Radiationless Transitions Research Articles

Quasi-intrinsic thiobase derivatives as potential targeted photosensitizers in two-photon photodynamic therapy

In this study, a set of potential quasi-intrinsic photosensitizers for two-photon photodynamic therapy (PDT) are proposed based on the unnatural 2-amino-8-(1′-β-ᴅ-2′-deoxyribofuranosyl)-imidazo[1,2-ɑ]-1,3,5-triazin-4(8H)-one (P), which is paired with the 6-amino-5-nitro-3-(1′-β-ᴅ-2′-deoxyribofuranosyl)-2(1H)-pyridone (Z) and can specifically recognize breast and liver cancer cells. Herein, the effects of sulfur substitution and electron-donating/electron-withdrawing groups on the photophysical properties in aqueous solution are systematically investigated. The one- and two-photon absorption spectra evidence that the modifications could result in red-shifted absorption wavelength and large two-photon absorption cross-section, which contributes to selective excitation and provides effective PDT for deep-seated tissues. To ensure the efficient triplet state population, the singlet–triplet energy gaps and spin–orbit coupling constants were examined, which is responsible for a rapid intersystem crossing rate. Furthermore, these thiobase derivatives are characterized by the long-lived T1 state and the large energy gap for radiationless transition to ensure the generation of cytotoxic singlet oxygen.

Vibronic Coupling Drives the Ultrafast Internal Conversion in a Functionalized Free-Base Porphyrin.

Internal conversion (IC) is a common radiationless transition in polyatomic molecules. Theory predicts that molecular vibrations assist IC between excited states, and ultrafast experiments can provide insight into their structure-function relationship. Here we elucidate the dynamics of the vibrational modes driving the IC process within the Q band of a functionalized porphyrin molecule. Through a combination of ultrafast multidimensional spectroscopies and theoretical modeling, we observe a 60 fs Qy-Qx IC and demonstrate that it is driven by the interplay among multiple high-frequency modes. Notably, we identify 1510 cm-1 as the leading tuning mode that brings the porphyrin to an optimal geometry for energy surface crossing. By employing coherent wave packet analysis, we highlight a set of short-lived vibrations (1200-1400 cm-1), promoting the IC within ≈60 fs. Furthermore, we identify one coupling mode (1350 cm-1) that is responsible for vibronic mixing within the Q states. Our findings indicate that porphyrin-core functionalization modulates IC effectively, offering new opportunities in photocatalysis and optoelectronics.

Final ion charge spectra upon cascade decay of inner-shell vacancies in atomic Au

Final ion charge spectra produced by the cascade de-excitations of 1s- to 5p 3/2-vacancies of the gold atom are calculated by direct construction and analysis of the cascade decay trees. The energies of multivacancy configurations arising in the course of the cascade development, and the partial widths of radiation and radiationless cascade transitions are calculated using the Pauli–Fock approximation. The energies of the cascade transitions are calculated as the differences of total Pauli–Fock energies of initial and final ionic configurations which allowed excluding energy-forbidden radiationless transitions in numerous multivacancy configurations. Partial widths of transitions are expressed in the form that allowed accounting for the effect of electron subshell populations on the transition widths. The partial widths of the transitions between the states of the overlapping initial and final state multiplets are corrected so as to exclude energy-forbidden transitions between the multiplet states. It is demonstrated that accurate accounting for possible forbiddance of transitions between cascade configurations and the exclusion of energy-forbidden term-to-term transitions between the multiplets’ states are crucial in deep-initial-vacancy cascade simulations.

Key of Suppressed Triplet Nonradiative Transition-Dependent Chemical Backbone for Spatial Self-Tunable Afterglow.

Highly efficient persistent (lifetime > 0.1 s) room-temperature phosphorescence (pRTP) chromophores are important for futuristic high-resolution afterglow imaging for state-of-the-art security, analytical, and bioimaging applications. Suppression of the radiationless transition from the lowest triplet excited state (T1) of the chromophores is a critical factor to access the high RTP yield and RTP lifetime for desirable pRTP. Logical explanations for factor suppression based on chemical structures have not been reported. Here we clarify a strategy to reduce the radiationless transition from T1 based on chemical backbones and yield a simultaneous high RTP yield and high RTP lifetime. Yellow phosphorescence chromophores that contain a coronene backbone were synthesized and compared with yellow phosphorescent naphthalene. One of the designed coronene derivatives reached a RTP yield of 35%, which is the best value for chromophores with a RTP lifetime of 2 s. The optically measured rate constant of a radiationless transition from T1 was correlated precisely with a multiplication of vibrational spin–orbit coupling (SOC) at a T1 geometry and with the Franck–Condon chromophore factor. The agreement between the experimental and theoretical results confirmed that the extended two-dimensional fused structure in the coronene backbone contributes to a decrease in vibrational SOC and Franck–Condon factor between T1 and the ground state to decrease the radiationless transition. A resolution-tunable afterglow that depends on excitation intensity for anticounterfeit technology was demonstrated, and the resultant chromophores with a high RTP yield and high RTP lifetime were ideal for largely changing the resolution using weak excitation light.

Vibrational Radiationless Transition from Triplet States of Chromophores at Room Temperature.

The radiationless transition rate based on intramolecular vibrations from the lowest excited triplet state (T1) at room temperature [knr(RT)] is crucial for triplet energy harvesting in optoelectronics and photonics applications. Although a decrease of knr(RT) of chromophores with strong intermolecular interactions is often proposed, scientific evidence for this has not been reported. Here we report a method to predict knr(RT). We optically estimated knr(RT) of various molecularly dispersed chromophores with a variety of transition characteristics from T1 to the ground state (S0) under appropriate inert liquid or solid host conditions. Spin-orbit coupling (SOC) without considering molecular vibrations was not correlated with the estimated knr(RT). However, the estimated knr(RT) was strongly correlated with a multiplication of SOC considering vibrations freely allowed at room temperature and the Franck-Condon factor. This correlation revealed that knr(RT) of many heavy-atom-free chromophores with a visible T1-S0 transition energy and local excited T1-S0 transition characteristics is intrinsically less than 100 s-1 even when vibrations freely occur. This information will assist researchers to appropriately design materials without limitations regarding intermolecular interactions to control T1 lifetime at room temperature and facilitate triplet energy harvesting.

Manipulating the Dynamics of Dark Excited States in Organic Materials for Phototheranostics.

Manipulating the dynamics of dark excited states (DESs), such as higher excited singlet or excited triplet states with no or small radiative decay, are of both fundamental and practical interests, an important application being photoactivated diagnosis and therapy (phototheranostics), which include photoacoustic (PA) imaging, photodynamic therapy (PDT), and photothermal therapy (PTT). However, the current understanding of DESs in organic structures is rather limited, thus making any rational manipulation of DES in organic materials very challenging.A DES decays primarily by radiationless transition through two pathways: (i) singlet-to-triplet intersystem crossing (ISC) and (ii) internal conversion (IC) relaxation. The deactivation of a DES via ISC can generate cytotoxic reactive oxygen species (ROS) for PDT, while IC could convert photons into heat for PA imaging and PTT. In this Account, we highlight our research on developing a fundamental understanding of structure-property relationships for manipulation of DESs in organic materials in relation to phototheranostic applications. We describe the application of femtosecond transient absorption (fs-TA) spectroscopy for obtaining valuable insights into the DES dynamics. Afterward, we present our work on DESs in nonrigid molecules that revealed greatly enhanced ISC through geometry twisting, which leads to an innovative pathway to develop organic materials exhibiting external stimuli-responsive reversible switching of ISC. We introduce the concept of smart PDT where highly efficient ISC imparted by geometry twisting in the acidic environment specific to tumors leads to very efficient and highly localized PDT, thus leaving surrounding healthy tissues at a different pH unaffected. This insightful understanding of ISC can lead to the development of more advanced photosensitizers for PDT. Two other emergent concepts from our work presented here are (1) significantly enhanced IC producing strong local heating by combining two-photon absorption with excited state absorption for cumulative multiphoton absorption, thus greatly increasing the strength of the PA signal for nonlinear PA imaging, and (2) shown by an example of an organic molecule, BODIPY, nanoscale charge-transfer state mediated strong IC in aggregate nanoparticles resulting in exceptionally high photothermal conversion efficiency of 61% for both PA and PTT. Some in vivo results of the phototheranostic studies using BODIPY are presented, providing an elegant example of nanoscale manipulation of the excited state dynamics.This Account concludes with a summary and discussion of future perspectives. We hope this Account will deepen the understanding of molecular and nanoscale control of excited state dynamics in organic materials, hopefully enticing a broad range of scientists within different disciplinary areas.

Structure of Kα1,2 - and Kβ1,3 -emission x-ray spectra for Se, Y, and Zr

The $K\ensuremath{\alpha}$ and $K\ensuremath{\beta}$ x-ray spectra of Se, Y, and Zr were studied experimentally and theoretically in order to obtain information on the $K{\ensuremath{\alpha}}_{1}$ line asymmetry and the spin doublet in $K{\ensuremath{\beta}}_{1,3}$ diagram lines. Using a high-resolution antiparallel double-crystal x-ray spectrometer, we obtained the line shapes, that is, asymmetry index and natural linewidths. We found that the corrected full width at half maximum of the $K{\ensuremath{\alpha}}_{1}$ and $K{\ensuremath{\alpha}}_{2}$ lines as a function of $Z$ is in good agreement with the data in the literature. Furthermore, satellite lines arising from shake-off appear in the low-energy side of the $K{\ensuremath{\alpha}}_{1}$ and $K{\ensuremath{\alpha}}_{2}$ lines in Se but, in Y and Zr, it was very difficult to identify the contribution of the shake process to the overall lines. The $K{\ensuremath{\beta}}_{1,3}$ natural linewidth of these elements was also corrected using the appropriate instrumental function for this type of x-ray spectrometer, and the spin doublet energies were obtained from the peak positions. The corrected full width at half maximum (FWHM) of the $K{\ensuremath{\beta}}_{1}$ x-ray lines increases linearly with $Z$, but this tendency was found to be, in general, not linear for $K{\ensuremath{\beta}}_{3}$ x-ray lines. This behavior may be due to the existence of satellite lines originated from shake processes. Simulated line profiles, obtained using the multiconfiguration Dirac-Fock formalism, accounting for radiative and radiationless transitions and shake-off processes, show a very good agreement with the high-resolution experimental spectra.

Sydney Leach. 11 April 1924—24 December 2019

Sydney Leach will be remembered as an outstanding and inspirational scientist, an irreplaceable friend to many—artists and musicians as well as academic colleagues. He encouraged and influenced numerous scientists as a mentor. After graduating from King's College London and carrying out war work at Farnborough, he spent all his scientific life based in Paris, working principally at Orsay and, in his later years, at the Observatoire de Paris–Meudon. Sydney was a major influence in establishing chemical physics in France after World War II, founding the highly influential Laboratoire de Photophysique Moléculaire (LPPM) at Orsay, where much of his pioneering work was performed. The ‘Sydney lab’ lives on in the newly created Institut des Sciences Moléculaires d'Orsay. Early experiments often took place at the synchrotron source (ACO, Super-ACO), just a few hundred yards from LPPM. He was a pioneering advocate of synchrotron radiation, and a driving force for its use in spectroscopy and photodynamics, along with free-electron lasers, supersonic jets, coincidence spectroscopy and matrix isolation—techniques that were applied and refined over decades and used to explore fundamental processes such as photoionization, vibronic coupling and radiationless transitions. Sydney's seminal studies of polyatomic molecular ions led him towards fresh horizons in planetary atmospheric and space science. His work opened new vistas in cometary spectroscopy, polycyclic aromatic hydrocarbons, fullerenes and their possible roles in the chemistry of the interstellar medium and, finally, biologically relevant species, helping to instigate the newly developing subject of astrobiology—a perfect example of his sustained prescience in the world of science.

Lise Meitner, β-decay and non-radiative electromagnetic transitions

The current activities in detecting neutrinos as carriers of information from far out in our Universe prompt us to look back on the research activities a century ago that led to the discovery of these weakly interacting particles. One of the leading researchers was Lise Meitner, who observed electrons with well-defined energy, besides the continuous energy spectrum emitted in β decay. These electron lines are well understood as radiationless nuclear transitions competing with γ-ray emission. It is proposed to name the electrons resulting out of this so-called internal conversion process after Lise Meitner and Charles D. Ellis. The equivalent process within the electronic (atomic) shell is the Auger effect , competing with X-ray emission. In this context, the radioactive decay of UX1 or 234 Th, well studied a century ago by Lise Meitner and Charles Ellis, is re-visited, and the mono-energetic electrons are ascribed entirely to the internal conversion process.

Electron transfer in nonpolar media.

The Marcus theory of electron transfer views fluctuating orientations of permanent dipoles as the nuclear mode bringing the donor and acceptor into the tunneling resonance. Electronic polarization of the solvent is excluded as the fast mode adiabatically following the electronic density. This view, valid for solids, does not apply to molecular liquids where molecular translations (density fluctuations) modulate the induction interaction of the donor-acceptor complex with the solvent. This mechanism of promoting radiationless electronic transitions is considered here in the framework of the perturbation liquid-state theory. The reorganization energy of electron transfer in nonpolar solvents is nonzero and reaches the values of 0.1-0.3 eV for typical molecular sizes and solvents used in applications. The reorganization energy scales quadratically with the molecular polarizability of the solvent and decays as the inverse fifth power with the size of the donor and acceptor. The combination of the entropic character of the density fluctuations, driven by re-packing of molecular cores, with the short range of induction solute-solvent interactions leads to the violation of the fluctuation-dissipation theorem for the variance of the donor-acceptor energy gap. An explicit, approximately hyperbolic, dependence of the reorganization energy on temperature is predicted. It leads to a non-Arrhenius kinetic law for the rate constant of electron transfer in nonpolar liquid solvents.

The elusive dynamics of aqueous permanganate photochemistry.

Despite decades of investigation, mechanistic details of aqueous permanganate photo-decomposition remain unclear. Here we follow photoinduced dynamics of aqueous permanganate with femtosecond spectroscopy. Photoexcitation of KMnO4(aq) in the visible unleashes a sub-picosecond cascade of non-radiative transitions, leading to a distinct species which relaxes to S0 with a lifetime of 16 ps. Tuning excitation to the UV shows increasing formation of a metastable intermediate, which outlives our ∼1 ns window of detection. Guided by electronic structure calculations and observations from three pulse excitation experiments, we assign the 16 ps species as the lowest Jahn-Teller component of the 3T1 triplet state and suggest a plausible sequence of radiationless transitions, which rapidly populate it. In conjunction with photodecomposition quantum yields obtained from the literature, these results demonstrate that aqueous permanganate photo-decomposition proceeds through a long-lived intermediate which is formed in parallel to the triplet in less than one ps upon UV absorption. The possibility that this is the postulated highly oxidative peroxo species, a fraction of which leads to the stable (MnO2- + O2) fragments, is discussed. Finally, periodic modulations detected in the pump-probe signal are assigned to ground-state vibrational coherences excited by impulsive Raman. Their wavelength-dependent absolute phases outline the borders between adjacent electronic transitions in the linear spectrum of permanganate.

Singlet fission in a hexacene dimer: energetics dictate dynamics.

Singlet fission (SF) is an exciton multiplication process with the potential to raise the efficiency limit of single junction solar cells from 33% to up to 45%. Most chromophores generally undergo SF as solid-state crystals. However, when such molecules are covalently coupled, the dimers can be used as model systems to study fundamental photophysical dynamics where a singlet exciton splits into two triplet excitons within individual molecules. Here we report the synthesis and photophysical characterization of singlet fission of a hexacene dimer. Comparing the hexacene dimer to analogous tetracene and pentacene dimers reveals that excess exoergicity slows down singlet fission, similar to what is observed in molecular crystals. Conversely, the lower triplet energy of hexacene results in an increase in the rate of triplet pair recombination, following the energy gap law for radiationless transitions. These results point to design rules for singlet fission chromophores: the energy gap between singlet and triplet pair should be minimal, and the gap between triplet pair and ground state should be large.

High resolution vibronic state-specific dissociation of NO2+ in the 10.0-15.5 eV energy range by synchrotron double imaging photoelectron photoion coincidence.

Vacuum ultraviolet (VUV) photoionization and dissociative photoionization of NO2 in the 10.0-15.5 eV energy range have been investigated in detail by using high-resolution double imaging photoelectron photoion coincidence (i2PEPICO) at synchrotron SOLEIL. Five low-lying electronic states of the NO2+ cation, X1Σg+, a3B2, b3A2, A1A2 and B1B2, are prepared with well-resolved vibronic structures and their state-specific dissociation mechanisms are unraveled and discussed. The present experimental results clarify that except the X1Σg+ ground electronic state and the first three vibrational levels of the a3B2 electronic state, the other cationic states of NO2+ within the present energy range are totally dissociative towards the NO+(X1Σ+) + O(3P) and/or NO+(X1Σ+) + O(1D) dissociation limits. An energy barrier exists along the direct dissociation route of the a3B2 state, and the b3A2 electronic state is a quasi-bound state with a very shallow well, both of which adiabatically correlate to the NO+(X1Σ+) + O(3P) dissociation limit. The a3B2 state mainly with bending vibration excitations undergoes a non-adiabatic transition to the 23A''(3B1) repulsive state along its bending potential energy curve and then predissociates into the NO+(X1Σ+) + O(3P) products. Our experimental results firstly demonstrate that the NO+(X1Σ+, v) fragment ions produced from individual vibronic levels of the dissociative NO2+(a3B2, b3A2) states are produced at the v = 0 ground vibrational level with a high rotational population due to the excitation of the vibrational bending mode of NO2+ and the associated imparted torque upon dissociation. The slower predissociations of the A1A2 and B1B2 electronic states via their spin-orbit couplings with the repulsive 23A''(3B1) state produce the NO+(X1Σ+) and O(3P) fragments with a long vibrational progression. In addition, the B1B2 state can also undergo a radiationless transition such as internal conversion into the hot X1Σg+ state and then dissociate into the second dissociation channel correlated to the NO+(X1Σ+) and O(1D) products.

Multiconfiguration Dirac–Fock calculations of Zn K‐shell radiative and nonradiative transitions

Zinc K‐shell radiative and radiationless transition rates are calculated using the multiconfiguration Dirac–Fock method. Correlation up to the 4p orbital is included in almost all transition rate calculations. Calculated radiative transition rates and transition probabilities are compared with Scofield's Dirac–Hartree–Slater and Dirac–Hartree–Fock calculations, presenting good agreement with the later. Radiative transition intensity ratios involving the strongest lines are compared with theoretical, experimental, and empirical‐fit values. Most ratios are in close agreement with the empirical‐fit values from NIST's Fundamental Parameters database. Calculated radiationless transition rates and ratios are compared with Chen et al.'s Dirac–Fock values and Safronova et al.'s Dirac–Fock values. The K‐LL transition rates are overall lower than Chen et al.'s values, whereas the K‐LX and K‐XY transition rates are overall higher. Calculated K‐LX/K‐LL and K‐XY/K‐LL ratios are relatively close to the experimental values compared. Some calculated intensities relative to K‐L are in good agreement with the experimental values, whereas others present worse agreement. The calculated fluorescence yield is higher than all theoretical, experimental, and empirical‐fitted values compared, probably because the total radiationless transition rate value calculated in the present work is relatively low.

Nonequilibrium vibrational population and donor-acceptor vibrations affecting rates of radiationless transitions.

An analytical theory is developed for radiationless transitions in molecules characterized by nonequilibrium populations of their vibrational modes. Several changes to the standard transition-state framework follow from nonequilibrium conditions: (i) non-Arrhenius kinetics, (ii) the violation of the fluctuation-dissipation theorem (FDT), and (iii) the breakdown of the detailed balance. The violation of the FDT is reflected in the breakdown of relations between the first (Stokes shift) and second (inhomogeneous band-width) spectral moments and of similar relations between reorganization parameters for radiationless transitions. The detailed balance between the forward and backward rates is not maintained, requiring a lower effective free energy of the reaction relative to the thermodynamic limit. The model suggests that strong control of radiationless transitions can be achieved if a nonequilibrium population of vibrations modulating the donor-acceptor distance is produced.

Structure and photoluminescence properties of Ca0.99−xSrxAlSiN3:0.01Ce3+ solid solutions

AbstractNitride phosphors of Ca0.99−xSrxAl1.01Si0.99N3:0.01Ce3+ (0 ≤ x ≤ 0.9) were synthesized by conventional solid‐state method. XRD data analysis shows that all samples are single phase with CaAlSiN3‐type structure. Under blue or near‐ultraviolet (~400 nm) light excitation, the emission peak can be tuned largely from 615 to 568 nm by increasing Sr content, and the emission intensity is maximal at x = 0.8. With the Sr content increase, the emission band blue‐shifts due to the decreased crystal field splitting and the reduced centroid shift; while the thermal luminescence quenching resistance is almost unchanged. The quenching temperature (T50) is well above 500 K for all samples, which satisfies the requirement of commercial application. The quenching process is mainly attributed to the radiationless transitions by thermally activated crossover from the 5d excited state to 4f ground state in the configurational coordinate diagram. The luminescence properties show that the (Ca,Sr)AlSiN3:Ce3+ phosphors are very promising for use in blue and near‐ultraviolet light excited white‐light‐emitting diodes.

Ultrafast excited state intramolecular proton/charge transfers in novel NIR-emitting molecules

The unusual large bathochromic shift from a novel near-infrared (NIR)-emitting molecule, 2-[3-(benzo[d]thiazol-2-yl)-2-hydroxy-5-methylstyr-yl]-3-ehtylbenzo[d]thiazol-3-ium iodide (named cyanine 1) with combination of intramolecular charge transfer (ICT) and intramolecular proton transfer (IPT) process in one molecular framework, is systematically investigated using ultrafast transient absorption (TA) spectroscopy and quantum chemical calculations. In order to understand the synergetic coupling effect of the excited state intramolecular proton/charge transfers (ESIPT/ESICT) for the intense near-infrared emission of cyanine 1, an analogue non-ESIPT molecule, 2-[5-(benzo[d]thiazol-2-yl)-2-hydroxystyryl]-3-ehtylbenzo[d]thiazol-3-ium iodide (named cyanine 2) has also been investigated as comparison. Steady-state spectra and theoretical calculations suggest that the large Stokes shift and high fluorescence quantum yield in cyanine 1 originate from the ultrafast ESIPT, which leads to the efficient extension of π-conjugation in the molecular backbone in its excited states. Femtosecond transient absorption spectra further confirm above-mentioned conclusion that an extremely fast ESIPT process occurs in cyanine 1 upon excitation, followed by a solvent reorganization process (ca. 1.5 ps). This solvation is obviously slower compared to cyanine 2 (ca. 0.8 ps), indicating the extent of ESICT concerned ESIPT in keto* form of cyanine 1 is slightly weaker than that of ESICT in cyanine 2, where the fast ESIPT plays an important role in extending the efficient π-conjugation in the molecular backbone by adjusting the electronic charge distribution in keto* form. Such an effect can reduce the radiationless transition due to weak solvation process in keto* form, and then promotes the quantum yield of the large red-shifted fluorescence in cyanine 1.

Strain induced optical properties of perovskite LaFeO3

Tuning the properties of perovskite metal oxide by varying the crystal structure leads to many functional applications. Here strain induced by an external matrix like reduced graphene oxide (RGO) in the crystal structure of LaFeO3 (LFO) and its related optical properties are investigated. X-ray diffraction technique is used to find the strain in the crystal structure of graphene-LFO (GL) composite. The band gap and band tailing energy of the composites are determined by applying modified Kubelka–Munk function on UV–Vis DRS data. The steady-state fluorescence emission spectra of the samples show a monotonic decrease with increase in concentration of RGO. GL composites show enhanced photocatalytic degradation of RhodamineB (RhB) dye under solar spectrum irradiation. Variations in lifetime, fluorescence intensity and photocatalytic activity of GL composites are explained in terms of intermediate bands produced by RGO and associated radiation-less transitions of photoexcited electrons. RGO induced strain and tuning the optical properties of perovskites can provide a pathway to engineer new multifunctional materials in optoelectronic applications.

Spin-Vibronic Mechanism for Intersystem Crossing.

Intersystem crossing (ISC), formally forbidden within nonrelativistic quantum theory, is the mechanism by which a molecule changes its spin state. It plays an important role in the excited state decay dynamics of many molecular systems and not just those containing heavy elements. In the simplest case, ISC is driven by direct spin-orbit coupling between two states of different multiplicities. This coupling is usually assumed to remain unchanged by vibrational motion. It is also often presumed that spin-allowed radiationless transitions, i.e. internal conversion, and the nonadiabatic coupling that drives them, can be considered separately from ISC and spin-orbit coupling owing to the vastly different time scales upon which these processes are assumed to occur. However, these assumptions are too restrictive. Indeed, the strong mixing brought about by the simultaneous presence of nonadiabatic and spin-orbit coupling means that often the spin, electronic, and vibrational dynamics cannot be described independently. Instead of considering a simple ladder of states, as depicted in a Jablonski diagram, one must consider the more complicated spin-vibronic levels. Despite the basic ideas being outlined in the 1960s, it is only with the advent of high-level theory and femtosecond spectroscopy that the importance of the spin-vibronic mechanism for ISC in both fundamental as well as applied research fields has been revealed with significant impact across chemistry, physics, and biology. In this review article, we present the theory and fundamental principles of the spin-vibronic mechanism for ISC. This is followed by empirical rules to estimate the rate of ISC within this regime. The most recent developments in experimental techniques, theoretical methods, and models for the spin-vibronic mechanism are discussed. These concepts are subsequently illustrated with examples, including the ISC mechanisms in transition metal complexes, small organic molecules, and organic chromophores.

Theoretical Insights into the Switching Off/On of 1 O2 Photosensitization in Chemicontrolled Photodynamic Therapy.

Density Functional Theory and time-dependent (TD) DFT calculations were carried out for recently reported 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-based photosensitizers (PSs) that could be activated by reactive oxygen species (ROS) to generate 1 O2 specifically in target tissues. To assess the applicability of the compounds as activatable PSs (a-PSs) in photodynamic therapy, absorption wavelengths; singlet-triplet energy gaps; and spin-orbit matrix elements for the radiationless transition, Sn →Tm , were investigated. A TD-DFT qualitative analysis indicated that only a Br-substituted BODIPY derivative with the chromanol ring of α-tocopherol linked by methylene functioned as an a-PS. The chromanol ring promotes photoinduced electron transfer to the BODIPY unit that reduces the probability of intersystem crossing and triplet-state population, and can turn off 1 O2 photosensitization. Therefore, 1 O2 photosensitization can be switched on only in target cells in which the chromanol ring is oxidized by ROS. The oxidation reaction pathways of the most promising derivative, by either 1 O2 or cumyloxyl radical as typical ROS, have been examined to reveal that oxidation by the cumyloxyl radical is more effective than that by 1 O2 .