c 6 optionally substituted but not containing fluorine or silicon b phenyl or benzyl both optionally substituted or c a hetaromatic residue and if p is equal to 1 and r 2 is hydrogen or alkyl c 1 c 6 then r 1 can also be another residue r 3 o ch 2 m o n x u2014 a u2014 s u2014 z u2014 where r 3 is hydrogen or alkyl c 1 c 6 and z is selected from ch 2 2 ch 2 3 ch 2 4 ch 2 ch ch 2 oh ch 2 ch oh ch oh ch 2 ch 2 ch 2 och 2 ch 2 1 2 phenylene 1 3 phenylene 1 4 phenylene 2 5 1 3 4 thiadiazolyl 1 3 xylylene 1 3 5 triazinyl optionally substituted and r 2 is hydrogen or alkyl c 1 c 6 optionally substituted or another residue r 1 u2014 s u2014 a u2014 x u2014 examples of x are u2014 c u2550 o u2014 u2014 ch 2 c u2550 o o u2014 u2014 c u2550 o nh u2014 u2014 s u2550 o 2 u2014 or a direct bond examples of suitable groups r 1 include methyl ethyl ch 3 oc u2550 o ch 2 u2014 ch 3 ch 2 oc u2550.