Novel Excited Quintet State in Porphyrin: Bis(quinoline−TEMPO)−yttrium−tetraphenylporphine Complex

New mono- and bis[4-(3-hydroxy-2-methyl-4-quinolinoyloxy)-2,2,6,6-tetramethylpiperidin-1-oxyl](meso-tetraphenylporphyrinato)yttrium(III) complexes have been synthesized, and the properties of the
excited states generated by photoexcitation of porphyrin were
studied by time-resolved (TR) and pulsed two-dimensional electron
paramagnetic resonance (EPR) spectroscopy. A TR-EPR spectrum
was observed in the quartet (S = 3/2) or quintet (S = 2) states
generated from interactions of one or two radicals with the
photoexcited triplet state of the porphyrin. The zero-field splitting
D values of these states were analyzed in terms of those of the
triplet and the radical-triplet pair. The spin states of the excited
states were definitely assigned by measuring the nutation frequencies with pulsed EPR.