Nitrophenols are Photoacids?

TLDR

The ultrafast dynamics of 4-nitrocatechol suggest highly-efficient proton transfer in the excited state. This process occurs within a few picoseconds in water. Interestingly, this process also occurs in 2-propanol, a less efficient proton acceptor. These excited-state proton transfer processes may be the reason why 4-nitrocatechol has low photochemical yields.

Motivations

Nitrophenols are strong light-absorbers in both the UV and Visible. This gives NPs the ability to absorb sunlight and undergo chemical transformations in the atmosphere.

Fig. 1. Molecule used (4NC) and simplistic Jablonski diagram

They are known to photodegrade, but the electronic states responsible for photochemical reactions are not well characterized.

Methods

Pump-probe experiments were conducted with a 340 nm pump pulse. Transient absorption was measured by varying the time delay between pump and probe pulses with a delay line. Spectra were produced via a spectrograph with a CCD detector.

Fig. 2. Experimental setup

Theoretical calculations

Fig. 3.Energy surfaces calculated with TD/TDA-PBE0/6-311+G(d)

Density functional theory calculations suggest that 4NC may reorient itself in the first excited singlet state (S₁).

Solvent-dependent dynamics

Fig. 4. Transient absorption spectra in water and 2-propanol

Timeline in Water

1.) Sub-ps molecular rearrangement (twisting)

2.) Deprotonation around 1-2 ps

3.) Relaxation of the anion

4.) Long-term build-up of anion in solution

Timeline in 2-propanol

1.) Sub-ps twisting

2.) Intersystem crossing (~0.7-2.6 ps)

3.) Triplet population maxes out around 7 ps

4.) Deprotonation happens from triplet state (~9-35 ps)

5.) Relaxation of the anion

6.) Long-term build-up of anion in solution

Excited-state proton transfer in both solvents!

Fig. 5. Transient absorption and molar extinction spectra

The transient absorption spectra at long delay times resemble the absorption by the ground-state anion 4NC−. The spectrum is less pronounced in water due to the fact the water is more likely to re-protonate 4NC− than 2-propanol.

Fig. 6. Jablonksi-like diagram summarizing observed dynamics

Although both solvents appear to allow for 4NC to undergo excited-state proton transfer, the pathways seem to differ in the two solvents.