Proton Transfer and Photoluminescence Intermittency of Single Emitters in Dyed Crystals

The role of proton transfer in the photoluminescence
intermittency
(PI) of single molecules of violamine R (VR) overgrown in potassium
acid phthalate (KAP) crystals is evaluated in comparisons of protonated
(KAP) and deuterated (DKAP) mixed crystals between 23 and 60 °C.
The PI is analyzed by the construction of cumulative distribution
functions that are statistically compared. We find that the on- and off-interval duration distributions
change with isotopic substitution consistent with proton transfer
contributing to the PI of VR. The on- and off-interval duration distributions have distinct temperature
dependencies consistent with different mechanisms for dark state production
and decay. Additional evidence for proton-transfer is provided by
distributions of single molecule emission-energy maxima that reflect
emission from protonated and deprotonated VR. A mechanism for the
PI of KAP is presented, where the dark state is assigned to formation
of the colorless, leuco form of VR, formed by proton transfer from
VR to the KAP lattice, and decay of the dark state involves ring-opening
promoted by proton transfer from KAP to VR. The distributed kinetics
for dark-state production and decay are modeled using a log-normal
distribution for the PI data in preference to a power-law previously
assumed. A discussion of the log-normal distribution with regards
to PI and proton transfer is presented.