Generation of singlet oxygen by irradiation
Rose bengal (pictured below) is a member of the fluorescein family.


(image source: wikipedia)

There are 3 steps in the process where rose bengal sensitizes the formation of singlet oxygen..

\begin{gather}
\ce{RB -&gt;[$h\nu$]\  ^1RB}\tag1\\
\ce{^1RB -&gt;T[ISC]\  ^3RB}\tag2\\
\ce{^3RB + ^3O2 -&gt;T[sensitization]\  ^1RB + ^1O2}\tag3\\
\end{gather}

In the first step, ground state rose bengal (a ground state singlet) absorbs light producing an excited singlet state.  Because of the heavy iodine atoms in the molecule intersystem crossing (ISC) to the excited triplet state rapidly occurs (heavy atom effect). In the presence of ground state triplet dioxygen, a process called "sensitization" occurs. In this step the excited triplet rose bengal transfers its energy to the dioxygen promoting it to the excited singlet state while the rose bengal is deactivated and returns to its single ground state.  In this sensitization step, 2 triplets are converted to 2 singlets so, importantly, spin is conserved.