Multiple Anti-Kasha Emissions in Transition-Metal Complexes

Conformational Locking of the Geometry in Photoluminescent Cyclometalated N^C^N Ni(II) Complexes

In our research aimed at replacing precious transition metals like platinum with abundant base metals such as nickel for efficient triplet emitters, we synthesized and studied Ni(II) complexes [Ni(LNHR)Cl]. These complexes containing the N^C^N cyclometalating dipyridyl-phenide ligand, equipped with pending H-bonding amine groups (NH(C₆H₅) (LNHPh) and NH(C₆H₅CH₂), ClLNHBn). Molecular structures determined from experimental X-ray diffractometry and density functional theory (DFT) calculations in the ground state showed marked deviation of the Cl- coligand (ancillary ligand) from the ideal planar coordination, with τ4 values of 0.35 and 0.33, respectively, along with hydrogen bonding interactions of the ligand NH function with the Cl- coligand. The complexes exhibit long-wavelength absorption bands at approximately 425 nm in solution, with the experimental spectra being accurately reproduced through time-dependent density functional theory (TD-DFT) calculations. Vibrationally structured emission profiles and steady-state photoluminescence quantum yields of 30% for [Ni(LNHPh)Cl] and 40% for [Ni(LNHBn)Cl] (along with dual excited state lifetimes in the ns and in the ms range) were found in frozen 2-methyl-tetrahydrofuran (2MeTHF) glassy matrices at 77 K. Furthermore, within a poly(methyl methacrylate) matrix, the complexes showed emission bands centered at around 550 nm within a temperature range from 6 K to 300 K with lifetimes similar to 77 K. Based on TD-DFT potential scans along the metal-ligand (Ni-N) coordinate, we found that in a rigid environment that restricts the geometry to the Franck-Condon region, either the triplet T5 or the singlet S4 state could contribute to the photoluminescence.

Luminescent N^C^N Pincer Ni(II), Pd(II), and Pt(II) Complexes with a Pendant Coumarin Group: The Role of Auxiliary Ligands and Environments

Square-planar d8-configured metal complexes and their excited states play a key role in photocatalysis, sensing, and optoelectronic devices. However, metal-to-ligand charge-transfer (MLCT) excited states involving transition metals, particularly those with a 3d8 electronic configuration, present challenges due to rapid nonradiative relaxation via low-lying metal-centered (MC) states. In this work, an isoelectronic and isostructural series of cyclometalated complexes [MX(dpb)] with M = Ni(II), Pd(II), and Pt(II), dpb = 1,3-di(2-pyridyl)phenide, and auxiliary ligand X = chlorido, azido, and triazolato were studied by UV/vis absorption, steady-state, and time-resolved photoluminescence spectroscopy in solution and frozen glassy matrix at 77 K, along with DFT calculations. Consistently, the Pd(II) and Pt(II) complexes exhibited a characteristic emission from their triplet ligand-centered (3LC) excited states. In contrast, Ni(II) complexes with auxiliary chlorido, azido, and triazolatoCOOCH3,COOCH3 ligands were nonemissive at low and room temperatures, due to the presence of low-lying MC-type ligand-field excited states. On the other hand, [Ni(triazolatoCoumarin,COOCH3)(dpb)] showed phosphorescence from the T5 state in a frozen glassy matrix at 77 K, since the restrictive environment limits structural relaxation, while at room temperature, the primary emission is due to singlet LC excited states from the coumarin moiety of the free ligand.

Ruthenium-catalyzed C-H bond activation and annulation of phenothiazine-3-carbaldehydes: facile access to dual-emission materials

Reported herein is the first example of a ruthenium-catalyzed C-H activation/annulation of phenothiazine-3-carbaldehydes to construct structurally diverse pyrido[3,4-c]phenothiazin-3-iums with dual-emission characteristics. Novel organic single-molecule white-light materials based on pyrido[3,4-c]phenothiazin-3-iums with dual-emission and thermally activated delayed fluorescence (TADF) characteristics have been developed for the first time herein. Furthermore, the dual-emission molecule could be fabricated as water-dispersed NPs, which could be applied in two-channel emission intensity ratio imaging to observe the intercellular structure and can specifically target the cell membrane.

Photodissociation of Cr(CO) 4 bpy $$ \mathrm{Cr}{\left(\mathrm{CO}\right)}_4\mathrm{bpy} $$ : A Non-Adiabatic Dynamics Investigation

Carbonyl complexes ofd 6 $$ {d}^6 $$ metals with an α-diimine ligand exhibit both emission and ligand-selective photodissociation from MLCT states. Studying this photodissociative mechanism is challenging for experimental approaches due to an ultrafast femtosecond timescale and spectral overlap of multiple photoproducts. The photochemistry of a prototypical systemCr(CO) 4 bpy $$ \mathrm{Cr}{\left(\mathrm{CO}\right)}_4\mathrm{bpy} $$ is investigated with non-adiabatic dynamic simulations. Obtained 86 fs lifetime of the brightS 3 $$ {S}_3 $$ state and 13% quantum yield are in good agreement with experimental data. The present simulations suggest a ballistic mechanism of photodissociation, which is irrespective of the occupied electronic state. This is in contrast to the previously established mechanism of competitive intersystem crossing and dissociation. Selectivity of axial photodissociation is shown to be caused by the absence of an avoided crossing in the equatorial direction.

Photocatalytic Substrate Oxidation Catalyzed by a Ruthenium(II) Complex with a Phenazine Moiety as the Active Site Using Dioxygen as a Terminal Oxidant

We have developed photocatalytic oxidation of aromatic substrates using O2 as a terminal oxidant to afford only 2e--oxidized products without the reductive activation of O2 in acidic water under visible-light irradiation. A RuII complex (1) bearing a pyrazine moiety as the active site in tetrapyrido[3,2-a:2',3'-c:3″,2″-h:2‴,3‴-j]phenazine (tpphz) as a ligand was employed as a photocatalyst. The active species for the photocatalysis was revealed to be not complex 1 itself but the protonated form, 1-H+, protonated at the vacant diimine site of tpphz. Upon photoexcitation in the presence of an organic substrate, 1-H+ was converted to the corresponding dihydro-intermediate (2-H+), where the pyrazine moiety of the ligand received 2e- and 2H+ from the substrate. 2-H+ was facilely oxidized by O2 to recover 1-H+. Consequently, an oxidation product of the substrate and H2O2 derived from dioxygen reduction were obtained; however, the H2O2 formed was also used for oxidation of 2-H+. In the oxidation of benzyl alcohol to benzaldehyde, the turnover number reached 240 for 10 h, and the quantum yield was determined to be 4.0%. The absence of ring-opening products in the oxidation of cyclobutanol suggests that the catalytic reaction proceeds through a mechanism involving formal hydride transfer. Mechanistic studies revealed that the photocatalytic substrate oxidation by 1-H+ was achieved in neither the lowest singlet excited state nor triplet excited state (S1 or T1) but in the second lowest singlet excited state (S2), i.e., 1(π-π*)* of the tpphz ligand. Thus, the photocatalytic substrate oxidation by 1-H+ can be categorized into unusual anti-Kasha photocatalysis.

mer-M(CO)3(PNP)0/+ pincer complexes (M = W(0) or Re(I); PNP = 4,5-bis(diphenylphosphino)acridine): synthesis, spectroscopy and anti-Kasha emission

Two isoelectronic and isostructural W(0) and Re(I) complexes mer-W(CO)3(PNP) (1) and [mer-Re(CO)3(PNP)]Cl (2) (PNP = 4,5-bis(diphenylphosphino)acridine) were synthesized and characterized by X-ray diffraction, infrared, electronic absorption and emission spectroscopy, and cyclic voltammetry. Structures of these complexes show a metal center bonded to the pincer ligand and two axial CO and one equatorial CO ligands. DFT calculations showed that the LUMOs of both complexes are the lowest energy π* orbitals localized in the acridine part of the ligand. The HOMO of 1 is dominated by the dπ orbital of W(0) while the HOMO of 2 has a substantial contribution from the highest energy π orbital of the acridine ring. TD-DFT calculations were performed to assist assignment of the UV-vis absorption spectra. The UV-vis absorption spectrum of 1 shows a very low energy W → π* (acridine) metal-to-ligand-charge-transfer (MLCT) absorption band that ranges from visible (500 nm) to near-infrared (>900 nm) regions and an intense acridine π → π* absorption band at 410 nm. There is a blue-green window in the ∼450-500 nm range between the π → π* and W → π*(acridine) MLCT absorptions. The absorption spectrum of 2, dominated by intense π → π* absorptions, shows no distinct low energy MLCT band. Complex 1 is luminescent, displaying acridine-based ππ* fluorescence at 501 nm which is anti-Kasha as it is higher in energy than the lowest energy excited state.

Triplet-Singlet Emission of d-Block Metal Complexes Characterized by Spin-Orbit Natural Transition Orbitals

Spin-orbit natural transition orbital (SO-NTO) methodology, recently developed in our group for complete and restricted active space (CAS/RAS) wavefunction calculations, is applied to analyze triplet-to-singlet emission in transition metal complexes. The lowest-energy (longest-wavelength) spin-forbidden transitionT 1 → S 0 ${{{\rm T}}_{1}\to {{\rm S}}_{0}}$ is studied for for [Ir(pbt)2(acac)] and [Re(CO)4(pbt)] and the complexes [W(CO)4(bpy)] and [Mo(CO)4(bpy)]. For the latter complexes, spin-forbidden transitions from higher spin-triplet levels are additionally analyzed. SO-NTOs are compared with spin-free NTOs for the transitions under consideration. The major assignment of a spin-forbidden transition is obtained from the spin-free NTO analysis, while the source of intensity of the electronic transition is revealed by the SO-NTOs.

Influence of Donor-Acceptor Interactions on MLCT Hole Reconfiguration in {Ru(bpy)} Chromophores

In MLCT chromophores, internal conversion (IC) in the form of hole reconfiguration pathways (HR) is a major source of dissipation of the absorbed photon energy. Therefore, it is desirable to minimize their impact in energy conversion schemes by slowing them down. According to previous findings on {Ru(bpy)} chromophores, donor-acceptor interactions between the Ru ion and the ligand scaffold might allow to control HR/IC rates. Here, a series of [Ru(tpm)(bpy)(R-py)]2+ chromophores, where tpm is tris(1-pyrazolyl)methane, bpy is 2,2'-bipyridine and R-py is a 4-substituted pyridine, were prepared and fully characterized employing electrochemistry, spectroelectrochemistry, steady-state absorption/emission spectroscopy and electronic structure computations based on DFT/TD-DFT. Their excited-state decay was monitored using nanosecond and femtosecond transient absorption spectroscopy. HR/IC lifetimes as slow as 568 ps were obtained in DMSO at room temperature, twice as slow as in the reference species [Ru(tpm)(bpy)(NCS)]+.

Neutral vs Charged Luminescent Radicals: Anti-Kasha Emission and the Impact of Molecular Surrounding

Organic luminescent materials attract growing interest as an elegant solution for sustainable and inexpensive light-emitting devices. Most of them are neutral-emitting molecules with an implicit restriction of 25% internal quantum efficiency due to a spin-forbidden nature of the T1 → S0 transition. Utilizing organic radicals allows one to overcome such limits by theoretically boosting quantum yield up to 100%. Recently, different light-emitting radicals based on carbonyl- and carboxyl-substituted benzenes were synthesized and stabilized in different polymer matrices or ionic liquids. While some of them were proved to be suitable luminescent materials, the exact theoretical explanation of the nature of their emission is missing. There are two main hypotheses proposed in the literature. The first one suggests that the origin of luminescence is D2 → D0 anti-Kasha emission from anion radicals, while the second theory is based on D1 → D0 Kasha emission from neutral protonated radicals. In this work, we investigate both hypotheses and compare their derivatives with the available experimental data. We used density functional theory and complete-active space perturbation theory to investigate the absorption and emission properties in various aromatic carbonyl radicals. We found that both emission mechanisms can coexist simultaneously, with a dominant emission contribution made by anion radicals because of better agreement between oscillator strengths and radiative rate constants. Our numerical simulations agree with the experimental data and provide theoretical foundations for the fabrication of next-generation light-emitting devices based on luminescent radicals.

Bifurcation of Excited-State Population Leads to Anti-Kasha Luminescence in a Disulfide-Decorated Organometallic Rhenium Photosensitizer

We report a rhenium diimine photosensitizer equipped with a peripheral disulfide unit on one of the bipyridine ligands, [Re(CO)3(bpy)(S-Sbpy4,4)]+ (1+, bpy = 2,2'-bipyridine, S-Sbpy4,4 = [1,2]dithiino[3,4-c:6,5-c']dipyridine), showing anti-Kasha luminescence. Steady-state and ultrafast time-resolved spectroscopies complemented by nonadiabatic dynamics simulations are used to disclose its excited-state dynamics. The calculations show that after intersystem crossing the complex evolves to two different triplet minima: a (S-Sbpy4,4)-ligand-centered excited state (3LC) lying at lower energy and a metal-to-(bpy)-ligand charge transfer (3MLCT) state at higher energy, with relative yields of 90% and 10%, respectively. The 3LC state involves local excitation of the disulfide group into the antibonding σ* orbital, leading to significant elongation of the S-S bond. Intriguingly, it is the higher-lying 3MLCT state, which is assigned to display luminescence with a lifetime of 270 ns: a signature of anti-Kasha behavior. This assignment is consistent with an energy barrier ≥ 0.6 eV or negligible electronic coupling, preventing reaction toward the 3LC state after the population is trapped in the 3MLCT state. This study represents a striking example on how elusive excited-state dynamics of transition-metal photosensitizers can be deciphered by synergistic experiments and state-of-the-art calculations. Disulfide functionalization lays the foundation of a new design strategy toward harnessing excess energy in a system for possible bimolecular electron or energy transfer reactivity.

Photoswitchable Molecular Units with Tunable Nonlinear Optical Activity: A Theoretical Investigation

The first-, second-, and third-order molecular nonlinear optical properties, including two-photon absorption of a series of derivatives, involving two dithienylethene (DTE) groups connected by several molecular linkers (bis(ethylene-1,2-dithiolato)Ni- (NiBDT), naphthalene, quasilinear oligothiophene chains), are investigated by employing density functional theory (DFT). These properties can be efficiently controlled by DTE switches, in connection with light of appropriate frequency. NiBDT, as a linker, is associated with a greater contrast, in comparison to naphthalene, between the first and second hyperpolarizabilities of the "open-open" and the "closed-closed" isomers. This is explained by invoking the low-lying excited states of NiBDT. It is shown that the second hyperpolarizability can be used as an index, which follows the structural changes induced by photochromism. Assuming a Förster type transfer mechanism, the intramolecular excited-state energy transfer (EET) mechanism is studied. Two important parameters related to this are computed: the electronic coupling (VDA) between the donor and acceptor fragments as well as the overlap between the absorption and emission spectra of the donor and acceptor groups. NiBDT as a linker is associated with a low electronic coupling, VDA, value. We found that VDA is affected by molecular geometry. Our results predict that the linker strongly influences the communication between the open-closed DTE groups. The sensitivity of the molecular nonlinear optical properties could assist with identification of molecular isomers.

Calculation of Excited State Internal Conversion Rate Constant Using the One-Effective Mode Marcus-Jortner-Levich Theory

In this article, the one-effective mode Marcus-Jortner-Levich (MJL) theory and the classical Marcus theory for electron transfer were applied to estimate the internal conversion rate constant, k_IC, of organic molecules and a Ru-based complex, all belonging to the Marcus inverted region. For this, the reorganization energy was calculated using the minimum energy conical intersection point to account for more vibrational levels, correcting the density of states. The results showed good agreement with experimental and theoretically determined k_IC, with a small overestimation by the Marcus theory. Also, molecules less dependent on the solvent effects, like benzophenone, presented better results than molecules with an expressive dependence, like 1-aminonaphthalene. Moreover, the results suggest that each molecule possesses unique normal modes leading to the excited state deactivation that does not necessarily match the X-H bond stretching, as previously suggested.

Photoactive Metal-to-Ligand Charge Transfer Excited States in 3d6 Complexes with Cr0, MnI, FeII, and CoIII

Many coordination complexes and organometallic compounds with the 4d⁶ and 5d⁶ valence electron configurations have outstanding photophysical and photochemical properties, which stem from metal-to-ligand charge transfer (MLCT) excited states. This substance class makes extensive use of the most precious and least abundant metal elements, and consequently there has been a long-standing interest in first-row transition metal compounds with photoactive MLCT states. Semiprecious copper(I) with its completely filled 3d subshell is a relatively straightforward and well explored case, but in 3d⁶ complexes the partially filled d-orbitals lead to energetically low-lying metal-centered (MC) states that can cause undesirably fast MLCT excited state deactivation. Herein, we discuss recent advances made with isoelectronic Cr⁰, Mn^I, Fe^II, and Co^III compounds, for which long-lived MLCT states have become accessible over the past five years. Furthermore, we discuss possible future developments in the search for new first-row transition metal complexes with partially filled 3d subshells and photoactive MLCT states for next-generation applications in photophysics and photochemistry.

Anti-Kasha Fluorescence in Molecular Entities: Central Role of Electron-Vibrational Coupling

According to Kasha's rule, the emission of a photon in a molecular system always comes from the lowest excited state. A corollary of this rule (i.e., the Kasha-Vavilov rule) states that the emission spectra are independent of the excitation wavelength. Although these rules apply for most of the molecular systems, violations of these rules are often reported. The prototypical case of a Kasha's rule violation is the fluorescence observed from S₂ in azulene. Thanks to the advances in both theoretical and experimental research, other types of anomalous fluorescence (e.g., excitation energy transfer (EET)-based dual emissions, thermally activated fluorescence, etc.) are more recurrently reported in the literature. Sometimes, these anomalous processes involve higher-lying excited states but are mechanistically different from the azulene-like anomalous fluorescence. However, the underlying mechanisms leading to these anomalous emissions can be numerous and are not yet well understood.In order to shed some light on the above phenomena, this Account provides a comprehensive review of this topic. We herein report quantum chemical investigations in target molecular systems breaking Kasha's rule. The latter molecules were chosen because they were unambiguously reported to display anti-Kasha fluorescence. Our studies highlight three different types of anti-Kasha scenarios. Specifically, (i) the strong electronic, weak vibrational nonadiabatic coupling (NAC) regime (here named the type I case, i.e., azulene-like); (ii) the strong electronic, strong vibrational NAC regime (type II case, i.e., thermally activated S₂ fluorescence); and the (iii) very weak electronic NAC regime (type III case, i.e., EET dyads). In addition, by combining state-of-the-art quantum chemical calculations with excited-state decay rate theories and appropriate excited-state kinetic models, we provide semiquantitative estimations of photoluminescence quantum yields for the most rigid molecular entities. Finally, we propose the use of simple theoretical descriptors relying on calculations of the excited-state density difference and the electron-vibrational coupling to classify anomalous emissions according to their coupling scenario.Besides the fundamental interest of the above investigations, the herein developed computational protocols and descriptors will be useful for the tailored design of dyes with tunable and unconventional fluorescence properties and their exploitation in a wide range of areas (i.e., from organic light-emitting diodes (OLEDs) to bioimaging, small-molecule fluorescent probes, and photocatalysis). Finally, our theoretical framework enables the attainment of a holistic understanding of the interconversion processes between excited states, where the electron-vibrational coupling is shown to play a central role in determining the efficacy.

Anti-Kasha emissions of single molecules in a plasmonic nanocavity

Kasha's rule generally holds true for solid-state molecular systems, where the rates of internal conversion and vibrational relaxation are sufficiently higher than the luminescence rate. In contrast, in systems where plasmons and matter interact strongly, the luminescence rate is significantly enhanced, leading to the emergence of luminescence that does not obey Kasha's rule. In this work, we investigate the anti-Kasha emissions of single molecules, free-base and magnesium naphthalocyanine (H2Nc and MgNc), in a plasmonic nanocavity formed between the tip of a scanning tunneling microscope (STM) and metal substrate. A narrow-line tunable laser was employed to precisely reveal the excited-state levels of a single molecule located under the tip and to selectively excite it into a specific excited state, followed by obtaining STM-photoluminescence (STM-PL) spectrum to reveal energy relaxation from the state. The excitation to higher-lying states of H2Nc caused various changes in the emission spectrum, such as broadening and the appearance of new peaks, implying the breakdown of Kasha's rule. These observations indicate emissions from the vibrationally excited states in the first singlet excited state ( S1) and second singlet excited state ( S2), as well as internal conversion from S2 to S1. Moreover, we obtained direct evidence of electronic and vibronic transitions from the vibrationally excited states, from the STM-PL measurements of MgNc. The results obtained herein shed light on the energy dynamics of molecular systems under a plasmonic field and highlight the possibility of obtaining various energy-converting functions using anti-Kasha processes.

Luminescent First-Row Transition Metal Complexes

Precious and rare elements have traditionally dominated inorganic photophysics and photochemistry, but now we are witnessing a paradigm shift toward cheaper and more abundant metals. Even though emissive complexes based on selected first-row transition metals have long been known, recent conceptual breakthroughs revealed that a much broader range of elements in different oxidation states are useable for this purpose. Coordination compounds of V, Cr, Mn, Fe, Co, Ni, and Cu now show electronically excited states with unexpected reactivity and photoluminescence behavior. Aside from providing a compact survey of the recent conceptual key advances in this dynamic field, our Perspective identifies the main design strategies that enabled the discovery of fundamentally new types of 3d-metal-based luminophores and photosensitizers operating in solution at room temperature.

Computational Protocol to Calculate the Phosphorescence Energy of Pt(II) Complexes: Is the Lowest Triplet Excited State Always Involved in Emission? A Comprehensive Benchmark Study

The reliable calculation of phosphorescence energies of phosphor materials is at the core of designing efficient phosphorescent organic light-emitting diodes (PhOLEDs). Therefore, it is of paramount importance to have a robust computational protocol to perform those calculations in a black-box manner. In this work, we use Domain-Based Local Pair Natural Orbital Coupled Cluster theory with single, double, and perturbative triple excitation (DLPNO-CCSD(T)) calculations to attain the phosphorescence energies of a large pool of Pt(II) complexes. Several approaches to incorporate relativistic effects in our calculations were tested. In addition, we have used the DLPNO-CCSD(T) values (i.e., our best theoretical values) to assess the performance of different flavors of density functional theory including pure, hybrid, meta-hybrid, and range-separated functionals. Among the tested functionals, the M06HF functional provides the best values compared with the DLPNO-CCSD(T) ones, with a mean absolute deviation (MAD) value of 0.14 eV. In its turn, and thanks to the increased accuracy achieved in the calculation of phosphorescence energies, we also demonstrate that not all of the investigated complexes emit from their lowest-lying triplet state (T₁). The outlier complexes include different complex photophysical scenarios and both Kasha and anti-Kasha types of complexes. Finally, we provide a general computational protocol to pre-screen whether T₁ is actually the emissive state and to accurately calculate the phosphorescence energies of Pt(II) complexes.

Ab initio nonadiabatic molecular dynamics of charge carriers in metal halide perovskites

Photoinduced nonequilibrium processes in nanoscale materials play key roles in photovoltaic and photocatalytic applications. This review summarizes recent theoretical investigations of excited state dynamics in metal halide perovskites (MHPs), carried out using a state-of-the-art methodology combining nonadiabatic molecular dynamics with real-time time-dependent density functional theory. The simulations allow one to study evolution of charge carriers at the ab initio level and in the time-domain, in direct connection with time-resolved spectroscopy experiments. Eliminating the need for the common approximations, such as harmonic phonons, a choice of the reaction coordinate, weak electron-phonon coupling, a particular kinetic mechanism, and perturbative calculation of rate constants, we model full-dimensional quantum dynamics of electrons coupled to semiclassical vibrations. We study realistic aspects of material composition and structure and their influence on various nonequilibrium processes, including nonradiative trapping and relaxation of charge carriers, hot carrier cooling and luminescence, Auger-type charge-charge scattering, multiple excitons generation and recombination, charge and energy transfer between donor and acceptor materials, and charge recombination inside individual materials and across donor/acceptor interfaces. These phenomena are illustrated with representative materials and interfaces. Focus is placed on response to external perturbations, formation of point defects and their passivation, mixed stoichiometries, dopants, grain boundaries, and interfaces of MHPs with charge transport layers, and quantum confinement. In addition to bulk materials, perovskite quantum dots and 2D perovskites with different layer and spacer cation structures, edge passivation, and dielectric screening are discussed. The atomistic insights into excited state dynamics under realistic conditions provide the fundamental understanding needed for design of advanced solar energy and optoelectronic devices.

Resonance in Chirogenesis and Photochirogenesis: Colloidal Polymers Meet Chiral Optofluidics

Metastable colloids made of crystalline and/or non-crystalline matters render abilities of photonic resonators susceptible to chiral chemical and circularly polarized light sources. By assuming that μm-size colloids and co-colloids consisting of π- and/or σ-conjugated polymers dispersed into an optofluidic medium are artificial models of open-flow, non-equilibrium coacervates, we showcase experimentally resonance effects in chirogenesis and photochirogenesis, revealed by gigantic boosted chiroptical signals as circular dichroism (CD), optical rotation dispersion, circularly polarized luminescence (CPL), and CPL excitation (CPLE) spectral datasets. The resonance in chirogenesis occurs at very specific refractive indices (RIs) of the surrounding medium. The chirogenesis is susceptible to the nature of the optically active optofluidic medium. Moreover, upon an excitation-wavelength-dependent circularly polarized (CP) light source, a fully controlled absolute photochirogenesis, which includes all chiroptical generation, inversion, erase, switching, and short-/long-lived memories, is possible when the colloidal non-photochromic and photochromic polymers are dispersed in an achiral optofluidic medium with a tuned RI. The hand of the CP light source is not a determining factor for the product chirality. These results are associated with my experience concerning amphiphilic polymerizable colloids, in which, four decades ago, allowed proposing a perspective that colloids are connectable to light, polymers, helix, coacervates, and panspermia hypotheses, nuclear physics, biology, radioisotopes, homochirality question, first life, and cosmology.

Bifurcation of excited state trajectories toward energy transfer or electron transfer directed by wave function symmetry

This work explores the concept that differential wave function overlap between excited states can be engineered within a molecular chromophore. The aim is to control excited state wave function symmetries, so that symmetry matches or mismatches result in differential orbital overlap and define low-energy trajectories or kinetic barriers within the excited state surface, that drive excited state population toward different reaction pathways. Two donor-acceptor assemblies were explored, where visible light absorption prepares excited states of different wave function symmetry. These states could be resolved using transient absorption spectroscopy, thanks to wave function symmetry-specific photoinduced optical transitions. One of these excited states undergoes energy transfer to the acceptor, while another undertakes a back-electron transfer to restate the ground state. This differential behavior is possible thanks to the presence of kinetic barriers that prevent excited state equilibration. This strategy can be exploited to avoid energy dissipation in energy conversion or photoredox catalytic schemes.

Behind the scenes of spin-forbidden decay pathways in transition metal complexes

The interpretation of the ultrafast photophysics of transition metal complexes following photo-absorption is quite involved as the heavy metal center leads to a complicated and entangled singlet-triplet manifold. This opens up multiple pathways for deactivation, often with competitive rates. As a result, intersystem crossing (ISC) and phosphorescence are commonly observed in transition metal complexes. A detailed understanding of such an excited-state structure and dynamics calls for state-of-the-art experimental and theoretical methodologies. In this review, we delve into the inability of non-relativistic quantum theory to describe spin-forbidden transitions, which can be overcome by taking into account spin-orbit coupling, whose importance grows with increasing atomic number. We present the quantum chemical theory of phosphorescence and ISC together with illustrative examples. Finally, a few applications are highlighted, bridging the gap between theoretical studies and experimental applications, such as photofunctional materials.

Anti-Kasha Conformational Photoisomerization of a Heteroleptic Dithiolene Metal Complex Revealed by Ultrafast Spectroscopy

We investigated the anti-Kasha photochemistry and anti-Kasha emission of d⁸-metal donor-acceptor dithiolene with femtosecond UV-vis transient absorption spectroscopy and molecular modeling. Experimentally, we found a lifetime of 1.4 ps for higher excited states, which is exceptionally long when compared to typical values for internal conversion (IC) (10 s of fs or less). Consequently, a substantial emission originates from the second excited state. Molecular modeling suggests this to be a consequence of the spatially separated molecular orbitals of the first and second excited states, which gives a charge transfer character to the IC. More surprisingly, we found that the inherent flexibility of the molecule allows the metal complex to access different configurations depending on the photoexcited state. We believe that this unique manifestation of anti-Kasha photoinduced conformational isomerization is facilitated by the exceptionally long lifetime of the second excited state.

Wave-Function Symmetry Control of Electron-Transfer Pathways within a Charge-Transfer Chromophore

Despite a diverse manifold of excited states available, it is generally accepted that the photoinduced reactivity of charge-transfer chromophores involves only the lowest-energy excited state. Shining a visible-light laser pulse on an aqueous solution of the chromophore-quencher [Ru(tpy)(bpy)(μNC)Os^III(CN)₅]^- assembly (tpy = 2,2';6,2''-terpyridine and bpy = 2,2'-bipyridine), we prepared a mixture of two charge-transfer excited states with different wave-function symmetry. We were able to follow, in real time, how these states undergo separate electron-transfer reaction pathways. As a consequence, their lifetimes differ in 3 orders of magnitude. Implicit are energy barriers high enough to prevent internal conversion within early excited-state populations, shaping isolated electron-transfer channels in the excited-state potential energy surface. This is relevant not only for supramolecular donor/acceptor chemistry with restricted donor/acceptor relative orientations. These energy barriers provide a means to avoid chemical potential dissipation upon light absorption in any molecular energy conversion scheme, and our observations invite to explore wave-function symmetry-based strategies to engineer these barriers.

Dual Emission: Classes, Mechanisms, and Conditions

There has been much interest in dual-emission materials in the past few years for materials and life science applications; however, a systematic overview of the underlying processes is so-far missing. We resolve this issue herein by classifying dual-emission (DE) phenomena as relying on one emitter with two emitting states (DE1), two independent emitters (DE2), or two correlated emitters (DE3). Relevant DE mechanisms for materials science are then briefly described together with the electronic and/or geometrical conditions under which they occur. For further reading, references are given that offer detailed insight into the complex mechanistic aspects of the various DE processes or provide overviews on materials families or their applications. By avoiding ambiguities and misinterpretations, this systematic, insightful Review might inspire future targeted designs of DE materials.

Semiclassical Approach to Photophysics Beyond Kasha's Rule and Vibronic Spectroscopy Beyond the Condon Approximation. The Case of Azulene

Azulene is a prototypical molecule with an anomalous fluorescence from the second excited electronic state, thus violating Kasha's rule, and with an emission spectrum that cannot be understood within the Condon approximation. To better understand the photophysics and spectroscopy of azulene and other nonconventional molecules, we developed a systematic, general, and efficient computational approach combining the semiclassical dynamics of nuclei with ab initio electronic structure. First, to analyze the nonadiabatic effects, we complement the standard population dynamics by a rigorous measure of adiabaticity, estimated with the multiple-surface dephasing representation. Second, we propose a new semiclassical method for simulating non-Condon spectra, which combines the extended thawed Gaussian approximation with the efficient single-Hessian approach. S₁ ← S₀ and S₂ ← S₀ absorption and S₂ → S₀ emission spectra of azulene, recorded in a new set of experiments, agree very well with our calculations. We find that accuracy of the evaluated spectra requires the treatment of anharmonicity, Herzberg-Teller, and mode-mixing effects.