Determination of trap energy levels in poly(fluorenylene phthalide) films using thermostimulated luminescence

Abstract

Long lasting afterglow emission of UV irradiated samples of some poly(arylene phthalide)s (PAP's) was observed during investigation of spectral and lumines cent physicochemical features of solutions and films of these polymers [1,2]. Thermal treatment of the sam ples after completion of the emission resulted in the re emergence of glow. Analysis of the photolumines cence, thermostimulated luminescence (TSL), and EPR spectra of model compounds and PAP films, as well as quantum chemical calculations using the TD DFT formalism aimed to identify luminescence emit ters made it possible to specify the set of potential emitters in the observed luminescence spectra of these polymers [3,4]. From the cited studies, it follows that UV irradiation of the polymers with light leads to ion ization and charge separation, with the polyaromatic backbone-which is in particular fluorenylene in the case of poly(fluorenylene phthalide) (PFP)-becom ing an electron donor and the array of oxygen con taining entities of PAP being acceptors, i.e., electron traps. In the order of decreasing acceptor properties, these are the terminal groups (anthraquinone, acidic), defective anthronic group, and phthalide side frag ments. In addition, UV irradiation of PFP results in the emergence of new long lived absorption centers comprising of polymer chain segments in the triplet state due to intersystem crossing, which facilitate the formation of polymer matrix stabilized radical ions [5]. Radical recombination in PFP films is accompa nied by the emergence of a band at ~500 nm that coin cides with the long wavelength fluorescence band of PFP [1]. The traps stabilized by polymer matrix can be activated by heating, which results in freezing out of molecular mobility of polymer chain segments [6]. Furthermore, voids and other structural defects can act as physical electron traps in amorphous polymers.Along with radiothermoluminescence (RTL), TSL methods are among the most sensitive techniques for investigation of relaxation and structural transitions in polymers and, being relatively simple from the techni cal point of view, make it possible to obtain additional information on the physicochemical features of poly mer materials. In this communication, we present the results of a TSL study of traps in PFP films.Thermally stimulated luminescence was recorded with a photometer operating in the current mode as described in [7]. The TSL spectra were measured using edge filters according to [8]. Temperature was moni tored with a copper constantan thermocouple and a digital millivoltmeter. Poly(fluorenylene phthalide) films were irradiated with unfiltered light from a LOS 2 powerful UV source (xenon lamp with P = 1 kW) for 20 min at Т of ~ 20°C. The TSL intensity measured after irradiation decayed according to the second order rate law for parallel reactions [3], thereby indi cating the complexity of chemical composition of active species involved in the interaction of UV light with PFP. After luminescence decay to a background level at room temperature, the sam...