Three new Re(CO) 3 Cl complexes (ReL1-ReL3) containing the N,N-bidentate ligands 1-(2-pyridyl)-3-phenylimidazo[1,5-a]pyridine (L1), 1-(2-pyridyl)-3-(4-tert-butylphenyl)imidazo[1,5-a]pyridine (L2), and 1-(2-pyridyl)-3-(4-dimethylaminophenyl)imidazo[1,5-a]pyridine (L3) were synthetized and fully characterized. Photophysical properties of L1-L3 and ReL1-ReL3 were studied with absorption and emission spectroscopy. The X-ray structure of ReL3 was determined. Time-dependent DFT (TDDFT) calculations were performed in order to elucidate the electronic structures and the excited states of ligands and complexes. Ligands L1 and L2 show 1 π-π* emission with limited charge-transfer character (CT), while L3 emits from an excited state with higher CT character due to the presence of a dimethylamino group. No emissive metal-to-ligand charge-transfer (MLCT) states are found for the rhenium complexes. ReL1 and ReL2, although similar to their ligands, display a ligand-centered 1 π-π*/ 3 π-π* dual emission; singlet emissions fall at 21.8 × 10 3 (458 nm) and 22.6 × 10 3 cm -1 (443 nm), respectively, and the structured triplet emissions have two peaks at 17.9 × 10 3 (558 nm) and 16.3 × 10 3 cm -1 (613 nm) in both complexes. ReL3 emits from a ligand-centered CT state at 18.9 × 10 3 cm -1 (530 nm). Finally, the complex [Re(L1)(CO) 3 py]PF 6 (ReL1py) (where py ) pyridine) was prepared and studied by spectroscopy and computational methods. The complex has high-energy emission centered at 22.9 × 10 3 cm -1 (437 nm). DFT calculations show that dual fluorescence almost disappears due to the reduced spin-orbit coupling. Finally, electrochemical properties of ligands and rhenium complexes have been investigated.

This work reports on the positive impact of (i) attaching methoxy groups at the ortho position of the bipyridine ligand (6,6'-dimethoxy-2,2'-bipyridine) in heteroleptic copper(I) complexes belonging to the [Cu(bpy)(POP)] family, and (ii) a new device design comprising a multilayered architecture to decouple hole/electron injection and transport processes on the performance of light-emitting electrochemical cells (LECs). In short, the substituted complex showed enhanced thermal- and photostability, photoluminescence, and ionic conductivity features in thin films compared to those of the archetypal complex without substitution. These beneficial features led to LECs outperforming reference devices in terms of luminance, stability, and efficacy. Furthermore, a new device design resulted in a 10-fold enhancement of the stability without negatively affecting the other figures of merit. Here, hole/electron injection and transport processes are performed at two different layers, while electron injection and electron-hole recombination occur at the copper(I) complex layer. As such, this work provides further insights into a smart design of N^N ligands for copper(I) complexes, opening the path to a simple device architecture toward an enhanced electroluminescence response.

a This work provides the synthesis, structural characterization, electrochemical and photophysical features, as well as the application in light-emitting electrochemical cells (LECs) of a novel heteroleptic copper(I) complex -[Cu(impy)(POP)] [PF 6 ], where impy is 3-(2-methoxyphenyl)-1-(pyridine-2-yl)imidazo[1,5-a]pyridine and POP is bis{2-(diphenylphosphanyl)phenyl}ether. This compound shows blue photoluminescence (PL, λ = 450 nm) in solution and solid-state and excellent redox stability. Despite these excellent features, the electroluminescence (EL) response is located at ∼550 nm. Although the EL spectrum of LECs is typically red-shifted compared to the PL of the electroluminescent material, a shift of ca. 100 nm represents the largest one reported in LECs. To date, the large shift phenomena have been attributed to (i) a change in the nature of the lowest emitting state due to a concentration effect of the films, (ii) a reversible substitution of the ligands due to the weak coordination to the Cu(I), and (iii) a change in the distribution of the excited states due to polarization effects. After having discarded these along with others like the irreversible degradation of the emitter during device fabrication and/or under operation conditions, driving conditions, active layer composition, and changes in the excited states under different external electrical stimuli, we attribute the origin of this unexpected shift to a lack of a thermally activated delayed fluorescence (TADF) process due to the solely ligand-centered character of the excited states. As such, the lack of a charge transfer character in the excited states leads to a blue-fluorescence and yellowphosphorescence photo-and electro-responses, respectively. This corroborates recent studies focused on the design of TADF for heteroleptic copper(I) complexes. Overall, this work is a clear insight into the design of new copper(I) complexes towards the preparation of blue LECs, which are still unexplored.

Keywords: Rhenium / X-ray structure / TDDFT / UV/Vis spectroscopy / Luminescence (L3), were studied by spectroscopy, X-ray diffraction, and computa-

Excited-state characters and dynamics of [ReCl(CO)3(3-R-1-(2-pyridyl)-imidazo[1,5-α]pyridine)] complexes (abbreviated ReGV-R, R = CH3, Ph, PhBu(t), PhCF3, PhNO2, PhNMe2) were investigated by pico- and nanosecond time-resolved infrared spectroscopy (TRIR) and excited-state DFT and TD-DFT calculations. Near UV excitation populates the lowest singlet state S1 that undergoes picosecond intersystem crossing (ISC) to the lowest triplet T1. Both states are initially formed hot and relax with ∼20 ps lifetime. TRIR together with quantum chemical calculations reveal that S1 is predominantly a ππ* state localized at the 1-(2-pyridyl)-imidazo[1,5-α]pyridine (= impy) ligand core, with impy → PhNO2 and PhNMe2 → impy intraligand charge-transfer contributions in the case of ReGV-PhNO2 and ReGV-PhNMe2, respectively. T1 is predominantly ππ*(impy) in all cases. It follows that excited singlet and corresponding triplet states have to some extent different characters and structures even if originating nominally from the same preponderant one-electron excitations. ISC occurs with a solvent-independent (CH2Cl2, MeCN) 20-30 ps lifetime, except for ReGV-PhNMe2 (10 ps in CH2Cl2, 100 ps in MeCN). ISC is 200-300 times slower than in analogous complexes with low-lying MLCT states. This difference is interpreted in terms of spin-orbit interaction and characters of orbitals involved in one-electron excitations that give rise to S1 and T1 states. ReGV-R present a unique case of octahedral heavy-metal complexes where the S1 lifetime is long enough to allow for separate spectroscopic characterization of singlet and triplet excited states. This study provides an insight into dynamics and intersystem crossing pathways of low-lying singlet and triplet excited states localized at bidentate ligands bound directly to a heavy metal atom. Rather long (1)IL lifetimes indicate the possibility of photonic applications of singlet excited states.

Luminescent ligands in Ir(III) cyclometalated complexes. The photophysical and photochemical properties of Ir-cyclometalated complexes containing luminescent ligands are evaluated (see figure). Significant admixture between Ir and ligand orbitals induces an efficient intersystem crossing. Photochemical reactions performed in the presence of oxygen lead to new Ir-cyclometalated complexes containing N(amido) groups directly bound to Ir.A series of phosphorescent cyclometalated heteroleptic iridi um(III) phenylpyridinato (ppy) complexes containing luminescent 1-pyridylimidazo[1,5-alpha]pyridine (pip) ligands, namely [Ir(ppy)(2)(pip)](+), have been synthesised, characterised and their electrochemical, photophysical and electronic properties studied. Seven X-ray structures have been resolved. Excitation of [Ir(ppy)(2)(pip)](+) in acetonitrile at room temperature results in a dual luminescence, strongly quenched by O(2). Four complexes show, in absence of O(2), a high-energy emission (assigned to a (3)MLLCT transition) with two maxima in the blue region of the visible spectra, and a second structured emission (assigned largely to a (3)LC pi-pi* transition) centred around lambda=555 nm. Lifetimes of high-energy emissions are between 0.6 and 1.3 mus. Time-dependent density functional calculations combined with the conductor-like polarisable continuum model method, with acetonitrile as solvent, have been used to calculate a series of ground and excited states of the derivatives under investigation, and the transitions compared with the experimental UV/Vis absorption spectra. A quick and efficient photochemical reaction has been observed for these iridium derivatives that leads to the formation of a new class of cyclometalated iridium complexes containing a stable deprotonated amide unusually coordinated to the metal through a nitrogen bond. The synthesis of a (15)N enriched selected ligand has been performed to investigate, by means of NMR, the particular facile route to these new set of derivatives. The electrochemical behaviour of all complexes is also reported.

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