Excited states articles within Nature Chemistry

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  • Research Briefing |

    Trans–cis photoisomerization is a fundamental photochemical reaction that is thought to proceed through an intermediate with a perpendicular conformation. However, unambiguous identification of this state has proved challenging. The combination of state-of-the-art ultrafast spectroscopy and quantum chemical calculations now provides evidence for its structural observation in stilbene photoisomerization.

  • Article |

    The prototypical trans–cis photoisomerization of stilbenes is thought to occur via a transient intermediate with a perpendicular conformation—often called ‘the phantom state’—but its unambiguous identification has thus far proved difficult. Now, using ultrafast ultraviolet Raman spectroscopy and ab initio molecular dynamics simulation, evidence for its existence and its perpendicular conformation has been obtained.

    • Hikaru Kuramochi
    • , Takuro Tsutsumi
    •  & Tahei Tahara

  • Article |

    Exciton–exciton annihilation is conventionally assumed to be limited by diffusion. Now, using time-resolved photoluminescence microscopy to determine exciton diffusion constants and annihilation rates in two substituted perylene diimide aggregates, along with a microscopic model, it has been shown that annihilation can be suppressed through quantum interference of the spatial phase of delocalized excitons.

    • Sarath Kumar
    • , Ian S. Dunn
    •  & Libai Huang

  • Article |

    Hybrid structures made up of quantum dots functionalized with molecules are highly tunable platforms for light-driven applications; however, the interaction between their components is often weak. Now it has been shown that by connecting molecules to silicon quantum dots via p-conjugated tethers, strongly coupled exciton states can be generated that prove advantageous for photon upconversion.

    • Kefu Wang
    • , R. Peyton Cline
    •  & Ming Lee Tang

  • In Your Element |

    Rajeev K. Dubey and Frank Würthner discuss the colourful character of perylene bisimides and the impact of aggregation on their functional properties.

    • Rajeev K. Dubey
    •  & Frank Würthner

  • Article
    | Open Access

    Electronic spin influences chemistry profoundly, but its role in surface chemistry is poorly established. Now the spin-dependent reaction probabilities of oxygen atoms with a graphite surface have been studied. Molecular dynamics simulations help elucidate the mechanism for spin-flipping, which is observed to occur with low probability in surface scattering experiments.

    • Zibo Zhao
    • , Yingqi Wang
    •  & G. Barratt Park

  • Article
    | Open Access

    Noble metals dominate the field of photosensitizers and luminophores. Now, an approach incorporating cyclometalating and carbene functions into FeIII complexes has been shown to enable dual emission from the opposing ligand-to-metal and metal-to-ligand charge-transfer states. The latter shows an exceptionally long lifetime of 4.6 ns and is quenched by oxygen and other quenchers.

    • Jakob Steube
    • , Ayla Kruse
    •  & Matthias Bauer

  • Article |

    Intermolecular Coulombic decay (ICD) is a process whereby a photoexcited molecule relaxes while ionizing a neighbouring molecule. ICD is efficient when intermolecular interactions are active and consequently it is usually observed in weakly bound systems. Now, an efficient ICD is shown to occur even between unbound pyridine molecules excited at ambient-light intensities.

    • Saroj Barik
    • , Saurav Dutta
    •  & G. Aravind

  • News & Views |

    Photoredox catalysts offer a promising approach to performing reactions with high energetic requirements, however, the influence of solvent and counter ions is not fully understood. Now, a microwave-based technique is shown to give direct insight into their effects on charge reorganization during catalysis.

    • Ferdinand C. Grozema

  • In Your Element |

    The tris(2,2′-bipyridine)ruthenium(II) cation, or ‘rubipy’ to its friends, has had a significant influence on our understanding of the photophysics of transition metal complexes, and has also helped revolutionize organic photochemistry, explains Daniela M. Arias-Rotondo.

    • Daniela M. Arias-Rotondo

  • Article |

    Despite much research, the high-spin-state relaxation mechanism of Fe(II) spin-crossover complexes is unresolved. Using ultrafast circular dichroism spectroscopy it has now been revealed that the spin relaxation is driven by a torsional twisting mode, which breaks the chiral symmetry of a prototypical Fe(II) compound. Stereocontrolling the configuration of the complex can thus be used to slow down the spin relaxation.

    • Malte Oppermann
    • , Francesco Zinna
    •  & Majed Chergui

  • Article |

    Ion pairing is ubiquitous in low-dielectric-constant solvents, but whether it influences the reactivity of common cationic photoredox catalysts has been unclear. However, it has now been shown that ion pairing is responsible for a 4-fold modulation in reactivity in a prototypical Ir(III) complex and is explained by excited-state ion-pair reorganization.

    • J. D. Earley
    • , A. Zieleniewska
    •  & G. Rumbles

  • Article |

    Aromatic systems that interact non-covalently are important in many settings, such as base-pair stacking and DNA–drug interactions; however, their excited-state molecular dynamics are not fully understood. Now, intermolecular Coulombic decay in benzene dimers has been observed. The process is initiated by electron-impact ionization and proceeds through ultrafast energy transfer between the benzene molecules.

    • Xueguang Ren
    • , Jiaqi Zhou
    •  & Alexander Dorn

  • Article
    | Open Access

    The breakdown of the Born–Oppenheimer approximation is omnipresent in chemistry and detailed understanding of non-adiabatic dynamics is still incomplete. Now, the non-adiabatic quenching of electronically excited OH(A2Σ+) molecules by H2 has been investigated using full-dimensional quantum dynamics calculations and a high-quality diabatic-potential-energy matrix, providing insight into the branching ratio of the two electronic quenching channels.

    • Bin Zhao
    • , Shanyu Han
    •  & Hua Guo

  • News & Views |

    Excited anion states provide doorways for molecular electron capture. Now, two-dimensional photoelectron spectroscopy of cluster anions has been shown to be a powerful tool for revealing the role of the local environment in facilitating the process.

    • C. Annie Hart
    •  & Richard Mabbs

  • Article |

    Although electron-driven chemistry is ubiquitous, how molecular electron capture is altered by solvent remains poorly understood. Now, using anion two-dimensional photoelectron spectroscopy, it is shown that the presence of water molecules can enhance electron capture and that considering the mechanism from the perspective of the anion offers further understanding.

    • Aude Lietard
    • , Golda Mensa-Bonsu
    •  & Jan R. R. Verlet

  • Article |

    Electronic–vibrational interplay can enable electron and energy transfer processes to be regulated. Now, coherence spectroscopy has been used to disentangle two vibrational pathways that control an electron transfer reaction. It has been shown that a fast, effectively ballistic, electron transfer along one vibrational path acts like a pulse to generate a coherent wavepacket along another vibrational pathway.

    • Shahnawaz R. Rather
    • , Bo Fu
    •  & Gregory D. Scholes

  • Article |

    The role of the biexcitonic triplet-pair state 1(TT) during triplet–triplet annihilation events in singlet-fission materials has been the subject of recent debate. Now, emissive 1(TT) states have been shown to be direct products of triplet–triplet annihilation in both endothermic and exothermic singlet-fission materials.

    • David G. Bossanyi
    • , Maik Matthiesen
    •  & Jenny Clark

  • Article |

    Understanding the photophysical properties of transition-metal complexes is paramount to advances in photocatalysis, solar energy conversion and light-emitting diodes. Now, long-lived emission via thermally activated delayed fluorescence has been demonstrated from an air- and water-stable zirconium complex featuring excited states with significant ligand-to-metal charge transfer character.

    • Yu Zhang
    • , Tia S. Lee
    •  & Carsten Milsmann

  • Article |

    Controlling single-molecule magnets (SMMs) with ultrashort laser pulses could be key to future data storage devices, however, the photophysics of SMMs is complex. Now, using a monomer model system, it has been shown that optical excitation of Mn(iii)-SMMs leads to a modulation of the Jahn–Teller distortion, which is important for its magnetic anisotropy.

    • Florian Liedy
    • , Julien Eng
    •  & J. Olof Johansson

  • Article |

    Generating high-energy triplet excitons from singlet fission without excess energy loss is a critical goal for potential applications. Now it is shown that molecular chromophores that are connected covalently can harbour multiple long-lived and high-energy triplets—created from one photon—only if more than two chromophoric units are present and they have sufficient flexibility to isolate the excitations upon torsional motion.

    • Nadezhda V. Korovina
    • , Christopher H. Chang
    •  & Justin C. Johnson

  • Article |

    Although they are synthetically tunable, organic molecules that undergo singlet fission (the generation of two excitons from one photon) have not demonstrated the excited-state properties necessary to improve optoelectronic devices. Now, a general ‘energy cleft’ molecular design scheme has been demonstrated that enables rapid generation and long lifetimes of multiple triplet excitons that are for device applications.

    • Andrew B. Pun
    • , Amir Asadpoordarvish
    •  & Matthew Y. Sfeir

  • Article |

    Quantum coherence and dephasing in molecular motions determine the behaviour of many chemical reactions and are the fundamental basis for the concept of coherent control. Now, ultrafast X-ray scattering combined with a detailed structural determination analysis precisely measures the coherent vibrational motions of a polyatomic organic molecule following photoexcitation.

    • Brian Stankus
    • , Haiwang Yong
    •  & Peter M. Weber

  • News & Views |

    Light is often used to trigger reactions, energetically exciting the reactant(s) to kick them over the intrinsic reaction barrier. Now, however, the reaction between an excited atom and a charged molecule at very low temperatures has been shown not to adhere to this paradigm, instead undergoing a reaction blockading effect.

    • Roland Wester

  • Article |

    Measurements of vector correlations provide insight into the forces acting during molecular collisions, and are a stringent test of electronic-structure calculations. Now, non-intuitive dynamics of molecular collisions have been revealed by measuring the correlation between the relative velocities of the colliders and the molecular rotational angular momentum—before and after the collision—for NO(A 2Σ+) + Ne.

    • Thomas R. Sharples
    • , Joseph G. Leng
    •  & Matthew L. Costen

  • Article |

    Molecular collisions can lead to the absorption of incident light even for transitions that are spectroscopically forbidden for the isolated molecules. Now the electronic–vibrational transitions of O2 have been theoretically studied and, contrary to textbook knowledge, it is shown that the absorption mechanism and the spectral line shape depend on the collision partner, oxygen or nitrogen.

    • Tijs Karman
    • , Mark A. J. Koenis
    •  & Gerrit C. Groenenboom

  • Article |

    Isotope effects provide deep insight into mechanisms of chemical and biochemical processes. Now, it has been shown that the pattern of isotopic substitution of the isomerizing bond of the retinal chromophore in the visual pigment rhodopsin significantly alters the reaction quantum yield—revealing a vibrational phase-dependent isotope effect.

    • C. Schnedermann
    • , X. Yang
    •  & R. A. Mathies

  • Article |

    Singlet fission may one day allow solar cells to produce two excited electrons with one photon. Now, by comparison of the time-resolved photoluminescence and sensitized triplet–triplet annihilation of a tetracene derivative, it has been shown that—contrary to previous reports—the excimer state is a trap, and not a necessary intermediate for singlet fission.

    • Cameron B. Dover
    • , Joseph K. Gallaher
    •  & Timothy W. Schmidt

  • Article |

    According to the entatic-state principle, distortions of the coordination geometry of a reacting compound can optimize the energies of starting and final states to improve reaction rates. Here, the entatic-state principle is observed to apply to photoactive copper complexes that have a constrained ligand geometry, resulting in very short charge-transfer state lifetimes.

    • B. Dicke
    • , A. Hoffmann
    •  & S. Herres-Pawlis

  • Article |

    The capture of an electron by a molecule represents one of the most fundamental chemical transformations, but its mechanism at very low energies remains unclear. Now, it is shown that low-energy electron attachment to hexafluorobenzene is mediated by a non-valence correlation-bound state of the anion.

    • Joshua P. Rogers
    • , Cate S. Anstöter
    •  & Jan R. R. Verlet

  • Article |

    Singlet fission — the conversion of one singlet exciton into two triplet excitons, could improve the efficiency of photovoltaic devices — but its mechanism is still to be fully understood. Now, in films of TIPS-tetracene, it has been shown that the formation of the triplet pair state, which has been proposed to mediate singlet fission, is ultrafast and vibronically coherent in this endothermic fission system.

    • Hannah L. Stern
    • , Alexandre Cheminal
    •  & Richard H. Friend

  • Article |

    Photoprotection is crucial for the fitness of organisms that carry out oxygenic photosynthesis. LHCSR, a photosynthetic light-harvesting complex, has been implicated in photoprotection in green algae and moss. Now, single-molecule studies of LHCSR have revealed that multi-timescale protein dynamics underlie photoprotective dissipation of excess energy.

    • Toru Kondo
    • , Alberta Pinnola
    •  & Gabriela S. Schlau-Cohen

  • Article |

    With recent and improved understanding of how nuclear and electronic degrees of freedom can interact with each other comes the opportunity to directly control electronic processes. Now it has been shown that ultrafast vibrational excitation can direct light-induced intramolecular electron transfer along a specific path.

    • Milan Delor
    • , Stuart A. Archer
    •  & Julia A. Weinstein

  • Article |

    Singlet fission — converting a singlet exciton to two triplet excitons — may be useful for improving photovoltaic efficiency. Ultrafast spectroscopic measurements and quantum chemical calculations have now uncovered aspects of the process critical to it occurring efficiently, including the role of intermolecular vibrations and symmetry breaking, and the location of a conical intersection on the excited-state potential-energy surface.

    • Kiyoshi Miyata
    • , Yuki Kurashige
    •  & Yoshiyasu Matsumoto

  • Article |

    Photoreceptors play an essential role in determining the fate of subsequent biological reactions, however, tracking their structural evolution on ultrafast timescales has been challenging. Now, photoactive yellow protein has been studied using time-domain Raman spectroscopy with sub-7-femtosecond pulses, revealing the ultrafast rearrangement of its hydrogen-bonding structure and also the structure of the first photocycle intermediate.

    • Hikaru Kuramochi
    • , Satoshi Takeuchi
    •  & Tahei Tahara

  • Article |

    Difficulties in experimentally achieving simultaneous structural sensitivity and time resolution have hindered the real-time mapping of the vibrational energy relaxation pathways in biomacromolecules. Now, using ultrashort light pulses to locally deposit excess energy in a protein-bound haem, the temporal evolution of the subsequent energy flow has been monitored, unravelling vibrational couplings that lead to mode-specific temperature changes.

    • C. Ferrante
    • , E. Pontecorvo
    •  & T. Scopigno

  • Article |

    Singlet fission in assemblies of molecular chromophores offers a promising route to improving solar cell efficiencies, but its mechanism is not fully understood. Now, a series of covalently bound π-stacked terrylenediimide dimers have been studied to elucidate the role of interchromophore charge-transfer states in the mechanism of singlet fission.

    • Eric A. Margulies
    • , Claire E. Miller
    •  & Michael R. Wasielewski

  • Article |

    Photoexcited holes in CdS nanocrystals rapidly trap to the surface and although they are integral to nanocrystal photophysics and photochemistry, their dynamics have remained elusive. Time-resolved spectroscopy and theoretical modelling have now revealed that trapped holes in CdS nanorods are mobile and undergo a random walk on the nanocrystal surface.

    • James K. Utterback
    • , Amanda N. Grennell
    •  & Gordana Dukovic

  • Article |

    Nature's speciality is to direct and control the reactivity of species, which are otherwise fatally destructive. However, the processes and design rules required to achieve such precise control are not clear. A de novo designed metalloprotein that stabilizes an otherwise unstable organic radical has now been developed to guide our understanding.

    • Gözde Ulas
    • , Thomas Lemmin
    •  & William F. DeGrado

  • News & Views |

    Vision is initiated by photoisomerization of 11-cis retinal in the visual pigment rhodopsin — a fast and efficient process. Spectroscopic studies now demonstrate that the transition from the reactant photoexcited-state to the ground-state photoproduct, which mediates this important reaction, occurs on a sub-50-fs timescale and is vibrationally coherent.

    • Richard A. Mathies

  • Article |

    The isomerization of the retinal chromophore of rhodopsin is the photochemical process that initiates the sense of vision. Now, heterodyne-detected transient grating spectroscopy has been used to resolve coherent vibrational dynamics during this process, helping to identify strictly local vibrational motions as the origin of the coherent surface crossing, which occurs on a sub-50-fs timescale.

    • Philip J. M. Johnson
    • , Alexei Halpin
    •  & R. J. Dwayne Miller

  • News & Views |

    Electron transfer is ubiquitous across both life and modern technologies, and thus being able to control it is an attractive goal. Now, targeted infrared excitation has been used to modulate the efficiency of electron transfer in a series of donor–bridge–acceptor molecules.

    • Igor V. Rubtsov

  • Article |

    The ultrafast and mode-specific infrared excitation of several donor–bridge–acceptor (DBA) assemblies in solution has been shown to modulate their light-induced electron transfer properties. New insights are afforded into the role of vibrational processes immediately following light absorption in charge-transfer molecules and a recipe for efficient ‘vibrational control’ of electron transfer is proposed.

    • Milan Delor
    • , Theo Keane
    •  & Julia A. Weinstein

  • Article |

    Fe(II) complexes display transitions between spin states that can be triggered externally. Now the light-induced ΔS = 2 transition upon excitation of the metal-to-ligand charge-transfer states of Fe(II)-polypyridine complexes has been investigated at high time-resolution in the visible and the ultraviolet range. It has been shown to occur in less than 50 fs — that is, on a sub-vibrational timescale.

    • Gerald Auböck
    •  & Majed Chergui

  • Article |

    Roaming — a new and unusual reaction mechanism in gas-phase chemical transformations — is now shown to occur in solution. Following ultraviolet excitation of geminal tribromides, what initially seems to be the simple fission of a bond is in fact isomerization occurring through the roaming of molecular fragments.

    • Andrey S. Mereshchenko
    • , Evgeniia V. Butaeva
    •  & Alexander N. Tarnovsky

  • News & Views |

    In 1972, Baird showed theoretically that the electron counting rule for aromaticity and antiaromaticity in the lowest ππ* triplet state is opposite to that in the electronic ground state. A pair of compounds that manifests this reversal in character has now been identified and characterized experimentally for the first time.

    • Henrik Ottosson
    •  & K. Eszter Borbas

  • Article |

    In the ground state, aromatic rings contain [4n + 2] π electrons whereas antiaromatic systems have [4n] π-electrons. Baird's rule states that this situation is reversed in the lowest triplet excited state. It has now been shown using a combination of spectroscopy and quantum chemical calculations that two closely related bis-rhodium hexaphyrins exhibit properties consistent with Baird's rule.

    • Young Mo Sung
    • , Min-Chul Yoon
    •  & Dongho Kim

  • Article |

    The adenine analogue 2-aminopurine has been considered as intrinsically fluorescent and is widely used in biochemical assays to probe DNA and RNA structure. It is now shown that the molecule alone is nearly non-fluorescent, however, its fluorescence is increased by up to 95 times through hydrogen bonding to a single water molecule.

    • Simon Lobsiger
    • , Susan Blaser
    •  & Samuel Leutwyler

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