An environment-sensitive fluorophore can change its maximum emission wavelength (λem), fluorescence quantum yield (Φf), and fluorescence lifetime in response to the surrounding environment. We have developed two new intramolecular charge-transfer-type environment-sensitive fluorophores, DBThD-IA and DBSeD-IA, in which the oxygen atom of a well-established 2,1,3-benzoxadiazole environment-sensitive fluorophore, DBD-IA, has been replaced by a sulfur and selenium atom, respectively. DBThD-IA is highly fluorescent in n-hexane (Φf=0.81, λem=537 nm) with excitation at 449 nm, but is almost nonfluorescent in water (Φf=0.037, λem=616 nm), similarly to DBD-IA (Φf=0.91, λem=520 nm in n-hexane; Φf=0.027, λem=616 nm in water). A similar variation in fluorescence properties was also observed for DBSeD-IA (Φf=0.24, λem=591 nm in n-hexane; Φf=0.0046, λem=672 nm in water). An intensive study of the solvent effects on the fluorescence properties of these fluorophores revealed that both the polarity of the environment and hydrogen bonding with solvent molecules accelerate the nonradiative relaxation of the excited fluorophores. Time-resolved optoacoustic and phosphorescence measurements clarified that both intersystem crossing and internal conversion are involved in the nonradiative relaxation processes of DBThD-IA and DBSeD-IA. In addition, DBThD-IA exhibits a 10-fold higher photostability in aqueous solution than the original fluorophore DBD-IA, which allowed us to create a new robust molecular nanogel thermometer for intracellular thermometry.