Femtosecond Relaxation of Photoexcitations in a Poly(Para-Phenylene)-Type Ladder Polymer

Abstract

Time-resolved femtosecond pump and probe experiments were conducted on films of poly( paraphenylene)-type ladder homopolymers featuring a high degree of intrachain order. We find stimulated emission in the region of the interband luminescence which is of the same magnitude as the photoinduced absorption. The latter is located energetically below the interband luminescence and therefore does not compete with the stimulated emission. We conjecture this to be an intrinsic property of conjugated polymers of high intrachain order and a narrow distribution of long conjugation lengths. From the decay dynamics we conclude that stimulated emission and photoinduced absorption originate from different species. PACS numbers: 78.47.+p, 42.65.Re, 78.55.Kz The yield and fate of the various photoexcitations in conjugated polymers and their implications for electroluminescent, photoconductive, and nonlinear optical devices attracted a lot of interest recently [1]. The increase of conjugation and the reduction of defects was the main motivation to incorporate a poly( para-phenylene) (PPP) backbone into a ladder polymer structure (LPPP) [2,3] (Fig. 1). The high intrachain order [4], the small Stokes shift [5] coupled with a high quantum yield [6], and the excellent solubility due to the large sidegroups made these materials promising candidates for application in light-emitting devices, which were in fact built [7,8]. Both for the fundamental understanding of the states involved and for potential applications the mutual interdependence of absorbing and emitting species is vital. Nevertheless, the electronic properties of the excited states of these compounds have not yet been investigated. Previous studies on poly(paraphenylene vinylene) (PPV) have shown that upon photoexcitation a competition between stimulated emission and photoinduced absorption (PA) can be observed [9] which is attributed to singlet excitons and spatially indirect excitons, respectively. Substantial differences in the photophysics of LPPP's with regard to conjugated polymers with a broad distribution of conjugation lengths [10] have to be expected. The narrow distribution of long conjugation lengths in LPPP's suppresses the exciton migration process, which is considered dominant at small time scales in PPV [11]. The high intrachain order (low defect concentration and planarity) leads to an effective one-dimensional semiconducting behavior, which can support ultrafast intrachain charge separation.The objective of the present study is to shed light on the question of initially formed photoexcitations in LPPP's. Since the electronic properties within the group of LPPP's are known to vary slightly as a function of the substituents and chain length [8,12,13] as well as ring position [5], we chose the material with the highest intramolecular order presently available, which we denote as m-LPPP due to the methyl substituent at the methin bridge (symbol Y in Fig. 1). The temporal evolution of the photogenerated states is monitored by their contribution to transient ...

Localized triplet excitations and the effect of photo-oxidation in ladder-type poly(p-phenylene) and oligo(p-phenylene)

“…8͒ and time resolved spectroscopic techniques. 9 Simple spin-statistical considerations predict that 25% of the recombining nongeminate polaron pairs generated by carrier injection in an OLED will yield singlet excitons ͑SE's͒ whereas the other 75% will yield nonradiative triplet excitons ͑TE's͒, which theoretically limits the quantum yield of the OLED to 25%. However, it was suggested that the strong difference in the nature of the SE and TE states influences the formation cross-section of SE's and TE's from polarons.…”

Influence of star-like iridium complexes in the graphoepitaxy of polyfluorene thin films

“…Hairy-rod polymers 1 are one important example and, among those with -conjugated backbones 2 ͓e.g., poly͑p-phenylenes͒, 3 polyfluorenes ͑PFs͒, 4 and ladder derivatives 5 ͔, the nature of the main chain conformation and interchain packing are crucial factors in ultimately determining optical and transport properties. The structural characteristics at larger length scales are equally as important, and this has been the main emphasis in a number of recent studies.…”

Femtosecond hot-exciton emission in a ladder-typeπ-conjugated rigid-polymer nanowire

“…[1][2][3][4][5][6][20][21][22][23][24][25][26][27][28][29][30][31][32][33] Femtosecond pump-probe techniques are of little use here due to the superposition of stimulated emission from the hot exciton with photoinduced absorption to high-lying excited states; [2][3][4][5]23,[34][35][36][37][38][39] there are also limitations on the availability of the deep-blue spectral component from the white-light supercontinuum needed as a probe. [34][35][36][37][38][39] 045308-1 1098-0121/2013/87(4)/045308 (9) ©2013 American Physical Society In pristine thin films, π -conjugated polymers are typically amorphous glasses, having a statistical distribution of chain lengths and random orientations, and each chain usually is composed of multiple segments due to chain folding, twisting, kinks, or chain defects, etc., and all of these yield poorly resolved absorption and emission spectra and provide a large heat bath to enable excitons to migrate nondispersively over relatively large distance during their lifetimes. [1][2][3][4]…”

“…[34][35][36][37][38][39] 045308-1 1098-0121/2013/87(4)/045308 (9) ©2013 American Physical Society In pristine thin films, π -conjugated polymers are typically amorphous glasses, having a statistical distribution of chain lengths and random orientations, and each chain usually is composed of multiple segments due to chain folding, twisting, kinks, or chain defects, etc., and all of these yield poorly resolved absorption and emission spectra and provide a large heat bath to enable excitons to migrate nondispersively over relatively large distance during their lifetimes. [1][2][3][4][5][6][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] Moreover, the rich vibronic modes and the strong electron-phonon coupling on each single chain greatly broaden the overall optical transition cross sections in absorption and emission. [1][2][3][4][5][6][20][21][22][23][24]…”

“…[1][2][3][4][5][6][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] Moreover, the rich vibronic modes and the strong electron-phonon coupling on each single chain greatly broaden the overall optical transition cross sections in absorption and emission. [1][2][3][4][5][6][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38]…”