pH-Dependent absorption spectra of aqueous 4-imidazolecarboxaldehyde: theoretical and experimental insights into a marine chromophore proxy

The sea surface microlayer (SSML) contains light-absorbing organic chromophores known to initiate daytime aqueous-phase chemistry in the environment. These compounds, referred to as marine chromophoric dissolved organic matter (m-CDOM), are known to partition from the sea surface into sea spray aerosol (SSA), where they undergo substantial changes in acidity, transitioning from the slightly basic SSML to the more acidic SSA. Recent studies suggest that the photosensitizing efficiency of m-CDOM is pH-dependent, but its chemical complexity hinders direct molecular-level investigation. Because m-CDOM contains more nitrogen than its terrestrial counterpart, 4-imidazolecarboxaldehyde (4IC)-a nitrogen-containing chromophore and known photosensitizer-has served as an effective model system form m-CDOM. Yet, little is known about its p<i>K</i>_a values or how its optical properties respond to the variable pH conditions characteristic of the marine atmosphere. Understanding the pH-dependent behavior of 4IC is essential for modeling the photochemical behavior of nitrogen-rich marine chromophores. Here, we characterize the speciation and optical properties of 4IC across a wide pH range, reporting its p<i>K</i>_a and the pH effect in its absorption features. The diol-aldehyde equilibrium of 4IC, along with the optical properties of the species involved, was also investigated in the context of daytime chemistry in the marine environment. Using theoretical and experimental techniques, we show that the protonation state of 4IC significantly affects its electronic transitions. Under acidic conditions typical of submicron SSA, 4IC exhibits low-energy absorption bands that overlap with the solar spectrum, with more intense high-energy features extending into the actinic region. As pH increases to values characteristic of the SSML, the more intense band redshifts towards the solar spectrum. Theoretical results closely match experimental trends, reproducing the redshift and accurately predicting the energies of key transitions.