A series of photochemically active, "axial-bonding" type phosphorus(V) porphyrin-azoarene (P-A) conjugates, viz., [(TTP)Pv(-O-AZM)(2)](+), [(TTP)P-V(-O-AZT)(2)](+), [(TTP)P-V(-O-AZB)(2)](+), and [(TTP)P-V(-O-AZN)(2)](+) (TTP is 5,10,15,20-tetra(4-methylphenyl)porphyrinato dianion and O-AZM, O-AZT, O-AZB, and O-AZN are the axially coordinated 4-methoxy azophenoxo, 4-methyl azophenoxo, azophenoxo, and 4-nitro azophenoxo ligands, respectively) have been synthesized and fully characterized by mass (FAB), infrared, UV-visible, proton nuclear magnetic resonance (1D and H-1-H-1 COSY), and electrochemical methods. The spectroscopic data suggest that the two azoarene subunits in these hexa-coordinated systems exist in their trans isomeric forms and that there is minimal ground-state interaction between the axial ligands and the basal porphyrin pi plane in each case. Continuous irradiation of CH3CN solutions containing these new P-A conjugates at 345 +/- 5 nm results in isomerization of the phosphorus-bound azoarene subunits to produce the respective cis forms, as revealed by the UV-visible spectral data. The reverse, thermal back reactions have also been spectrally monitored, and the trans forms could be recovered quantitatively. Rate measurements and detailed analysis of the kinetic data have been made for both the forward and the reverse reactions. Fluorescence due to the porphyrin components of the "trans forms" of [(TTP)P-V(-O-AZM)(2)](+), [(TTP)P-V(-O-AZT)(2)](+), and [(TTP)P-V(-O-AZB)(2)](+) are quenched in comparison with that due to a porphyrin reference compound ([(TTP)P-V(OH)(2)](+)). Moreover, fluorescence intensities of the respective "cis forms" are less than the corresponding "trans forms" in each case. The photochemical/thermal trans <----> cis isomerization reactions of the axial azoarene subunits and the consequent modulation of the fluorescence intensities have been repeated several times with minimal loss of the material. Based on the results of detailed photochemical studies carried out with these new P-A conjugates, it is argued that distance dependence of the photoinduced electron transfer occurring between the axial azoarene subunit and the singlet porphyrin is responsible for their effective and stable photoswitching function.