Diblock copolymers, consisting of a photoaddressable mesogen-containing dispersed phase and a polystyrene matrix, have been synthesized and characterized as holographic data storage materials, The photoaddressable phase contains p-methoxy-substituted azobenzene side groups and benzoylbiphenyl mesogenic side groups in a statistical distribution. Three-ring mesogenic groups in combination with photoaddressable azobenzene side groups have been introduced for the first time in block copolymers in order to increase the local refractive-index difference between illuminated and non-illuminated volume elements, as well as to improve the stability of the orientation, while, at the same time, decreasing the optical density. To this end, a series of block copolymers with different azobenzene/mesogen ratios have been synthesized. The light-induced reorientation dynamics of the chromophores and mesogens and the long-term stability of the orientation have been studied with volume holographic measurements. A remarkable increase in stability of the holographic gratings has been achieved.