Ultrafast Excited-State Intermolecular Proton Transfer in Indigo Oligomer

It has been a long-lasting debate whether indigo undergoes excited-state proton transfer and how this contributes to its photostability. A prevailing point of view is that a sub-picosecond excited-state intramolecular single proton transfer occurs; however, it has been studied mostly under dilute solution conditions. In this work, excited-state structural dynamics of indigo oligomers formed at millimolar concentration in dimethyl sulfoxide is investigated using femtosecond visible pump spectroscopy, infrared and visible probe spectroscopies, and steady-state infrared and fluorescence spectroscopies. Experimental evidence indicates the presence of transient intermolecular electronic excited-state proton transfer, which is supported by quantum-chemistry computations. The formed enol species disappears with a time constant of 200-300 fs, followed by a relatively slow nonradiative relaxation to the electronic ground state. Our results reveal new photochemistry of indigo particularly in its oligomeric state.