Artificial light-harvesting systems in aqueous media which mimic nature are of significant importance; however, they are often restrained by the solubility and the undesired aggregation-caused quenching effect of the hydrophobic chromophores. Here, we report a generalized strategy toward the construction of efficient artificial light-harvesting systems based on supramolecular peptide nanotubes in water. By molecularly aligning the hydrophobic chromophores along the nanotubes in a slipped manner, an artificial light-harvesting system with a two-step sequential Förster resonance energy transfer process is successfully fabricated, showing an energy transfer efficiency up to 95% and a remarkably high fluorescence quantum yield of 30%, along with high stability. Furthermore, the spectral emission could be continuously tuned from blue through green to orange, as well as outputted as a white light continuum with a fluorescence quantum yield of 29.9%. Our findings provide a versatile approach of designing efficient artificial light-harvesting systems and constructing highly emissive organic materials in aqueous media.

Ultrafast pump–probe spectroscopies and computational chemistry unravel the excited state photophysics responsible for the photostability of molecules in natural and commercial sunscreens.

In an effort to illuminate why nature has chosen a particular set of bio-molecular ‘building-blocks’ for life, recent years have seen an up-surge of gas-phase spectroscopy experiments aimed at understanding why the DNA/RNA nucleobases, aromatic amino acids and their corresponding chromophore subunits, exhibit a resistance to photochemical damage (photostability) following the absorption of ultraviolet (UV) radiation. The research considered in this Perspective article specifically focuses on the role of electronically excited dissociative ¹πσ* states (formed through the photo-induced promotion of an electron in a σ* ← π molecular orbital transition), which have been implicated to contribute to this photostable behaviour. In particular, we review the application of gas-phase femtosecond pump-probe spectroscopies to gain insights into ¹πσ* state driven relaxation dynamics in a number of heteroaromatic biomolecules and their UV chromophore subunits. This Perspective article also discusses how the information obtained from these studies can be used as a ‘stepping-stone’ for extending this research to larger, more complex biomolecules and ultimately, more realistic systems in solution.

The heteroaromatic ultraviolet chromophore pyrrole is found as a subunit in a number of important biomolecules: it is present in heme, the non-protein component of hemoglobin, and in the amino acid tryptophan. To date there have been several experimental studies, in both the time- and frequency-domains, which have interrogated the excited state dynamics of pyrrole. In this work, we specifically aim to unravel any differences in the H-atom elimination dynamics from pyrrole across an excitation wavelength range of 250–200 nm, which encompasses: (i) direct excitation to the (formally electric dipole forbidden) 1¹πσ* (¹A₂) state; and (ii) initial photoexcitation to the higher energy ¹ππ* (¹B₂) state. This is achieved by using a combination of ultrafast time-resolved ion yield and time-resolved velocity map ion imaging techniques in the gas phase. Following direct excitation to 1¹πσ* (¹A₂) at 250 nm, we observe a single time-constant of 126 ± 28 fs for N–H bond fission. We assign this to tunnelling out of the quasi-bound 3s Rydberg component of the 1¹πσ* (¹A₂) surface in the vertical Franck–Condon region, followed by non-adiabatic coupling through a 1¹πσ*/S₀ conical intersection to yield pyrrolyl radicals in their electronic ground state (C₄H₄N()) together with H-atoms. At 238 nm, direct excitation to, and N–H dissociation along, the 1¹πσ* (¹A₂) surface is observed to occur with a time-constant of 46 ± 22 fs. Upon initial population of the ¹ππ* (¹B₂) state at 200 nm, a rapid ¹ππ* (¹B₂) → 1¹πσ* (¹A₂) → N–H fission process takes place within 52 ± 12 fs. In addition to ultrafast N–H bond cleavage at 200 nm, we also observe the onset of statistical unimolecular H-atom elimination from vibrationally hot S₀ ground state species, formed after the relaxation of excited electronic states, with a time-constant of 1.0 ± 0.4 ns. Analogous measurements on pyrrole-d₁ reveal that these statistical H-atoms are released only through C–H bond cleavage.