The radical anions of perylene diimides (PDI˙-*) are increasingly used photocatalysts in consecutive photoinduced electron transfer (conPET) for aryl-halide reduction. Despite this, the mechanism behind this reduction by PDI˙-* remains unclear since no dynamic quenching was observed for aryl halides. Here, we combine stationary and ultrafast transient absorption spectroscopy from the UV to the near IR to reinvestigate the photoreactivity of PDI˙- in the presence of bromoacetophenone (BAP). Independently of the method used to generate PDI˙-, the solvent, and the excitation wavelength, a prompt decrease of the excited-state population without any change in lifetime is observed upon addition of BAP. This effect coincides with a similar decrease of the stationary fluorescence intensity of PDI˙-, pointing to the occurrence of static quenching by BAP. As no photoproduct is detected, this static quenching is associated with a sub-100 fs photoinduced electron transfer, followed by ultrafast recombination by back electron transfer. It does not involve any pre-association of the reactants in the ground state but is simply due to the significant probability of a BAP molecule to be at an adequate distance and orientation relative to PDI˙-* for electron transfer to occur without diffusion. Using molecular dynamics simulations, we show that such condition is easily fulfilled at high quencher concentrations. Despite this, the overall quantum yield of the conPET mechanism is vanishingly small, because of several fundamental limitations related to the two photoinduced electron transfer steps and to the use of organic radical ions as photocatalysts.

Localisation of the electronic excitation via excited-state symmetry breaking (ESSB) is a characteristic property of many quadrupolar dyes in polar environments, and was shown to depend on the nature of the electron donor (D) and acceptor (A) subunits and their separation distance. Here, we compare the excited-state properties of two centrosymmetic D-π-A-π-D dyes with a dipyrrolonaphthyridinedione (DPND) acceptor and N,N-dimethylaniline donors, which only differ by the position of the -π-D arms on the DPND core. Time-resolved IR spectroscopy reveals that ESSB and hence exciton trapping on single A-π-D branch is facilitated with the donor arms in 1 and 7 positions on the DPND core compared to 3 and 9 positions, given that it occurs in a medium polar solvent for the former and only in highly polar media for the latter. This a priori unexpected difference is explained by the significantly larger dipole moment generated upon ESSB for the 1,7 isomer, as reflected by its much larger fluorescence solvatochromism compared to the other isomer. In this respect, the DPND core with its C2h symmetry allows for a much finer tuning of the excited-state properties of quadrupolar dyes than most previously used D or A cores of higher symmetry.

Herein, we reinvestigate the photophysics of ovalene, a prototypical nanographene for which conflicting spectroscopic results have been reported. Owing to its structural similarity and its identical D2h point-group symmetry, ovalene can essentially be viewed as a larger pyrene. We show that its optical transitions can be understood using the same model that is invoked to explain the excited states of pyrene. Absorption and (polarized)-emission measurements reveal that the S1 ← S0 (1B3u ← 1Ag) transition is forbidden, whereas the first prominent absorption band can be assigned to the allowed S2 ← S0 (1B2u ← 1Ag) transition, in contrast to recent reassignments. Temperature and time-dependent spectroscopic measurements show that the S1 and S2 states quickly establish a thermal pre-equilibrium, giving rise to thermally activated S2 → S0 emission at room-temperature. As a result, the fluorescence lifetime of ovalene decreases with increasing temperature while its fluorescence quantum yield increases. Contrary to the frequently cited small energy gap of ∼400 cm-1, our measurements reveal a significantly larger S2-S1 gap of approximately 1200 cm-1.

Ionic liquids are increasingly discussed as alternatives to conventional organic solvents for applications based on photoinduced electron transfer. For the rational design of such applications, reliable estimates of electron-transfer driving forces are essential. Based on the Born model of solvation, the moderate dielectric constants of ionic liquids (εr ≈ 8 - 15) suggest that they should resemble medium-polarity solvents such as dichloromethane or pyridine in photoinduced electron transfer and exhibit comparable solvation energies. Here, we test this assumption by experimentally comparing the solvation energies of three small organic solutes relevant to photochemistry in several imidazolium-based ionic liquids and in conventional dipolar solvents. Solvation energies were inferred from shifts of half-wave reduction potentials obtained from cyclic voltammetry. We find that, for the investigated solutes, ionic liquids provide solvation energies comparable to those of strongly polar solvents such as acetonitrile or dimethyl sulfoxide. While the organic solvents follow the qualitative trend predicted by the Born equation, ionic liquids deviate from it and yield much larger solvation energies compared to dipolar solvents of the same dielectric constant. This behavior is attributed to the intrinsically high ionic strength of ionic liquids, which enhances electrostatic screening and results in substantially larger solvation energies and consequently much larger driving forces for photoinduced electron transfer than would be expected based on their dielectric constants alone.

Photoinduced symmetry-breaking charge separation (SB-CS) between two identical molecules has been reported with an increasing number of chromophores, but is sometimes difficult to detect when reversible. Using a perylenediimide-based molecular cage, we demonstrate that the combination of electronic transient absorption and time-resolved fluorescence enables the detection of SB-CS even when no clear spectral signature of the ionic product is visible in the transient spectra. This is possible thanks to the presence of the easily observable delayed fluorescence and the global target analysis of both transient absorption and time-resolved fluorescence data using a modified exciplex model. We found that SB-CS in this cage occurs reversibly in a broad range of solvent polarities, even in highly polar media. On the basis of molecular dynamics simulations, the weak driving force of SB-CS is explained by the limited mobility of the solvent molecules inside the cage, and the reduced solvation energy of the charge-separated state that results.

Abstract Proton-coupled electron transfer (PCET) is foundational to catalysis, bioenergetics, and energy conversion, yet directly observing the interplay between electronic redistribution, protonation, and solvent reorganization remains challenging. We combine femtosecond optical spectroscopy, ultrafast N K-edge X-ray absorption spectroscopy, and time-resolved X-ray solution scattering to capture the steps of a sequential PCET reaction in water with atomic-site specificity. Using a ruthenium polypyridyl model complex, we resolve the electron redistribution upon photoinduced metal-to-ligand charge transfer and subsequent ( ~ 460 ps) protonation at a ligand nitrogen, as well as the concomitant rearrangement of the first-solvation-shell. Combined with advanced electronic structure and molecular dynamics simulations, our measurements reveal a marked localization of the excited-state electron density at the protonated N site, together with a switch from N···HO to NH···O hydrogen-bonds. These results establish a multimodal X-ray framework for mechanistic insight into PCET and its control in catalysis, artificial photosynthesis, and biological energy flow.

In electron donor-acceptor (D-A) molecules, the relative orientation of constituents has a dramatic influence over their performance. However, the D and A subunits are generally composed of planar aromatic backbones, and the effect of curvature is rarely explored. Here, we investigate how the twist of the aromatic core of a symmetric double-branched D-π-A molecule affects the nature and dynamics of its lower singlet excited state. We show that the twisting of the central donor not only affects the chiroptical properties, and increases the triplet yield, but also facilitates excited-state symmetry breaking (ESSB) and the trapping of the exciton on one D-π-A branch of the molecule. This enhancement is attributed to the decrease in the interbranch coupling upon distortion. Because of this, the loss of the coupling upon ESSB requires a smaller gain in solvation energy to be compensated for and, thus, exciton trapping occurs in a less polar solvent. Consequently, distortion can be viewed as an additional tuning knob for controlling the localisation of electronic excitation in large conjugated systems.

Electron donor-acceptor (D-A) polymers are emerging as promising candidates for the development of solid materials with tunable emission. Herein, we investigate the excited-state dynamics of polymers consisting of a central naphthalenediimide (NDI) acceptor with two polystyrene donor chains and copolymers with various secondary donors incorporated. We find strong differences in the dynamics when going from diluted polymer solutions to pure polymer films. In liquids, ultrafast intrachain electron transfer from a styrenic donor to the excited NDI, followed by sub-nanosecond charge recombination to the ground state is observed. Because of the tight packing in the film, ultrafast electron transfer occurs between donors and acceptors of different polymer chains. Emission is found to originate from the most strongly coupled D-A pairs, for which electron transfer is so fast that it leads to a lifetime broadening of the NDI absorption band. Because of this, these highly coupled pairs can be photoselected upon red-edge excitation. The charge-transfer state decays on the tens of ns timescale via radiative and non-radiative charge recombination to the ground state as well as via charge recombination to the triplet state of NDI. This latter pathway, which is detrimental to the fluorescence quantum yield, is almost suppressed with the strongest secondary donor. Finally, we show that excitation of the secondary donor instead of the NDI acceptor does not lead to the population of the charge-transfer state and thus does not contribute to the luminescence of the films.

The excited radical anion of perylene diimide (PDI) dyes is broadly used in thermodynamically challenging synthetic reactions via a consecutive photoinduced electron transfer (ConPET) mechanism. This approach relies on two sequential single-photon absorptions, in which the first triggers photoinduced electron transfer (PET) while the second populates the doublet excited state of the ensuing radical anion. Herein, we employ pump-pump-probe spectroscopy to investigate all of the elementary steps of this mechanism. We show that radical anion formation upon direct quenching of photogenerated 1PDI* is an inefficient strategy due to ultrafast charge recombination of the generated geminate ion pair (GIP). Alternatively, we demonstrate that 3PDI* can be populated via triplet charge recombination with a halogenated electron donor and, from there, it can be quenched with a second donor, generating a triplet GIP and rendering charge recombination spin-forbidden to efficiently accumulate PDI• - in solution. This anion can then be selectively excited by a second laser pump pulse, potentially triggering a catalytic reaction.

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.

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, ΦT. 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 ΦT, 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 ΦT with increasing excitation wavelength reported earlier can be explained by the neglect of the early relaxation dynamics.

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 ϕ (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.

Proton-coupled electron transfer (PCET) is foundational to catalysis, bioenergetics, and energy conversion, yet capturing and disentangling the coupled motions of electrons, protons, and solvent has remained a major experimental challenge. We combine femtosecond optical spectroscopy, site-specific ultrafast soft X-ray absorption spectroscopy, and time-resolved X-ray scattering with advanced calculations to disentangle the elementary steps of PCET in solution. Using a ruthenium polypyridyl model complex, we directly resolve photoinduced electron redistribution, ligand-site protonation within 100 ps, and the accompanying solvent reorganization. This unified multi-modal approach provides an orbital-level, atomistic picture of PCET, showing how electronic, nuclear, and solvation degrees of freedom can be separated experimentally. Our results establish a general X-ray framework for understanding and ultimately controlling PCET in catalysis, artificial photosynthesis, and biological energy flow.

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.

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.

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.

So far, symmetry-breaking charge separation (SB-CS) has been observed with a limited number of chromophores and is usually inhibited by the formation of an excimer. We show here that thanks to fine-tuning of the interchromophore coupling via structural control, SB-CS can be operative with pyrene, despite its high propensity to form an excimer. This is realized with a bichromophoric system consisting of two pyrenes attached to a crown ether macrocycle, which can bind cations of different sizes. By combining stationary and time-resolved spectroscopy together with molecular dynamics simulations, we demonstrate that the excited-state dynamics can be totally changed depending on the binding cation. Whereas strong coupling leads to rapid excimer formation, too weak coupling results in noninteracting chromophores. However, intermediate coupling, achieved upon binding of Mg2+, allows for SB-CS to be operative.

Fluorescent flippers have been introduced as small-molecule probes to image membrane tension in living systems. This study describes the design, synthesis, spectroscopic and imaging properties of flippers that are elongated by one and two alkynes inserted between the push and the pull dithienothiophene domains. The resulting mechanophores combine characteristics of flippers, reporting on physical compression in the ground state, and molecular rotors, reporting on torsional motion in the excited state, to take their photophysics to new level of sophistication. Intensity ratios in broadened excitation bands from differently twisted conformers of core-alkynylated flippers thus report on mechanical compression. Lifetime boosts from ultrafast excited-state planarization and lifetime drops from competitive intersystem crossing into triplet states report on viscosity. In standard lipid bilayer membranes, core-alkynylated flippers are too long for one leaflet and tilt or extend into disordered interleaflet space, which preserves rotor-like torsional disorder and thus weak, blue-shifted fluorescence. Flipper-like planarization occurs only in highly ordered membranes of matching leaflet thickness, where they light up and selectively report on these thick membranes with red-shifted, sharpened excitation maxima, high intensity and long lifetime.