Hybrid polysilsesquioxanes for fluorescence resonance energy transfer

Dyes and Pigments 170 (2019) 107622

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Hybrid polysilsesquioxanes for fluorescence resonance energy transfer a,∗

a

b

Maria Nowacka , Anna Kowalewska , Damian Plażuk , Tomasz Makowski a b

T

a

Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363, Lodz, Poland University of Lodz, Faculty of Chemistry, Department of Organic Chemistry, Tamka 12, 91-403, Lodz, Poland

A R T I C LE I N FO

A B S T R A C T

Keywords: Pyrene poly(silsesquioxanes) Fluorescence Light harvesting

New hybrid polymers containing pyrene units bound to linear silsesquioxane chains (LPSQ-Py) through rigid ethene linkers exhibit almost exclusively excimer fluorescence emission, both in very diluted solutions (10−8 mol/L in THF and CHCl3) and in the solid state. They are also thermally stable and exhibit good thin film forming ability. Comparative studies with their octahedral silsesquioxane analogue (POSS-Py) elucidated the role of the specific double-strand architecture of LPSQ-Py backbone and distribution of side Py groups for their unique photo-luminescent properties. The rigid fluorescent rods of LPSQ-Py make interesting candidates for optoelectronic devices. Their ability to act as antennae for energy transfer to red-emitting acceptors has been studied.

1. Introduction

excited states can limit self-quenching [7]. Various strategies have been employed to alleviate the energy dissipation, including the use of tailored antenna chromophores [8] or energy cascades [9]. A recent approach to the problem is based on the phenomenon of fluorescence emission induced or enhanced by aggregation of molecules (AIE and AIEE phenomena) [10,11]. Many polymeric systems with Py moieties attached as side groups to the main backbone are known [12–16]. Such materials can be processed in solution and fabricated into thin films via simple spin-coating or inkjet printing deposition. Unfortunately, those coating techniques involve quite rapid evaporation of solvents and most often fail to provide materials organized enough to show high excimer emission due to random packing of Py. A careful design of macromolecules is required for an effective overlap of chromophores incorporated into backbones or grafted as pendant moieties [17,18]. In this report we provide structure-property analysis for new welldefined hybrid poly(2-(pyren-1-yl)vinyl)silsesquioxanes (LPSQ-Py) (Scheme 1). The key feature of LPSQ-Py is the rigidity of the double backbone, which can significantly hinder the formation of random coils and allows retaining an appropriate distribution of side pyrene groups for the formation of intramolecular excimers. The improved efficiency of interactions between the side chromophores results in an exceptionally high intensity of intramolecular excimer fluorescence emission and resistance to AIQ effects. Polysilsesquioxanes exhibit valuable physicochemical properties due to their polymeric nature and highly ordered double-chain structures, e.g. high thermal stability, good solubility and miscibility with

Extraordinary optoelectronic properties of organic π-conjugated molecules make them useful materials for organic light-emitting diodes, organic photovoltaics, field effect transistors or biosensors [1]. Derivatives of pyrene (Py) are frequently used in such systems due to their characteristic Stokes shift, strong absorbance, high quantum yields and good photochemical stability [2]. Those properties may be of advantage for phenomena based on energy transfer. Single molecules of pyrene are efficient blue light emitters (λem = 380 nm) when dissolved or molecularly dispersed in solvents. Pyrene excimer emissions (λem = 450–500 nm) are more suited for the energy transfer to redemitting acceptors. Crystalline pyrene and Py-derivatives emit almost exclusively the excimer fluorescence, providing an appropriate organization of molecules in the solid state [3,4]. Unfortunately, preparation of thin fluorescent films with high excimer emission can be a problem. The emission is often reduced or completely quenched at high concentrations in solution and in the solid state due to the formation of nanoaggregates or clusters. In such cases the free movement of molecules is hindered by the environment and the geometry required for an effective overlap cannot be attained. Juxtaposed aromatic rings may experience strong intermolecular π-π stacking, inducing relaxation (or decay) of the excited states to the ground state via non-radiative channels. The phenomenon known as “aggregation-induced quenching” (AIQ) [5,6], can be extremely challenging for organic light-emitting devices and artificial light-harvesting systems. Restriction of nonradiative decay of the

∗

Corresponding author. E-mail address: [email protected] (M. Nowacka).

https://doi.org/10.1016/j.dyepig.2019.107622 Received 8 April 2019; Received in revised form 3 June 2019; Accepted 4 June 2019 Available online 05 June 2019 0143-7208/ © 2019 Elsevier Ltd. All rights reserved.

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of POSS-Py of much more open structure, the main product consisted of octahedral cage bearing eight pyrene moieties in side chains, admixed only with traces of species bearing one additional Py group. LPSQ-Py and POSS-Py were also characterized with 13C and 29Si NMR (in solution and solid state) (Fig. S1) and FTIR spectroscopy. The latter technique was found to be a valuable tool to verify the yield of coupling between 1-bromopyrene and LPSQ-Vi. The assignment of IR characteristic bands is presented on Fig. 1 and Table S1. Vinyl groups of LPSQ-Vi gave rise to a moderate ν(C=C) band at 1597 cm−1 and vibration modes corresponding to ν(C–H) (including the band characteristic to stretching of terminal C–H at 3064 cm−1), ρ(C–H), δ(H–C–H) and ω(C–H) [38–40]. Those bands disappeared with the coupling progress, and new vibrations characteristic to ethenyl linkers and ring modes of aryl groups, appeared in the spectrum of the LPSQ-Py as expected. An intense peak at 840 cm−1 was assigned to out-of-plane C–H bending vibrations (wags) of trans structural isomers of the formed alkene side groups. Thermal characteristics of the obtained silsesquioxanes were assessed with TGA and DSC techniques (Fig. 2). LPSQ-Vi is a viscous liquid that exhibits a glass transition at −26 °C. The rigidity of macromolecules increased enormously after the modification and devitrification of LPSQ-Py occurred at very high temperatures (Tg = 158 °C). The rod-like LPSQ-Py do not show any mesomorphic properties within the studied temperature range. Grafting pyrene rings improves the thermal stability of both the polymer and octasilsesquioxane. Both products were fairly thermally stable at ∼400 °C in N2 atmosphere whereas Td (5%) of the both substrates was below 200 °C [32].

Scheme 1. Structure of LPSQ-Py and POSS-Py studied herein.

other polymers [19–22] LPSQ-type macromolecules were used as additives/modificators in polymer blends and composite materials [23,24], polymer electrolytes [25] and materials for organoelectronic applications [26]. They can also form supramolecular nanostructures [27–29]. The unique morphology of LPSQ makes them interesting precursors for photoluminescent materials. For example, LPSQ bearing side carbazole groups exhibited enhanced monomer emission both in solution and in solid state [30]. The effect was attributed to the separation of carbazole moieties induced by the chain structure that limited the AIQ effect. The specific structure of cis-isotactic LPSQ-Ph control the suitable face-to-face parallel arrangement of fluorophores and the formation of intramolecular excimers [31]. We have shown that the fixed positions and restricted rotation of side 2-(pyren-1-yl)vinyl groups make LPSQ-Py highly green emissive not only in extremely diluted solutions (10−8 mol/L in THF and CHCl3) but also in the solid state. The role of LPSQ architecture (ladder backbone and specific distribution of side groups) for the efficient interactions between chromophores was confirmed by comparative studies with a 3D model - 1,3,5,7,9,11,13,15-octa((2-(pyren-1-yl)vinyl))pentacyclo-[9.5.1.13,9.15,15.17,13]-octasiloxane (POSS-Py). LPSQ-Py were also applied as light harvesting antennae for energy transfer to organic dyes absorbing within 450–550 nm.

2.2. Optical and photophysical properties – comparison of LPSQ-Py and POSS-Py Optical and photophysical properties of the obtained LPSQ-Py and POSS-Py were studied at room temperature with absorption and fluorescence spectroscopy both in diluted solutions and in the solid state, in comparison to 1-ethynylpyrene in THF and chloroform at a concentration range of c = 10−8 ─ 10−3 mol/L of Py units. UV/vis spectra of 1-ethynylpyrene (Fig. 3a) show well-resolved vibronic bands in the 250–370 nm range with four main peaks at 272, 283, 341 and 359 nm attributed to the S0 → S3 and S0 → S2 transitions, which are characteristic of pyrene residues. Such clear-structured absorption bands were not obtained for LPSQ-Py or POSS-Py. Their spectra (very similar for silsesquioxanes of both kinds) consist two sets of major peaks. The absorption wavelengths maxima of S0 → S2 transitions were identical for LPSQ-Py and POSS-Py dissolved in the same solvent (347 nm in THF and 353 nm in CHCl3). However, the spectral features are broad and slightly red shifted comparing to those of 1-ethynylpyrene, which suggests pre-association of chromophores in the ground state [41]. The ratio of the intensities of the vibronic bands nor the peak-to-valley ratio could not be measured for both LPSQ-Py and POSS-Py due to the broadness of the spectral features, pointing to the presence of self-assembled pyrene structures even in extremely diluted solutions (10−8 mol/L). The photophysical properties of all the studied materials were analyzed with steady-state fluorescence measurements, carried out using for the excitation wavelengths of the S0 → S2 transition. Solutions of 1-ethynylpyrene in THF and CHCl3 (Fig. S2) within the concentration range 10−8 ─ 10−3 mol/L of Py units become less emissive without formation of excimers on concentration increase. The structure of silsesquioxanes has a strong effect on their fluorescence (Fig. 3b). FL spectra of dilute solutions of LPSQ-Py (both in THF and CHCl3) exhibited almost exclusively a bright green excimer emission band with a maximum of 486 nm. Increase of the concentration resulted in a slight bathochromic shift to 491 nm. Traces of monomeric vibronic bands could be observed only in the most diluted solutions in THF (c < 10−7 mol/L). High intensity of the excimer emission band in the

2. Results and discussion 2.1. Synthesis and characteristics of LPSQ-Py The synthesis of LPSQ of high molecular mass and regular backbone structure and conveniently functionalized to allow for derivatization with Py substituents is not trivial. We have recently found that welldefined oligomeric LPSQ bearing side vinyl groups can be prepared by in-situ condensation of cyclic 1,3,5,7-tetravinyl-1,3,5,7-tetrasilanols [32]. Macromolecules of LPSQ-Vi are terminated with trimethylsilyl groups that assist in hindering of chain coiling. The presence of vinyl substituents allows for a variety of chemical modifications, following addition or coupling pathways such as hydrosilylation [33], metathesis and silylative coupling [34]. The studied LPSQ-Py were thus obtained via palladium-catalyzed Mizoroki-Heck coupling between the respective vinyl-functionalized precursors (Mn = 1000, PDI = 1.23) and 1-bromopyrene (Scheme 2) (for details see Electronic Supporting Information). The coupling method was published before for the preparation of poly(methyl-ethenepyrenylsiloxanes) [16] and for grafting iodobenzene [35] to octahedral silsesquioxanes. The 3D octahedral model silsesquioxane (POSS-Py) was synthesized following the Mizoroki-Heck procedure published earlier by Lo et al. [36] and Chanmungkalakul et al. [37]. The reaction progress was monitored by 1H NMR and it was carried out until the complete conversion of vinyl groups. Owing to the steric hindrance, it seems unlikely that the internal alkenes in the product could undergo a subsequent coupling with another large molecule of 1bromopyrene. MALDI ToF analysis have shown (Fig. S5) that in the case 2

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Scheme 2. Synthesis of LPSQ-Py via palladium-catalyzed Mizoroki-Heck coupling procedure.

than in THF. The effect can be explained by the influence of solvent properties (polarity, dipole moment and dielectric constant). Pyrene derivatives are well-known for their solvatochromism due to differences in dipole moment between excited and ground state [42–44]. Nevertheless, the shape of the FL spectra in both solvents and the relative intensity of the component bands are very close. At higher concentrations spectra of LPSQ-Py both in THF and CHCl3 contain a residual 400 nm band that was tentatively ascribed to fluorescent species of geometry different than symmetrical excimers emitting at around 486 nm. The poly(methyl-ethenepyrenylsiloxanes) described by Dong et al. [16], owing to the flexibility of siloxane bonds in the single chain backbone, contained much larger amounts of species emitting at 400 and 420 nm. The corresponding FL spectra of the polyhedral model POSS-Py show within all the studied concentration range, both in THF and CHCl3 (Fig. 5), the emission vibronic bands at 400 and 422 nm along a broad excimer emission centred at about 450 nm but clearly constructed of several components. The ratio of bands at 486 nm and 400 nm becomes almost the same on the concentration increase, which points to the presence of intramolecular excimers. The solvent induced solvatochromism of POSS-Py is not much pronounced, except for a slight bathochromic shift of the highest energy bands in CHCl3 (Fig. S4). It was earlier shown that POSS-Py can generate fluorescence of the pyrene–pyrene excimer through space when dissolved in DMSO, while ππ* fluorescence emission was dominant in THF (λem = 421 nm and ΦF = 0.65 in THF) [38]. A hypsochromic shift of the excimer emission

Fig. 1. FTIR spectra of poly(vinylsilsesquioxanes) (LPSQ-Vi) and LPSQ-Py.

most diluted solutions points to its intramolecular nature due to the congestion of chromophores along the polymer chain (Fig. 4). In case of the earlier described hybrid poly(methyl-ethenepyrenylsiloxanes), the increased excimer FL intensity was attributed to prevention of the excessive π–π stacking in Py domains by polysiloxane chains and restricted intramolecular rotation (RIR) of the ethene linkers between the main chain and Py groups [16]. The intensity of FL emission of LPSQ-Py in chloroform was higher

Fig. 2. Thermal analysis of LPSQ-Py and LPSQ-Vi (a) DSC traces recorded at heating rate 10 °C/min (3rd run); (b) thermogravimetric analysis (N2 atmosphere, 10 °C/ min). 3

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Fig. 3. (a) UV/vis absorption and (b) fluorescence emission (λex = 347 nm) spectra recorded for 10−6 mol/L solutions of LPSQ-Py, POSS-Py and 1-ethynylpyrene in THF.

Py, in dilute THF solution, were in nanoseconds range with two or three components (Table 1). Similar results were obtained by Dong et al. for poly(methyl-ethenepyrenylsiloxanes) [16]. Fluorescence lifetimes of LPSQ-Py are distinctively higher than those of POSS-Py (solution at 10−6 mol/L in THF). This result is in agreement with a general observation that systems of aggregated chromophore molecules usually have longer fluorescence lifetimes. For example, pyrene derivatives (1acetyl-3-phenyl-5-(1-pyrenyl)-pyrazoline) arranged in well-organized linear structures showed FL lifetime of 28 ns. [46]. 2.3. Aggregation of LPSQ-Py and POSS-Py in solution The mode of supramolecular packing of organic π-conjugated molecules can play a crucial role in the solid state and impact any potential optoelectronic applications. The local environment determines whether an excimer or a non-fluorescent complex is formed in the aggregated state [2,51]. We have carried out an analysis of the propensity of LPSQPy towards aggregation (Fig. 6). The decrease of fluorescence intensity on the increase of concentration in solution occurred more rapidly in CHCl3 (at c ≥ 5x10−6 mol/L) than in THF (c ≥ 10−4 mol/L). The same effect, but much less pronounced, was also observed for POSS-Py (FL quenching at c ≥ 5x10−6 mol/L in CHCl3 and 10−6 mol/L in THF). The aggregation of LPSQ-Py was also investigated in THF/water mixtures (Fig. 6). UV/vis absorption and FL spectra were recorded on addition of water to the THF solution of LPSQ-Py. At 10−6 mol/L of pyrene units the absorbance at 347 nm was about 0.45. The absorption spectra did not change much, which suggests that the organization of chromophores was largely retained. Addition of small amounts of H2O resulted only in a negligible reduction of fluorescence intensity. Yet, any new emission bands were not observed. The rigidity of the double backbone, steric repulsion and restricted intramolecular rotation of side 2-(pyren-1-yl)vinyl groups in LPSQ-Py prevented too close intermolecular π−π stacking. Larger amounts of water induced a slight increase of FL intensity. The effect can be ascribed to the formation of intramolecular excimers on aggregation.

Fig. 4. Fluorescence spectra of LPSQ-Py in THF.

was also observed in THF. The excitation spectra of LPSQ-Py and POSS-Py in THF and CHCl3 were recorded at the emission wavelength of the excimer (λem = 486 nm) (Fig. 5). The observed degree of correlation with the respective absorption spectra was slightly different. Excitation spectra were also recorded in THF at the emission wavelength of 400 nm and 423 nm. Their overlap after normalization also was not exact. Both hybrids LPSQ-Py and POSS-Py, in dilute THF solution (c = 10−6 mol/L), emit with fluorescence quantum yield of ΦF = 0.126 and 0.187, respectively, which is close to the value of 1-ethynylpyrene (ΦF = 0.15). Similar values of ΦF were reported for other systems containing pyrene molecules organized in linear π-stacks [45], being a part of cocrystals [46] or linked to DNA chains [47] and were attributed to molecular aggregation of chromophores. The effect of intermolecular interactions and the type of supramolecular packing on the performance of emissive materials built of π-conjugated molecules is well known [48]. For example, aggregates of bis(pyrene) derivatives showed low fluorescence quantum yields in solutions (ΦF = 0.011) due to the twisted intramolecular charge transfer (TICT) [49]. Sandwich-type Haggregates were also poorly emissive in the solid state (ΦF ≤ 0.031), yet formation of J-type aggregates resulted in almost 30-fold fluorescence enhancement. It was also reported that cis- and trans-poly(1-ethynylpyrenes) have dissimilar optical and photophysical properties due to differences in geometry and interactions between the pendant pyrene groups [50]. The fluorescence lifetimes of 1-ethynylpyrene, LPSQ-Py and POSS-

2.4. Organization of LPSQ-Py and POSS-Py in solid state Good solubility of LPSQ-Py allowed for the preparation of thin films on silicon supports. Solvents of significantly different vapor pressure were chosen to study the effect of their evaporation rate on the organization of molecules in film layers. Solid state organization was studied by wide angle X-ray scattering analysis (Fig. 7). 1-ethynylpyrene crystallizes in the monoclinic system with a unit cell containing four molecules aligned in two planes [52]. 4

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Fig. 5. Emission and excitation fluorescence spectra with absorbance spectra of LPSQ-Py (a,b) and POSS-Py (c,d) in THF (a,c) and CHCl3 (b,d). All measurements for solutions at concentration 10−6 mol/L.

toluene covered completely the support (Fig. 8b and c). The fibrous structures were present in large amounts, but they were much thicker and longer due to slower solvent evaporation. Thin layers of POSS-Py were prepared in an analogous way. AFM analysis revealed the presence of globular morphologies in films prepared from both solvents (Fig. 8d, Fig. S6b). The size of the observed structures (Fig. 8d), around 55 nm, suggests that they are agglomerates of POSS molecules. Despite the structural differences both LPSQ-Py and POSS-Py are emissive in the solid state (Fig. 9). Larger intensity of POSS-Py FL emission than that of LPSQ-Py can be ascribed to the matrix separation by 3D inorganic cores of the former compound. Similar effect solid state photo-optical properties were described for other 3D polyhedral silsesquioxanes decorated with luminophore groups [36,55,56]. The λmax in the FL spectrum of LPSQ-Py is slightly blue shifted in comparison to POSS-Py which illustrates the difference in supramolecular organization of Py moieties in the solid state for both materials. The maxima in the absorption spectrum of LPSQ-Py are also hypsochromically shifted, suggesting a slightly higher order of Py packing mode for LPSQ. Additional measurements showed decrease in photoluminescence quantum yields in the solid state for both LPSQ-Py and POSS-Py. However, due to the specific structure of LPSQ-Py, their quantum yield (ΦF = 0.018 at λem = 521 nm) was higher than for POSS-Py (ΦF < 0.01 at λem = 522 nm).

The respective diffractogram shows three diffraction peaks at 2Θ = 8.5°, 12.5°, 13.2° corresponding to d spacing of 1.0, 0.7 and 0.67 nm that relate to the distances between planes with the same orientation in the crystal lattice. Characteristic arrangement of molecules in the unit cell leads to the presence of a double peak with maxima at around 0.7 and 0.67 nm. A similar effect was observed on diffractograms obtained for polysiloxanes with phthalocyanine moieties in side chains [53]. WAXS analysis revealed differences in crystal structure of 1-ethynylpyrene, LPSQ-Py and POSS-Py. Diffractogram of LPSQ-Py shows three very wide peaks with maxima at around 2Θ = 4.2°, 9.5°, 21.0° (d = 2.1, 0.9 and 0.4 nm). Wide diffraction peaks are typical for ladder-like silsesquioxanes [32,54]. According to HyperChem calculations signals at around 2 and 1 nm correspond to the ladder width and approximate size of the substituent group, respectively (Fig. 7). A wide peak with maximum at around 2Θ = 21.0° (0.4 nm) is characteristic for the length of Si–O–Si bond [32]. Broad diffraction peak within 2Θ = 8.0–17.0° (1.1–0.5 nm) can be associated with organization mode of pyrene substituents along the polymer chain. Diffractogram of POSSPy shows two wide peaks with maxima at around 2Θ = 5.0° and 9.5° (1.7 and 0.9 nm) that correspond to the size of molecule (similarly to LPSQ-Py). Differences between diffraction patterns of LPSQ-Py and POSS-Py result from less organized packing of the substituents around the inorganic core of POSS-Py and different organization of pyrene groups in the solid state. The surface structure of thin films of LPSQ-Py, POSS-Py and 1ethynylpyrene was analyzed with AFM. Films obtained from chloroform (Fig. 8a) had irregular morphology. Islands of LPSQ-Py with granular surface contained thin twisted fibers of helical structure. Their presence can be tentatively associated with intermolecular interactions between pyrene molecules in the side chains. Thin films obtained from

2.5. Energy transfer and light harvesting with LPSQ-Py Natural light-harvesting complexes collect sunlight and deliver it by cascade processes based on transfer of energy (EnT) and electrons (ElT) to the reaction centres where it is stored as chemical energy [57,58]. The design of artificial light-harvesting assemblies is a hot topic and 5

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Table 1 Summary of fluorescence lifetime measurements in THF solutions. Compound 1-ethynylpyrene

c [mol/L] –6

10

λem(max)

τ [ns] (contribution, %)

383

τ1 = 4.32 (2.22) τ2 = 17.8 (97.78) τaverage = 16.6 τ1 = 0.83 (1.32) τ2 = 16.5 (98.68) τaverage = 13.2 τ1 = 7.93 (28.4) τ2 = 15.3 (3.56) τ3 = 21.4 (68.0) τaverage = 11.0 τ1 = 16.3 (64.8) τ2 = 51.1 (19.4) τ3 = 2.85 (15.8) τaverage = 10.9 τ1 = 4.38 (90.9) τ2 = 23.0 (9.1) τaverage = 4.72 τ1 = 2.56 (93.5) τ2 = 18.8 (5.6) τaverage = 2.68 τ1 = 2.56 (94.5) τ2 = 21.2 (6.5) τaverage = 2.71 τ1 = 8.21 (36.3) τ2 = 21.2 (57.0) τ3 = 1.75 (6.7) τaverage = 9.1 τ [ns] (contribution, %) τ1 = 0.28 (6.9) τ2 = 1.89 (9.8) τ3 = 0.80 (83.3) τaverage = 0.93 τ1 = 0.69 (10.2) τ2 = 4.38 (86.0) τ3 = 2.38 (3.8) τaverage = 2.9 τ1 = 2.58 (95.4) τ2 = 17.2 (4.6) τaverage = 2.68 τ1 = 2.58 (94.7) τ2 = 20.3 (5.3) τaverage = 2.71 τ1 = 0.12 (3.57) τ2 = 4.20 (96.4) τaverage = 1.88 τ1 = 2.38 (94.7) τ2 = 8.84 (2.13) τaverage = 2.41

403

LPSQ-Py

LPSQ-Py + NR

10–6

a

501

486

600

LPSQ-Py + Cou

a

492

511

POSS-Py

10–6

487

Compound POSS-Py + NR

c [mol/L]

λem(max) 511

a

600

POSS-Py + Cou

a

492

511

NR

10–6

600

Cou

10–6

492

Fig. 6. Change of fluorescence intensity with concentration of LPSQ-Py and POSS-Py and change of fluorescence intensity of LPSQ-Py (10−6 mol/L solution in THF) in the presence of various amount of water. Fluorescence intensity measured at λem = 487 nm.

[66-68]. We have found that LPSQ-Py are able to act as energy donors to redemitting Nile Red (Fig. 10). As required for an efficient resonance transfer of energy, the emission of LPSQ-Py overlaps well with absorption spectrum of NR in solution in THF. Excitation at 350 nm involves mainly Py units and the emission in solutions is observed from the NR acceptor. A decrease of the Py excimer emission was observed due to fluorescence resonance energy transfer in the presence of NR. A slight change of the shape of Py excimer emission band was also noted on increase of the concentration of NL. It is doubtful that the change of symmetry is the result of a hypsochromic shift of the excimer emission band. The effect rather suggests a selective fluorescence resonance energy transfer from a defined range of spectrally suitable excimers. The fluorescence resonance energy transfer was also observed from POSS-Py to NR (Fig. S4). Coumarin 6 was also tested for the resonance energy transfer from LPSQ-Py. The characteristic emission of Cou increased with quenching the pyrene excimer band. The obtained results suggest a pathway for fluorescence resonance energy transfer from LPSQ-Py to the acceptor dye. The energy transfer phenomenon was also confirmed through fluorescence lifetime measurements carried out for the mixtures of LPSQ-Py and POSS-Py with NR and Cou (Table 1, Fig. 11). An increase of the NR fluorescence lifetime was observed for mixtures with LPSQ-Py and POSS-Py along with decrease of the FL lifetime of the donors, which is typical for energy transfer. Results obtained for mixtures with Cou showed more subtle changes in the lifetime of the dye emission. Interestingly, one of LPSQ-Py and POSS-Py FL lifetimes (τ3) disappeared.

a Measurements for mixtures of LPSQ/POSS-Py and appropriate dye (NR = Nile Red or Cou = Coumarin 6) (mixtures contain 3 nmol of each compound).

several synthetic antenna systems were described [59-61]. Antenna materials absorb photons and the excited state energy is subsequently transferred to acceptor molecules in the ground state. The nonradiative excited state energy transfer processes in this case occur through fluorescence resonance energy transfer [62-64]. The antenna chromophores should not exhibit self-quenching and be able to generate a longrange charge separation [65]. An appropriate arrangement and geometrical overlap of chromophores is also required to provide a suitable emission band of high quantum yield. Thus, the properties and high stability of the prepared hybrid LPSQ-Py makes them interesting candidates for such advanced photoluminescence systems. The efficiency of fluorescence resonance energy transfer depends on the distance between donor and acceptor molecules and the overlap of the emission spectrum of the donor and the absorbance spectrum of the acceptor. It is thus important to choose spectrally suitable species to transfer energy from green-emitting pyrene excimers. Organic dyes such as 9-diethylamino-5-benzo[α]phenoxazinone (Nile Red, NR) and 3-(1,3-Benzothiazol-2-yl)-7-(diethylamino)-2H-chromen-2-one (Coumarin 6, Cou) can be applied in artificial energy transfer systems

3. Conclusions Specific structure of linear ladder-like poly(silsesquioxanes) with rigid double-strand main chain grafted with side pyrene groups favours the formation of green emitting intramolecular excimers, both in very diluted solutions and in the solid state. The observed optical properties arise from well-defined spacing between chromophores and restriction of their motion with respect to the polymer backbone. Parallel arrangement of pyrene moieties can be connected with the fluorescence lifetimes and a quantum yield of 0.13 observed for the studied system. The unique photo-optical properties and processability of LPSQ-Py makes them an interesting hybrid material for possible applications in optoelectronics (e.g. large area display lighting devices). It was found that LPSQ-Py is capable of an efficient fluorescence resonance energy transfer to suitable red-emitting dyes. The obtained data provide 6

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Fig. 7. WAXS diffractograms of thin films of LPSQ-Py, POSS-Py and 1-ethynylpyrene cast on silicon plates. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 8. AFM analysis (phase images) of thin films of LPSQ-Py (a–c) and POSS-Py (d) on silicon plates cast from chloroform (a) and toluene (b,c,d) solutions. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 9. Fluorescence and absorption (insert) spectra of LPSQ-Py, POSS-Py and 1-ethynylpyrene in the solid state.

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Fig. 10. Energy transfer from LPSQ-Py to Nile Red (a) and Coumarin (b) with dye concentration dependence and spectral overlap of LPSQ-Py and dyes. Dye concentration range between 3.3x10−7 mol/L and 1.6x10−5 mol/L. Measurements for 3 mL of LPSQ-Py solutions at concentration 10−6 mol/L. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 11. Fluorescence lifetime decay profiles for LPSQ-Py and POSS-Py (solutions in THF, 10−6 mol/L) and their mixtures with dyes (NR or Cou).

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important information for the use of polymeric materials for innovative light harvesting systems and composite light emitting diodes.

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