Luminescence Behavior Research Articles

Heteroleptic Eu(III) emissive complexes: Luminescent, optoelectronic and theoretical investigation

The synthesis of four red emissive complexes of europium with formula [Eu(Hfodo)3L] based on 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione (Hfodo) and 1,10-phenanthroline derivatives (L) is described. The coordination sphere of europium ion is formed by the oxygen and nitrogen atoms of three bidentate fluorinated diketones and one bidentate auxiliary moiety, respectively. The outcome of the luminescence analysis has confirmed the intense monochromatic red emission of the synthesized complexes in the powder and solution form. From the emission spectral data, colorimetric parameters were calculated which also affirm the luminescent behavior of prepared complexes. The range of band gap, high color purity and branching ratio (∼70%) analogous to 5D0 → 7F2 confirm the usefulness of bright emissive materials in displays and laser diodes.

Self-Assembly of Mechanoluminochromic Ladder-Shaped Gold(I) Clusters Promoted Using Cooperative Aurophilicity.

The self-assembly of mechanoluminochromic polynuclear gold(I) complexes has attracted more and more attention in the field of supramolecular gold(I) chemistry. In this work, we adopted a stepwise self-assembly strategy to precisely synthesize two polynuclear gold(I) supramolecular clusters. Through cooperative AuI···AuI and Au-N interactions, the gold(I) clusters 1+•BF4- and 24+•4BF4- with Au4 and Au16 cores, respectively, were successfully constructed. In these supramolecular clusters, (dppm)Au2Cl2 coordination motifs and trithiocyanuric linkers were stepwise assembled via sequential thiolate-chloride/phosphine coordination substitution and Au-S/Au-N coordination bond rearrangement. Two well-defined gold(I) supramolecular clusters displayed intense emission both in the solid state and in solution. Furthermore, the ladder-shaped cluster 24+•4BF4- exhibited reversible mechanochromic luminescence behavior in the solid state as well as aggregation-caused redshifted emission in solution. Upon mechanical grinding, the emission of the cluster 24+•4BF4- changed from yellow at 582 nm to red at 612 nm. The initial emission could be fully recovered by treatment with acetonitrile.

Branched dodecaynes: synthesis and their optical properties

Two homologous branched dodecaynes 1,3,5-{(4- tBu)[2,3-(RC≡C)2]Ph(C≡C–C≡C)}3Ph (2a, R = H; 2b, R = Ph) were synthesized and characterized in this work. According to steady-state spectroscopy, their electronic absorption and luminescent behaviors were investigated and compared. Simultaneously, as a typical structure of this class of dodecaynes, the molecular configuration of dodecayne 2a was depicted by density functional theory (DFT) simulation and the maximal absorption were supported by the subsequent TD-DFT calculations. In addition, the electrochemiluminescence (ECL) emission was also investigated. The experiment results showed that π-extended dodecayne 2b with more distorted trigonal–planar possessed strong and stable ECL emission, indicating that it has potential application in ECL and luminescence fields.

Investigation on luminescence behavior of Mg4Ta2O9 through experimental and first-principles calculation methods

In this work, Mg4Ta2O9 single crystal was synthesized by optical floating zone method in N2 atmosphere. The luminescence behavior of Mg4Ta2O9 single crystal was investigated through experimental and first-principles calculation methods. Its photoluminescence, scintillation and thermal stimulated luminescence properties, including photoluminescence excitation, photoluminescence, photoluminescence decay, thermal stimulated luminescence and X-ray excited luminescence spectra, were measured. In addition, the temperature dependence of photoluminescence, photoluminescence decay and photoluminescence afterglow curves of Mg4Ta2O9 were measured and discussed. Combined with the experimental and density functional theory calculations result, the energy transfer schematic diagrams of the Mg4Ta2O9 were proposed to explain the photoluminescence, scintillation, and thermal stimulated luminescence behaviors.

Exciton Luminescence and Optical Properties of Nanocrystalline Cubic Y2O3 Films Prepared by Reactive Magnetron Sputtering.

This paper presents a comprehensive study of the energy structure, optical characteristics, and spectral-kinetic parameters of elementary excitations in a high-purity nanocrystalline cubic Y2O3 film synthesized by reactive magnetron sputtering. The optical transparency gaps for direct and indirect interband transitions were determined and discussed. The dispersion of the refractive index was established based on the analysis of interference effects. It was found that the refractive index of the Y2O3 film synthesized in this study is higher in order of magnitude than that of the films obtained with the help of other technologies. The intrinsic emission of Y2O3 film has been discussed and associated with the triplet–singlet radiative relaxation of self-trapped and bound excitons. We also studied the temperature behavior of the exciton luminescence of Y2O3 for the first time and determined thermal activation barriers. The optical energy and kinetic parameters of cubic Y2O3 films were analyzed in comparison with those of the monoclinic films of yttrium oxide. The main difference between the optical properties of cubic and monoclinic Y2O3 films was established, which allowed for a supposition of their application prospects.

Coordination Polymers from Biphenyl-Dicarboxylate Linkers: Synthesis, Structural Diversity, Interpenetration, and Catalytic Properties.

The present work explores two biphenyl-dicarboxylate linkers, 3,3'-dihydroxy-(1,1'-biphenyl)-4,4'-dicarboxylic (H4L1) and 4,4'-dihydroxy-(1,1'-biphenyl)-3,3'-dicarboxylic (H4L2) acids, in hydrothermal generation of nine new compounds formulated as [Co2(μ2-H2L1)2(phen)2(H2O)4] (1), [Mn2(μ4-H2L1)2(phen)2]n·4nH2O (2), [Zn(μ2-H2L1)(2,2'-bipy)(H2O)]n (3), [Cd(μ2-H2L1) (2,2'-bipy)(H2O)]n (4), [Mn2(μ2-H2L1)(μ4-H2L1)(μ2-4,4'-bipy)2]n·4nH2O (5), [Zn(μ2-H2L1)(μ2-4,4'-bipy)]n (6), [Zn(μ2-H2L2)(phen)]n (7), [Cd(μ3-H2L2)(phen)]n (8), and [Cu(μ2-H2L2) (μ2-4,4'-bipy)(H2O)]n (9). These coordination polymers (CPs) were generated by reacting a metal(II) chloride, a H4L1 or H4L2 linker, and a crystallization mediator such as 2,2'-bipy (2,2'-bipyridine), 4,4'-bipy (4,4'-bipyridine), or phen (1,10-phenanthroline). The structural types of 1-9 range from molecular dimers (1) to one-dimensional (3, 4, 7) and two-dimensional (8, 9) CPs as well as three-dimensional metal-organic frameworks (2, 5, 6). Their structural, topological, and interpenetration features were underlined, including an identification of unique two- and fivefold 3D + 3D interpenetrated nets in 5 and 6. Phase purity, thermal and luminescence behavior, as well as catalytic activity of the synthesized products were investigated. Particularly, a Zn(II)-based CP 3 acts as an effective and recyclable heterogeneous catalyst for Henry reaction between a model substrate (4-nitrobenzaldehyde) and nitroethane to give β-nitro alcohol products. For this reaction, various parameters were optimized, followed by the investigation of the substrate scope. By reporting nine new compounds and their structural traits and functional properties, the present work further outspreads a family of CPs constructed from the biphenyl-dicarboxylate H4L1 and H4L2 linkers.

Charge Transfer-Triggered Bi3+Near-Infrared Emission in Y2Ti2O7for Dual-Mode Temperature Sensing

Trivalent bismuth is a popular main group ion showing versatile luminescent behaviors in a broad spectral range from ultraviolet to visible, but barely in the near-infrared (NIR) region. In this study, we have observed unexpected NIR emission at ∼744 nm in a Bi3+-doped pyrochlore, Y2Ti2O7 (YTOB). Our first-principles electronic structure calculation and analysis of the Bi local structure via extended X-ray absorption fine structure indicate that only Bi3+ species appears in YTOB and it has a similar local environment to that of Y3+. The NIR emission is assigned to a Ti4+ → Bi3+ metal-to-metal charge transfer process. Moreover, we have demonstrated dual-mode luminescence thermometry based on the luminescence intensity ratio (LIR) and lifetime (τ) in 0.5% Bi3+ and 0.5% Pr3+ co-doped Y2Ti2O7 (YTOB0.5P0.5). It exhibits high thermometric sensitivity simultaneously in the cryogenic temperature range from 78 to 298 K based on τ of the NIR emission of Bi3+ at 748 nm and in the temperature range of 278-378 K based on the LIR of Bi3+ to Pr3+ emissions (I748/I615). As a novel LIR-τ dual-mode thermometric material over a wide temperature range, the maximum relative sensitivities of the YTOB0.5P0.5 reach 3.53% K-1 at 298 K from the τ mode and 3.52% K-1 at 318 K based on the LIR mode. The dual-mode luminescence thermometry with high responsivity from our Bi3+-based pyrochlore Y2Ti2O7 phosphor opens a new avenue for more luminescent materials toward multi-mode thermometry applied in complex temperature-sensing conditions.

The roles of packing modes in manipulating gel stability and luminescence behavior in fluorescent supramolecular gels

Fluorescent supramolecular gel is a large group in luminescent gels. In the scope of practical applications, excellent stability and robust luminescence are complementary and indispensable for high-performance fluorescent supramolecular gels. Therefore, in this Perspective, the proceedings of precursor explorations of stabilizing and boosting the luminescence behavior of the gels will be briefly summarized. The challenges and possible development frontiers will also be pondered to inspire future scientific research achievements.

Physico-chemical characterization studies of collagen labelled with Ru(II) polypyridyl complex

The rich luminescence behaviour exerted by transition metal complexes has found significant role in the development of biomolecular and cellular probes. The conjugation of fluorophore to a protein has its own advantage over the label-free system due to its high sensitivity. While numerous proteins have been labelled with either organic or inorganic fluorophores, the conjugation of luminescent transition metal complexes with collagen has not yet been attempted. Here, in this study, the conjugation of a Ru(II) polypyridyl complex with collagen was carried out and its physico-chemical characterization was studied. The conjugation of Ru(II) to collagen was characterized by UV-Visible, fluorescence and ATR-FT-IR spectroscopy. The conjugation of Ru(II) did not alter the triple helical structure of the collagen as evidenced from CD spectral data. The luminescence behaviour of the Ru-tagged collagen was found to be similar to that of the commercially available fluorescein isothiocyanate (FITC) tagged collagen with increase in luminescence upon addition of collagenase. Gel-based collagenase assay showed that the digestion of collagen can be vizualized using UV light due to intrinsic fluorophore tag without carrying out the staining-destaining processes. Energy dispersive X-Ray analysis (EDAX) confirms the presence of Ru in Ru-collagen fibrils. To the best of our knowledge, this is the first report on the conjugation of a Ru(II) complex with the fibrous protein collagen that exhibits similar property as of FITC-collagen and can be used as an alternative.

Influence of the fluorine ion content on luminescence properties of the EuII+/III+-doped silicate-substituted apatite

In this study the preparation, characterization, and photoluminescence properties of the Eu3+-doped apatite compounds have been presented. The Eu3+ ion doped silicate-substituted apatite, Ca9.9Eu0.1(PO4)2(SiO4)4(OH)2−xFx where x = 0; 0.8; 1.2; 2.0, was obtained by a microwave-assisted hydrothermal method. A detailed investigation on the heat-treated samples was done by using X-ray diffraction (XRD), scanning electron microscopy energy-dispersive spectrometry (SEM-EDS), transmission electron microscopy (TEM) and Fourier-transform infrared spectroscopy (FT-IR). The luminescence behavior of Eu3+ ion was explored based on excitation and emission spectra as well as luminescence decay profiles. The emission spectra under excitation to the charge-transfer (CT) transition shows the broad band associated with Eu2+ ion emission. The asymmetry parameter (R parameter) and CIE color coordinates were also discussed.

Detailed luminescence dating of dust mass accumulation rates over the last two glacial-interglacial cycles from the Irig loess-palaeosol sequence, Carpathian Basin

Mineral dust records distant from dust sources are crucial in establishing wider atmospheric dust loads in the past. However, detailed, independent chronologies for loess-palaeosol sequences distant from local dust sources are still rare in Europe. In this study we present a high-resolution OSL and pIRIR290 chronology and multi-proxy investigation of the Irig loess-palaeosol sequence (LPS), Vojvodina, Serbia. Our results show that 4–11 μm quartz is a reliable dosimeter only up to ~59 ka (179 Gy), after which the OSL ages underestimate the expected ages based on stratigraphy. The pIRIR290 signal displays generally good luminescence behaviour and resultant ages show good agreement with the expected stratigraphic ages, although with apparent 15 kyr and 35 kyr age underestimations at the L2-S2 and S2-L3 boundaries, respectively. The calculated dust mass accumulation rates (MAR) reveal large fluctuations during the last two glacial-interglacial cycles. During the penultimate glacial, the highest MARs were observed between 171 and 181 ka, subsequently decreasing rapidly during MIS 5. During the last glacial period, dust MARs display maximum values during MIS 2 and MIS 4, while the lowest dust input was observed during the Holocene, followed by MIS 3. Dust MAR and grain-size shifts appear to lead changes in magnetic susceptibility by several kyr. Our results imply that changes in dust availability and wind dynamics occurred prior to changes in climate-controlled soil moisture conditions at the Irig LPS. Moreover, our results reveal a close coupling of the dust MAR and grain-size variations during the Mid-Late Pleistocene. These patterns suggest that the Irig site is likely representative of regional dust dynamics, which separates it from more source proximal loess sites, such as those close to major rivers, which are likely more affected by local dust availability and river discharge.

Optimization of Eu3+-to-Host Emission Ratio in Double-Perovskite Molybdenites for Highly Sensitive Temperature Sensors

Double-perovskite phosphors exhibit unusual thermal and luminescent behavior and could potentially be applied as efficient and sensitive luminescent temperature sensors. Four double perovskite hosts─Ba2MgMoO6 (BMM), Sr2MgMoO6 (SMM), Ba2ZnMoO6 (BZM), and Sr2ZnMoO6 (SZM)─are studied for the correlations between the symmetry, energetic structure, luminescence, and thermometric performance. The quantum yield of Eu3+ emission in those hosts is higher for the samples with high cubic symmetry and ranges up to over 20% for Ba2MgMoO6. The Eu3+ ions can be excited indirectly by the host absorption bands in the UV and charge transfer band (CTB) in the blue range. The symmetry of the host plays a crucial role in the thermal stability of the host emission, which together with varying activation energy of Eu3+ luminescence provides a basis for thermometric sensitivity optimization. The relative sensitivities (Sr) obtained by the Eu-doped molybdenite hosts are 3.2% at 75 °C for Sr2MgMoO6 and 9.2% at −196 °C for Ba2ZnMoO6. It is also demonstrated that the sensing performance is higher in hosts with a uniform quenching profile of host luminescence and steep quenching of Eu3+ luminescence.

Preparation and luminescence behaviour of perovskite LaAlO3:Tb3+ nanophosphors for innovative displays

Herein, we report the preparation, structural and luminescent behaviour of the perovskite LaAlO3:Tb3+ nanophosphors. The low temperature gel-combustion method was employed for the preparation of the present La1−xAlO3:xTb3+ (x = 0.01–0.09) nanomaterials. The crystalline nature and other unit cell parameters were calculated by diffraction pattern and refined data of Rietveld. The present perovskite nanophosphor was found to be cubic crystal having space group Pm3̅m with space number 221. Transmission electron microcopy image has affirmed the smooth morphology, uniform distribution of particles (20–60 nm) with some agglomeration. The analysis of chemical bonding with vibrational characteristics was done by Fourier transform infrared (FTIR) spectroscopy. The information about elemental composition of the selected phosphor was obtained with the help of EDX analysis. In PL spectra recorded at λex= 266 nm, the dominating emission was obtained on 543 nm for 5D4→7F5 in the greenish spectral region. The appropriate color coordinates in the intense green region confirmed the potentiality of LaAlO3:Tb3+ nanophosphors in the display devices and other lighting applications.

Hydrogen bonding-induced oxygen clusters and long-lived room temperature phosphorescence from amorphous polyols

Developing non-conjugated luminescent polymers (NCLPs) with fluorescence and long-lived room-temperature phosphorescence is of great significance for revealing the essence of NCLPs luminescence, which has gradually attracted the attention of researchers in recent years. Herein, polymethylol (PMO) and poly(3-butene-1,2-diol) (PBD) with polyhydroxy structures were prepared and their luminescence behaviors were investigated to reveal the clusteroluminescence (CL) mechanism. Compared with polyvinyl alcohol with non-luminescent behavior, PMO and PBD exhibit cyan-blue fluorescence with quantum yields of ca. 12% and green room-temperature phosphorescence with lifetimes of ca. 89 ms in the solid state. Both fluorescence and phosphorescence exhibit typical excitation-dependent CL behavior. Experimental and theoretical analyses show that the strong hydrogen-bonding interaction of PMO and PBD greatly promotes the formation of oxygen clusters and the through-space n-n interaction of oxygen atoms, enabling fluorescence and phosphorescence emission. Our results have enormous implications for understanding the CL mechanism of NCLPs and provide a new polymer design strategy for the rational design of novel NCLPs materials.

Ultra‐Wide Vis–NIR Mg2Al4Si5O18:Eu2+,Cr3+ Phosphor Containing Unusual NIR Luminescence Induced by Cr3+ Occupying Tetrahedral Coordination for Hyperspectral Imaging

AbstractCr3+‐activated broadband near‐infrared (NIR) phosphors, featured by the octahedral coordination preference and strong absorption in visible (Vis) region, have great potential application in nondestructive assessment. It is still a challenge to develop Cr3+‐doped phosphors with the tetrahedrally coordinated Cr3+ NIR emission behavior and ultra‐wide Vis–NIR luminescence. Herein, an unusual NIR‐emitting Mg2Al4Si5O18:Cr3+ phosphor offers emission peak at 867 nm with full‐width at half‐maximum of 237 nm due to preferential occupation of the AlO4 tetrahedra by Cr3+ as demonstrated by structural and optical properties studies. Eu2+–Cr3+ energy transfer is proposed to induce ultra‐wide Vis–NIR Mg2Al4Si5O18:Eu2+,Cr3+ phosphors with more efficient NIR emission and lower thermal quenching behavior of Cr3+. The fabricated Vis–NIR phosphor‐converted light‐emitting diode is expected to be an alternative to halogen lamp in hyperspectral imaging. This work reveals the luminescence behavior of Cr3+ in tetrahedra and demonstrates the application of Mg2Al4Si5O18:Eu2+,Cr3+ phosphor in hyperspectral imaging, which will facilitate further research on NIR and Vis–NIR phosphors.

Fluorescent Calixarene-Schiff as a Nanovehicle with Biomedical Purposes

Gene therapy is a technique that is currently under expansion and development. Recent advances in genetic medicine have paved the way for a broader range of therapies and laid the groundwork for next-generation technologies. A terminally substituted difluorene-diester Schiff Base calix[4]arene has been studied in this work as possible nanovector to be used in gene therapy. Changes to luminescent behavior of the calixarene macrocycle are reported in the presence of ct-DNA. The calixarene macrocycle interacts with calf thymus DNA (ct-DNA), generating changes in its conformation. Partial double-strand denaturation is induced at low concentrations of the calixarene, resulting in compaction of the ct-DNA. However, interaction between calixarene molecules themselves takes place at high calixarene concentrations, favoring the decompaction of the polynucleotide. Based on cytotoxicity studies, the calixarene macrocycle investigated has the potential to be used as a nanovehicle and improve the therapeutic efficacy of pharmacological agents against tumors.

Activating Ultrahigh Thermoresponsive Upconversion in an Erbium Sublattice for Nanothermometry and Information Security.

Thermal activation of upconversion luminescence in nanocrystals opens up new opportunities in biotechnology and nanophotonics. However, it remains a daunting challenge to achieve a smart control of luminescence behavior in the thermal field with remarkable enhancement and ultrahigh sensitivity. Moreover, the physical picture involved is also debatable. Here we report a novel mechanistic design to realize an ultrasensitive thermally activated upconversion in an erbium sublattice core-shell nanostructure. By enabling a thermosensitive property into the intermediate 4I11/2 level of Er3+ through an energy-migration-mediated surface interaction, the upconverted luminescence was markedly enhanced in the thermal field together with a striking thermochromic feature under 1530 nm irradiation. Importantly, the use of non thermally coupled red and green emissions contributes to the thermal sensitivity up to 5.27% K-1, 3 times higher than that obtained by using conventional thermally coupled green emissions. We further demonstrate that the controllable surface interaction is a general approach to the thermal enhancement of upconversion for a series of lanthanide-based nanomaterials. Our findings pave a new way for the development of smart luminescent materials toward emerging applications such as noncontact nanothermometry, information security, and anticounterfeiting.

Potential Anticancer Activities and Catalytic Oxidation Efficiency of Platinum(IV) Complex.

The treatment of an aqueous acetonitrile solution of chloroplatinic acid hydrate H2PtCl6.xH2O and pyridine-2-carbaldehyde-oxime (paOH) in the presence of potassium thiocyanate at room temperature (25°) led to the formation of a new Pt(IV) complex with the formula [Pt(SCN)2(paO)2], (1). Complex 1 was fully characterized by FT-IR, UV-vis and NMR spectroscopic techniques as well as elemental analysis. The crystallographic structure of complex 1 was obtained by single-crystal X-ray diffraction. The structure of complex 1 consists of a distorted octahedral geometrical environment around the platinum center in which the coordination sites are occupied by two terminal thiocyanate ligands in trans arrangement and two bidentate paO ligands through four nitrogen atoms. In addition, the in vitro evaluation of the cytotoxicity of platinum complex 1 against four different cancer cell lines was performed. The IC50 values for colon (HCT116), liver (HepG2), breast (MCF-7) and erythroid (JK-1) treated with complex 1 are 19 ± 6, 21 ± 5, 22 ± 6, and 13 ± 3 μM, respectively. In HCT116 cells treated with the IC50 dose of our title compound, apoptosis and necrosis were increased by 34% and 27.8%, respectively. Cells halted in the proliferative phase (S phase) to 21.7 % and 29.8% in HCT116 and HepG2 cells treated with complex 1 have anti-proliferative actions. Furthermore, the catalytic activity of synthesized complex 1 was examined in the oxidation reaction of benzyl alcohols in the presence of an oxidant. Finally, the luminescence behavior of complex 1 was investigated.

Eu3+ ions doped lithium aluminium gadolinium borophosphate glasses: Energy transfer, optical and luminescence behaviors for red emission material

Eu3+ ions doped Li2O–Al2O3–Gd2O3–B2O3–P2O5 glasses have been prepared by melt quenching and characterized for their physical, optical and luminescence properties. Judd-Ofelt (JO) theory is used to evaluate the oscillator strengths and the three intensity parameters (Ωλ, λ = 2, 4 and 6) of the glasses. The radiative properties such as radiative transition probabilities (AR), radiative lifetimes (τ), branching ratios (β) and emission cross-section (σe) have been calculated from the absorption spectra according to the JO theory. The scintillation properties of glasses could be measured by comparing emission spectrum under the X-ray induced luminescence with bismuth germanate (BGO). The Commission Internationale de l’Elcairage (CIE) chromaticity coordinates (x,y) were also evaluated from the emission spectra of glasses and values of glasses take part in the red region.

Drug Ambroxol-based luminescent materials with sulfonate group: Synthesis, crystal structure and Hirshfeld surface analysis

Two new salts with ambroxol moieties, namely, n[HAX]+ nRT– H2O (RT ​= ​MBS (n ​= ​2, 1) and SBS (n ​= ​1, 2)) [AX ​= ​4-((2-amino-3,5-dibromobenzyl)amino) cyclohexan-1-ol, MBS ​= ​p-toluenesulfonic acid, SBS ​= ​sulfosalicylic acid], which were obtained through the mixture of AX and two phenyl sulfonic acid, respectively. The emission peaks located at 526 and 471 ​nm can be observed for compounds 1 and 2 in the solid state at room temperature, respectively. Compared with the AX itself, the red-shifted values can be observed to be 140 and 85 ​nm for compounds 1 and 2, respectively, revealing that the luminescent maxima are proportional to the intermolecular stacking distances. The decay lifetimes of AX, 1 and 2 can be found to be 0.643, 0.671 and 0.589 ns, respectively. Additionally, the quantum yields of 14.7%, 1.2% and 37.1%, respectively. Compounds 1 and 2 were structurally characterized by X–ray single crystal diffraction, EA, UV–Vis and FT-IR spectra. Single crystal X-ray diffraction reveals that there exists an order of packing distances, which is in a good agreement with that of luminescent maxima. The results calculated by Hirshfeld surface show that the close contacts of C⋯H/H⋯C can be responsible for the luminescent behaviors. The O⋯H/H⋯O close contacts can account for the melting points of compounds. Additionally, the thermal properties for compounds 1 and 2 have been discussed in detail.