Transient absorption spectra of triplet states and charge carriers of conjugated polymers

Abstract

The use of pulse radiolysis to study transient species in conjugated polymers in solutions is discussed, with particular reference to poly(2-methoxy-5-(2 0 -ethylhexoxy)-p-phenylenevinylene) (MEH-PPV). The excited triplet state is speci®cally generated by energy transfer on radiolysis of benzene solutions in the presence of suitable sensitisers, and its spectrum is reported. The presence of vibronic structure suggests a localised triplet state. By varying the energy of the sensitiser, it is possible to determine the energy of the lowest triplet state. Results are also presented for the triplet±triplet absorptions and energies of ®ve other common p-conjugated polymers. The MEH-PPV triplet absorption, produced on radiolysis, does not increase linearly with either polymer concentration or radiation dose. This is suggested to be due to the presence of multiple triplet excitations on the isolated polymer chains. Intrachain triplet±triplet annihilation is observed, and is seen to lead to delayed¯uorescence. Spectra are also reported for the positive and negative charge carriers of MEH-PPV and the other polymers speci®cally produced by pulse radiolysis of the polymers in chloroform or tetrahydrofuran solutions. q

Photomodulation spectrum of phenyl-substituted ladder-type poly(para-phenylene) under hydrostatic pressure

“…The triplet-triplet absorption of Ph-LPPP shows an indefinite vibronic structure, which is probably indicated for the coupling of the C-C stretch modes [16] of the conjugated backbone. The PIA of Ph-LPPP is very similar to what have been reported for other conjugated polymers [17,18] at low temperatures using the same technique. Triplet state has more localized character due to restrictions of transition from singlet to triplet state due to spin conservation [19,20].…”

Triplet state dynamics on isolated conjugated polymer chains

“…We have previously reported spectra and energies of the triplet states of these polymers generated in this way [26,27,31,41]. The technique has also been used to study some p-phenylenevinylene oligomers [51] and broken conjugation polymers [52].…”

“…Knowledge of the formation and decay pathways of these triplets is therefore important for a full understanding of the electroluminescence behaviour of conjugated organic polymers. We report results of polymer triplet state dynamics in dilute solution, under conditions where they can be considered to be isolated chains [31], i.e., where there are no interchain interactions, and where the time between collisions of two polymer molecules is longer than the lifetimes of the excited states being studied. Preparation of triplet states by both photoexcitation and energy transfer will be considered.…”

Interchain effects in the ultrafast photophysics of a semiconducting polymer:THztime-domain spectroscopy of thin films and isolated chains in solution

“…18 It has also been demonstrated, however, that such optical studies may be complicated by the fact that different types of excitons and charges may have very similar spectral signatures. [18][19][20][21][22] THz time-domain spectroscopy ͑THz-TDS͒ therefore ideally complements optical studies, as it allows one to directly probe both exciton and free charge species simultaneously: it is sensitive to the real and imaginary components of frequency-dependent ͑0.2-1.5 THz͒ photoconductivity and, as such, can monitor free and bound charges on a subpicosecond timescale. Our initial THz-TDS studies 5 of semiconducting polymers have indicated that excitons are the primary photoexcitation product, while only a small number of free charges are formed on ultrafast times, most likely from dissociation of "hot" excitons.…”