Spectral manifestations of the splitting of the lowest triplet levels in Pd complexes of nonsymmetric porphyrins

Based on the study of the phosphorescence and phosphorescence excitation spectra of Pd(II) tetramethylporphyrin (PdTMP) and Pd(II) tetramethyldiethylporphyrin (PdTMDEP) in solutions in 2-methyltetrahydrofuran and dodecane in the temperature range 77–283 K, the occurrence of the splitting of the lowest degenerate singlet (S ₁) and triplet (T ₁, T ₂) levels of porphyrin molecules is established. In the absorption of molecules of the compounds studied, two components, S ₁ and S ₂, are revealed in the range of allowed long-wavelength Q(0,0) transitions (530–550 nm) and four components, T ₁–T ₄, are observed in the range of spin-forbidden intersystem crossing transitions S ₀ → T _n (560–670 nm), with all the triplet levels T ₁-T ₄ being located below the S ₁ level on the energy scale. It is shown that an increase in the degree of deformation of the porphyrin macrocycle caused by steric interactions between β alkyls and hydrogen meso atoms is accompanied by an increase in the splitting in the system of singlet levels δE(S ₂–S ₁) from 120 cm^?1 for PdTMDEP to 215 cm^?1 for PdTMP, as well as by an increase in the splitting in the system of triplet levels δE(T ₂–T ₁) from 190 cm^?1 for PdTMDEP to 250 cm^?1 for PdTMP.     

Fine-structure spectra of metal complexes of porphin in tetrahydrofuran at low temperatures: Manifestation of nonplanarity effects

The fine-structure fluorescence spectra of Mg and Zn porphins in solid tetrahydrofuran matrices at the liquid helium temperature are recorded for the first time. The fluorescence spectra of Mg porphin molecules deposited from the gas phase on a sapphire substrate simultaneously with tetrahydrofuran molecules are measured. Based on the data obtained from the fluorescence and fluorescence excitation spectra, it is ascertained that there are two noninteracting spectrally different long-and short-wavelength forms of metal complexes of porphins in the ground state. In the case of Mg porphin, the spectral gap for these two forms at 4.2 K amounts to 330 cm^?1. The short-wavelength form is attributed to the nonplanar saddle conformation of a porphin molecule, while the long-wavelength form is associated with the nonplanar domed conformation, in which the central metal ion is displaced out of the plane of the porphyrin ligand. The fine-structure fluorescence spectra of both forms of Mg and Zn porphins are measured in tetrahydrofuran at 4.2 K upon selective laser excitation. The frequencies of the normal vibrations in the ground electronic state are determined and the reasons for the differences in the vibrational frequencies of the forms are discussed.     

Spectral manifestations of nonradiative processes in azulene

The linewidths for S₁ ← S₀ (at 14 652 cm⁻¹) and S₂ ← S₀ (at 28 048 cm⁻¹) of azulene in a naphthalene host crystal at 1.2°K are presented along with measurements of the line narrowing due to perdeuteration of the azulene. The results are related to current ideas of nonradiative processes in azulene and azulene-d₈.     

Spectral intensities of lanthanide complexes

The effects of solvents and complex formation with nitrogen donor ligands on the spectral intensities of the f→f transitions in Nd(III) ion have been reported. The intensity parameters T_λ have been calculated for 14 systems and are compared with oxygen donor ligands.     

Spectroscopic manifestations of the intramolecular hydrogen bond in ordinary and sterically hindered porphyrins

Spectroscopic manifestations of the intramolecular hydrogen bond (IHB) in ordinary and sterically hindered porphyrins were investigated by the methods of IR spectroscopy and molecular modeling. It is shown that the IHB plays a part in the formation of the νNH and γNH vibrations of porphyrins. By changing the type and value of the distortion of the macroring at the expense of steric effects of interaction of volumetric substituents, the parameters and spectroscopic manifestations of the IHB can be varied in a significantly larger range as compared with ordinary tetrapyrroles having nearly quadratic and planar macrorings. Reported at the VIIIth International Conference on Spectroscopy of Porphyrins and Their Analogs, Minsk, September 22–26, 1998. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 4, pp. 522–527, July–August, 1999.     

Spectral manifestations of association of unlike dye molecules in polyelectrolyte solutions

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 52, No. 3, pp. 424–429, March, 1990.     

Isomerism in dihalogenometal porphyrins and phthalocyanines: a theoretical study of dihalogenotitanium complexes

Trans- and cis-isomers of dihalogenotitanium porphyrins and phthalocyanines, TiX₂P and TiX₂Pc (X = F, Cl, Br, I; P = porphyrin, Pc = phthalocyanine), which provide a good example of isomerism in dihalogenometal porphyrin and phthalocyanine complexes, have been studied by density functional theory calculations using the B3LYP hybrid method and triple-ζ valence basis sets. All eight complexes have two isomers, trans-isomer of D _4h symmetry and cis-isomer of C _2v symmetry with halogen atoms located in the same plane with two meso-atoms. In the case of difluorotitanium phthalocyanine the cis-isomer is preferable in the gas phase, while in all porphyrin complexes and remaining three phthalocyanine complexes the trans-isomers have lower energies than cis-isomers. Electrostatic repulsion between the halogen atoms and central nitrogen atoms of the macroheterocycle seems to play the major role in determining the preference of the trans- or cis-isomer.     

Spectral processing of two concurrent harmonic complexes

In a concurrent profile analysis task, each of the two observation intervals was the sum of two harmonic complexes. In the first interval one of the harmonic complexes had a flat spectrum and the other had a broad spectral peak at 1 kHz. In the second interval, the association between the spectral profiles and the complexes was either consistent with the first interval, or inconsistent so that profile changes (flat versus peaked) could be created in both of the complexes. In two experiments, thresholds and psychometric functions for detecting the profile change were measured in terms of the spectral peak's magnitude as functions of three types of segregation cues: Difference in fundamental frequency, onset asynchrony, and difference in interaural time difference between the two complexes. Decreasing the magnitude of each cue led to higher thresholds, and shallower psychometric functions whose upper asymptotes often failed to reach 100% correct. The patterns of the threshold and psychometric functions varied across cue types and across individual listeners. The results suggest that informational masking is present in the concurrent profile analysis task. Segregation cues appear to contribute to the release from informational masking, but the process depends on listening strategies adopted by individual listeners.     

Spectral luminescence properties of hydroporphyrazine metallo complexes

We have studied the spectral luminescence properties of a zinc complex of dibenzobarreleno-substituted tetraazaisobacteriochlorin (a synthetic tetrapyrrole pigment which is the structural analog of the biologically important chromophore isobacteriochlorin) and a palladium complex of dibenzobarreleno-substituted tetraazachlorin. We have shown that despite the substantial effect of aza-substitution on the electronic structure and the electronic spectra of the tetrapyrrole molecules, the effect of hydrogenation of adjacent pyrrole rings on the spectral luminescence properties is similar in compounds with nitrogen and carbon bridges. As in the free base case, the fluorescence of the studied zinc complex is strongly quenched at 293 K but flares up at 77 K (temperature and viscosity-dependent quenching). To study the substituted Zn-tetraazaisobacteriochlorin, we determined the position of the 0-0 band of the low-intensity transition S₀ → S₂ and showed that the S₂ → S₁ energy difference increases with bridge aza substitution. For the Pd complex of dibenzobarreleno-substituted tetraazachlorin, we could not detect phosphorescence in the near IR region (to 1150 nm); we detected weak fluorescence. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 5, pp. 700–707, September–October, 2008.     

Spectral manifestations of intramolecular interactions in acid chlorides of unsaturated phosphorus acids

The UV absorption spectra of acid dichlorides of aryl phosphonic and phosphinic acids in the 185–340 nm region were investigated. Hypothetical conclusions are made concerning the nature of the electronic transitions in phosphorus-containing benzenes. The interaction of phosphorus-containing group with a -electron system of the aromatic radical is characterized on the basis of the data obtained. The role of vacant orbitals of phosphorus and chlorine atom, unshared pair p^III, and p–d bond of the phosphoryl group in the investigated effects is examined.Izvestiya VUZ. Fizika, No. 3, pp. 31–36, March, 1973.     

Spectral and luminescent specific features of phenylpyridine ethylenediamine complexes of Pt(II), Pd(II), and Au(III)

The orbital nature and basic photophysical parameters of phenylpyridine ethylenediamine complexes of Pt(II), Pd(II), and Au(III) are found from the comparative analysis of absorption and luminescence spectra and the luminescence decay kinetics. The orbital nature and properties of the lowest excited states responsible for luminescence are shown to depend on the type of metal and the energy of singlet-triplet splitting.     

Spectral and luminescent properties of ammonia cyclopalladated complexes

The synthesis of ammonia cyclometalated palladium(II) complexes [Pd(NH₃)₂C^N]ClO₄ (C^N is the deprotonated form of 2-phenylpyridine, 2-(para-tolyl)pyridine, 7,8-benzo(h)quinoline, 2,6-diphenylpyridine, and 4-phenylpyrimidine) is developed. The IR and electronic absorption and emission spectra of these complexes are studied. It is found that the ammine and analogous ethylenediamine cyclometalated Pd(II) complexes have similar spectral and luminescent properties and the same nature of the electronically excited ³(π-π*)-type state responsible for the long-lived luminescence, the π and π* orbitals being localized on the corresponding cyclometalating ligand. The efficient temperature quenching of the luminescence of Pd(II) complexes at room temperature is assigned to the thermally activated population of metal-centered electronically excited states with subsequent nonradiative deactivation.     

Spectral manifestations of intramolecular interactions in acid chlorides of unsaturated acids of phosphorus

The mutual effect of atomic groups in acid chlorides of threeand four-coordination phosphorus was investigated by the nuclear quadrupole resonance of Cl³⁵ (NQR Cl³⁵) and Raman spectroscopy methods. On the basis of the NQR data it is suggested that in the ground state the interaction of unshared pairs of chlorine with the d orbitals of phosphorus is displayed weakly and the interaction of the indicated atoms is accomplished predominantly by an inductive mechanism. The integral intensities of the Raman lines of vibrations of the P-Cl and P=Cl bonds were measured. The Raman scattering data and the results of quantummechanical calculations by the molecular orbital method indicate some additional multiplicity of the P-Cl bond.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 75–80, December, 1972.The authors thank V. T. Aleksanyan and G. Kuz'yants for help in measuring the depolarization ratio.     

Manifestation of nonplanarity effects and charge-transfer interactions in spectral and kinetic properties of triplet states of sterically strained octaethylporphyrins

Properties of the triplet states of octaethylporphyrins with the steric hindrance (free bases and Pd complexes) are studied by the methods of stationary and kinetic spectroscopy in the temperature range from 77 to 293 K. The mono-mesophenyl substitution results in a decrease in the quantum yield and shortening of the phosphorescence lifetime of Pd complexes by 250–3500 times in degassed toluene at 293 K. The phosphorescence quenching is caused by nonplanar dynamic conformations of the porphyrin macrocycle in the T ₁ state, which also lead to the appearance of new bands at λ～1000 nm in the T-T absorption spectra. As the number of meso-phenyls (Pd-octaetyltetraphenylporphyrin) increases, the quantum yield of phosphorescence further decreases (<10^?5) at 293 K, the lifetime of the T ₁ state shortens (<50 ns), and the efficiency of the singlet oxygen generation abruptly decreases (<0.01). The intense bathochromic emission of this compound at 705 nm with a lifetime of 1 ms at 77 K is assigned to the phosphorescence of a nonplanar conformation. Upon meso-orthonitrophenyl substitution, the quenching of phosphorescence of Pd complexes (by more than 10⁴ times at 293 K) is caused by direct nonadiabatic photoinduced electron transfer from the T ₁ state to the nearest charge-transfer state with the probability k _et ^T =(1.5–4.0)×10⁶ s^?1. The induced absorption of ortho-nitro derivatives in the region between 110 and 1400 nm is caused by mixing of pure ππ* states with charge-transfer states.     

Spectral features of formation of H-bonded complexes in coumarins and psoralens

Spectral features of H-bonded complexes formed by molecules of coumarin 1 and psoralen with methanol (1∶1) have been studied by the methods of quantum chemistry [a semiempirical method with intermediate neglect of differential overlap (INDO) with special spectroscopic parametrization] and by the method of electrostatic potential (MEP). The MEP using INDO wave functions was used to find ways of approach to the methanol molecules and the structure of H-bonded complexes. The direction of spectral shifts of the different (with respect to the orbital nature) electronic excited states in the formation of the complexes and ways of approach to the solvent molecules in the formation of the first solvation shell are established. Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 88–92, March, 1999.     

Twining effects in the magnetism of small Pd clusters

We report a theoretical study of the magnetic behavior of symmetrical twined Pd_N (N≤220) clusters. The twined Pd_N particles were built from two equal Pd_M seed-clusters with fcc-like structure for M=38, 55, 79 and 116. The optimized geometrical structures of Pd_N (with N<2M) were obtained from an uniform relaxation of the fcc-like twined configurations using the embedded atom method (EAM). The spin-polarized electronic structure and related magnetic properties of those optimized geometries were calculated by solving self-consistently a spd tight-binding Hamiltonian. We observe that, in some cases the twining process may induce and/or enhance the magnetic moment of the clusters even in the case when the seed-clusters are non-magnetic. Our results also suggest a strong dependence on the twining orientation, providing further support to the influence of symmetry effects on the magnetic properties of finite transition-metal systems. We discuss our results in comparison with some recent experimental observations for Pd nanoparticles [Shinohara et al., Phys. Rev. Lett. 91 (2003) 197201. [14]; Sampedro et al., Phys. Rev. Lett. 91 (2003) 237203. [13]].