Diogen Laertije - Životi i mišljenja istaknutih filozofa

The vast majority of polyhedral assemblies prepared by combining organic bent ligands and “photophysically innocent” palladium­(II) metal ions are nonemissive. We report here a simple strategy to switch on the luminescence properties of a polyhedral assembly by combining a thermally activated delayed fluorescence (TADF) organic emitter based on a dipyridylcarbazole ligand scaffold with Pd²⁺ ions, giving rise to a luminescent Pd₆L₁₂ molecular cube. The assembly is capable of encapsulating within its cavity up to three molecules per cage of fluorescein, in its neutral lactone form, and up to two molecules of Rose Bengal in its dianionic quinoidal form. Photoinduced electron transfer (PeT) between the photoactive cage and the encapsulated Fluorescein and photoinduced energy transfer (PET) from the cage to encapsulated Rose Bengal have been observed by steady-state and time-resolved emission spectroscopy

Revising Intramolecular Photoinduced Electron Transfer (PET) from First-Principles

ConspectusPhotoinduced electron transfer (PET) plays relevant roles in many areas of chemistry, including charge separation processes in photovoltaics, natural and artificial photosynthesis, and photoluminescence sensors and switches. As in many other photochemical scenarios, the structural and energetic factors play relevant roles in determining the rates and efficiencies of PET and its competitive photodeactivation processes. Particularly, in the field of fluorescent sensors and switches, intramolecular PET is believed (in many cases without compelling experimental proof) to be responsible of the quench of fluorescence. There is an increasing experimental interest in fluorophore’s molecular design and on achieving optimal excitation/emission spectra, excitation coefficients, and fluorescence quantum yields (importantly for bioimaging purposes), but less efforts are devoted to fundamental mechanistic studies. In this Account, I revise the origins of the fluorescence quenching in some of these systems with state-of-the-art quantum chemical tools. These studies go beyond the common strategy of analyzing frontier orbital energy diagrams and performing PET thermodynamics calculations. Instead, the potential energy surfaces (PESs) of the lowest-lying excited states are explored with time-dependent density functional theory (TD-DFT) and complete active space self-consistent field (CASSCF) calculations and the radiative and nonradiative decay rates from the involved excited states are computed from first-principles using a thermal vibration correlation function formalism. With such a strategy, this work reveals the real origins of the fluorescence quenching, herein entitled as dark-state quenching. Dark states (those that do not absorb or emit light) are often elusive to experiments and thus, computational investigations can provide novel insights into the actual photodeactivation mechanisms. The success of the dark-state quenching mechanism is demonstrated for a wide variety of fluorescent probes, including proton, cation and anion targets. Furthermore, this mechanism provides a general picture of the fluorescence quenching which englobes intramolecular charge-transfer (ICT), ratiometric quenching, and those radiationless mechanisms believed to be originated by PET. Finally, this Account provides for the first time a computational protocol to quantitatively estimate this phenomenon and provides the ingredients for the optimal design of fluorescent probes from first principles

Exploring the Triplet Excited State Potential Energy Surfaces of a Cyclometalated Pt(II) Complex: Is There Non-Kasha Emissive Behavior?

In this Article, we address the complexity of the emissive processes of a square-planar heteroleptic Pt­(II) complex bearing 2-phenylpyridine (ppy) as cyclometalated ligand and an acetylacetonate derivative (dbm) as ancillary ligand. The origins of emission were identified with the help of density functional theory (DFT) and quadratic response (QR) time-dependent (TD)-DFT calculations including spin–orbit coupling (SOC). To unveil the photodeactivation mechanisms, we explored the triplet potential energy surfaces and computed the SOCs and the radiative decay rates (<i>k</i>_r) from possible emissive states. We find that emission likely originates from a higher-lying ³MLCT/³LLCT state and not from the Kasha-like ³MLCT/³LC_dbm state. The temperature-dependent nonradiative deactivation mechanisms were also elucidated. The active role of metal-centered (³MC) triplet excited states is confirmed for these deactivation pathways

RASPT2/RASSCF vs Range-Separated/Hybrid DFT Methods: Assessing the Excited States of a Ru(II)bipyridyl Complex

The excited states of the <i>trans</i>(Cl)-Ru(bpy)Cl₂(CO)₂ (bpy = bypyridyl) transition-metal (TM) complex are assessed using the newly developed second-order perturbation theory restricted active space (RASPT2/RASSCF) method. The delicate problem of partitioning the RAS subspaces (RAS1, RAS2, and RAS3) is addressed, being the choice of the RAS2 the bottleneck to obtain a balanced description of the excited states of different nature when TMs are present. We find that the RAS2 should be composed by the correlation orbitals involved in covalent metal–ligand bonds. The level of excitations within the RAS1 and RAS3 subspaces is also examined. The performance of different flavors of time-dependent density functional theory including pure, hybrid, meta-hybrid, and range-separated functionals in the presence of solvent effects is also evaluated. It is found that none of the functionals can optimally describe all the excited states simultaneously. However, the hybrid M06, B3LYP, and PBE0 functionals seem to be the best compromise to obtain a balanced description of the excited states of <i>trans</i>(Cl)-Ru(bpy)Cl₂(CO)₂, when comparing with the experimental spectrum. The conclusions obtained in this molecule should pave the road to properly treat excited states of larger Ru–polypyridyl complexes, which are of particular interest in supramolecular chemistry

Modeling the Photochrome–TiO₂ Interface with Bethe–Salpeter and Time-Dependent Density Functional Theory Methods

Hybrid organic–inorganic semiconductor systems have important applications in both molecular electronics and photoresponsive materials. The characterizations of the interface and of the electronic excited-states of these hybrid systems remain a challenge for state-of-the-art computational methods, as the systems of interest are large. In the present investigation, we present for the first time a many-body Green’s function Bethe–Salpeter investigation of a series of photochromic molecules adsorbed onto TiO₂ nanoclusters. On the basis of these studies, the performance of time-dependent density functional theory (TD-DFT) calculations is assessed. In addition, the photochromic properties of different hybrid systems are also evaluated. This work shows that qualitatively different conclusions can be reached with TD-DFT relying on various exchange–correlation functionals for such organic–inorganic interfaces and paves the way to more accurate simulation of many hybrid materials

Electronic Structure of N₂P₂ Four-Membered Rings and the Effect of Their Ligand Coordination to M(CO)₅ (Cr, Mo, and W)

In this article the biradicaloid character of the ground-state structures of N₂P₂R₂ (R = CH₃) rings is studied using the DFT and CASSCF methods, and a satisfactory agreement of the B3LYP functional and CASSCF­(6,6) ab initio method has been found. In order to obtain an adequate description of the biradicaloid character, we have combined two criteria: (i) singlet–triplet energy gaps and (ii) relative values of the occupation numbers for bonding and antibonding orbitals associated with the radical sites. We have analyzed how the biradicaloid character of the N₂P₂R₂ ring changes upon coordination to M­(CO)₅ (M = Cr, Mo, and W) at the B3LYP/6-311+G* level of theory. Interestingly, in some cases the biradicaloid character increases dramatically upon complexation of the N₂P₂R₂ ligands

General Approach To Compute Phosphorescent OLED Efficiency

Phosphorescent organic light-emitting diodes (PhOLEDs) are widely used in the display industry. In PhOLEDs, cyclometalated Ir­(III) complexes are the most widespread triplet emitter dopants to attain red, e.g., Ir­(piq)₃ (piq = 1-phenylisoquinoline), and green, e.g., Ir­(ppy)₃ (ppy = 2-phenylpyridine), emissions, whereas obtaining operative deep-blue emitters is still one of the major challenges. When designing new emitters, two main characteristics besides colors should be targeted: high photostability and large photoluminescence efficiencies. To date, these are very often optimized experimentally in a trial-and-error manner. Instead, accurate predictive tools would be highly desirable. In this contribution, we present a general approach for computing the photoluminescence lifetimes and efficiencies of Ir­(III) complexes by considering all possible competing excited-state deactivation processes and importantly explicitly including the strongly temperature-dependent ones. This approach is based on the combination of state-of-the-art quantum chemical calculations and excited-state decay rate formalism with kinetic modeling, which is shown to be an efficient and reliable approach for a broad palette of Ir­(III) complexes, i.e., from yellow/orange to deep-blue emitters

Controlling Triplet–Triplet Annihilation Upconversion by Tuning the PET in Aminomethyleneanthracene Derivatives

Activatable triplet–triplet annihilation upconversion was achieved using aminomethyleneanthracene derivatives. The molecular structures of the anthracene derivatives were varied by changing the number of phenyl substituents on the anthracene core (<b>A-1</b>, <b>A-2</b>, and <b>A-3</b> containing no phenyl and one and two phenyl substituents, respectively). The structural modifications tune the intersystem crossing (ISC), the fluorescence, as well as the distance between the electron donor (amino group) and the fluorophore by using methylene (<b>A-1</b> and <b>A-2</b>) or a benzyl moiety (<b>A-3</b>) as a linker. Triplet–triplet annihilation upconversion is mainly tuned by photoinduced electron transfer (PET). Hence, the fluorescence of <b>A-1</b> and <b>A-2</b> can be switched on by protonation or acetylation of the amino group, whereas <b>A-3</b> gives persistent strong fluorescence. Determination of the Gibbs free energy changes indicated significantly different PET driving forces for the three compounds. The mechanism of the fluorescence switching was studied with steady state UV–vis absorption, fluorescence emission spectroscopy, nanosecond transient absorption spectroscopy, and <i>ab initio</i> computations. We found that the PET exerts different quenching effects on the <i>singlet</i> and <i>triplet</i> excited states of the anthracene derivatives. The triplet–triplet annihilation upconversion using these compounds as triplet acceptors/emitter was studied as well, and it was found that upconversion can be switched on by inhibition of the PET through acetylation and protonation

Phosphorescent Properties of Heteroleptic Ir(III) Complexes: Uncovering Their Emissive Species

In this contribution, we assess the computational machinery to calculate the phosphorescence properties of a large pool of heteroleptic [Ir(C^N)2(N^N)]+ complexes (where N^N is an ancillary ligand and C^N is a cyclometalating ligand) including their phosphorescent rates and their emission spectra. Efficient computational protocols are next proposed. Specifically, different flavors of DFT functionals were benchmarked against DLPNO-CCSD(T) for the phosphorescence energies. The transition density matrix and decomposition analysis of the emitting triplet excited state enable us to categorize the studied complexes into different cases, from predominant triplet ligand-centered (3LC) character to predominant charge-transfer (3CT) character, either of metal-to-ligand charge transfer (3MLCT), ligand-to-ligand charge transfer (3LLCT), or a combination of the two. We have also calculated the vibronically resolved phosphorescent spectra and rates. Ir(III) complexes with predominant 3CT character are characterized by less vibronically resolved bands as compared to those with predominant 3LC character. Furthermore, some of the complexes are characterized by close-lying triplet excited states so that the calculation of their phosphorescence properties poses additional challenges. In these scenarios, it is necessary to perform geometry optimizations of higher-lying triplet excited states (i.e., Tn). We demonstrate that in the latter scenarios all of the close-lying triplet species must be considered to recover the shape of the experimental emission spectra. The global analysis of computed emission energies, shape of the computed emission spectra, computed rates, etc. enable us to unambiguously pinpoint for the first time the triplet states involved in the emission process and to provide a general classification of Ir(III) complexes with regard to their phosphorescence properties

Molecular Structure–Intersystem Crossing Relationship of Heavy-Atom-Free BODIPY Triplet Photosensitizers

A thiophene-fused BODIPY chromophore displays a large triplet-state quantum yield (Φ_T = 63.7%). In contrast, when the two thienyl moieties are not fused into the BODIPY core, intersystem crossing (ISC) becomes inefficient and Φ_T remains low (Φ_T = 6.1%). First-principles calculations including spin–orbit coupling (SOC) were performed to quantify the ISC. We found larger SOC and smaller singlet–triplet energy gaps for the thiophene-fused BODIPY derivative. Our results are useful for studies of the photochemistry of organic chromophores