A triplet–triplet annihilation based up-conversion process investigated in homogeneous solutions and oil-in-water microemulsions of a surfactant

Abstract

The triplet–triplet annihilation based up-conversion process, involving a platinum octaethyl-porphyrin (PtOEP) as a sensitizer and tetraphenyl-pyrene (TPPy) as an emitter, has been investigated in homogeneous solutions of toluene, bromobenzene and anisole, and oil-in-water microemulsions of the TX-100 surfactant, where toluene constitutes the non-polar phase. In homogeneous solutions, the highest up-conversion quantum yield (of the order of 20%) has been achieved in toluene, being the solvent that has the lowest viscosity among those explored. The up-conversion emission from the PtOEP–TPPy pair has been then investigated in a toluene based oil-in-water microemulsion at three different concentrations of the solutes, showing quantum yields up to the order of 1%, under the same irradiation conditions, but different deoxygenating procedures. The results herein reported might represent a good starting point for a future investigation in microheterogeneous systems. An optimization of the microemulsion composition, in terms of surfactant, co-surfactant and toluene concentrations, could allow us to increase the sensitizer and emitter concentrations and set up the best operative conditions to obtain even higher up-conversion efficiencies.

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Introduction

Up-conversion of solar radiation represents an interesting phenomenon and one of the most promising solutions to the issue of low energy photon loss in the process of solar light harvesting by the common large band-gap photocatalysts.^1–6 Even though the transformation of low frequency coherent light into higher energy radiation is a process which dates back to the 1960s⁷ and that has been achieved in several ways afterwards,⁸ its fulfilment with non-coherent radiation, such as the light coming from the Sun, is more difficult to pursue. Nevertheless, after the early pioneering experiments carried out by Parker and Hatchard,^9–10 several important attempts have been made and reported in the literature and, in this scenario, triplet–triplet annihilation (TTA) as a mechanism to accomplish the up-conversion of non-coherent radiation has drawn notable attention over the last decade.^11–44 In these studies, quite often the essential ingredients for TTA have been organic molecules such as metallated porphyrins and aromatic polycyclic hydrocarbons. The former plays the role of sensitizers, owing to their red-shifted absorption spectra and significant quantum yields of triplet formation, while the latter, characterized by quantum yields of fluorescence often close to unity, act as higher-energy emitters. In the light of possible applications in the field of solar energy exploitation and practical realization of devices with technological interest, such as dye-sensitized solar cells, TTA has been investigated not only in fluid solution but also in rigid systems.^{5,21,25,30,31,42,45–50} The sensitizers and the emitters are usually dispersed in rubber-like polymeric materials of various compositions and rigidity. Unfortunately, one of the major drawbacks of this technique is a decreased efficiency of the up-conversion activity with respect to the fluid solution, due to a reduced diffusion rate of the molecules in the rigid environment.⁴⁶ In these media the intensities of the up-conversion signals are strongly affected by the properties of the rigid host, such as the T_g of the polymeric material, and they are even weaker than those in inorganic materials based on up-converter rare earth ions.³⁰ One of the possible ways to overcome these issues might be either by the use of soft polymeric hosts²¹ or by the synthesis of a multichromophoric system, containing both the donor and the acceptor species, thus avoiding the requirement of a diffusion controlled bimolecular process.⁵¹ Another interesting alternative, which we present in this work, might be by dissolving the organic molecules for TTA into a microheterogeneous fluid system, such as an oil-in-water microemulsion, where cage and segregation effects can make the sensitizer–emitter encounters easier, without a significant decrease of the diffusion rate of the solutes. Recently, Baluschev^52,53 and co-workers demonstrated the possibility to perform a TTA-assisted photon up-conversion in a predominantly aqueous environment with appreciable efficiencies. In ref. 52 they employed a polyoxyethanyl–tocopheryl sebacate micellar solution to obtain a highly efficient up-conversion emission from a meso-tetraphenyl-tetrabenzoporphine palladium–dibenz[de,kl]anthracene (perylene) couple, whose photon flux in the water environment was estimated to be only four-fold less than that of the same system in toluene, while in ref. 53 the up-converting dye system was enclosed in a synthesized nanocapsule, in an aqueous dispersion. In another paper,⁵⁴ Chujo and co-workers studied a sensitizer–emitter upconverting pair accumulated into water-soluble dendrimers; moreover, the possibility to use an aqueous medium would no doubt be of great importance in the search for biocompatible sensitizer–emitter couples⁵⁵ as well. The role of microheterogeneous systems in photophysical and photochemical processes has been thoroughly investigated and it is well documented in the literature.^56–62 Microemulsions are rather complex systems and their peculiar structure and composition has been the subject of several studies^56,63–71 with a two-fold interest: on the one hand, the characterization of their morphology and structure is carried out using molecular probes; on the other hand, the microemulsions represent a particular microheterogeneous medium where interesting relaxation processes after photoexcitation can be investigated.

Recently, we were synthesizing and characterizing solid solutions of metal oxides to be employed as heterogeneous photo-catalysts for hydrogen production from water.^72–74 Since our photocatalysts do not absorb wavelengths longer than 500 nm, to reduce the waste of solar visible light, we turned to the TTA-UC process.⁷⁵ In this work, we present the first study of the up-conversion emission from a 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum(II) (PtOEP)–1,3,6,8-tetraphenylpyrene (TPPy) sensitizer–emitter pair in an oil-in-water microemulsion of 4-(1,1,3,3-tetramethylbutyl)phenyl-polyethylene glycol (TX-100) (Scheme 1), which has been reported to be the best TX-series surfactant for enhancing toluene solubilization.⁷⁶ To understand the effects of the microheterogeneous environment on the up-conversion emission of the PtOEP–TPPy system, our investigation was first carried out in homogeneous solvents having different viscosities, such as toluene, which also constitutes the non-polar component of the oil-in-water microemulsions prepared, bromobenzene, that also bears Br as a heavy atom, and anisole.

Scheme 1 Structures of PtOEP, TPPy and the TX-100 surfactant.

Experimental

Materials

The compounds 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum(II) (PtOEP) (98%, purchased from Sigma), 1,3,6,8-tetraphenylpyrene (TPPy) (Sigma), 4-(1,1,3,3-tetramethylbutyl)phenyl-polyethylene glycol (TX-100) (Fluka) and sodium sulphite (anhydrous, ≥99%, Carlo Erba Reagents) were used as received after checking their degree of purity by recording their absorption, emission and excitation spectra and/or by HPLC. The solvents toluene, bromobenzene, anisole (all of high purity, from Fluka) and 1-pentanol (≥99%, Sigma) were used as received. Water was deionized and distilled before use.

Instruments

The absorption spectra were recorded using a Perkin-Elmer Lambda 800 double-beam spectrophotometer.

The emission spectra were collected using a Spex Fluorolog-2 1680/1 equipped with a system for spectral correction; the quantum yields were determined using 9,10-diphenylanthracene (DPA) in cyclohexane (quantum yield: Φ_F = 0.90⁷⁷) as a standard. When recording the up-conversion emission signals, a cut-off filter was always inserted between the excitation monochromator and the sample holder, to avoid direct excitation of the emitter molecule due to the second order component passing through the monochromator. Measurements were performed in right angle (RA) geometry, by collecting the up-conversion emission at 90° with respect to the direction of the incident excitation beam, which was always focused on the centre of the cuvette (1 cm optical path).

Emission lifetimes were measured by means of different experimental set-ups, depending on the lifetime range of the species involved.

The PtOEP phosphorescence decay, in the range of tens of microseconds, was measured using a Varian Cary Eclipse spectrofluorimeter provided with an 80 Hz pulsed xenon lamp, whereas the fluorescence lifetime of the TPPy emitter was recorded using an Edinburgh Instruments 199S fluorescence spectrometer, based on the time-correlated single-photon counting technique.

Methods

The oil-in-water microemulsions were prepared by using water, toluene (as the internal organic phase for solubilizing the sensitizer–emitter couple), a surfactant, to reduce the interface tension between the two media, and a co-surfactant, to ensure flexibility to the interfacial layer of the supramolecular structure. TX-100 was chosen as the surfactant agent, because of its aforementioned ability to form microemulsions with a large toluene content,⁷⁶ characterized by relatively wide non-polar cavities with respect to those obtained from other surfactants of the same family. 1-Pentanol was used as the co-surfactant agent. These optically transparent oil-in-water microemulsions, which turned out to be thermodynamically and chemically stable, contained an excess of water (71.1 wt%), toluene (1.4 wt%), TX-100 (24.2 wt%) and 1-pentanol (3.3 wt%).

The emission quantum yields (Φ(X)) were obtained by comparing corrected areas of the sample (Area_X) and the standard (Area_St) emissions, using eqn (1):

which accounts for the differences in absorbance and refraction index of the sample (A_X, n_X) and standard (A_St, n_St) solutions, where Φ_F(St) is the fluorescence quantum yield of the standard. The accuracy in the Φ(X) values is estimated within 10%.

In the up-conversion experiments in the right angle geometry, where highly absorbing solutions were used, the up-conversion quantum yield (Φ_UC) was calculated using the modified eqn (2):

since the absorption of two photons is needed to observe the emission of one up-converted photon.^22,30 The factor 2 normalizes the maximum value attainable of Φ_UC (i.e. 0.5) to 1. Area^corr_UC is the corrected integrated intensity of the up-conversion emission: it has been obtained by correcting the recorded up-conversion band for the inner filter and self-absorption effects. These effects arise from the partial absorption of the incident light before it reaches the centre of the cuvette (primary inner filter effect) and the re-absorption of the emitted light from the sample (secondary inner filter and self-absorption effects).⁷⁸ More details of this procedure can be found in our previous work.⁷⁵

Potassium ferrioxalate actinometry was used to determine the radiation intensity of the excitation source; the light was focused on an area of 0.3 cm².

The deoxygenating procedure is a crucial point in these measurements, given the well-known quenching effects of molecular oxygen on the excited triplet states of the molecules. In the case of homogeneous solutions the deoxygenating procedure consisted of argon bubbling, whilst in the case of microheterogeneous media, sodium sulphite⁷⁹ was employed to avoid foam production in the cell, affecting the optical transparency of the sample. Na₂SO₃ allows sample deoxygenation without altering the spectral properties of the solutions in the visible region. It was added to the microemulsions in concentrations equal to 0.1 mol dm⁻³ and left to react for a reasonable time (about 15 min), until the emission signal reached a stable intensity maximum. The concentration of sodium sulphite was chosen taking into account the concentration of oxygen in a toluene air-equilibrated solution at room temperature and atmospheric pressure (about 10⁻³ mol dm⁻³⁸⁰).

The luminescence decay curves, I(t), were analysed by two methods: the non-linear least-squares method and the maximum entropy method (MEM) using the MemExp Software available online.^81,82 In the maximum entropy method the experimental decay I(t) is fitted by the following function:

where g(log τ) and h(log τ) are the lifetime distributions that correspond to decay and rise kinetics, respectively, and the polynomial term accounts for the baseline. Scattered excitation light can be accounted for with positive values of the ξ parameter. The instrument response function R(t) is peaked at zero time and is appreciable only in the interval [−t₀, t_f]. The fit procedure entails the maximization of the function Q defined in eqn (4):

Q = S − λC − αI (4)

S is entropy defined as where f is the image that includes both the g and the h lifetime distributions, whereas F is the MEM prior distribution used to incorporate prior knowledge into the solution. C is a measure of the quality of the fit F to the data. When we analyzed the data of phosphorescence decays with normally distributed noise, C is the χ². For Poisson-distributed data like those collected by the single photon counting technique, C is the Poisson deviance. I is a normalization factor; λ and α are Lagrange multipliers. In our group, we have proved that the maximum entropy method is a valuable tool in describing the poly-exponential character of the luminescence decays for samples consisting of different conformers dissolved in solution,³⁸ fluorophores embedded in polymers^84,85 and for microcrystalline compounds^86,87 at room temperature.

Results and discussion

The pair of compounds used as a sensitizer and an emitter for the up-conversion study was chosen after a series of preliminary measurements carried out in our laboratory, where several potential candidates for these roles had been tested. In particular, the choice of PtOEP allowed us to optimize the conditions for the up-conversion process, the triplet energies of the sensitizer (PtOEP) and the emitter (TPPy) being very close to each other (221 cm⁻¹ = 1kT). Furthermore, the presence of a heavy atom such as Pt favours the spin–orbit coupling, thereby increasing the kinetic constant of the collisional Dexter energy transfer involving the triplet states of PtOEP and TPPy. Both these issues result in an enhancement of the up-conversion quantum yield.⁷⁵

PtOEP–TPPy in homogeneous solution

The compounds PtOEP and TPPy were first investigated in the three aromatic solvents: toluene, bromobenzene and anisole. These solvents have closely related structures but different viscosities: toluene is the least viscous, whereas anisole is the most viscous, respectively (see Table 3). Bromobenzene has also a heavy atom (Br) in its structure. Our intention was to test the possible action of these parameters on the up-conversion quantum yield.

The molar extinction coefficients of the porphyrin have been measured in the three solvents and they are very similar (Fig. 1), indicating that the excitation of the sensitizer to its excited singlet state occurs with the same probability in the three cases. The lifetimes and quantum yields of phosphorescence have also been determined in argon de-aerated solutions (Table 1).

Fig. 1 Quantitative absorption spectra of PtOEP in toluene (black), bromobenzene (grey) and anisole (light grey).

Table 1 Molar extinction coefficients (ε), lifetimes (τ_P), quantum yields (Φ_P) and rate constants (k_P) of phosphorescence of PtOEP in the three solvents

Solvent	ε/dm^3 mol^−1 s^−1		τ P/μs	Φ P	k P/×10^4 s^−1
	λ = 382 nm	λ = 536 nm
Toluene	2.93 × 10^5	63 760	90	0.41	0.45
Bromobenzene	2.74 × 10^5	62 570	84	0.98	1.16
Anisole	3.12 × 10^5	64 780	75	0.87	1.16

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The quantum yield of phosphorescence determined in toluene is in agreement with the value reported in the literature.⁸⁸ From Table 1, it can be inferred that both the Φ_P and the k_P values in toluene are smaller than those found in the other two solvents. The quantum yields of fluorescence of the TPPy acceptor alone have also been measured in the three solvents explored, where unitary values were found in all cases.

The study of the up-conversion features of the PtOEP–TPPy pair has then been carried out in toluene, bromobenzene and anisole.

A plausible mechanism for the up-conversion process is the following (eqn (5)–(9)):

S₀(PtOEP) + hν → S₁(PtOEP) (5)

S₁(PtOEP) → T₁(PtOEP) (6)

T₁(PtOEP) + S₀(TPPy) → S₀(PtOEP) + T₁(TPPy) (7)

T₁(TPPy) + T₁(TPPy) → S₀(TPPy) + S₁(TPPy) (8)

S₁(TPPy) → S₀(TPPy) + hν′ (9)

where step (5) represents the red photon absorption by PtOEP, (6) the intersystem crossing of PtOEP, (7) the intermolecular energy transfer, (8) the TTA process and (9) the up-converted emission (with hν′ > hν). It has to be noted that step (8) refers to the case of a homo-TTA involving the emitter triplets, which is the main channel for annihilation when the concentration of the emitter is at least an order of magnitude higher than that of the sensitizer, as in most of our experimental cases. However, the possibility of additional homo-TTA on the sensitizer or hetero-TTA involving the triplet states of both species cannot be completely ruled out.²⁶ In the following, we will analyze Stern–Volmer data for our up-converting pair, with the aim to find the most suitable solvent for the energy transfer process (7).

Toluene. The normalized absorption and emission spectra of PtOEP and TPPy in toluene are shown in Fig. 2. It can be seen that the fluorescence spectrum of the emitter (TPPy, emission λ_max = 419 nm) does not exhibit any significant overlap either with the Soret (λ_max = 382 nm) or the Q (λ_max = 536 nm) singlet–singlet absorption bands of PtOEP and this feature avoids the up-conversion emission intensity to be significantly damped by filter effects.

Fig. 2 Normalized absorption (full line) and emission (dashed line) spectra of PtOEP (black) and TPPy (grey) in toluene.

The excitation of the PtOEP–TPPy system was carried out in de-aerated (after bubbling with argon) toluene solution at 536 nm, and the emission of the up-converted photons was collected in the violet-blue region. The concentration of the sensitizer was kept constant ([PtOEP] = 10⁻⁵ mol dm⁻³), whilst that of the acceptor (TPPy) was varied within the range 2 × 10⁻⁵–2 × 10⁻³ mol dm⁻³. The emission spectra, corrected for the inner filter effects, based on the procedure reported in the Experimental section, are shown in Fig. 3.

Fig. 3 Up-conversion emission spectra as a function of the TPPy acceptor concentration, in toluene; [PtOEP] = 1.1 × 10⁻⁵ mol dm⁻³.

The spectra reported in Fig. 3 were measured regulating the incident power by means of a grey filter on excitation, thus obtaining an excitation intensity similar to the reference value: I_exc = 194 W m⁻². This value is close to the solar intensity integrated over the absorption range of the porphyrin “Q band” at A.M. = 1.5.

For the concentrations of PtOEP (10⁻⁵ mol dm⁻³) and TPPy (6 × 10⁻⁴ mol dm⁻³) giving the highest value of Φ_UC, we studied the effect of excitation intensity to verify when Φ_UC becomes independent of the I_exc (Fig. 4a). The plot of the integrated up-conversion emission intensity as a function of the absorbed power is shown on the logarithmic scale in Fig. 4b.

Fig. 4 (a) Dependence of up-conversion emission on the intensity of the excitation beam at 536 nm, obtained by using grey filters at 9.8, 20.4, 40.6, 54.9, 65.2, 88.9, 100% transmittance values. The data have been collected for a solution of PtOEP 10⁻⁵ mol dm⁻³ and TPPy 6 × 10⁻⁴ mol dm⁻³; (b) a logarithmic plot of the integrated up-conversion emission intensity vs. the excitation intensity.

From a minimum least squares method treatment of the data in Fig. 4b, a slope of (1.63 ± 0.04; R² = 1) was obtained. Such a value is intermediate between the two regimes of quadratic (slope = 2) and linear (slope = 1) dependence of the integrated emission area on I_exc, respectively. The first case is normally encountered in a low intensity range, due to the two-photon mechanism involved in the up-conversion process. In contrast, upon increasing the excitation intensity, a progressive shift towards a linear dependence of the integrated emission area on the I_exc value is observed. In the latter situation, the quantum yield of up-conversion emission Φ_UC is independent of the excitation intensity and its plot vs. I_exc would exhibit a plateau.³⁶ The results shown in Fig. 4 were obtained under excitation intensity values which did not allow us to reach this limit.

Particular attention was devoted to step (7) of the overall mechanism of up-conversion, that is, the energy transfer involving the low-lying triplet states of the sensitizer and the emitter molecules. This spin-forbidden process, adequately described by a Dexter collisional mechanism,³⁹ can be treated in terms of a Stern–Volmer equation (eqn (10)):

where I₀ and I are the phosphorescence intensities of PtOEP in the absence and presence of TPPy, respectively, and K_SV is the Stern–Volmer constant, which is the product of the pure PtOEP triplet lifetime (τ₀) and the quenching kinetic constant (k_q). The value of K_SV was determined by collecting the phosphorescence intensity of PtOEP as a function of the TPPy concentration (Fig. 5).

Fig. 5 (a) Quenching of PtOEP phosphorescence upon increasing the TPPy concentration, in the range 2 × 10⁻⁵ ÷ 2 × 10⁻³ mol dm⁻³ and (b) the corresponding Stern–Volmer diagram.

From the slope of the Stern–Volmer plot (eqn (10) and Fig. 4b), a K_SV = 29 300 ± 100 mol⁻¹ dm³ was found. The corresponding k_q, which in the present case can be identified with the rate constants of energy transfer, due to the low excitation power and high emitter–sensitizer concentration ratio used, could then be obtained once the phosphorescence lifetimes (τ₀, that is, τ_P of Table 1) was measured. The phosphorescence decay, well fitted by a monoexponential function, gave a τ₀ of 90 ± 3 μs, which is in fair agreement with the value 80 ± 5 μs reported in the literature.⁸⁹ The rate constant for the energy transfer process was then found to be k_q = 3.24 × 10⁸ mol⁻¹ dm³ s⁻¹ (Table 2).

Table 2 Up-conversion quantum yields (Φ_UC) at various concentrations of TPPy in the three solvents used. [PtOEP] = 10⁻⁵ mol dm⁻³, I_exc = 194 W m⁻²

[TPPy]/mol dm^−3	Φ UC
	Toluene	Bomobenzene	Anisole
2 × 10^−3	7.34 × 10^−2	1.87 × 10^−2	1.80 × 10^−3
1.5 × 10^−3	1.03 × 10^−1	2.78 × 10^−2	2.57 × 10^−3
1 × 10^−3	1.33 × 10^−1	3.32 × 10^−2	3.42 × 10^−3
6 × 10^−4	1.91 × 10^−1	4.08 × 10^−2	5.76 × 10^−3
2 × 10^−4	1.72 × 10^−1	2.28 × 10^−2	1.82 × 10^−3
6 × 10^−5	3.57 × 10^−2	7.56 × 10^−3	1.03 × 10^−3
2 × 10^−5	8.68 × 10^−3	1.64 × 10^−3	3.31 × 10^−4

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To achieve a better comparison among the three solvents used, the measurements were carried out under the same experimental conditions, in terms of the concentrations of both the sensitizer ([PtOEP] = 10⁻⁵ mol dm⁻³) and the emitter ([TPPy] = 2 × 10⁻⁵ ÷ 2 × 10⁻³ mol dm⁻³) and in terms of excitation intensity (194 W m⁻²) (see below, Fig. 6 and Table 2).

Fig. 6 Up-conversion emission quantum yields as a function of the TPPy acceptor concentration, for the three solvents explored: toluene (black dots); bromobenzene (grey squares); anisole (light grey triangles).

Bromobenzene. Following the same procedure already described above for toluene, the excitation of the PtOEP–TPPy system was carried out in de-aerated (after bubbling with argon) bromobenzene at 536 nm, and the emission of the up-converted photons was collected in the violet-blue region. The areas of the emission spectra, corrected for the inner filter effects, allowed the corresponding up-conversion quantum yield values to be measured, as reported in Table 2.

The values of K_SV were determined by collecting the phosphorescence intensity of PtOEP as a function of the TPPy concentration. From the slope of the Stern–Volmer equation (eqn (10)) a K_SV = 17 000 ± 100 mol⁻¹ dm³ was found. The phosphorescence decay, well fitted by a monoexponential function, gave a τ₀ of 84 ± 3 μs and the resulting rate constant for the energy transfer process was then found to be k_q = 2.01 × 10⁸ mol⁻¹ dm³ s⁻¹.

Anisole. The up-conversion process for the pair PtOEP–TPPy was investigated also in anisole because this solvent exhibits a viscosity value only slightly higher than bromobenzene, without the concomitant presence of a heavy atom. The excitation of the PtOEP–TPPy system was carried out in de-aerated (after bubbling with argon) anisole at 536 nm, and the emission of the up-converted photons was collected in the violet-blue region. The areas of the emission spectra, corrected for the inner filter effects, allowed the corresponding up-conversion quantum yield values to be measured, as reported in Table 2.

The values of K_SV were determined by collecting the phosphorescence intensity of PtOEP as a function of the TPPy concentration. From the slope of the Stern–Volmer treatment (eqn (10)), a K_SV = 22 200 ± 100 mol⁻¹ dm³ was found. The phosphorescence decay, well fitted by a monoexponential function, gave a τ₀ of 75 ± 3 μs, while the resulting rate constant for the energy transfer process was k_q = 2.96 × 10⁸ mol⁻¹ dm³ s⁻¹.

The higher Φ_UC obtained in toluene with respect to bromobenzene and anisole can be explained based on various parameters. A factor which is no doubt favourable is the low viscosity of toluene, which renders the molecular diffusion easier, thereby enhancing the efficiency of the energy transfer between the triplet state of the PtOEP and that of the TPPy. Moreover, both the Φ_P and the k_P values in toluene are smaller than those found in the other two solvents (see Table 1). This is an indication that the radiative deactivation of the triplet state is unfavoured with respect to other non-radiative paths, such as the energy transfer process to an acceptor molecule. This fact, already reported by other authors²² who documented the detrimental effect of the sensitizer phosphorescence on the triplet–triplet energy transfer and the up-conversion emission process, represents a further explanation for the higher Φ_UC measured in toluene. In all of the solvents explored, the highest quantum yield value was achieved for a concentration of the acceptor [TPPy] = 6 × 10⁻⁴ mol dm⁻³. The observed trends of Φ_UCvs. [TPPy] are not so common in the literature, where a monotonic increase of the quantum yield with increasing the acceptor concentration is generally reported.³⁶ However, the experimental data we present are reproducible and have already been obtained in a previous investigation in our laboratory⁷⁵ and by S. Baluschev and co-workers,²⁴ who reported a decrease of the up-conversion quantum yield after the acceptor concentration had reached the “threshold” value of 10⁻³ mol dm⁻³. A possible explanation for this issue might be the formation of TPPy complexes, since this concentration has been reported as the onset value at which aggregation of TPPy takes place.⁹⁰

The efficiency of the energy transfer process is quantified in terms of the rate constant of bimolecular quenching, k_q, obtained from the Stern–Volmer treatment (Fig. 7 and Table 3).

Fig. 7 Stern–Volmer plots for the three solvents explored: black dots, toluene; grey squares, bromobenzene; light grey triangles, anisole ([PtOEP ∼ 10⁻⁵ mol dm⁻³]).

Table 3 Stern–Volmer (K_SV) and quenching kinetic constants for the pair PtOEP and TPPy in the three solvents having different viscosity (η)

Solvent	η (Pa s)	[PtOEP] (mol dm^−3)	K SV (dm^3 mol^−1)	k q (dm^3 mol^−1 s^−1 × 10^8)
Toluene	0.5859	1.1 × 10^−5	29 300	3.24
Bromobenzene	1.196	1.1 × 10^−5	17 000	2.01
Anisole	1.32	1.0 × 10^−5	22 200	2.96

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The k_q value obtained in toluene, higher than those estimated in bromobenzene and anisole, as well as the longer lifetime of the PtOEP, plays a positive role in determining an increase of the up-conversion quantum yield with respect to the other two solvents. The differences found in bromobenzene and anisole, solvents having similar viscosity values, can be accounted for based on a comparison of the other parameters. From the K_SV, similar k_q values are obtained for these two solvents; therefore the larger Φ_UC exhibited in bromobenzene may be attributed to a heavy atom effect, which increases the probability of the spin-forbidden transition from the excited singlet state of the sensitizer, populated upon light absorption, to its triplet state, through inter-system crossing. In addition to that, the lifetime of PtOEP in bromobenzene is longer than that in anisole.

The PtOEP–TPPy couple in oil-in-water microemulsion

The possibility to confine both the sensitizer and the emitter species into a restricted environment, to favour the encounter process and therefore the energy transfer, with a possible increase of the up-conversion emission quantum yield, induced us to test the effect of the inclusion of PtOEP and TPPy in an oil-in-water microemulsion. Based on the results obtained in a homogeneous solution, we chose toluene as the organic phase, in light of the higher value of up-conversion quantum yield observed in this solvent. Owing to the necessity to work with optically transparent microemulsions and with the sensitizer and the acceptor species completely dissolved, microemulsions containing 6.5 × 10⁻⁶ mol dm⁻³ PtOEP and 6.5 × 10⁻⁵ mol dm⁻³ TPPy as the highest concentrations, respectively, were prepared. We chose to work with an excess of [TPPy] because in this condition we observed the largest Φ_UC in solution; moreover, a high ratio of [TPPy] over [PtOEP] guarantees that the TTA involves mainly pairs of TPPy triplets.

Photophysical properties of PtOEP and TPPy in microemulsion. The PtOEP sensitizer and the TPPy acceptor were firstly characterized individually in microemulsion before studying the up-conversion process. Therefore, the absorption (Fig. 8a) and phosphorescence emission spectra (Fig. 8b) of PtOEP were recorded in a microemulsion containing the sensitizer alone. The emission spectra were obtained after deoxygenation of the medium by addition of Na₂SO₃, in a concentration of 0.1 mol dm⁻³. The addition of the salt did not produce any spectral change.

Fig. 8 Quantitative absorption (a) and normalized phosphorescence emission (b) spectra of a toluene based microemulsion containing PtOEP (red). For the purpose of comparison, the spectra recorded on a homogeneous solution of PtOEP in toluene (black) are also reported. (c) Phosphorescence decay kinetics of PtOEP in toluene (black) and microemulsion (red) and (d) the lifetime distribution according to the MEM.

The spectra displayed in Fig. 8 do not show any significant modification in the PtOEP spectral features passing from a toluene homogeneous solution to a toluene based microemulsion. An increase in the phosphorescence quantum yield (Table 4) was observed in microemulsion with respect to the value found in toluene. This phenomenon might be ascribed to different environments where the solute molecules can be localized within the microemulsion structure. In most of these microenvironments, the triplet lifetime and the quantum yield undergo an increase with respect to the homogeneous solution. A further support of this hypothesis comes from the maximum entropy method (MEM) analysis performed on the PtOEP based microemulsions (Fig. 8); the results are summarized in Table 4. The analysis of the PtOEP phosphorescence decays by MEM reveals that the kinetics in toluene has a marked mono-exponential character (see Fig. 8d). Its lifetimes distribution appears like a Delta Dirac function peaked at 88 μs, in very good agreement with the value obtained by the least squares analysis (see Table 4). On the other hand, the kinetics of microemulsion is described by a much broader distribution of lifetimes (see Fig. 8d) centered at 104 μs (note that the least squares method gives a good result by a mono-exponential fitting function having a lifetime of 103 μs). Clearly, when PtOEP is in microemulsion, it experiences a large spectrum of micro-environments. In fact, the distribution includes lifetimes which are equal to but also longer and shorter than those found in pure toluene.

Table 4 PtOEP phosphorescence (Φ_P) and TPPy fluorescence (Φ_F) quantum yields; PtOEP phosphorescence (τ_P) and TPPy fluorescence (τ_F) lifetimes, calculated by means of the least squares method (LSM) and maximum entropy method (MEM) analyses, in toluene and oil-in-water microemulsion

Sensitizer/emitter	Medium	Φ P/F	τ P/F
			LSM	MEM
PtOEP	Toluene	0.41	90 μs	88 μs
	Microemulsion	0.71	103 μs	104 μs
TPPy	Toluene	1	2.0 ns	2.0 ns
	Microemulsion	0.62	0.17 ns; 2.5 ns	2.5 ns

---

The TPPy emitter was also studied in microemulsion without the presence of the PtOEP sensitizer. The quantitative absorption and fluorescence emission spectra are displayed in Fig. 9(a) and (b), respectively, along with the corresponding results obtained in toluene homogeneous solution.

Fig. 9 Quantitative absorption (a) and normalized fluorescence emission (b) spectra of a toluene based microemulsion containing TPPy (red). For the purpose of comparison, the spectra recorded on a homogeneous solution of TPPy in toluene (black) are also reported. Fluorescence decays (c) for TPPy in toluene (black) and microemulsion (red) and (d) lifetimes distribution according to MEM.

From a comparison of the results obtained in microemulsion and in toluene, no significant changes could be detected concerning the spectral shapes of both the absorption and emission spectra; nevertheless, a slight increase of the molar extinction coefficient was found in microemulsion. In contrast, the fluorescence quantum yields (Table 4) exhibited a decrease when compared with the value measured in a toluene homogeneous solution (Φ_F = 1). This fact may be probably be explained considering the possibility of TPPy aggregation.⁹⁰ By means of the single photon counting technique, the fluorescence lifetimes in microemulsion were also determined and well fitted by a double-exponential function (weights of 10% and 90% for the fast and the slow component decays, respectively). As already described for PtOEP, the presence of different environments within the microemulsion structure as possible sites of solubilization for TPPy can be the reason for these results. The maximum entropy method (MEM) analysis performed on the TPPy based microemulsions is displayed in Fig. 9 and the results are summarized in Table 4. The TPPy fluorescence decays in toluene and microemulsion are shown in Fig. 9c. When TPPy is dissolved in pure toluene, it gives rise to a pretty restricted and symmetrical distribution of lifetimes peaked at 2 ns (see Fig. 9d). This result is in agreement with the lifetime of the mono-exponential fitting function achieved by the least squares analysis (see Table 4). The presence of a TPPy lifetimes distribution even in solution of pure toluene may be due to different conformers of the rather flexible molecular skeleton (a similar effect due to conformers was found in ref. 83). In microemulsion, TPPy exhibits a much broader collection of lifetimes (see Fig. 9d). These lifetimes are equal but also longer and shorter than those estimated in pure solvent, as it occurs in the case of PtOEP. Even the least squares method gives the best fitting of the experimental data by a bi-exponential function with 0.17 ns and 2.5 ns as lifetimes. Therefore, when TPPy is dissolved in the microheterogeneous medium, it experiences a large distribution of micro-environments, as PtOEP does. These spreading effects exerted by micro-heterogeneous environments on the electronic excited states lifetimes have been already observed also in other cases (see ref. 84–86).

Microemulsions containing the up-converting couple PtOEP–TPPy. After having investigated the sensitizer and emitter properties in microemulsion, the PtOEP–TPPy pair was dissolved in the microheterogeneous medium for the up-conversion study. To search for possible effects of the solute content on the up-conversion quantum yield, microemulsions containing PtOEP/TPPy in concentrations (mol dm⁻³): (A) 6.5 × 10⁻⁶/6.5 × 10⁻⁵; (B) 3 × 10⁻⁶/3 × 10⁻⁵ and (C) 10⁻⁶/10⁻⁵, respectively, were prepared.

The absorption spectrum of a TX-100 based microemulsion containing the PtOEP and TPPy couple does not show substantial variations with respect to the spectrum recorded in toluene homogeneous solution, as can be inferred from Fig. 10a.

Fig. 10 (a) Normalized absorption spectra of a homogeneous toluene solution (black) and a toluene based microemulsion (grey) containing the PtOEP sensitizer and the TPPy emitter; (b) up-conversion emission profiles of the three microemulsions prepared: A (black), B (grey), C (light grey).

The up-conversion fluorescence spectra originating from 536 nm excitation of three microemulsions containing different combinations of the sensitizer and the emitter and deoxygenated by Na₂SO₃ are shown in Fig. 10b. The up-conversion intensity exhibits an expected increase upon raising the solute concentrations.

The effect of the excitation intensity on the up-conversion emission was also analyzed by using various grey filters with different values of transmittance (Fig. 11).

Fig. 11 (a) Dependence of the intensity of up-conversion band on the absorbed intensity of the excitation beam at 536 nm, obtained by using grey filters at 4.39, 9.78, 20.35, 40.01, 47.03, 55.15, 64.79, 89.31, 100% transmittance values. The data have been collected for a microemulsion A containing PtOEP 6.5 × 10⁻⁶ mol dm⁻³ and TPPy 6.5 × 10⁻⁵ mol dm⁻³; (b) a plot of the integrated up-conversion emission intensity vs. the absorbed power in logarithmic scale.

From the plot of the integrated up-conversion emission intensity as a function of the absorbed power, on the logarithmic scale (Fig. 11b), a slope of 0.97 ± 0.06 (R² = 1) was found, thereby indicating that the regime of linearity with I_exc was reached and, therefore, the region of independence of Φ_UC from the excitation intensity was also attained. This finding was further confirmed when values of Φ_UC very similar to those found at I_exc = 194 W m⁻² were obtained employing two different excitation intensities for irradiation (42 and 87 W m⁻², see Table 5).

Table 5 Up-conversion quantum yields (Φ_UC) obtained with the toluene based microemulsion A, containing the highest concentrations of both the PtOEP sensitizer and the TPPy emitter, at three different values of excitation intensities (I_exc)

Microemulsion	[PtOEP] (mol dm^−3)	[TPPy] (mol dm^−3)	Φ UC
			I exc = 42 W m^−2	I exc = 87 W m^−2	I exc = 194 W m^−2
A	6.5 × 10^−6	6.5 × 10^−5	9.54 × 10^−3	1.02 × 10^−2	9.48 × 10^−3

---

As already pointed out above, the substantial agreement among the Φ_UC measured with different excitation intensities nicely confirms the attainment of the region where the up-conversion quantum yields are independent of I_exc. The lower up-converting efficiency with respect to the corresponding toluene homogeneous solution might have been foreseen, considering the difficulty of dissolving organic molecules in a prevalently aqueous environment and the results of Baluschev and co-workers, who observed a four-fold decrease of the up-conversion photon flux in a surfactant aqueous solution with respect to toluene.⁵² It has to be noted that another previous literature example concerning a TTA-based up-conversion process in a water environment⁵⁴ reported a marked decrease of four orders of magnitude in the up-conversion efficiency when comparing the results in the aqueous solution with those obtained in an organic solvent. Moreover, it should also be emphasized that different deoxygenation procedures were employed in homogeneous (argon bubbling) and microheterogeneous (Na₂SO₃) media and this could contribute to the discrepancy observed. However, the use of oil-in-water microemulsions demonstrated a significant possibility to pave the way for a TTA-based up-conversion process even under these unfavourable conditions. The increase of the sensitizer and emitter concentrations in a TX-100 microemulsion could provide an interesting way to achieve high solute concentrations in a restricted space, thus favouring the TTA process, even in comparison with homogenous environments. Such an increase of the solute content would require either a change in the nature of the sensitizer–emitter couple or a modification of the oil-in-water microemulsion structure, where the size of the cavities could be regulated, for instance, upon changing the co-surfactant/surfactant ratio. This could allow us to reduce the well known drawbacks observed in completely rigid media, such as solids, polymers, films, rubbers, etc., where the drastic decrease of the diffusion rate strongly affects the up-conversion process.

Conclusions

A TTA based up-conversion study of the couple PtOEP–TPPy was carried out in homogeneous and microheterogeneous media. The highest quantum yield for the up-conversion emission in homogeneous solution was found for a concentration of the TPPy acceptor of 6 × 10⁻⁴ mol dm⁻³ in toluene (Φ_UC = 0.19), the solvent having the lowest viscosity among those explored. The PtOEP in toluene exhibits the longest lifetime (90 μs), the highest k_q (3.24 × 10⁸ dm³ mol⁻¹ s⁻¹) and the lowest phosphorescence quantum yield (Φ_P = 0.40). All these factors have a positive influence on the resulting Φ_UC value observed.

PtOEP and TPPy were first characterized separately in a toluene based oil-in-water microemulsion. The sensitizer showed an increase of both the Φ_P and the τ_P values with respect to the homogeneous case, probably due to the presence of different environments where the solute molecules can be localized within the microemulsion structure. In most of these microenvironments, the triplet lifetime and the quantum yield show an increase with respect to the homogeneous solution. A support for this hypothesis comes from the maximum entropy method (MEM) analysis. In contrast, TPPy exhibited a decrease of the Φ_F, which can probably be ascribed to aggregation processes favoured by the confinement effect exerted by the microheterogeneous medium.

The up-conversion emission from the PtOEP–TPPy couple has then been investigated in microemulsion at three different concentrations. The molecular pair constituted by PtOEP as a sensitizer and TPPy as an emitter demonstrated significant up-conversion efficiency in microheterogeneous media. Microemulsions of the surfactant turned out to be an interesting environment where the TTA-based up-conversion can be accomplished, with quantum yields up to the order of 10⁻², upon irradiation at 536 nm with an excitation intensity of 194 W m⁻². The results herein reported might represent a good starting point for further investigation of the TTA based up-conversion process in these microheterogeneous systems. An optimization of the relative concentrations of the surfactant, the co-surfacting agent, and the water and the toluene content in such an environment, could allow us to increase the sensitizer and emitter concentrations and set up the best operative conditions to obtain high up-conversion efficiencies. This study might well be extended to other possible sensitizer–acceptor couples.

Acknowledgements

The authors gratefully acknowledge the financial support of the Ministero per l'Università e la Ricerca Scientifica e Tecnologica (Rome, Italy), the University of Perugia [PRIN 2010–2011, 2010FM738P] and the Progetto Nazionale FISR “Vettore Idrogeno: Sistemi Innovativi di Produzione di Idrogeno da Energie Rinnovabili”.

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