The present invention relates to triarylmethane compounds of the formula (I), which suitable as monomers for preparing thermoplastic resins having beneficial optical properties and which can be used for producing optical devices. R1, R2 are e.g. hydrogen; Y is an alkylene group having 2, 3 or 4 carbon atoms, Ar is selected from mono- or polycyclic aryl and mono- or polycyclic hetaryl; X1, X2, X3, X4 are CH, C—Rx or N, provided that in each ring at most two of X1, X2, X3, X4 are N; Rx is e.g. halogen, CN or CH═CH2. The invention also relates to thermoplastic resins comprising a polymerized unit of the compound of formula (I).