolecular state, the PL band at 500 nm is followed by a vibronic band at 535 nm, suggesting that the intermolecular DPA state has a ground-state character. However, no detection of low-energy absorption features was possible in the UV−vis spectra of the PS:DPA:PtOEP film that could confirm the presence of ground-state dimer species. (See Figure S4 in the Supporting Information.) It is very likely that the nature of the DPA intermolecular state is very similar to the excimer species that are formed in the aggregates of the disk-shaped perylene bisimide dyes molecules.48 Future work on the theoretical study of electronic coupling between DPA monomers49 in an environment of short-range order is expected to provide useful information. At present, when DPA is dispersed in PS, the monomer-like PL emission and the PL emission of the intermolecular state are less clearly resolved, suggesting a higher energetic disorder for DPA excited-state manifold in the PS:DPA film that broadens the PL of both monomeric and dimer DPA species. In the light of these results, we attribute the PL band II to an intermolecular DPA excimeric state that is activated by the TTA-generated DPA singlet state, following the TTA process in the DPA triplet manifold. Figure 2a−c shows that with respect to room temperature, the overall PL intensity of all samples increases when measured at 100 K. We have performed temperature-dependent PL measurements in a range of 100−290 K for all studied systems. Figure 2d−f presents the evolution of the spectrally integrated intensity of PtOEP phosphorescence at 650 nm (spectral range of 630−660 nm) and DPA up-converted luminescence at 450 nm (spectral range 400−480 nm) for the samples of PS:PtOEP 2 wt % (Figure 2d), DPA:PtOEP 2 wt % (Figure 2e), and PS:DPA:PtOEP 2 wt % (Figure 2f). Starting from 100 K and by increasing the temperature, the PtOEP phosphorescence and DPA up-converted luminescence intensities reduce. For the reference sample of PS:PtOEP 2 wt %, the PtOEP phosphorescence intensity at 100 K is stronger by a factor of 2.0 than when measured at 290 K. For the case of the DPA:PtOEP 2 wt % system and with respect to 290 K, at 100 K the PtOEP phosphorescence intensity is stronger by a factor of 8.5, whereas the DPA up-converted emission intensity is stronger by a factor of 2.3 and is maximized. Similar temperature-dependent effects are seen in the PL properties

known that PL band III originates from triplet dimers formed in PtOEP clusters.32−36 Figure 3 presents the normalized PL

Figure 3. Normalized time-integrated photoluminescence spectra of the PtOEP dimer at (a) 290 and (b) 100 K for films of PS:PtOEP 2 wt % (circles), DPA:PtOEP 2 wt % (squares), and PS:DPA:PtOEP 2 wt % (triangles). The spectra are normalized in the PtOEP monomer phosphorescence peak at 650 nm, and they are magnified in the region of 750−900 nm for clarity. In all cases, the photoexcitation of the samples was at 532 nm.

spectra of the PS:PtOEP 2 wt %, DPA:PtOEP 2 wt % and PS:DPA:PtOEP 2 wt % systems at 290 and at 100 K, magnified in the spectral region of 730−900 nm, where the spectral signature of the emissive PtOEP aggregates can be clearly observed. The PL spectra shown in Figure 3a,b, suggest that the strength of the PtOEP triplet dimers is negligible in the PS:PtOEP 2 wt % sample, but it increases substantially in the DPA:PtOEP 2 wt % film. For the case of the ternary PS:DPA:PtOEP 2 wt % system, the PtOEP triplet dimer is still present, albeit in less concentration than in the DPA:PtOEP 2 wt % binary sample. Interestingly, at 100 K, the PL spectra of the PtOEP aggregate in the ternary PS:DPA:PtOEP 2 wt % composite exhibit sharper spectral features in contrast with the corresponding spectrum of the binary DPA:PtOEP 2 wt % composite that is found to be broader. (See also Figure S1 in the Supporting Information.) This suggests that in the ternary composite the concentration of the PtOEP aggregates is lower but the aggregates are better ordered, resulting in a smaller energetic disorder in their excited states. Most likely, this is due to a narrower distribution of intermolecular configurations in the adjacent PtOEP monomers comprising the PtOEP triplet dimers. We have measured the PL spectra of an additional reference sample with increased PtOEP content (PS:PtOEP 6 wt %), but no sign of increased PtOEP triplet dimer concentration was detected. (See Figure S2 in the Supporting Information.) This finding suggests that in amorphous matrices such as PS the packing of PtOEP is random, resulting in weak electronic coupling of adjacent PtOEP monomers. In contrast, in the presence of molecules such as DPA with a tendency for longrange packing, the PtOEP monomers adapt an ordered configuration that leads to the formation of PtOEP triplet dimers in larger-sized PtOEP clusters. Therefore, in the ternary composite of PS:DPA:PtOEP, fewer PtOEP aggregates are formed that are still well-associated. In fact, it is very likely that D

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Figure 4. Intensity-dependent pulsed photoexcitation at 532 nm (10 ns pulse) at (a) 290 K for PtOEP phosphorescence in PS:PtOEP 2 wt % (solid red circles), PtOEP phosphorescence in DPA:PtOEP 2 wt % (solid red squares), and PtOEP phosphorescence in PS:DPA:PtOEP 2 wt % (solid red triangles); (b) at 290 K for DPA up-converted luminescence in DPA:PtOEP 2 wt % (solid blue squares) and at 100 K (solid blue triangles); (c) at 100 K for PtOEP phosphorescence in PS:PtOEP 2 wt % (open red circles), PtOEP phosphorescence in DPA:PtOEP 2 wt % (open red squares), and PtOEP phosphorescence in PS:DPA:PtOEP 2 wt % (open red triangles); and (d) at 100 K for DPA up-converted luminescence in DPA:PtOEP 2 wt % (open blue squares) and at 100 K (open blue triangles). The dashed black lines correspond to power law fits on the data, and m corresponds to the exponent of the power law IPL∝ Iexcm. The vertical dashed line indicates the pulse energy used (40 μJ) for recording the data shown in Figures 2 and 5

of the PS:DPA:PtOEP 2 wt % ternary system; both the PtOEP phosphorescence intensities and the DPA up-converted emission intensity at 100 K are enhanced by a factor of 2.7. We note that for the DPA:PtOEP and PS:DPA:PtOEP samples an optimum temperature Tmax is found at which the PtOEP phosphorescence is maximized. For the DPA:PtOEP 2 wt % system, it is Tmax = 180 K, where PtOEP phosphorescence intensity is increased by 12.7, and for the PS:DPA:PtOEP system it is Tmax = 140 K, where an increase of 6.8 is found. Concerning the DPA up-converted luminescence of the PS:DPA:PtOEP system, it is optimized at Tmax = 140 K, where the up-converted PL emission intensity increases by a factor of 3.1 with respect to room temperature. The observed maximum at 140 K of the up-converted DPA PL emission intensity of the ternary PS:DPA:PtOEP system could be rationalized on the basis of energetic disorder effects in the DPA excited-state manifold.42 The dispersion of the DPA and PtOEP components in the inert PS matrix results in a broad distribution of different environments that surround the DPA and the PtOEP molecules. In an ensemble of DPA molecules such as the one in the solid-state environment of the PS:DPA:PtOEP 2 wt %, the effective electronic coupling of adjacent DPA monomers varies, and it results in a distribution of DPA excited states. This is clearly seen in the comparison of the PL spectra of films made by DPA and PS:DPA (see Figure S3 in the Supporting Information), and it is in line with the

recorded AFM images of the DPA:PtOEP and PS:DPA:PtOEP systems (Figure 1c,d). By resolving the PL contribution of the DPA intermolecular state with the use of Gaussian fits, it is found that with respect to the DPA films the PS:DPA sample exhibits a broader PL band of the intermolecular state (FWHMDPA = 193 ± 2 meV and FWHMPS:DPA = 242 ± 5 meV). Consequently, a larger energetic disorder in the DPA excites states is expected in the ternary PS:DPA:PtOEP films than in the DPA:PtOEP 2 wt % system. In addition, the use of the PS binder results in weaker intermolecular interactions between the DPA monomers in the ternary composite and the concentration of the DPA intermolecular states is reduced. This manifests in the PL spectra of PS:DPA:PtOEP, in which the monomer-like DPA up-converted luminescence intensity (PL band I) increases on the expense of the DPA intermolecular state PL intensity (PL band II). 3.3. Photoexcitation Intensity Dependence. We have performed photoexcitation intensity-dependent measurements of the TTA-UC process for the samples of PS:PtOEP 2 wt %, DPA:PtOEP 2 wt %, and PS:DPA:PtOEP films at 100 and at 290 K. Similar experiments have been previously reported for the TTA-UC systems in different environments and in different photoexcitation regimes by steady-state or pulsed photoexcitation.8−10,23,50−52 In our experiment, time-integrated PL spectra were recorded after using a train of 10 ns pulses of 532 E

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nm at a repetition rate of 10 Hz for photoexciting the samples in a range of average pulse energies of 20−700 μJ/pulse. Figure 4 shows that both PL intensities (IPL) of the PtOEP phosphorescence and the DPA up-converted luminescence increase with increasing the photoexcitation intensity (Iexc), following a power law IPL∝ Iexcm. In the photoexcitation regime of 40 μJ/pulse, the PtOEP phosphorescence dependence of the DPA:PtOEP 2 wt % and PS:DPA:PtOEP 2 wt % samples is linear with a slope of m = 0.87 to 0.98, suggesting the negligible contribution of TTA among photoexcited PtOEP triplet states. In contrast, in the PS:PtOEP 2 wt % sample where no triplet quenchers are present to deplete the PtOEP triplet excited-state population, TTA events between PtOEP triplet states are substantial, and at 100 K, where PtOEP triplets are less mobile, a nearly squareroot dependence of PtOEP phosphorescence on Iexc with a slope of m = 0.65 is found. Concerning the dependence of the DPA up-converted luminescence on Iexc, at both 290 and 100 K, the slope in the ternary composite is higher than in the binary composite. Additionally, in the binary DPA:PtOEP 2 wt % system, the change in temperature has no effect on the slope of the DPA emission intensity. In contrast, for the ternary PS:DPA:PtOEP 2 wt % composite, at 100 K, the slope of the DPA up-conversion intensity increases by 10% reaching close to the square of Iexc with m = 1.8. 3.4. Time-Gate Photoluminescence. To gain more information on the temperature-dependent character of the PtOEP TET events, we have performed time-gated spectroscopic measurements on the nanosecond time-scale. Timegated PL spectra of the PS:PtOEP 2 wt %, DPA:PtOEP 2 wt %, and PS:DPA:PtOEP 2 wt % systems were recorded in a range of temperatures between 100 and 290 K after photoexciting the samples at 532 nm with a photoexcitation intensity of 40 μJ/ pulse. The spectral integration of the recorded time-gated spectra in the region of 650 nm (630−660 nm) and 450 nm (400−480 nm) provided the kinetics of PtOEP phosphorescence and DPA up-converted luminescence, respectively. The nanosecond time-gated PL measurements enabled the study of the DPA up-converted blue emission kinetics of the DPA:PtOEP 2 wt % sample, but for the PS:DPA:PtOEP 2 wt % ternary system no DPA up-converted PL signal was detected on this time scale. Figure 5a shows that with respect to the monoexponential kinetics of the PS:PtOEP 2 wt % system, the PtOEP phosphorescence kinetics of the DPA:PtOEP and PS:DPA:PtOEP films is accelerated and contains an additional decay component that verifies the occurrence of PtOEP triplet quenching by TET. On the basis of the biexponential decay fit of the time-gated phosphorescence data, the determined rates kTET for the PtOEP triplet quenching were determined. These rates represent the combined effect of PtOEP triplet quenching by TET to DPA triplets and to PtOEP triplet dimers. No TTA events were considered to participate in the depletion of the PtOEP triplet states for the photoexcitation intensity of 40 μJ/ pulse used in the time-gated PL measurements. The validity of this assumption is confirmed by the photoexcitation intensity experiments on the same samples (Figure 4), and it is in line with recent transient femtosecond absorption studies of the DPA:PtOEP 2 wt % system.40 For the binary DPA:PtOEP system at room temperature, it is found that the 70% of the PtOEP triplet population decays via a fast relaxation channel, resulting in a TET rate of kTET‑bin[290K] = 1/8.6 ns−1. The corresponding PtOEP triplet population of the

Figure 5. Time-gated photoluminescence kinetics (on the nanoscale time scale) for PtOEP phosphorescence in a PS:PtOEP 2 wt % film (open squares) and in a DPA:PtOEP 2 wt % film (open triangles) and up-converted DPA luminescence in a DPA:PtOEP 2 wt % film (open circles) measured at (a) 100 and (b) 290 K. For these two temperatures, the respective PtOEP phosphorescence of a PS:DPA:PtOEP 2 wt % film (filled triangles) is also presented. The solid lines are monoexponential (for the PS:PtOEP system) and biexponential (for the DPA:PtOEP and PS:DPA:PtOEP systems) fits on the data. In all cases, the photoexcitation of the samples was at 532 nm, and timegated spectra were recorded with a 10 ns gate step and a 10 ns gatetime. No up-converted DPA luminescence was detected in the PS:DPA:PtOEP 2 wt %, on the nanoscale time scale.

ternary PS:DPA:PtOEP 2 wt % is 58%, and it undergoes a slower TET with a rate of kTET‑tern[290K] = 1/18.6 ns−1. By lowering the temperature, the kTET rates in the two systems are not changed significantly, but the proportion of the PtOEP population that is transferred is affected for the case of the ternary composite. At 100 K, 70% of the PtOEP triplets of the binary system are transferred with a rate of kTET‑bin[100K]= 1/7.9 ns−1, whereas in the ternary composite the fraction of the PtOEP triplet that is transferred is reduced to 49% and the triplet transfer rate remains unchanged with kTET‑tern[100K]= 1/ 17.4 ns−1. By directly photoexciting the Q-band of the PtOEP component in the PS:DPA:PtOEP 2 wt % photon upconverting composite, thermally activated PtOEP triplet diffusion occurs in the PtOEP domains, followed by an energy transfer of the PtOEP triplets to DPA triplet manifold. The electronic coupling between the triplet states of the PtOEP and the DPA components in the ternary composite is not affected by temperature variations, as suggested by the determined TET rates that remain unchanged. However, the magnitude of the TET events is temperature-dependent, indicating that the migration of triplets in the PtOEP domains is thermally activated. Recent temperature-dependent phosphorescence studies in ternary poly(methyl methacrylate) (PMMA):DPA:PtOEP composite films photoexcited at 375 nm have reported similar behavior of the PtOEP triplet decay routes.40 In contrast, direct PtOEP TET to DPA takes place in the binary DPA:PtOEP 2 wt % owing to the higher concentration of PtOEP/DPA interfaces that renders PtOEP triplet migration within the PtOEP domains unnecessary. The high fraction of the PtOEP triplets that relax via the fast decay channel in the binary DPA:PtOEP system offers evidence for the high concentration of the PtOEP dimers and of the many PtOEP/DPA interfaces in the binary system. The dispersion of PtOEP and DPA in the optically inert PS matrix minimizes the PtOEP/DPA interfaces and reduces the strength F

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The Journal of Physical Chemistry C

Article

Figure 6. Jablonski-type energy diagram that depicts the photophysical processes that dictate the TTA-UC process in the DPA:PtOEP solid-state photon up-converting composites. Photoexcitation of PtOEP at 532 nm (Iexc) results in the population of the first singlet excited state of PtOEP (S1‑PtOEP) and is instantly40 followed by an intersystem crossing step (kisc) to the first triplet excited state of PtOEP (T1‑PtOEP). Depopulation of the T1‑PtOEP takes place via (i) radiative transition (kPh) to the ground state (S0‑PtOEP), (ii) nonradiative transition (knr) to S0‑PtOEP, (iii) triplet energy transfer (kq,Ph) to a PtOEP triplet dimer state (T1‑PtOEP,D), (iv) triplet energy transfer (kTET) to a DPA triplet state (T1‑DPA), and (v) triplet energy migration (kd) in the PtOEP aggregate phase. Following the activation of T1‑DPA states by the kTET step, triplet−triplet annihilation (TTA) in the DPA phase leads to the population of the first DPA singlet state (S1‑DPA) from where the up-converted delayed luminescence (kUC) is generated. Quenching of the DPA up-converted delayed luminescence takes place by the activation (kq,UC) of the DPA intermolecular state (DPAIMS), by energy recycling (kET) to S1‑PtOEP38 and by a nonradiative transition to the ground state (S0‑DPA). Both species of T1‑PtOEP,D and DPAIMS are radiative coupled to S0‑DPA (kPh,D and kIMS,UC), and they exhibit a characteristic luminescence centered at 780 and 500 nm, respectively. No hetero-TTA events between T1-PtOEP and T1-DPA38 are considered.

migration of DPA excitons in the DPA domains is not possible, this fraction is 46%. Therefore, almost half of the total DPA photoexcitations is prone to the instant activation of DPA intermolecular states in the binary DPA:PtOEP 2 wt % composite. At low temperatures, the buildup of the DPA dimer population is expected to take place at early times after the generation of the DPA singlet states and much faster than the time resolution of our delayed luminescence setup. At room temperature, the TTA-generated DPA singlet states can escape their capture by the DPA intermolecular states via thermally activated diffusion. Consequently the fraction of the DPA photoexcitation that follows this decay route is reduced to 37%. The temperature-dependent nanosecond time-scale kinetics of the DPA up-converted luminescence verify the high concentration of the DPA intermolecular states in the binary DPA:PtOEP 2 wt %. We note that the DPA:PtOEP 2 wt % system did not exhibit any DPA up-converted PL luminescence in the microsecond−millisecond time scale when time-gated PL spectra were recorded with a 10 μs time window. In contrast, the ternary PS:DPA:PtOEP 2 wt % composite film exhibited a long-lived DPA up-converted PL luminescence with an average lifetime of