ILCT Character Research Articles

A theoretical study of the structural and electronic properties of some titanocenes using DFT, TD-DFT, and QTAIM

Titanocenes are gradually proven to possess good anticancer activity. In this paper, we present a theoretical study of some properties of six promising anticancer titanocenes synthesized by the Tacke research group. The structural and electronic properties of the complexes have been investigated herein using the density functional theory (DFT) and the time-dependent DFT (TD-DFT) associated with a mixed basis sets comprising the 6-31 + G(d,p) basis set for C, H, and O atoms and the 6-31G(d) basis sets for the Ti4+ and Cl− ions. According to the results from topological analysis, even though there are strong Ti–C interactions in all complexes studied, the cyclopentadienyl derivatives are not covalently linked to the central Ti4+ ion through all of their carbon atoms. In addition, the Ti–Cl is the strongest metal-ligand bond. From the analyses of electronic spectra, it turns out that the complexes studied strongly absorb in the UV region of the electromagnetic spectrum and that the transitions are mainly LMCT, LLCT, and ILCT character. Our results are undoubtedly vital for further studies on the structure-activity relationship and mechanisms of interaction between the investigated titanocenes and DNA.

Unsymmetric Heteroleptic Ir(III) Complexes with 2-Phenylquinoline and Coumarin-Based Ligand Isomers for Tuning Character of Triplet Excited States and Achieving High Electroluminescent Efficiencies.

2-Phenylquinoline (PQ) and four coumarin-based ligand isomers with ease of synthesis have been selected to construct the unsymmetric heteroleptic [Ir(C1∧N)(C2∧N)(acac)]-type complex phosphors for organic light-emitting diodes (OLEDs). Six unsymmetric heteroleptic Ir(III) complexes have been obtained by employing four coumarin-based ligand isomers (L-C5/L-C6/L-C7/L-C8) in the [Ir(PQ)(C∧N)(acac)] structure due to two different coordinating carbon atoms in ligands L-C6 and L-C7 to form C-Ir bond. Through adopting unsymmetric heteroleptic [Ir(C1∧N)(C2∧N)(acac)] structure, these Ir(III) complexes can not only achieve impressive absolute quantum yield Φp (ca. 0.5-1.0), higher than that of complex [Ir(PQ)2(acac)] (ca. 0.4), but also realize a dual modulation of both emission color from orange (AIrC6out, λ = 578 nm) to red (AIrC5, λ = 622 nm) and the character of the lowest triplet excited states (T1), showing both 3MLCT character and 3ILCT (intraligand charge transfer) character in their T1 states. AIrC5, AIrC7out, and AIrC7in show MLCT character from Ir(III) center to ligand L-C5 or L-C7 and ILCT character in ligand L-C5 or L-C7 in their T1 states, while AIrC6out, AIrC6in, and AIrC8 show MLCT character from Ir(III) center to ligand PQ and ILCT character in ligand PQ in their T1 states. Moreover, the color-tuning mechanism and the lowest triplet state characters are investigated in detail. AIrC6in and AIrC8 were selected as emitters to evaluate the electroluminescent (EL) performance due to their high ΦP of nearly up to unity. Optimal orange-emitting device B2 based on AIrC8 can give a maximum external quantum efficiency (ηext) of 23.9%, a maximum current efficiency (ηL) of 70.9 cd A-1, and a maximum power efficiency (ηP) of 60.7 lm W-1. All these impressive results can definitely demonstrate the effectiveness of our simple approach for tuning character of the triplet excited states and achieving high-performance Ir-based phosphors in OLEDs.

An amine linker group modulates luminescent properties in a Rhenium(I) tricarbonyl complex. How can it be applied for ratiometric oxygen sensing?

The UV–Vis absorption spectrum of the rhenium(I) tricarbonyl complex [(NH2-phen)Re(CO)3Br] (ReNN-NH2Br, a deep-orange solid) in solution shows three bands located at 300, 370 and 445 nm. TD-DFT calculations and spectroscopic data indicate that the higher energy band corresponds to a ligand centered transition (LC), while the two lower energy bands have been ascribed to have major ILCT and MLCT character. Excitation at 370 nm leads to two emission bands centered at around 490 nm and 590 nm, which have different nature, 1ILCT and 3MLCT, respectively. Both show a low luminescent quantum yield, especially the lowest energy band. The emission quantum yields in argon-saturated solutions are largely increased, specially for the band at 590 nm. Our results demonstrate that adding the amino group has a non-innocent effect over the luminescent properties of the complex when is compared with those of [(phen)Re(CO)3Br] (ReNNBr). The capacity of these complexes to act as singlet oxygen sensitizers and their application as oxygen sensors were explored. Both complexes, ReNNBr and ReNN-NH2Br, were incorporated in silsesquioxane matrices and tested as ratiometric oxygen sensors using the intrinsic emission of the matrix as an emission internal reference signal. The SSO1-ReNN-NH2Br films was the most sensitive material for this application.

Photophysical Properties of Pt(II) Polypyridines with Five- versus Six-Membered Chelate Rings: Trade-Offs in Angle Strain.

This report describes the synthesis and characterization of a series of eight [Pt(NNN)X]+ complexes where the tridentate NNN ligand is (2,2'-bipyrid-6-yl)(pyrid-2-yl)sulfide (btp) or methyl(2,2'-bipyrid-6-yl)(pyrid-2-yl)amine (bmap) and X is OMe, Cl, phenylethynyl (C2Ph), or cyclohexylethynyl (C2Cy). The expectation was that inserting a heteroatom into the backbone of 2,2':6',2″-terpyridine (trpy) would expand the overall intraligand bite angle, introduce ILCT character into the excited states, and improve the photophysical properties. Crystal structures of [Pt(bmap)C2Ph]+ and [Pt(btp)Cl]+ reveal that atom insertion into the trpy backbone successfully expands the bite angle of the ligand by 8-10°. However, the impact on the photophysics is minimal. Indeed, of the eight systems investigated, only the [Pt(bmap)C2Ph]+ and [Pt(btp)C2Ph]+ complexes display appreciable emission in fluid solution, and they exhibit shorter emission lifetimes than [Pt(trpy)C2Ph]+. One reason is that the bond angle preferences of platinum and the inserted heteroatom induce the six-membered rings to deviate from planarity and adopt a boat-like conformation, impairing charge delocalization within the ligand. In addition, angle strain induces the donor atoms about platinum to assume a pseudotetrahedral arrangement, which offsets any benefit due to the increase in overall bite angle by promoting deactivation via d-d excited states. The results reveal that, in order to improve the luminescence of a [Pt(NNN)X]+ system, one must take care to avoid trading one kind of angle strain for another.

Luminescent Re(I) terpyridine complexes for OLEDs: what does the DFT/TD-DFT probe reveal?

The electronic structure and spectroscopic properties of a series of rhenium(I) terpyridine complexes were investigated using density functional theory (DFT) and time dependent density functional theory (TD-DFT) methods. The influence of different substituent groups on the optical and electronic properties of Re(I) terpyridine complexes has also been explored. The reorganization energy calculations show that the substituted Re(I) terpyridine complexes are better electron transport materials with high quantum efficiency in OLED devices due to their high electron transport mobility and low λ(electron) values, whereas the unsubstituted complex shows relatively balanceable charge transfer abilities with the higher efficiency in organic light emitting devices (OLEDs). An NBO analysis reveals that n→σ* interactions are mainly responsible for the ground state stabilization of all the complexes. QTAIM results show that in all cases, Re-CO bonds are shared type transient interactions as reported in the other metal ligand complexes. The absorption is associated with (1)MLCT/(1)LLCT/(1)ILCT character while the emission transition has (3)MLCT/(3)LLCT/(3)ILCT character as revealed by a natural transition orbital (NTO) analysis. The higher quantum yields reported for the complexes 4-6 are found to be closely related to both its smaller ΔE(S1-T1), higher μ(S1), E(T1) and moderate (3)MLCT character. The calculated results show that Re(I) terpyridine complexes, particularly complexes 4-6, are suitable candidates for OLED materials.

Are Re(i) phenanthroline complexes suitable candidates for OLEDs? Answers from DFT and TD-DFT investigations

The electronic structure and spectroscopic properties of seven recently reported rhenium(i) phenanthroline complexes were investigated theoretically by density functional theory (DFT) and time dependent density functional theory (TD-DFT) methods. All the seven complexes are shown here to be better electron transport materials with high quantum efficiency in OLED devices due to their high electron transport mobility and low λ(electron) values. Particularly, among these seven chosen complexes the difference between λhole and λ(electron) for tricarbonyl Re(i) complexes is smaller, suggesting that these complexes have a better hole- and electron-transport balance in OLED devices. The absorption is associated with (1)MLCT/(1)LLCT transitions while the emission transition has (3)MLCT/(3)LLCT/(3)ILCT character as revealed by natural transition orbital (NTO) analysis. The calculated results show that the absorption and emission transitions and device efficiency can be changed by varying the nature and position of the axial ligands as revealed by the structural and bonding features of the complexes through natural bond orbital (NBO) and quantum theory of atoms in molecule (QTAIM) analysis.

Phosphorescent copper(I) complexes bearing 2-(2-benzimidazolyl)-6-methylpyridine and phosphine mixed ligands

Abstract Two 2-(2-benzimidazolyl)-6-methylpyridine (Hbmp) copper(I) complexes bearing PPh3 and 1,4-bis(diphenylphosphino)butane (dppb), namely, [Cu(Hbmp)(PPh3)2](ClO4) (1) and [Cu(Hbmp)(dppb)](ClO4) (2), have been synthesized. X-ray diffraction analysis reveals that the most significant influence of the phosphine ligands on the structures is on the P–Cu–P bond angle. Both two Cu(I) complexes exhibit a weak low-energy absorption at 360–450 nm, ascribed to the Cu(I) to Hbmp metal-to-ligand charge-transfer (MLCT) transition, perhaps mixed with some ILCT character inside Hbmp. The room-temperature luminescences are observed for 1 and 2, both in solution and in the solid state, which originate from the MLCT excited states and vary markedly with the phosphine ligands.

Luminescent triply-bridged dicopper(I) complex possessing 3,5-bis{6-(2,2′-dipyridyl)}pyrazole as a bis-chelating ligand

A new triply-bridged dicopper(I) complex, [Cu2(μ-dppm)2(μ-HL)](NO3)2 (1), has been prepared via successive treatment of cupric nitrate trihydrate with bis(diphenylphosphino)methane (dppm) and 3,5-bis{6-(2,2′-dipyridyl)}pyrazole (HL) in 2 : 4 : 1 molar ratio in dichloromethane. X-ray diffraction analysis of 1 reveals that the two Cu(I)s are in highly distorted tetrahedral environments with a distance of 4.2775(10) Å, triply bridged by two dppm ligands and one HL chelate as a bis-bidentate bridging ligand through two bipyridyl moieties. Intermolecular N···H–C hydrogen bonding and π···H–C interactions assemble the [Cu2(μ-dppm)2(μ-HL)]2+ cations into a 3-D supramolecular architecture with extended channels along the b-axis, filled with methanol and anions. Complex 1 shows weak low-energy absorptions at 350–425 nm, tentatively ascribed to Cu(I) to HL metal-to-ligand charge-transfer (MLCT) transition, probably mixed with some ILCT character inside HL. The emission is observed at ambient temperature for 1, both in solution and in the solid state, originating from the MLCT excited states.

Theoretical studies on structures and spectroscopic properties of 10-hydroxybenzo[h]quinoline zinc(II) and its derivatives substituted by cyano-groups

The geometric structures of 10-hydroxybenzo[h]quinoline zinc [Zn(BQ) 2] complex and its derivatives substituted by cyano-groups in the ground state and the lowest singlet excited state were optimized by B3LYP and ab initio CIS methods at LanL2DZ level, respectively. The absorption and emission spectra of these complexes were investigated by the time-dependent density functional theory (TD-DFT) level with PCM model on the basis of the optimized ground and excited states geometries, respectively. The calculated lowest-lying absorptions of all complexes were attributed to ILCT transitions mainly. The calculated emissions of complexes can be described as originated from an excited state with ILCT character mainly. The calculated results showed that the absorption and emission maximums could be tuned by changing the site of the substitutions. The Zn(BQ) 2 and its derivatives substituted by cyano-groups were potential electroluminescent materials with blue, green and yellow light emission.

Dinuclear platinum(ii) 4,6-diphenyl-2,2′-bipyridine complexes tethered by a rigid bridging ligand: synthesis and photophysics in solution and in LB film

Two dinuclear platinum(II) 4,6-diphenyl-2,2'-bipyridine (C^N^N) complexes (1 and 2) with a rigid bridging ligand cis-1,2-bis(diphenylphosphino)ethylene were synthesized and their photophysical properties were systematically investigated in solution for 1 and 2 and in LB film for 2. Similar to their corresponding mononuclear complexes, both complexes exhibit intense (1)π,π* absorption in the UV region and a broad, moderate absorption band in the visible region, which likely stems from the mixed (1)MLCT (metal-to-ligand charge transfer), (1)ILCT (intraligand charge transfer) and (1)π,π* transitions. Both complexes are emissive in solutions at room temperature and in glassy matrix at 77 K. The emitting state is tentatively assigned as (3)MLCT for 1 and (3)MLCT/(3)ILCT/(3)π,π* for 2 at room temperature. At 77 K, the emission observed for 1 is mainly from the emissive ground-state aggregates, which is concentration dependent; while in 2 the emission from the monomer dominates. Unlike the dinuclear platinum complex with flexible bridging ligand diphenylphosphinoethane, the electronic absorption and emission energies of 1 and 2 at room temperature are independent of their concentration, indicating a fixed conformation for these two complexes. In addition, the presence of alkoxyl substituents on the diphenylbipyridine ligands causes a bathochromic shift of the lowest-energy absorption band and the emission band at room temperature for 2, presumably due to the involvement of the ILCT character into the lowest excited states. The presence of alkoxyl substituents in 2 also makes 2 amphiphilic, allowing for the fabrication of LB films of 2. The electronic absorption and emission characteristics in the LB films of 2 are quite similar to those in solutions, indicating no intermolecular Pt-Pt interactions occur in the LB films. The dinuclear complex without alkoxyl substituent (1) exhibits vapochromic behavior to heteroatom-containing volatile organic compounds (VOC's).

DFT evaluation of the electronic structures and spectroscopic properties of the self-assembled [Pt2M4(C ≡ CH)8] (M=Cu, Ag) clusters

Electronic structures and spectroscopic properties of self-assembled [Pt2M4(C≡CH)8] (M=Cu, Ag) clusters have been studied by the TD-DFT (time-dependent density functional theory) calculations with the polarizable continuum model (PCM). The ground- and excited-state structures were optimized by the DFT (density functional theory) methods. The calculated structures and spectroscopic properties are in agreement with the corresponding experimental results. The [Pt2Ag4(C≡CH)8] clusters have two stable ground state geometries (D 4 and D 4h symmetry). The calculated Pt-M distances suggest only very weak interactions. The Cu-Cu distances are larger than the van der Waals radii of two Cu atoms and the Ag-Ag distances are analogous with the sum of van der Waals radii of two Ag atoms. Upon excitation, the interaction of Pt⋯M, Ag⋯Ag is strengthened, while the Cu⋯Cu distances are shortened but they are still larger than the sum of van der Waals radii of two Cu atoms. The lowest-energy absorptions are at 450, 365 and 375 nm and the emissions are at 611, 431 and 435 nm for [Pt2Cu4(C≡CH)8], [Pt2Ag4(C≡CH)8] (A) and (B), respectively. The transitions are all perturbed by the Cu or Ag composition through the UV-Vis spectra region; therefore, there are not pure ILCT or MPtLCT characteristics (ILCT: intraligand charge transfer; MLCT: metal-to-ligand charge transfer) in absorptions of heteropolynuclear [Pt2M4(C≡CH)8] clusters. Since the emissions and the lowest-absorptions have different transition characteristics for each complex, the emissions should not come from the lowest-energy absorptions. Because the M⋯M interactions in the excited state of [Pt2Ag4(C≡CH)8] are augmented, the emissions of [Pt2Ag4(C≡CH)8] clusters bear prominent ILCT character, which is the reason why the emission wavelengths of [Pt2Ag4(C≡CH)8] have a small hypsochromic shift relative to the emission wavelength of homoleptic [Pt(C≡CH)4]2− precursor.

Synthesis, structure, spectroscopic properties, and theoretical studies of a new blue/green luminescent complex: [Cd 2Cl 4(Hbm) 2] n· nH 2O (Hbm = 1 H-benzimidazol-2-ylmethanol)

A new blue/green luminescent complex, [Cd 2Cl 4(Hbm) 2] n · nH 2O (Hbm = 1 H-benzimidazol-2-ylmethanol), has been synthesized under hydrothermal condition, and characterized by X-ray crystallography, spectroscopy and thermoanalysis. The compound contains two isomeric polymeric chains with the difference in the coordination of the Hbm ligand to the central Cd(II) atom. The electronic structure and photoluminescent process was also investigated by means of density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. The calculation results demonstrate absorption peaks at 250, 261 and 287 nm are mainly (p, π) → π ∗ with ILCT character, (p, σ) → d with LMCT character and p → d with LMCT character, respectively, and luminescent emission at 462 nm is LMCT transition.

Mechanism of Ir(ppy)2(N^N)+ (N^N = 2-Phenyl-1H-imidazo[4,5-f][1,10]phenanthroline) Sensor for F−, CF3COOH, and CH3COO−: Density Functional Theory and Time-Dependent Density Functional Theory Studies

The geometries, electronic structures, and spectroscopic properties of Ir(ppy)2(N--N)(+) (1) (N--N = 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline, ppy = 2-phenylpyridine), Ir(ppy)2(N--N)(+) x F(-) (2), Ir(ppy)2(N--N)(+) x CF3COOH (3/3a), and Ir(ppy)2(N--N)(+) x CH3COO(-) (4) were investigated theoretically. The ground and the excited state geometries of 1-4 were optimized at the B3LYP/LANL2DZ and UB3LYP/LANL2DZ levels, respectively. The optimized geometries agree well with the corresponding experimental results. The HOMOs of 1-4 and 3a are composed of pi(ppy) and d(Ir), and the LUMOs of 1, 2, 3a, and 4 are contributed by pi*(N--N), whereas the LUMO of 3 is composed of pi*(N--N) and pi*(CF3COOH). Under the time-dependent density functional theory level with polarized continuum model model, the absorption and phosphorescence in CH2Cl2 media were calculated on the basis of the optimized ground and excited state geometries, respectively. The lowest-lying absorptions of 1 (412 nm) and 3/3a (409/419 nm) have MLCT/LLCT transition characters, and those of 2 (448 nm) and 4 (427 nm) are contributed by ILCT character. The calculated lowest-energy triplet excited states responsible for phosphorescence of 1 (519 nm) and 3/3a (661/702 nm) have mixing (3)MLCT/(3)LLCT/(3)ILCT characters, but those of 2 and 4 only have (3)ILCT but without (3)MLCT character, which is the reason for the no-emissive character of 2 and 4. Moreover, the phosphorescence character of 3 is hardly changed by different addition sites of CF3COOH group (3a). The calculated results also showed that complex 1 is more suitable for an F(-) sensor than for CF3COOH and CH3COO(-) sensors.

Synthesis, Structure, and Photophysical Properties of Luminescent Platinum(II) Complexes Containing Cyclometalated 4-Styryl-Functionalized 2-Phenylpyridine Ligands

A series of new luminescent cyclometalated platinum(II) complexes functionalized with various substituted styryl groups on the cyclometallating ligand [Pt(C/\N-ppy-4-styryl-R)(O/\O-(O)CCR'CHCR'C(O))] (ppy-4-styryl-R = E-4(4-(R)styryl-2-phenylpyridine) (3, R' = Me (acac); 4, R' = (t)Bu (dpm); R = H, OMe, NEt2, NO2) have been prepared. All complexes undergo an E-Z photoisomerization process in CH2Cl2 solution under sunlight, as monitored by 1H NMR. The solid-state structures of 3-OMe, 3-NEt2, 3-NO2, and 4-OMe have been determined by X-ray diffraction studies and compare well with optimized geometries obtained by density functional theory (DFT) calculations. The orbital pictures of 3-H, 3-OMe, and 3-NO 2 are very similar, the highest occupied molecular orbital (HOMO) being highly Pt(5d) metal-based. For 3-NMe2, an additional contribution from the amino-styryl fragment leads to a decreased metal parentage of the HOMO, suggesting a predominantly ILCT character transition. Complexes 3-H, 3-OMe, and 3-NO2 show a low-energy band (350-400 nm) assigned to predominantly charge-transfer transitions. The amino derivative 3-NEt2 displays a very strong absorption band at 432 nm, tentatively assigned to a mixture of ILCT (Et2N --> CH=CH) and metal-to-ligand charge-transfer (MLCT) (dpi(Pt) --> pi) transitions. Complexes 3 are weakly luminescent in CH2Cl2 solution at room temperature; the low intensity may be due to a competitive quenching through the E-Z photoisomerization process. All complexes exhibit similar structured emission bands under these conditions (around 520 nm), independent of the nature of the styryl-R group. In a frozen EPA glass (77 K), the spectrum of the representative complex 3-H exhibits two sets of vibronically structured bands (460-560, 570-800 nm; lambda(max) = 596 nm), due to the presence of two emitting species, the E and Z isomers, which have significantly different triplet excited-state energies. The other three complexes show similar behavior to 3-H at 77 K, but the lower-energy emission bands are progressively red-shifted in the order H < OMe < NO2 < NEt2 (e.g., for 3-NEt2, lambda(max)(em) = 658 nm; tau = 26 micros). The very large red-shift compared to related unsubstituted complexes (e.g., to [Pt(C/\N-ppy)(O/\O-acac)]) is the result of the extension of the pi-conjugated system and the electronic effects of substituent R.

CT excitation of [Ir(azulene)(1,5-cyclooctadiene)] +. Spectroscopy and photochemistry

The complex [Ir(azulene)(1,5-cyclocta-diene)] + shows long-wavelength absorptions which are assigned to spin-allowed ( λ max = 536 nm) and spin-forbidden (675 nm) transitions. They are of mixed MLCT (Ir → azulene) and ILCT (azulene) character. At 77 K the solid complex displays an emission ( λ max = 816 nm) which originates from the MLCT/ILCT triplet. At r.t. in solution the complex undergoes a photosubstitution of the azulene ligand. It is suggested that in the CT state the azulene ligand looses its coordinating ability.