Thermally Tunable Dual Emission of the d(8)-d(8) Dimer [Pt2(μ-P2O5(BF2)2)4](4)

A terpyridyl-imidazole based europium tris-(β-diketonate) complex as an efficient molecular luminescent thermometer and single component white light emitter via synergy in energy transfer between ligands and Eu3

The thermosensing and thermochromic behavior of one of our recently reported terpyridyl-imidazole based ternary europium tris-(β-diketonate) complexes of the composition [Eu(tta)3(tpy-HImzphen)] (tta = 2-thenoyltrifluoroacetone and tpy-HImzphen = 2-(4-[2,2':6',2''] terpyridin-4'-yl-phenyl)-1H-phenanthro[9,10-d]imidazole) has been thoroughly investigated in this work. The said Eu(III) complex exhibits magnificent thermosensing as well as thermochromic properties and can be recommended as an excellent temperature sensor in a wide temperature domain of 273-343 K in terms of both emission intensity ratio (Sm = 5.78% K-1 at Tm = 343 K, δT = 0.012 K) and lifetime values (Sm = 3.36% K-1 at Tm = 333 K, δT = 0.009 K) or even in terms of its emitting color (red at 268 K, violet at 303 K, and blue at 343 K). Additionally, it displays remarkable solvent-induced luminescence behavior by displaying various emitting colors instead of its sole characteristic red emission upon varying the nature of the solvent. Finally, amalgamating these two features, we are able to attain white light emission (Commission Internationale de l'Eclairage coordinates: x = 0.34, y = 0.38) at 283 K from a single component. A plausible energy transfer mechanism has also been proposed in light of the existence of the ligand-to-metal charge transfer (LMCT) state as the quencher.

Excitation-Wavelength-Dependent Photophysics of d8d8 Di-isocyanide Complexes

Binuclear Rh(I) and Ir(I) TMB (2,5-dimethyl-2,5-diisocyanohexane) and dimen (1,8-diisocyanomenthane) complexes possess dσ*pσ and dπpσ singlet and triplet excited states that can be selectively excited in the visible and UV spectral regions. Using perturbational spin-orbit TDDFT, we unraveled the detailed character and spin mixing of these electronic transitions and found that delocalization of pσ and dπ orbitals over C≡N- groups makes C≡N stretching vibrations sensitive reporters of electron density and structural changes upon electronic excitation. Picosecond time-resolved infrared spectra measured after visible light, 375 nm, and 316 nm excitation revealed excitation-wavelength-dependent deactivation cascades. Visible light irradiation prepares the ¹dσ*pσ state that, after one or two (sub)picosecond relaxation steps, undergoes 70-1300 ps intersystem crossing to ³dσ*pσ, which is faster for the more flexible dimen complexes. UV-excited ^1,3dπpσ states decay with (sub)picosecond kinetics through a manifold of high-lying triplet and mixed-spin states to ³dσ*pσ with lifetimes in the range of 6-19 ps (316 nm) and 19-43 ps (375 nm, Ir only), bypassing ¹dσ*pσ. Most excited-state conversion and some relaxation steps are accompanied by direct decay to the ground state that is especially pronounced for the most flexible long/eclipsed Rh(dimen) conformer.

The Observation of Interchain Motion in Self-Assembled Crystalline Platinum(II) Complexes: An Exquisite Case but By No Means the Only One in Molecular Solids

In organic and organometallic solids, upon electronic excitation, most intermolecular structural relaxations follow a pathway along the π-π stacking direction or metal-metal bond with significant coupling strength. Differently, we discovered that the self-assembled platinum(II) complexes, Pt(fppz)₂, exhibit an unusual interchain contraction. The ground-state and excited-state multiple local minima were distinguished by temperature-dependent excitation/emission spectra, indicating the existence of multiple local minima. The time-resolved emission decay revealed the excited-state structural relaxation lifetime with τ_obs = 41 ns at 298 K. Temperature-dependent X-ray diffraction analysis showed that the packing geometries contract 0.6 Å along the interchain direction (a-axis) at 50 K compared to the geometries at 298 K. Such structural displacements render the slow internal conversion rate in the excited states. We thus demonstrate the correlation between the packing geometries and the excited-state dynamics of the self-assembled Pt(II) complexes, shedding light on the unique direction of interchain structural deformation of the molecular aggregates.

Can Coumarins Break Kasha's Rule?

Coumarin C-2 was reported ( Signore et al., J. Am. Chem. Soc. , 2010 , 132 , 1276 and Brancato et al., J. Phys. Chem. B , 2015 , 119 , 6144 ) to break Kasha's rule. However, the two lowest excited singlet states of C-2 are separated by less than 0.5 eV. To slow down the S₂ → S₁ internal conversion and thus to enable the Kasha's rule-breaking S₂ fluorescence, a much larger energy separation seems to be necessary. Thus, the photophysical behavior reported for C-2 raised very basic questions concerning mechanisms of nonradiative transitions in organic molecules. Herein we reinvestigated luminescence of C-2 and found that thoroughly purified C-2 does not show any dual fluorescence in steady-state experiments, contrary to the previous findings. The higher-energy emission, previously erroneously assigned as S₂ → S₀ fluorescence of C-2, stems from persistent impurity of the synthetic precursor (C-1).

Sky-blue thermally activated delayed fluorescence (TADF) based on Ag(i) complexes: strong solvation-induced emission enhancement

Remarkable solvation-induced emission enhancement is discovered on a new Ag(i) complex showing sky-blue thermally activated delayed fluorescence (TADF).

Temperature dependence of photophysical properties of a dinuclear C^N-cyclometalated Pt(ii) complex with an intimate Pt–Pt contact. Zero-field splitting and sub-state decay rates of the lowest triplet

A dinuclear Pt(ii) complexes exhibits an unusually large zero field splitting in its metal–metal-to-ligand charge transfer triplet excited state.

Intersystem crossing rate dependent dual emission and phosphorescence from cyclometalated platinum complexes: a second order cumulant expansion based approach

Cumulant expansion technique has been used to unravel the microscopic origin of phosphorescence and dual emission from cyclometalated platinum complexes.

TADF Material Design: Photophysical Background and Case Studies Focusing on CuI and AgI Complexes

The development of organic light emitting diodes (OLEDs) and the use of emitting molecules have strongly stimulated scientific research of emitting compounds. In particular, for OLEDs it is required to harvest all singlet and triplet excitons that are generated in the emission layer. This can be achieved using the so-called triplet harvesting mechanism. However, the materials to be applied are based on high-cost rare metals and therefore, it has been proposed already more than one decade ago by our group to use the effect of thermally activated delayed fluorescence (TADF) to harvest all generated excitons in the lowest excited singlet state S₁ . In this situation, the resulting emission is an S₁ →S₀ fluorescence, though a delayed one. Hence, this mechanism represents the singlet harvesting mechanism. Using this effect, high-cost and strong SOC-carrying rare metals are not required. This mechanism can very effectively be realized by use of Cu^I or Ag^I complexes and even by purely organic molecules. In this investigation, we focus on photoluminescence properties and on crucial requirements for designing Cu^I and Ag^I materials that exhibit short TADF decay times at high emission quantum yields. The decay times should be as short as possible to minimize non-radiative quenching and, in particular, chemical reactions that frequently occur in the excited state. Thus, a short TADF decay time can strongly increase the material's long-term stability. Here, we study crucial parameters and analyze their impact on the TADF decay time. For example, the energy separation ΔE(S₁ -T₁ ) between the lowest excited singlet state S₁ and the triplet state T₁ should be small. Accordingly, we present detailed photophysical properties of two case-study materials designed to exhibit a large ΔE(S₁ -T₁ ) value of 1000 cm^-1 (120 meV) and, for comparison, a small one of 370 cm^-1 (46 meV). From these studies-extended by investigations of many other Cu^I TADF compounds-we can conclude that just small ΔE(S₁ -T₁ ) is not a sufficient requirement for short TADF decay times. High allowedness of the transition from the emitting S₁ state to the electronic ground state S₀ , expressed by the radiative rate k^r (S₁ →S₀ ) or the oscillator strength f(S₁ →S₀ ), is also very important. However, mostly small ΔE(S₁ -T₁ ) is related to small k^r (S₁ →S₀ ). This relation results from an experimental investigation of a large number of Cu^I complexes and basic quantum mechanical considerations. As a consequence, a reduction of τ(TADF) to below a few μs might be problematic. However, new materials can be designed for which this disadvantage is not prevailing. A new TADF compound, Ag(dbp)(P₂ -nCB) (with dbp=2,9-di-n-butyl-1,10-phenanthroline and P₂ -nCB=bis-(diphenylphosphine)-nido-carborane) seems to represent such an example. Accordingly, this material shows TADF record properties, such as short TADF decay time at high emission quantum yield. These properties are based (i) on geometry optimizations of the Ag^I complex for a fast radiative S₁ →S₀ rate and (ii) on restricting the extent of geometry reorganizations after excitation for reducing non-radiative relaxation and emission quenching. Indeed, we could design a TADF material with breakthrough properties showing τ(TADF)=1.4 μs at 100 % emission quantum yield.

Ultrafast Wiggling and Jiggling: Ir2(1,8-diisocyanomenthane)42

Binuclear complexes of d⁸ metals (Pt^II, Ir^I, Rh^I,) exhibit diverse photonic behavior, including dual emission from relatively long-lived singlet and triplet excited states, as well as photochemical energy, electron, and atom transfer. Time-resolved optical spectroscopic and X-ray studies have revealed the behavior of the dimetallic core, confirming that M-M bonding is strengthened upon dσ* → pσ excitation. We report the bridging ligand dynamics of Ir₂(1,8-diisocyanomenthane)₄²⁺ (Ir(dimen)), investigated by fs-ns time-resolved IR spectroscopy (TRIR) in the region of C≡N stretching vibrations, ν(C≡N), 2000-2300 cm^-1. The ν(C≡N) IR band of the singlet and triplet dσ*pσ excited states is shifted by -22 and -16 cm^-1 relative to the ground state due to delocalization of the pσ LUMO over the bridging ligands. Ultrafast relaxation dynamics of the ¹dσ*pσ state depend on the initially excited Franck-Condon molecular geometry, whereby the same relaxed singlet excited state is populated by two different pathways depending on the starting point at the excited-state potential energy surface. Exciting the long/eclipsed isomer triggers two-stage structural relaxation: 0.5 ps large-scale Ir-Ir contraction and 5 ps Ir-Ir contraction/intramolecular rotation. Exciting the short/twisted isomer induces a ∼5 ps bond shortening combined with vibrational cooling. Intersystem crossing (70 ps) follows, populating a ³dσ*pσ state that lives for hundreds of nanoseconds. During the first 2 ps, the ν(C≡N) IR bandwidth oscillates with the frequency of the ν(Ir-Ir) wave packet, ca. 80 cm^-1, indicating that the dephasing time of the high-frequency (16 fs)^-1 C≡N stretch responds to much slower (∼400 fs)^-1 Ir-Ir coherent oscillations. We conclude that the bonding and dynamics of bridging di-isocyanide ligands are coupled to the dynamics of the metal-metal unit and that the coherent Ir-Ir motion induced by ultrafast excitation drives vibrational dephasing processes over the entire binuclear cation.

The interplay between fluorescence and phosphorescence with luminescent gold(i) and gold(iii) complexes bearing heterocyclic arylacetylide ligands

The photophysical properties of a series of gold(i) [LAu(C 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 CR)] (L = PCy₃ (1a-4a), RNC (5a), NHC (6a)) and gold(iii) complexes [Au(C^N^C)(CCR)] (1b-4b) bearing heterocyclic arylacetylide ligands with narrow band-gap are compared. The luminescence of both series are derived from an intraligand transition localized on the arylacetylide ligand (ππ*(CCR)) but 1a-3a displayed prompt fluorescence (τ_PF = 2.7-12.0 ns) while 1b-3b showed mainly phosphorescence (τ_Ph = 104-205 μs). The experimentally determined intersystem crossing (ISC) rate constants (k_ISC) are on the order of 10⁶ to 10⁸ s^-1 for the gold(i) series (1a-3a) but 10¹⁰ to 10¹¹ s^-1 for the gold(iii) analogues (1b-3b). DFT/TDDFT calculations have been performed to help understand the difference in the k_ISC between the two series of complexes. Owing to the different oxidation states of the gold ion, the Au(i) complexes have linear coordination geometry while the Au(iii) complexes are square planar. It was found from DFT/TDDFT calculations that due to this difference in coordination geometries, the energy gap between the singlet and triplet excited states (ΔE_ST) with effective spin-orbit coupling (SOC) for Au(i) systems is much larger than that for the Au(iii) counterparts, thus resulting in the poor ISC efficiency for the former. Time-resolved spectroscopies revealed a minor contribution (<2.9%) of a long-lived delayed fluorescence (DF) (τ_DF = 4.6-12.5 μs) to the total fluorescence in 1a-3a. Attempts have been made to elucidate the mechanism for the origins of the DF: the dependence of the DF intensity with the power of excitation light reveals that triplet-triplet annihilation (TTA) is the most probable mechanism for the DF of 1a while germinate electron-hole pair (GP) recombination accounts for the DF of 2a in 77 K glassy solution (MeOH/EtOH = 4 : 1). Both 4a and 4b contain a BODIPY moiety at the acetylide ligand and display only ¹IL(ππ*) fluorescence with negligible phosphorescence being observed. Computational analyses attributed this observation to the lack of low-lying triplet excited states that could have effective SOC with the S₁ excited state.

Intersystem Crossing in Diplatinum Complexes

Intersystem crossing (ISC) in solid [(C₄H₉)₄N]₄[Pt₂(μ-P₂O₅(BF₂)₂)₄], abbreviated Pt(pop-BF₂), is remarkably slow for a third-row transition metal complex, ranging from τ_ISC ≈ 0.9 ns at 310 K to τ_ISC ≈ 29 ns below 100 K. A classical model based on Boltzmann population of one temperature-independent and two thermally activated pathways was previously employed to account for the ISC rate behavior. An alternative we prefer is to treat Pt(pop-BF₂) ISC quantum mechanically, using expressions for multiphonon radiationless transitions. Here we show that a two-channel model with physically plausible parameters can account for the observed ISC temperature dependence. In channel 1, ¹A_2u intersystem crosses directly into ³A_2u using a high energy B-F or P-O vibration as accepting mode, resulting in a temperature-independent ISC rate. In channel 2, ISC occurs via a deactivating state of triplet character (which then rapidly decays to ³A_2u), using Pt-Pt stretching (160 cm^-1) as a distorting mode to provide the energy needed. Fitting indicates that the deactivating state, ³X, is moderately displaced (S = 0.5-3) and blue-shifted (ΔE = 1420-2550 cm^-1) from ¹A_2u. Our model accounts for the experimental observation that ISC in both temperature independent and thermally activated channels is faster for Pt(pop) than for Pt(pop-BF₂): in the temperature independent channel because O-H modes in the former more effectively accept than B-F modes in the latter, and in the thermally activated pathway because the energy gap to ³X is larger in the latter complex.

Modulation of Intersystem Crossing Rate by Minor Ligand Modifications in Cyclometalated Platinum(II) Complexes

Photophysical properties of four new platinum(II) complexes comprising extended ppy (Hppy = 2-phenylpyridine) and thpy (Hthpy = 2-(2'-thienyl)pyridine) cyclometalated ligands and acetylacetonate (acac) are reported. Substitution of the benzene ring of Pt-ppy complexes 1 and 2 with a more electron-rich thiophene of Pt-thpy complexes 3 and 4 leads to narrowing of the HOMO-LUMO gap and thus to a red shift of the lowest energy absorption band and phosphorescence band, as expected for low-energy excited states of the intraligand/metal-to-ligand charge transfer character. However, in addition to these conventional spectral shifts, another, at first unexpected, substitution effect occurs. Pt-thpy complexes 3 and 4 are dual emissive showing fluorescence about 6000 cm(-1) (∼0.75 eV) higher in energy relative to the phosphorescence band, while for Pt-ppy complexes 1 and 2 only phosphorescence is observed. For dual-emissive complexes 3 and 4, ISC rates kISC are estimated to be in order of 10(9)-10(10) s(-1), while kISC of Pt-ppy complexes 1 and 2 is much faster amounting to 10(12) s(-1) or more. The relative intensities of the fluorescence and phosphorescence signals of Pt-thpy complexes 3 and 4 depend on the excitation wavelength, showing that hyper-intersystem crossing (HISC) in these complexes is observably significant.

Reduced and Superreduced Diplatinum Complexes

A d(8)-d(8) complex [Pt2(μ-P2O5(BF2)4](4-) (abbreviated Pt(pop-BF2)(4-)) undergoes two 1e(-) reductions at E1/2 = -1.68 and Ep = -2.46 V (vs Fc(+)/Fc) producing reduced Pt(pop-BF2)(5-) and superreduced Pt(pop-BF2)(6-) species, respectively. The EPR spectrum of Pt(pop-BF2)(5-) and UV-vis spectra of both the reduced and the superreduced complexes, together with TD-DFT calculations, reveal successive filling of the 6pσ orbital accompanied by gradual strengthening of Pt-Pt bonding interactions and, because of 6pσ delocalization, of Pt-P bonds in the course of the two reductions. Mayer-Millikan Pt-Pt bond orders of 0.173, 0.268, and 0.340 were calculated for the parent, reduced, and superreduced complexes, respectively. The second (5-/6-) reduction is accompanied by a structural distortion that is experimentally manifested by electrochemical irreversibility. Both reduction steps proceed without changing either d(8) Pt electronic configuration, making the superreduced Pt(pop-BF2)(6-) a very rare 6p(2) σ-bonded binuclear complex. However, the Pt-Pt σ bonding interaction is limited by the relatively long bridging-ligand-imposed Pt-Pt distance accompanied by repulsive electronic congestion. Pt(pop-BF2)(4-) is predicted to be a very strong photooxidant (potentials of +1.57 and +0.86 V are estimated for the singlet and triplet dσ*pσ excited states, respectively).