A significant part of our understanding of the excited-state properties of polythiophenes comes from studies of smaller oligothiophenes, which have a better-defined structure. Among them, terthiophene (3T) was reported to have an excitation-wavelength dependent triplet quantum yield, Φ_T. This was explained by the opening of a second intersystem crossing (ISC) pathway upon the high-energy excitation of distorted molecules. Here, we reinvestigate the excited-state dynamics of 3T in solvents of various viscosity and in a polymer film. Our results reveal that, although different subpopulations are excited upon high- and low-energy irradiation, the ISC dynamics, and hence Φ_T, are the same. We show that the distorted molecules excited at short wavelength undergo rapid planarization independent of the viscosity of the environment before significant ISC takes place. The apparent increase of Φ_T with increasing excitation wavelength reported earlier can be explained by the neglect of the early relaxation dynamics.

Although electron donor–acceptor complexes have been known for more than 70 years and are increasingly used in various applications, very little is still known about their structure in liquids. Here, we investigate the excited-state dynamics of a complex with two charge-transfer (CT) bands, which are usually discussed in terms of two distinct geometries, opening the possibility for photoselection. Apart from an initial ultrafast internal conversion to the lowest CT state upon high-energy band excitation, the ensuing dynamics do not depend on which CT transition has been excited, suggesting complexes of similar structures. The pure ground-state bleach dynamics, extracted using polarized transient absorption measurements, does not exhibit any hole-burning effect and is independent of the excitation wavelength, indicating an absence of photoselection. These results are rationalized using molecular dynamics simulations, which point to a broad distribution of structures with a significant oscillator strength for both transitions, contrary to the generally accepted picture.

Boramidine is a small water-soluble organic fluorophore that was recently introduced as a versatile building block of fluorescent probes. Herein, we show that boramidine is protonated in highly protic solvents. This behaviour explains the surprisingly large difference in the absorption spectrum reported previously when going from an organic to an aqueous environment. Transient absorption measurements reveal that the invariance of the fluorescence spectrum to the environment arises from an excited-state proton transfer to the solvent occurring a few ps after photoexcitation of the protonated boramidine. This photoacidity of boramidine is a further add-on to the polyvalence of this fluorophore.

Understanding how electronic energy is funnelled towards a specific location in a large conjugated molecule is of primary importance for the development of a site-specific photochemistry. To this end, we investigate here how electronic excitation redistributes spatially in a series of electron donor-acceptor (D-A) molecules containing two different donors, D and D', and organised in both linear D-A-D' and symmetric double-branch D'-A-D-A-D' geometries. Using transient IR absorption spectroscopy to probe the alkyne spacers, we show that for both types of systems in non-polar solvents, excitation remains delocalised over the whole molecule. In polar media, charge-transfer (CT) exciton in the linear D-A-D' systems localises rapidly at the end with the strongest donor. For the double-branch systems, excited-state symmetry breaking occurs and the CT exciton localises at the end of one of the two branches, even if the D' terminal donor is not the strongest one. This unexpected behaviour is explained by considering that the energy of a CT state depends not only on the electron donating and withdrawing properties of the donor and acceptor constituents, but also on the solvation energy. This study demonstrates the possibility to control the location of CT excitons in large conjugated systems by varying the nature of the donors and acceptors, the distance between them as well as the environment.

Boramidines are promising chromophores capable of circularly polarized luminescence (CPL). The synthesis, characterization, and photophysical analysis of novel BINOL- and H8-BINOL-tethered boramidines 1 and 2 are reported, leveraging the chiral perturbation strategy for CPL material design. These enantiopure compounds, prepared in a concise three-step synthesis, exhibit high fluorescence quantum yields (up to 95% in N2-saturated solutions) and luminescence dissymmetry factors (|glum| ~10-3). Transient absorption spectroscopy and quantum-chemical calculations provide insight into their singlet-triplet spin-orbit coupling and intersystem crossing mechanisms.