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Photoexcitation is well-known to trigger electronic metastable states andlead to phenomena like long-lived photoluminescence and photoconductivity. Incontrast, persistent photo-response due to ionic metastable states is rare. Inthis work, we report persistent structural and ferroelectric photo-responsesdue to proton metastable states via a nuclear quantum mechanism inferroelectric croconic acid, in which the proton-transfer origin offerroelectricity is important for the ionic metastable states. We show that,after photoexcitation, the changes of structural and ferroelectric propertiesrelax in about 1000 s, while the photoconductivity decays within 1 s,indicating the dominant ionic origin of the responses. The photogeneratedinternal bias field that survives polarization switching process suggestsanother proton transfer route and metastable state, in addition to themetastable states resulting from proton transfer along the hydrogen bondsproposed previously. Analysis based on Frank Condon principle reveals thequantum mechanical nature of the proton-transfer process both within thehydrogen bonds and out of the hydrogen bonds, where small mass of proton andsignificant change of potential landscape due to the excited electronic statesare the key. The demonstration of persistent photo-responses due to the protonmetastable states unveils a nuclear quantum mechanism for photo-tunability ofmaterials, which is expected to impact many material properties sensitive toionic positions.

Persistent Ionic Photo-responses and Frank-Condon Mechanism in Proton-transfer Ferroelectrics