Emission Color Tuning in Polythiophenes

These observations inspired Inganas and coworkers [107,497,498] to exploit the principle of steric hindrance to design PTs with emission colors spanning the full visible spectrum. A wide range of 3-substituted and 3,4-disubstituted thiophenes has been synthesized and successfully polymerized by FeCl3 in chloroform, affording polymers 411-424 [466, 498-501] (Chart 2.100). 

FIGURE 2.26 PL and absorption spectra of thin films of regioregular polythiophenes HT-396 (a) and HHTT-396 (b), spin coated on a fused silica substrate. (From Barta, P., Cacialli, F., Friend, R.H., and Zagorska, M., J. Appl. Phys., 84, 6279, 1998. With permission.) 

SCHEME 2.66 Synthesis of regioregular HHTT alkoxycarbonyl-PTs. (From Pomerantz, M., Cheng, Y., Kasim, R.K., and Elsenbaumer, R.L., J. Mater. Chem., 9, 2155, 1999.) 

SCHEME 2.67 Synthesis of regioregular HHTT octyl-PTs. (From Gill, R.E., Malliaras, G.G., Wilde-man, J., and Hadziioannou, G., Adv. Mater., 6, 132, 1994.) 

A certain balance should be kept in distortion of the thiophene planarity as a way to prevent the formation of interchain aggregates. Introducing two substituents at positions 3 and 4 of PT allows a shift of the emission through the entire visible range and prevents interchain interactions (resulting in a smaller decrease of the quantum yield in the solid state compared to solution). Highly crowded disubstituted PTs 418 421 show very low luminescence efficiency already in solution (Table 2.5) due to substantial distortion of the backbone [107,498], 

Naito and co-workers used a thienothiophene linker to improve the relatively poor PL of the polythiophene in the solid state [59]. Such linkers are well known in the design of high-mobility PTs for field effect transistors and photovoltaics (see Chapter 17). The pl value of thiophene polymer 33 is close to that for regioregular P3HT (HT-9f) in solution (27 and 33 %, respectively), but is much higher in the solid state (16 and 0.3 %, respectively), which was ascribed to intrachain disorder of this less symmetric structure. Both PL in films and EL spectra of an ITO/33/A1 device showed emission at 580 nm, although no other characteristics of device performance was reported. 

The emission spectrum of some PT-based bilayer devices could not be explained by a linear combination of emissions of the components. Thus, efficient white emission of the PLEDs ITO/30/PBD/A1 ( 1 el = 0.3 % at 7 V) consisted of blue (410 nm), green (530 nm) and red-orange (620 nm) bands. Whereas the 

End-capping of oligothiophenes with electron-deficient dimesitylboryl groups resulted in amorphous electron transporting materials 39 (n = 2, 3), for which extremely high brightness (up to 23,200 cd m ) OLEDs were fabricated [74]. 

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