fluorescent dyes and probes are key components in multiphoton based fluorescence microscopy imaging of biological samples . in order to address the demand for better performing dyes for two - photon based imaging , a new series of reactive fluorophores tailored for multiphoton imaging has been disclosed . these fluorophores are based upon the fluorene ring system , known to exhibit high fluorescence quantum yields , typically & gt ; 0 . 7 , and possess high photostability . they have been functionalized with moieties to act , e . g ., as efficient amine - reactive fluorescent probes for the covalent attachment onto , e . g ., proteins and antibodies . the synthesis and the single - photon spectral characteristics , as well as measured two - photon absorption cross sections of the reactive fluorophores in solution are presented . spectral characterizations of bovine serum albumin conjugated with the new reactive probe is presented .