Effect of cyano-addition on the photoacidity switch in 5-cyano-8-amino-2-naphthol

Cyano- or CN-additions are often utilized in the design of photoacids to enhance and/or enable excited state proton transfer (ESPT) from the protic site to aqueous and nonaqueous solvents. In diprotic photoacid 8-amino-2-naphthol (8N2OH), the protonation state of the amino group (NH₃⁺/NH₂) acts as an on-off switch for ESPT at the OH site in water. This study investigated whether the addition of CN in 5-cyano-8-amino-2-naphthol (5CN8) could override this switch and promote new ESPT pathways. Analysis of the steady-state and time-resolved emission data showed that in the presence of protonated NH₃⁺, CN enhances OH photoacidity (vs. in 8N2OH) and activates the ESPT pathway at NH₃⁺. Both protic sites, OH and NH₃⁺, can also donate a proton to methanol upon excitation. In contrast, in the presence of deprotonated NH₂, despite the addition of CN, ESPT is still not observed at the OH site for 5CN8. Thus, the addition of CN cannot override or negate the inhibiting effect of NH₂ on OH photoacidity. Potential causes for this inhibition are discussed, including electronic and antiaromaticity effects of CN and NH₂ substitution.