The present invention relates to a compound having the general formula (1) wherein in the general formula (1) the residues R1 to R8 and Ar are the same or different and are independently from each other selected from the group consisting of hydrogen, unsubstituted hydrocarbon groups, substituted hydrocarbon groups and inorganic groups, wherein at least one of the residues R1 to R8 and Ar is a hydrophilic group, such as for instance a group comprising one or more polyethylene groups.