A compound of formula (I):




wherein: each X is independently O or S; R1 in each occurrence is independently H a substituent; R2 in each occurrence is independently a substituent; B in each occurrence is independently a bridging unit; n in each occurrence is independently 0 or 1; A1 in each occurrence is independently an electron-accepting group; Z is a direct bond or, together with R1, forms a monocyclic or fused ring; and the compound of formula (I) is substituted with at least one substituent of formula (II):


-(L)p-[(OCH2CH2)r-R3]q   (II)

wherein: L is a linking group; p in each occurrence is independently is 0 or 1; r in each occurrence is independently an integer from 1 to 10; R3 is H or a substituent; and q is 1 if p is 0 and q is at least 1 if p is 1.