The present invention relates to compounds of formula (I)ArCH.sub.2 R.sup.1 (I)or a monomethyl or monoethyl ether thereof (the compound of formula (I) including these ethers may contain no more than 30 carbon atoms in total); ethers, esters, thereof; acid addition salts thereof; wherein Ar is a C.sub.15-18 fused tetracarbocyclic ring system containing 3 or 4 aromatic rings or a C.sub.17-22 fused pentacarbocyclic ring system containing 4, or 5 aromatic rings, or a substituted derivative thereof; the ring system Ar should be planar or deviate only slightly from planarity. Thus, the ring system contains a maximum of two non-aromatic carbon atoms which may be in the same ring, in which case they are adjacent, or in different rings;Ar is not perylene, fluoranthene, chrysene, pyrene, or triphenylene;R.sup.1 contains not more than eight carbon atoms and is a group ##STR1## wherein m is 0 or 1;R.sup.5 is hydrogen;R.sup.6 and R.sup.7 are the same or different and each is hydrogen or C.sub.1-5 alkyl optionally substituted by hydroxy;R.sup.8 and R.sup.9 are the same or different and each is hydrogen or C.sub.1-3 alkyl; ##STR2## is a five- or six-membered saturated carbocyclic ring; R.sup.10 is hydrogen, methyl or hydroxymethyl;R.sup.11, R.sup.12 and R.sup.13 are the same or different and each is hydrogen or methyl;R.sup.14 is hydrogen, methyl, hydroxy, or hydroxymethyl.