Vibrational relaxation by methylated xanthines in solution: Insights from 2D IR spectroscopy and calculations.

Two-dimensional infrared (2D IR) spectroscopy, infrared pump-infrared probe spectroscopy, and density functional theory calculations were used to study vibrational relaxation by ring and carbonyl stretching modes in a series of methylated xanthine derivatives in acetonitrile and deuterium oxide (heavy water). Isotropic signals from the excited symmetric and asymmetric carbonyl stretch modes decay biexponentially in both solvents. Coherent energy transfer between the symmetric and asymmetric carbonyl stretching modes gives rise to a quantum beat in the time-dependent anisotropy signals. The damping time of the coherent oscillation agrees with the fast decay component of the carbonyl bleach recovery signals, indicating that this time constant reflects intramolecular vibrational redistribution (IVR) to other solute modes. Despite their similar frequencies, the excited ring modes decay monoexponentially with a time constant that matches the slow decay component of the carbonyl modes. The slow decay times, which are faster in heavy water than in acetonitrile, approximately match the ones observed in previous UV pump-IR probe measurements on the same compounds. The slow component is assigned to intermolecular energy transfer to solvent bath modes from low-frequency solute modes, which are populated by IVR and are anharmonically coupled to the carbonyl and ring stretch modes. 2D IR measurements indicate that the carbonyl stretching modes are weakly coupled to the delocalized ring modes, resulting in slow exchange that cannot explain the common solvent-dependence. IVR is suggested to occur at different rates for the carbonyl vs ring modes due to differences in mode-specific couplings and not to differences in the density of accessible states.

Probing the heterogeneous structure of eumelanin using ultrafast vibrational fingerprinting.

Eumelanin is a brown-black biological pigment with sunscreen and radical scavenging functions important to numerous organisms. Eumelanin is also a promising redox-active material for energy conversion and storage, but the chemical structures present in this heterogeneous pigment remain unknown, limiting understanding of the properties of its light-responsive subunits. Here, we introduce an ultrafast vibrational fingerprinting approach for probing the structure and interactions of chromophores in heterogeneous materials like eumelanin. Specifically, transient vibrational spectra in the double-bond stretching region are recorded for subsets of electronic chromophores photoselected by an ultrafast excitation pulse tuned through the UV-visible spectrum. All subsets show a common vibrational fingerprint, indicating that the diverse electronic absorbers in eumelanin, regardless of transition energy, contain the same distribution of IR-active functional groups. Aggregation of chromophores diverse in oxidation state is the key structural property underlying the universal, ultrafast deactivation behavior of eumelanin in response to photoexcitation with any wavelength.

Effects of Intra- and Intermolecular Hydrogen Bonding on O-H Bond Photodissociation Pathways of a Catechol Derivative.

The catechol functional motif is thought to play both a structural and photochemical role in the ubiquitous natural pigment, eumelanin. Intramolecular and intermolecular hydrogen bonding interactions lead to a variety of geometries involving the two O-H groups in catechol, but its photophysical behavior in these situations has not been comprehensively characterized. Toward this end, we monitor the UV-induced O-H bond photodissociation reaction in an exemplar catechol derivative, 4- tert-butylcatechol, possessing different intramolecular and intermolecular hydrogen bonding geometries using femtosecond transient absorption spectroscopy measurements in the UV-visible and mid-infrared regions following 265 nm photoexcitation. Three different hydrogen bonding arrangements are obtained by tuning solution complexation equilibria of the catechol with the hydrogen bond acceptor, diethyl ether (Et2O), and are verified computationally. We find that intermolecular hydrogen bonding to the free O-H group in catechol increases its first excited singlet state (S1) lifetime by 2 orders of magnitude (i.e., ∼ 16 to 1410 ps), and that O-H bond dissociation is prevented because Et2O is a poor hydrogen atom acceptor. Complexation of both O-H groups with multiple Et2O molecules further elongates the S1 lifetime to 1670 ps due to shifting of the solution equilibria that describe complex formation. Weakening of the characteristic, intramolecular hydrogen bond of the catechol derivative by intermolecular hydrogen bonding to one or more Et2O molecules does not enhance the rate of O-H bond dissociation.