Laser desorption/ionization mass spectrometry, attenuated total reflection-Fourier transform infrared, and terahertz photoconductivity analyses reveal that the photodegraded ET signal originates from the N3 dye photodegradation product as bi-isonicotinic acid (4,4'-dicarboxylic acid-2,2'-bipyridine), which remains bonded to the TiO<sub>2</sub> surface via either bidentate chelation or bridging-type geometry.