p-tert-Buty lca lix[4]a rene-l i n ked t ris bi pyridy l rut heni u m (11) com plexes 4a and 4b show I u m i nescent pH sensor act ion, in 50% (v/v) aqueous methanol, by operation of an intramolecular photoinduced electron transfer (PET) process.Calixarenes and their derivatives have been extensively studied as hosts for both neutral and ionic species1,2 and calixarene-based chromionophores3 for Li+ , Na+, K+ and Ca*+, and fluorionophores4~5 for Na+ and Li+ have been prepared and shown to act as sensors. In addition to a calixarene-derived fluorescent sensor for neutral molecules6 and an electrochemical sensor for anions,7 luminescent lanthanide ion binding calixarenes are also known.8 We have recently reported pH PET9 sensors based on aminomethyltetraphenylporphyrin-tin(1v)lO and aminomethylbipyridylruthenium(r1) complexes. 11 We now report the first example of a calixarene-based pH (PET) sensor.The trisbipyridylruthenium(11) moiety was chosen as the luminophore with the free phenolic units of a calix[4]arene acting as the acid-base sites. These two units were coupled by a methylene spacer both to maximise the electron transfer rate HO 1 3a; R' = CH2bpy, R2 = Pr" b; R' = R2 = CHabpy Me Me 5