Short Polyenes Research Articles

Crystal Structures and Fluorescence Spectroscopic Properties of a Series of α,ω‐Di(4‐pyridyl)polyenes: Effect of Aggregation‐Induced Emission

Crystal structures and fluorescence spectroscopic properties were investigated for a series of all-(E) α,ω-di(4-pyridyl)polyenes (1-5) with different number of double bonds (n). Molecules 1 and 2 (n=1, 2) in crystals are arranged to form partially π-overlapped structures, whereas those of 3-5 (n=3-5) are stacked in a herringbone fashion. All these molecules, the shorter polyenes in particular, are almost nonfluorescent in solution. In the solid state, 1 and 2 are highly emissive as pure organic solids [fluorescence quantum yields (φf )=0.3-0.5], while 3 and 4 are only weakly fluorescent (φf <0.05). The strongly n-dependent fluorescence properties can be attributed to the largely different molecular arrangements in the crystals. Although 5 is nonfluorescent in the solid state, we observe a very clear structure-property relationship in 1-4. Compounds 1 and 2 become much more emissive in the solid state than in solution as a result of the aggregation-induced emission (AIE) effect.

Energies of selected conformers/rotamers of the unsubstituted linear polyenes C 2 n H 2 n +2 : Results at the B3LYP/6-311++G( d,p ) ( n = 2–14) and MP2(FC)/aug-cc-pVDZ ( n = 2–7) computational levels

Abstract Total molecular energies (Etot), zero point energies (EZPE), thermal corrections (Eth) to 298 K, and relative energies (Erel) are given for a data set of more than 200 conformers/rotamers/transition states of the unsubstituted linear polyenes, C2nH2n+2(n = 2–14), calculated at the B3LYP/6-311++G(d,p) level. For some shorter polyenes additional MP2(FC)/aug-cc-pVDZ and MP2(FC)/aug-cc-pVTZ optimizations, and single point MP4(SDQ)(FC)/aug-cc-pVTZ//MP2(FC)/aug-cc-pVTZ and MP4(SDTQ)(FC)/aug-cc-pVQZ//MP2(FC)/aug-cc-pVTZ calculations were performed. At all levels, Etot is lowest for the planar all-trans conformers. Planar mono-Cis (about a C C bond) conformers lie in a narrow energy range above the corresponding all-trans conformers at both the B3LYP/6–311++G(d,p) (∼1.7–2.1 kcal/mol) and the MP2(FC)/aug-cc-pVDZ (∼1.3 kcal/mol) levels. Di-Cis conformers are ∼1.3–1.5 kcal/mol higher in energy than mono-Cis conformers at the MP2(FC)/aug-cc-pVDZ level and mono-gauche (about a C C bond) rotamers are ∼0.5 kcal/mol higher in energy than di-Cis conformers. The energetic order of the di-Cis and mono-gauche forms is reversed at the B3LYP/6–311++G(d,p) level.

New Perspectives on an Old Issue: A Comparative MS-CASPT2 and OM2-MRCI Study of Polyenes and Protonated Schiff Bases.

Polyenic systems are involved in light perception of numerous living organisms. Although a π-conjugated backbone is a common feature of all polyenes, their photophysics may vary. We provide a comparative quantum mechanical study of low-lying S1 and S2 excited states in short (3-5 double bonds) symmetric all-trans linear polyenes and corresponding protonated Schiff bases. In our investigation, we use the well-established ab initio multireference CASPT2 approach and benchmark the efficient semiempirical OM2-MRCI approach against it. For all protonated Schiff bases, MS-CASPT2 results in two distinct S1 minima with inverted and noninverted bond length pattern, respectively. We find that OM2-MRCI is a computationally affordable and reliable alternative to MS-CASPT2 for investigations of polyenic systems, particularly when highly demanding calculations (e.g. excited-state dynamics) need to be performed.

Experimental and theoretical study on terahertz spectra of all-trans -carotene

The -carotene is a short chain polyene molecule containing nine -electron conjugated double-bonds. Because of its special molecular structure, -carotene has been used widely in many fields, including functional materials, optoelectronic devices and biological applications of light collection, light protection, anti-cancer, etc. Recently, new applications of -carotene in generation and detection of terahertz (THz) wave have also attracted great attention. In this work, all-trans -carotene films are prepared by spray coating, and the THz spectra in a wavenumber range of 30-400 cm-1 (a frequency range of 0.9-12 THz) of the as-prepared products are experimentally measured at room temperature by Fourier transform infrared spectroscopy. For comparison, the THz spectra in 0.5-3.0 THz are also characterized at the same temperature by THz time-domain spectroscopy. Based on these measurements, the fingerprint peaks of all-trans -carotene in the THz region are experimentally identified to be located at 54 cm-1 (1.62 THz), 57 cm-1 (1.71 THz), 64 cm-1 (1.91 THz), 77 cm-1 (2.32 THz), 90 cm-1 (2.69 THz), 98 cm-1 (2.95 THz), 115 cm-1 (3.45 THz), 124 cm-1 (3.72 THz), 134 cm-1 (4.02 THz), 170 cm-1 (5.11 THz), 247 cm-1 (7.42 THz), and 279 cm-1 (8.38 THz), respectively. It is worth noting that the recent results about the THz spectra of palm leaves are thus verified. Particularly, the B3 LYP method of density functional theory is further utilized in this work to theoretically simulate the THz spectra of all-trans -carotene molecule. It is revealed that the theoretical simulation results accord well with those experimentally measured data. In addition, we also find that the absorption peaks are caused by the torsion, deformation and rocking vibration of the molecules. Accordingly, the vibrational modes of the measured THz characteristic peaks at 148 cm-1 (4.44 THz), 132 cm-1 (3.96 THz), 115 cm-1 (3.45 THz), 76 cm-1 (2.28 THz) and 52 cm-1 (1.56 THz) are theoretically assigned, which provides a reference to explain the formation mechanism of the THz spectra. The valuable results presented in this work will be helpful for promoting the studies of the THz spectral features and response mechanisms of the organics.

Excited-state energy levels and photophysics of a short polyene 2-(4-phenyl-1,3-butadien-1-yl)thiophene

Abstract Emission, excitation and absorption spectra of a new and short polyene, 2-(4-phenyl-1,3-butadien-1-yl)thiophene (PBT), have been measured under different conditions by varying temperature and solvent and in the vapor phase, along with those of 2-(2-phenylethenyl)-thiophene (PET). The presence of a forbidden excited singlet state, located at energies slightly below the strongly absorbing state, is indicated for PBT, although the forbidden state was not identified for PET. The emission of PBT consists of fluorescence from the allowed S2 and forbidden S1 states in low polarizable solvents, while it consists of florescence mainly from forbidden S1 in the vapor phase and that from allowed S1 state in high polarizable solvents. Quantitative analysis of the temperature dependence of the S1 and S2 fluorescence spectra provides the S1 and S2 state energies and physical parameters that characterize the excited states of PBT.

Electron Correlation Effects on the Longitudinal Polarizabilities and Second Hyperpolarizabilities of Polyenes: A Finite Field Study

We carry out ab initio calculations, based on finite-field scheme, of the longitudinal polarizabilities (αL) and second hyperpolarizabilities (γL) of conjugated polyenes, and study the effects of electron correlation, with second-order Moller-Plesset perturbation theory and coupled cluster with singles and doubles method. Calculations with density functional theory are also made to compare with wave-function based methods. Our study shows that electron correlation reduces linear longitudinal polarizability and enhances longitudinal second hyperpolarizability for short polyenes, but the effects decrease as the chain increases; choosing appropriate basis sets is important when quantitative results are required.

Oligomerization of phenylferrocenylacetylene under the action of WCl6

Phenylferrocenylacetylene was found to be capable of oligomerizing in the presence of WCl6 (130 °C, 24 h) to afford short polyenes (with about 5 monomer units in the macromolecule) in up to ∼30% yield. The polyenes synthesized are promising reactive macromonomers and building blocks for high-tech materials.

Effects of O3 and NO2 on the natural weathering of plasticized poly(vinyl chloride)

AbstractThe aim of this work was to study the natural aging of plasticized poly(vinyl chloride) (PVC) stabilized with epoxidized sunflower oil and Ca/Zn stearates. For that purpose, PVC samples were exposed in Algiers (hot Mediterranean climate) in two sites where the concentration of atmospheric pollutants (NO2 and O3) was known for 2 years. Site I contained NO2 and O3, and site II contained only NO2. Samples were taken off every four months and characterized (mechanical properties and chemical modifications). The results showed a drop in tensile properties and an increase in Shore D hardness. The UV–Visible spectra evidenced the formation of short polyenes. Chemical structures due to the degradation were evidenced by ATR and transmission FTIR spectroscopy. It was found that the combination of O3 and NO2 exerted the most deleterious effect. J. VINYL ADDIT. TECHNOL., 2011. © 2011 Society of Plastics Engineers

Surface pinking in titanium dioxide/lead stabiliser filled PVC profiles

The “pinking” phenomenon which is known to occur in white PVC profiles has been investigated in detail in terms of test evaluations and analysis methods for determining the mechanism and nature of the species responsible. An artificial weathering test method using a Microscal unit with submersion of samples in water was established where colour measurements and FTIR analysis proved the similarity of functional groups present to those in naturally pinked samples. Analysis of the artificially pinked samples using hydroperoxide concentration analysis, fluorescence spectroscopy, microscopic FTIR and scanning electron microscopy showed that artificially pinked samples presented a more extended degree of degradation than samples that remained white during the exposure in the Microscal Unit. Short chain conjugated polyenes were also identified as the most likely functional groups responsible for the development of the pink colour and interactions between the titania pigment and the lead stabilisers would seem to have an influence in the pinking development. In addition, it was found that there was an increase in production of hydroperoxy radicals at the onset of colour development. Addition of trace lead dioxide accelerated the production of pinking. Different titanium dioxide grades used in the PVC profiles showed differences in pinking in the artificial test, also an increase in the titanium dioxide concentration decreased the formation of the pinking colouration in artificial conditions The use of different analysis methods (Microwave Dielectric Spectroscopy and ESR) showed differences in carrier radical production and carrier radical trapping between the pigments used suggesting that a small difference in the coatings could play an important part in the pinking phenomenon. Finally, a mechanism was proposed involving a redox reaction between the titanium dioxide and the lead stabiliser that could facilitate the production of short polyene chains in the PVC matrix producing the pink colouration.

X-ray Crystal Structure and Time-Resolved Spectroscopy of the Blue Carotenoid Violerythrin

Violerythrin, a blue-colored carotenoid, has been investigated by X-ray crystallography and steady-state and ultrafast time-resolved absorption spectroscopy. The X-ray crystal structure of violerythrin shows that the molecule is nearly planar with the terminal rings positioned in the s-trans conformation. The steady-state and time-resolved spectroscopic data of violerythrin do not differ significantly from those of other carbonyl carotenoids with long (N > 10) pi-electron conjugated chains. This indicates that while the four carbonyl groups in violerythrin are critical for generating the bathochromic shift that leads to the blue color of the molecule, no dramatic changes attributable to a charge-transfer state known to affect the excited-state properties of carotenoids with short polyene chains occur. This may be due to the symmetric distribution of the carbonyl groups, which would preclude such an effect. The structural requirements for a blue, neutral, carotenoid are a planar, symmetric, cross-conjugated chromophore, containing at least 30 pi-electrons, a central polyene chain with 9 or 10 conjugated carbon-carbon double bonds connected at each end by an s-trans or trans bond to two identical, cyclic end groups, each possessing a conjugated keto group further cross-conjugated to another keto group, or a double bond in a quinoid type structure.

How much double excitation character do the lowest excited states of linear polyenes have?

Doubly excited states play important roles in the low-energy region of the optical spectra of polyenes and their investigation has been subject of theoretical and experimental studies for more than 30 years now and still is in the focus of ongoing research. In this work, we address the question why doubly excited states play a role in the low-energy region of the optical spectrum of molecular systems at all, since from a naive point of view one would expect their excitation energy approximately twice as large as the one of the corresponding single excitation. Furthermore, we show that extended-ADC(2) is well suited for the balanced calculation of the low-lying excited 2 1 A g - , 1 1 B u - and 1 1 B u + states of long all- trans polyenes, which are known to possess substantial double excitation character. A careful re-investigation of the performance of TDDFT calculations for these states reveals that the previously reported good performance for the 2 1 A g - state relies heavily on fortuitous cancellation of errors. Finally, the title question is answered such that for short polyenes the lowest excited 2 1 A g - and 1 1 B u - states can clearly be classified as doubly excited, whereas the 1 1 A g - ground state is essentially represented by the (ground-state) HF determinant. For longer polyenes, in addition to increasing double excitation contributions in the 2 1 A g - and 1 1 B u - states, the ground state itself aquires substantial double excitation character (45% in C 22H 24), so that the transition from the ground state to these excited states should not be addressed as the excitation of two electrons relative to the 1 1 A g - ground state.

A Computational Study of End‐Group Conformational Energy Barriers in Carotenoids

Introduction Carotenoids possessing different end-groups are known to be selectively metabolized in biological systems and there is evidence demonstrating that end-groups affect the ability of carotenoids to adopt specific geometries within larger biological structures such as membranes. In order to further assess the importance of the end-group structure in carotenoids we have investigated the influence of the end-group type and functionality on the conformational energy barrier. Methods We used the density functional method and the program GAUSSIAN to calculate the conformational energy curves for rotation of the beta-ring or the epsilon-ring relative to short polyene chains around the C6-C7 single bond. Results Epsilon rings have been shown to possess a dramatically larger conformational energy barrier to rotation than beta rings. The rotational energy barrier of the epsilon ring relative to the polyene chain around the C6-C7 bond has been found to range from 8.65 to 9.05 kcal/mole whereas the barrier for rotation around the beta-ring varies from 3.06 to 3.40 kcal/mole. Conclusions Conjugation of the beta-ring has no influence on the conformational energy barrier to rotation in carotenoid systems possessing this end-group. The length of the polyene chain (extent of conjugation) and substitution on the ring have little influence on the rotational energy barrier. The most important factor influencing the conformational energy for ring rotation about the C6-C7 bond is the steric interaction between the ring and polyene methyl groups. The high barrier to rotation for the epsilon ring provides a physical basis for the observed differences in the metabolism and properties of carotenoids possessing this end-group in biological systems.

The first detection of the 3Ag− state in carotenoids using resonance-Raman excitation profiles

The singlet 3Ag− state that had been theoretically predicted in shorter polyenes [P. Tavan and K. Schulten J. Chem. Phys. 85 (1986) 6602; Phys. Rev. B 36 (1987) 4337] was first identified in bacterial carotenoids by measurements of resonance-Raman excitation profiles. It is almost overlapped with the 1Bu+ state in spheroidene (the number of conjugated double bonds, n=10), and located in-between the 1Bu+ and 1Bu− states in lycopene, anhydrorhodovibrin and spirilloxanthin (n=11–13). The slopes when the 2Ag−-, 1Bu−- and 3Ag−-state energies were expressed as linear functions of 1/(2n+1) exhibited the ratio of 2:3.1:3.8 in excellent agreement with that theoretically predicted, 2:3.1:3.7.

Study of the miscibility and the thermal degradation of PVC/PMMA blends

The aim of this paper is to study the miscibility and the thermal degradation of PVC/PMMA blends. For that purpose, blends of variable compositions from 0 to 100 wt% were prepared with and without plasticizer. Their physico-chemical characterization was carried out by differential scanning calorimetric analysis (DSC) and Fourier transform infrared spectroscopy (FTIR). Their thermal degradation under nitrogen at 185°C was studied and the HCl evolved from PVC was measured by the pH method. Degraded samples were characterized, after purification, by FTIR and UV-visible spectroscopy. The DSC analysis showed polymer miscibility up to 60 wt% of PMMA. This miscibility is due to a specific interaction of hydrogen bonding type between carbonyl groups (C=O) of PMMA and hydrogen from (CHCl) groups of PVC as evidenced by FTIR analysis. On the other hand, it was found that PMMA exerted a stabilizing effect on the thermal degradation of PVC by reducing the zip dehydrochlorination and by leading to the formation of short polyenes.

Using 1,3‐butadiene and 1,3,5‐hexatriene to model the cis‐trans isomerization of retinal, the chromophore in the visual pigment rhodopsin

AbstractThe short polyenes 1,3‐butadiene and 1,3,5‐hexatriene are used to model the cis‐trans isomerization of the protonated Schiff base of retinal (PSBR) in rhodopsin (Rh). We employed the complete active space self‐consistent field (CASSCF) method for calculation of the potential energy surfaces (PESs) in C2 symmetry. In the calculations, the central bond was twisted from 0 to 180° in the first singly excited singlet state (Sse), i.e., the state dominated by a configuration with one electron excited from HOMO to LUMO. It was found that the PES of 1,3‐butadiene has a maximum whereas the PES of 1,3,5‐hexatriene has a minimum for a twist angle of 90°. This is explained by a shift in border of single and double bonds in the Sse state. The first step in the cis‐trans isomerization of PSBR, which is the formation of the C6C7 (see Scheme 1 for numbering) twisted PSBR in the first excited singlet state (S1), inside the protein binding pocket of the visual pigment Rh is modeled using crystal coordinates and the calculations performed on 1,3‐butadiene and 1,3,5‐hexatriene. More specifically, a plausible approximate structure is calculated in a geometric way for the C6C7 90° twisted PSBR, which fits into the protein binding pocket in the best possible way. It has been shown earlier that PSBR has an energy minimum for this angle in S1. The CASSCF method was used to investigate the wave function of the calculated structure of PSBR. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002

SPECTROSCOPIC PROPERTIES OF SELECTIVELY DEUTERIUM-SUBSTITUTED RETINAL HOMOLOGUES

A set of retinal homologue molecules (C15 aldehyde, C17 aldehyde, C18 ketone, C20 aldehyde) in which 11H or 15H atom of their olefinic parts was selectively substituted by a deuterium (11D or 15D) was synthesized. Their spectroscopic properties were investigated by means of optical obsorption and FT-IR spectroscopies. Normal coordinate calculations were performed for all the set of molecules in order to establish the assignment of vibrational modes. Upon deuterium substitution of the 11H or 15H atom, apparent decrease of the oscillator-intensity of the absorption transition was observed in the case of shorter polyene molecules (C15 and C17aldehydes and C18ketone), while the decrease of the oscillator strength was negligible in the case of C20 aldehyde. Vibrational analysis suggested that the selective deuterium substitution induced the shift of electron distribution from double-bond site to single-bond site resulting in the decrease and increase in the bond orders of the former and the latter, respectively.

Optical properties of disubstituted polyacetylene thin films

We have studied the optical properties of thin films of novel disubstituted acetylene polymers such as poly(1-phenyl-2- p- n-butylphenylacetylene) (PDPA- nBu) and poly(1- n-hexyl-2-phenylacetylene) (PHxPA), that show high photoluminescence (PL) quantum efficiency in comparison with unsubstituted or monosubstituted polyacetylene. The polarization dependence of PL and Raman scattering of PDPA- nBu and PHxPA have been studied in order to clarify the emission mechanism. Assuming the model of short polyene conjugation length in the acetylene polymers, we estimate the average conjugation length from the respective stretching vibration frequency of the carboncarbon double bond. The emission mechanism of PHxPA and PDPA- nBu is discussed in relation with the electronic excited states of these polymers

Optical origin of efficient and selective trans↔cis isomerization in photoexcited polyenes: An ab initio calculation

An ab initio calculation is carried out to investigate the optical origin of highly-efficient and selective trans↔cis isomerizations in a short polyene. It is found that the highly-efficient isomerization is closely related to the highly-selective optical transitions. These dipole-allowed transitions occur selectively at two configurations with the dihedral angles θ11,12=60° and 120°, where the system can be excited or de-excited and is also able to undergo cis or trans isomerization effectively. For many configurations which are not favorable to the isomerization, the transition matrix elements are basically zero. This rules out many unfavorable transitions at these configurations, which is why photoinduced isomerization is so efficient.

The mode of action of metal stearate stabilisers in poly(vinyl chloride)—IV. The application of fluorescence spectroscopy to characterise chromophoric species

Fluorescence emission spectroscopy has been used to monitor the development of polyenes in poly(vinyl chloride) [PVC] which has been stabilised with mixtures of pre-heated (180 °C) calcium stearate (CaSt 2) and zinc stearate (ZnSt 2). Treatment with NaBH 4 has been used to distinguish polyenes attached to carbonylic groups. Emission from excitations at 245 and 360 nm enabled the development of short polyene sequences and carbonylic polyenes to be followed. CaSt 2 when pre-heated and added to ZnSt 2 and PVC produced high amounts of carbonylic polyenes and lead to secondary reactions yielding large amounts of black material attributed to the action of the prodegradant ZnCl 2 formed. Pre-heated ZnSt 2 produced small quantities of longer polyene sequences as shown by a slight yellowing of the PVC, and a pre-heated mixture of the soaps led to small quantities of carbonylic groups and an initial yellow discoloration. For the pre-heated mixture of soaps and pre-heated ZnSt 2 the formation of a complex is suggested which interacts with the PVC chain from the beginning of degradation; the nature of the interaction being ionic and determining the specific mechanism of the stabilisation.

Resonance Raman and quantum chemical studies of short polyene radical cations

The results of our investigations of the geometric and vibrational structures of some short conjugated polyene radical cations are reported. The radical cations of 1,3 — butadiene and three of its deuterated isotopomers, trans — and cis — 1,3 — pentadiene, 2 — methyl — 1,3 — butadiene, and E — and Z — 1,3,5 — hexatriene have been studied. The radical cations were generated radiolytically in a glassy Freon matrix and investigated by optical absorption and resonance Raman spectroscopy. Ab initio and density functional molecular-orbital calculations have been carried out to predict equilibrium structures and to assist assignment of the resonance Raman spectra. A new and improved scaled quantum mechanical force field for the butadiene radical cation was also determined. The presence of more than one rotamer was observed in all the polyene radical cations we investigated.