tokens: The experiment results showed : ( i ) not only 1O2 , but also free radicals ( O2 - . . OH and YHPD - . ) can be formed by the aid of YHPD ; and ( ii ) as to the ability of producing 1O2 , YHPD less than BHPD , while for generating O2 - . and . OH , YHPD greater than BHPD .