Fluorescence relaxation kinetics of poly(methylphenylsilane) film and nanocomposites
© Ostapenko et al. 2016
Received: 4 December 2015
Accepted: 10 March 2016
Published: 12 April 2016
A comparative study of fluorescence relaxation kinetics of σ-conjugated poly(methylphenylsilane) (PMPS) polymer film and nanocomposites has been performed by ultrafast time-gated fluorescence measurements at various temperatures. Investigations have revealed a fine structure of excitonic σ-σ* band. We attribute this structure to emission from two spatially independent states with different ordering of the polymer chain segments, type gauche and trans conformations. In contrary to a more ordered polymer poly(di-n-hexylsilane), no clear thermochromic transition has been detected in PMPS film; however, the trans band intensity increases with temperature and with excitation wavelength, but it is absent when polymer is incorporated into nanopores of small diameter.
Polysilanes belong to the class of silicon-organic polymers consisting of σ-conjugated Si backbone and organic side groups. Their electronic properties are attributed to the σ-conjugation originating from the overlap of Si sp3 orbitals . Polysilanes show remarkable fluorescence (FL) in the UV region  and high mobility of holes ; thus, these polymers are promising in construction of emitting or transporting layers for electroluminescence devices [3–5]. Evidently, structural organization of these polymers in solid state predetermines functioning of polymer-based devices; therefore, understanding of optical and electric features depending on the polymer structural arrangement is an important issue. Current trends in designing nanostructured materials for various applicative purposes can be achieved by means of different technological approaches. Embedding of polysilanes in nanoporous materials such as MCM-41 and SBA-15 is an effective way of producing and controlling nanostructured composites [6–9].
The shape of σ-σ* absorption band of polysilanes is determined by the trans-gauche isomerism of polymer chains and by the length distribution of conjugated segments . Such isomerism has been clearly observed in poly(di-n-hexylsilane) (PDHS) polymers with clear thermochromic transition from trans to gauche conformation at about 315 K temperature . Conjugated segments of different lengths have different transition energies; therefore, irradiation of the polymer into the blue edge of the absorption band preferentially excites the short higher-energy segments. Then, over their lifetime, excitons migrate to the longest segments, which have the smallest band gap and the lowest energy. Irradiation into the red edge of the absorption band addresses longer segments, increasing correlation between the absorbing and the emitting species. Thus, position of the FL band maximum in both cases is determined by the transition energies in the longest segments. FL quantum yields of polysilanes also depend on the excitation wavelength and reach their maximum values for the long-wavelength excitation . Temperature dependences of the FL spectra measured with high time resolution may provide information about intermediate stages. Relaxation processes are expected to be more complex in the case of coexistence of different conformeric forms.
This article presents ultrafast time-resolved fluorescence study of excited state dynamics of σ-conjugated poly(methylphenylsilane) (PMPS) films and PMPS confined within MCM-41 and SBA-15 silica nanopores of 2.8 and 9 nm diameter, respectively. Investigation reveals coexistence of spatially separated trans and gauche conformational forms of polymer chain, and we conclude that the lower energy trans states are partly populated by thermally stimulated exciton diffusion.
PMPS polymer was synthesized as described in . PMPS films (Mw = 11160) were prepared by drop-casting from the solution in toluene (1 wt.%) on metal substrates. Silicas MCM-41 and SBA-15 with the pore diameter of 2.8 and 9 nm, respectively, were synthesized by the techniques given in [11, 12]. The synthesized samples of MCM-41 were filtered and washed and then carefully calcined in air at heating rate of 1 K/min up to 813 K and held at this temperature for 5 h. In the case of SBA-15, the samples were calcined in an oven at 373 K with the heating rate of 1 K/min for 3 h and, subsequently, in dry air at 823 K for 4 h to remove the surfactant. The removal of the template was controlled by FTIR spectroscopy.
Mesoporous material was mixed with the polymer solution; afterwards, the mixture was placed into a dark vessel where it was slowly stirred for 2 days at 293 K and kept until the solvent evaporated. The obtained nanocomposite was double washed in fresh toluene to remove the polymer from the outer surface. Then, the obtained samples were dried for 12 h at room temperature for the residual moisture to be removed and then they were kept in a desiccator.
Steady-state FL spectra of PMPS films were measured with spectrometer based on streak camera described below, while FL spectrum of solution in toluene at 15 K temperature was measured with DFS-13 spectrometer under excitation at 313 nm. Ultrafast fluorescence dynamics of PMPS films and nanocomposites was measured by means of spectrometer based on a streak camera (Hamamatsu Photonics Ltd.) and femtosecond laser (Light Conversion Ltd.). The Yb:KGW laser produced 80-fs 1030-nm light pulses at 76 MHz repetition rate. Third (343 nm) and fourth (254 nm) harmonics of the laser radiation generated by HIRO harmonics generator (Light Conversion Ltd.) were used for the sample excitation. The excitation beam was focused to ∼100 μm spot on the sample, resulting in about 1 mW/mm2 average excitation power. The maximum time resolution of the whole system was about 3 ps, and spectral resolution was about 3 nm. All the measurements (except of solution in toluene) were performed in closed cycle cryostat (Janis Research Company Ltd.), which enabled temperature variation within 15–320 K. The samples were kept in vacuum during the measurements.
Results and Discussion
As temperature increases up to 200 K, a new UV band gradually appears at 357 nm, but a weak short-wavelength shoulder remains in the position of the band observed at 15 K. At room temperature, the UV band acquires clear doublet structure.
We suppose that the two excitonic bands are associated with the existence of spatially separated FL centers, which correspond to different ordering of the polymer chain segments, i.e., to different chain conformations, of gauche and trans types. Such polymorphism has been clearly observed for PDHS polymers causing the appearance of two fluorescence bands: the short-wavelength band attributed to the disordered gauche conformation and the long-wavelength band attributed to a more ordered trans conformation . Existence of gauche and trans conformational forms of PMPS film was also suggested in ref. ; however, properties of the two forms were not investigated in detail. Existence of similar two states in steady-state FL spectrum of thick PMPS film was also proposed in ref. . Our results show interplay between the fluorescence of the gauche and trans forms. Increase of intensity of the long-wavelength component with respect to the short-wavelength component as temperature increases from 15 to 320 K suggests that the trans species are at least partly populated via thermally activated energy transfer from the gauche species. Gauche species are apparently dominantly excited with 254 nm light and excitation energy transfer to trans species at low temperatures, up to about 200 K, is inefficient. Trans species, according to the ref. , have absorption band at about 341 nm; thus, they are efficiently excited with 343 nm light and, consequently, trans fluorescence is more intense at all temperatures (Fig. 3a).
Although PMPS, as well as PDHS polymer, reveals two conformeric states, their properties are rather different. PDHS embedded in nanoporous silica shows clear thermochromic transition from gauche to trans form in a (220-290) K range , while we do not see any evidence of thermochromism of PMPS. The difference is apparently related to the polymer structure. In contrast to PDHS, which is semicrystalline polymer, PMPS is an amorphous material; therefore, it has no distinct phase transitions, and the interplay between the two conformeric forms sufficiently clearly appears only in time-resolved fluorescence spectra. Consequently, our data reveal coexistence of gauche and trans conformeric forms in PMPS films independently of temperature. Although the gauche fluorescence in films decays up to two times faster than the trans fluorescence, energy transfer between the two conformers is not efficient suggesting that trans conformers are in minority and/or trans and gauche forms are spatially separated. Gauche conformers completely dominate when PMPS is incorporated in porous materials with small pore diameter, while in larger pores, the trans conformers and probably their aggregates are formed as well.
Conformational properties and excited state relaxation of σ-conjugated poly(methylphenylsilane) (PMPS) polymer film and nanocomposites have been investigated at different temperatures by ultrafast time-gated fluorescence measurements. PMPS shows exciton band in the UV region and a wide visible fluorescence band in the 400- to 500-nm regions. PMPS exciton fluorescence has doublet structure in nanocomposites and in low-temperature toluene matrix. Time-resolved investigations at different temperatures clear revealed the doublet structure and in PMPS film. We attribute this structure to the coexistence of gauche and trans conformations of polymer chain. Intensity of the long-wavelength component of the exciton band attributed to the trans conformation increases with the temperature and with excitation wavelength indicating partial population of this state via thermally activated energy transfer. The long-wavelength component attributed to the trans conformation is absent in the composite with small pore diameter hosting only one or two polymer chains where exciton diffusion is hindered. In the composite with larger pore diameters, the trans conformers and probably their aggregates are formed as well. In contrast with the semicrystalline PDHS, no thermochromic trans-gauche transitions are observed in PMPS because of its amorphous structure.
The authors are grateful to Prof. A. Watanabe for the polymer synthesis.
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