From what I have been able to research, Pope Valentine was our 100th Pope in 827 A.D. Saint Valentine’s Feast Day was established by Pope Gelasius in 496 A.D. This means that St. Valentine is not the same person as the one hundredth Bishop of Rome.
St. Valentine, although still recognized as a Saint, was removed from the General Roman Calendar in 1969, because of the lack of reliable information about him.
Feel free to do more research. I’m interested in knowing more about Pope Valentine now. He was never declared a Saint.
The below is taken from the Catholic Encyclopedia.
" At least three different Saint Valentines, all of them martyrs, are mentioned in the early martyrologies under date of 14 February. One is described as a priest at Rome, another as bishop of Interamna (modern Terni), and these two seem both to have suffered in the second half of the third century and to have been buried on the Flaminian Way, but at different distances from the city. In William of Malmesbury’s time what was known to the ancients as the Flaminian Gate of Rome and is now the Porta del Popolo, was called the Gate of St. Valentine. The name seems to have been taken from a small church dedicated to the saint which was in the immediate neighborhood. Of both these St. Valentines some sort of Acta are preserved but they are of relatively late date and of no historical value. Of the third Saint Valentine, who suffered in Africa with a number of companions, nothing further is known."