Rashid R. Valiev

Energy-Cascaded Upconversion in an Organic Dye-Sensitized Core/Shell Fluoride Nanocrystal

Lanthanide-doped upconversion nanoparticles hold promises for bioimaging, solar cells, and volumetric displays. However, their emission brightness and excitation wavelength range are limited by the weak and narrowband absorption of lanthanide ions. Here, we introduce a concept of multistep cascade energy transfer, from broadly infrared-harvesting organic dyes to sensitizer ions in the shell of an epitaxially designed core/shell inorganic nanostructure, with a sequential nonradiative energy transfer to upconverting ion pairs in the core. We show that this concept, when implemented in a core-shell architecture with suppressed surface-related luminescence quenching, yields multiphoton (three-, four-, and five-photon) upconversion quantum efficiency as high as 19% (upconversion energy conversion efficiency of 9.3%, upconversion quantum yield of 4.8%), which is about ~100 times higher than typically reported efficiency of upconversion at 800 nm in lanthanide-based nanostructures, along with a broad spectral range (over 150 nm) of infrared excitation and a large absorption cross-section of 1.47 × 10(-14) cm(2) per single nanoparticle. These features enable unprecedented three-photon upconversion (visible by naked eye as blue light) of an incoherent infrared light excitation with a power density comparable to that of solar irradiation at the Earth surface, having implications for broad applications of these organic-inorganic core/shell nanostructures with energy-cascaded upconversion.

Computational studies of photophysical properties of porphin, tetraphenylporphyrin and tetrabenzoporphyrin.

The molecular photonics of porphyrins are studied using a combination of first-principle and semi-empirical calculations. The applicability of the approach is demonstrated by calculations on free-base porphyrin, tetraphenylporphyrin, and tetrabenzoporphyrin. The method uses excitation energies and oscillator strengths calculated at the linear-response time-dependent density functional theory (TDDFT) or the corresponding values calculated at the linear-response approximate second-order coupled-cluster (CC2) levels. The lowest singlet excitation energies obtained in the TDDFT and CC2 calculations are 0.0-0.28 eV and 0.18-0.47 eV larger than the experimental values, respectively. The excitation energies for the first triplet state calculated at the TDDFT level are in excellent agreement with experiment, whereas the corresponding CC2 values have larger deviations from experiment of 0.420.66 eV. The matrix elements of the spin-orbit and non-adiabatic coupling operators have been calculated at the semi-empirical intermediate neglect of differential overlap (INDO) level using a spectroscopic parameterization. The calculations yield rate constants for internal conversion and intersystem crossing processes as well as quantum yields for fluorescence and phosphorescence. The main mechanism for the quenching of fluorescence in tetraphenylporphyrin and tetrabenzoporphyrin is the internal conversion, whereas for free-base porphyrin both the internal conversion and the intersystem crossing processes reduce the fluorescence intensity. The phosphorescence is quenched by a fast internal conversion from the triplet to the ground state.

Insights into Magnetically Induced Current Pathways and Optical Properties of Isophlorins

The magnetically induced current density of tetraoxa-isophlorin and dioxa-dithia-isophlorin have been studied at the density functional theory (DFT) level using the gauge including magnetically induced current method (GIMIC). The current density calculations show that the studied isophlorins with formally 28 π electrons are strongly antiaromatic, sustaining paratropic ring currents of -48.5 and -58.3 nA/T, respectively. All chemical bonds of the porphyrinoid macroring participate in the transport of the paratropic ring current. Calculations of excitation energies at the time-dependent density functional theory (TDDFT) level and at correlated ab initio levels show that tetraoxa-isophlorin and dithia-isophlorin have small optical gaps of 0.82-1.34 and 0.90-1.25 eV, respectively. The transition to the lowest excited state, which belongs to the Bg irreducible representation, is dipole forbidden and has therefore not been observed in the spectroscopical studies. For dioxa-dithia-isophlorin, the excitation energies of the Q and B bands calculated at the TDDFT level agree rather well with experimental values with deviations in the range of [-0.15, +0.27] eV, whereas corresponding excitation energies obtained at the ab initio levels are 0.14-0.51 eV too large as compared to experimental values. For tetraoxa-isophlorin, the deviations of the TDDFT excitation energies from experimental values are in the range [-0.10, +0.50] eV and the ab initio values are 0.53-0.97 eV larger than the experimental values due to the significant double excitation character of the excited states.

Ab initio simulation of pyrene spectra in water matrices

The absorption and emission spectra of free pyrene and pyrene in a water ice matrix were simulated ab initio. Water ice was mimicked by a large cluster of explicit water molecules. The optimum geometries of the ground and excited states, vibrational frequencies, and normal modes were calculated using DFT. The transition energies were calculated by XMCQDPT/CASSCF. Ab initio time-dependent Franck–Condon (FC) and time-independent Franck–Condon and Herzberg–Teller (FCHT) approximations were used for the vibronic profiles of the spectra. The absorption spectra of the free molecule and the molecule in the water cluster are well reproduced within the FC approximation. The fluorescence spectrum of the gas-phase pyrene cannot be satisfactorily reproduced within pure FC or within the FCHT approximation. However, the fluorescence spectrum of pyrene in the water cluster is satisfactorily reproduced within the FC approximation. Both absorption and emission spectra of pyrene in the water cluster are broadened due to the translations and torques of pyrene in the solvent cage even at low temperatures. This broad but well structured spectrum shape should be taken into account in the identification of PAHs in cometary ice.

Aromaticity of the completely annelated tetraphenylenes: NICS and GIMIC characterization

AbstractA series of heterocyclic and hydrocarbon [8]circulenes (also named completely annelated tetraphenylenes) were studied by the NICS and GIMIC methods in order to describe their aromatic properties from the magnetic criterion point of view. According to calculations all the hetero[8]circulene molecules demonstrate the bifacial aromatic/antiaromatic nature. The inner octatetraene core of the studied [8]circulenes is characterized by the presence of paratropic (“antiaromatic”) ring currents, whereas the outer macrocycle constructed from the five- and six-membered rings possesses the magnetically-induced diatropic (“aromatic”) ring current. The hydrocarbon [8]circulenes studied in this work consist of a similar planar cyclooctatetraene core but they exhibit a rather different balance of magnetically-induced ring currents. Graphical AbstractAromaticity of the completely annelated tetraphenylenes

Aromatic Pathways in Carbathiaporphyrins

Magnetically induced current densities and current pathways have been calculated for carbaporphyrins and carbathiaporphyrins using the gauge including magnetically induced current (GIMIC) method. The aromatic character and current pathways are obtained from the calculated current density susceptibilities. The current-density calculations show that five of the studied carbaporphyrinoids are aromatic, two are antiaromatic, and one is nonaromatic. The analysis of the current pathways of the investigated molecules reveals some general trends for the current flow in carbaporphyrinoids. Insertion of a CH2 group into the all-carbon ring generally cuts or restricts the current flow, leading to a stronger current of the alternative pathway of the ring. No obvious trends regarding the current strengths and pathways of the thiophene and cyclopentadienyl rings were obtained. The present study shows that it is indeed difficult to predict the electron delocalization pathways of general carbaporphyrinoids. Thus, a careful analysis of the current density is necessary for determining their electron delocalization pathways.

Benzoannelated aza-, oxa- and azaoxa[8]circulenes as promising blue organic emitters

In the present work, we studied the synergetic effect of benzoannelation and NH/O-substitution for enhancing the absorption intensity in a series of novel designed benzoannelated aza- and oxa[8]circulenes. Semi-empirical estimations of the fluorescence rate constants allowed us to determine the most promising fluorophores among all the possible benzoannelated aza-, oxa- and mixed azaoza[8]circulenes. Among them, para-dibenzoannelated [8]circulenes demonstrated the most intense light absorption and emission due to the prevailing role of the linear acene chromophore. Calculated φfl values are in complete agreement with experimental data for a number of already synthesized circulenes. Thus, we believe that the most promising circulenes designed in this study can demonstrate an intensive fluorescence in the case of their successful synthesis, which in turn could be extremely useful for the fabrication of future blue OLEDs. Special attention is devoted to the aromaticity features and peculiarities of the absorption spectra for the two highly-symmetrical (D4h ground state symmetry) π-isoelectronic species as well as the so-called tetrabenzotetraaza[8]circulene and tetrabenzotetraoxa[8]circulene molecules. Both of them are characterized by rich electronic spectra, which can be assigned only by taking into account the vibronic coarse structure of the first electronic absorption band; the 0-1 and 0-2 transitions were found to be active in the absorption spectrum in complete agreement with experimental data obtained for both energy and intensity. The corresponding promotive vibrational modes have been determined and their vibronic activity estimated using the Franck-Condon approximation.

Predicting the degree of aromaticity of novel carbaporphyrinoids

Magnetically induced current densities have been calculated for dioxaporphyrin, dithiaporphyrin, true carbaporphyrins, and N-confused porphyrins using the gauge-including magnetically induced current (GIMIC) method. The current-strength susceptibilities (current strengths) have been obtained by numerically integrating the current flow passing selected chemical bonds. The current strength calculations yield very detailed information about the electron delocalization pathways of the molecules. The strength of the ring-current that circles around the porphyrinoid macroring is used to estimate the degree of molecular aromaticity. The studied porphyrinoid structures have been obtained by replacing the NH and N groups of porphin with formally isoelectronic moieties such as O, S, CH and CH2. Replacing an NH moiety of trans-porphin with isoelectronic O and S does not significantly change the current strengths and pathways, whereas substitution of N with an isoelectronic CH group leads to significant changes in the current pathway and current strengths. CH2 groups cut the flow of diatropic currents, whereas in strongly antiaromatic molecules a significant fraction of the paratropic ring-current is able to pass the sp(3) hybridized inner carbons. N-confused porphyrinoids sustain a ring current whose strength is about half the ring-current strength of porphin with the dominating current flow along the outer pathway via the NH moiety. When no hydrogen is attached to the inner carbon of the inverted pyrrolic ring, the current prefers the inner route at that ring.

Theoretical and experimental investigation of photophysical properties of Zn(DFP SAMQ)2.

Theoretical calculations and experimental measurements were carried out for the investigation of spectroscopic and photophysical properties of Zn(DFP SAMQ)2 complex. The rate constant of intersystem crossing and the radiative rate constant were calculated using ab initio method. The rate constant of the internal conversion was estimated using the received calculated values and the experimental fluorescence quantum yield. It was shown that the main mechanism for the deactivation of the excited electronic energy of the first singlet excited state is the process of internal conversion.

Aromaticity of the doubly charged [8]circulenes

Magnetically induced current densities and current pathways have been calculated for a series of fully annelated dicationic and dianionic tetraphenylenes, which are also named [8]circulenes. The gauge including magnetically induced current (GIMIC) method has been employed for calculating the current density susceptibilities. The aromatic character and current pathways are deduced from the calculated current density susceptibilities showing that the neutral [8]circulenes have two concentric pathways with aromatic and antiaromatic character, respectively. The inner octatetraene core (the hub) is found to sustain a paratropic (antiaromatic) ring current, whereas the ring current along the outer part of the macrocycle (the rim) is diatropic (aromatic). The neutral [8]circulenes can be considered nonaromatic, because the sum of the ring-current strengths of the hub and the rim almost vanishes. The aromatic character of the doubly charged [8]circulenes is completely different: the dianionic [8]circulenes and the OC-, CH-, CH2-, SiH-, GeH-, SiH2-, and GeH2-containing dicationic species sustain net diatropic ring currents i.e., they are aromatic, whereas the O-, S-, Se-, NH-, PH- and AsH-containing dicationic [8]circulenes are strongly antiaromatic. The present study also shows that GIMIC calculations on the [8]circulenes provide more accurate information about the aromatic character than that obtained using local indices such as nuclear-independent chemical shifts (NICSs) and (1)H NMR chemical shifts.