Photoluminescence ladder polymers

Keywords Electroluminescence Ladder polymers Lasing Light-emitting diodes Photoluminescence... 

Surprisingly, if a solution of the polyalcohol was treated with boron trifluoride etherate in, e.g., dichloro-methane or chloroform, the ring closure to give the ladder polymer took place within seconds and was quantitative. The resulting new type of ladder polymer, a planar methylene-bridged poly(p-phenylene), is deep yellow, and solutions thereof display an extremely intense blue photoluminescence. 

In the cyclization to the ladder structure LPPP, the color of the reaction solution changes from colorless to an intense yellow, connected with the appearance of a considerably intense blue photoluminescence. The absorption spectrum of the resulting ladder polymer substantiates the drastic changes in the electronic structure. A very sharp, structured absorption band appears on the long-wave absorption maximum of about 450 nm (tt-tt transition) [19] (see Fig. 14.1). 

Figure 9-1. Materials overview a few selected conjugated polymers and their properties have been compiled and the following abbreviations are used DO-PPP... Poly(2-decyloxy-l,4-phenylene), EHO-PPP... Poly(2-(2 -ethylhexyloxy)-l,4-phenylene), CN-PPP...Poly(2-(6 -cyano-6 -methyl-heptyloxy)-l,4-phenylene), m-LPPP...methyl-substituted ladder-type Poly(l,4-phenylene), and PLQY = photoluminescence quantum yield.

FIGURE 5.7 Absorption, photoluminescence emission, and photoinduced absorption spectrum of a polymer film of ladder-type polymer with pentaphenyl segments. The inset shows the actual chemical structure. 

The photoluminescence spectrum of MeLPPP as depicted in Figure 5.8 is characterized by a steep onset at 2.69 eV and by the well-resolved vibrationally split maxima which are homologous to the excitation spectrum. The dominant photoluminescence maximum is only very slightly Stokes-shifted by -35 meV, which is due to the ladder-type structure of the polymer hindering geometrical relaxations. The steep onset of the absorption spectrum reflects the high intrachain order of MeLPPP in the film. 

The fast kinetics of the photoluminescence emission, picosecond and femtosecond relaxation, in ladder-type polyparaphenylenes is notable [142,143,186], since such measurements reveal stimulated emission and therefore promising aspects for LPPPs as a suitable material for optically pumped solid-state homopolymer lasers as well as for the realization of electrically pumped solid-state polymer lasers [144]. To illustrate the situation we depict the photoluminescence intensity as a function of the excitation power (excitation wavelength A = 295 nm) for... 

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