Optical and photophysical properties of indolocarbazole derivatives

We present a study of the optical and photophysical properties of five ladder indolo[3,2-b]carbazoles, namely, M1, M2, M3, M4, and M5. The ground-state optimized structures were obtained by B3LYP/6-31G* density functional theory (DFT) calculations, whereas the optimization (relaxation) of the first singlet excited electronic state (S1) was performed using the restricted configuration interaction (singles) (RCIS/6-31G*) approach. The excitation to the S1 state does not cause important changes in the geometrical parameters of the compounds, as corroborated by the small Stokes shifts. The excitation and emission energies have been obtained by employing the time-dependent density functional theory (TDDFT). For all the compounds, excitation to the S1 state is weakly allowed, whereas the S2 <-- S0 electronic transition of each oligomer possesses a much larger oscillator strength. The absorption and fluorescence spectra of the compounds have been recorded in chloroform. A reasonable agreement is obtained between TDDFT vertical transition energies and the (0,0) absorption and fluorescence bands. On one hand, the pattern of the aliphatic side chains does not affect the absorption and fluorescence maxima of the compounds. On the other hand, the replacement of aliphatic chains by phenyl or thiophene rings induces hypsochromic shifts in the absorption and fluorescence spectra. Finally, the fluorescence quantum yield and lifetime of the compounds in chloroform have been obtained. From these data, the radiative and nonradiative rate constants of the deactivation of the S1 state have been determined.     

A study of substituent effect on 1H and 13C nmr spectra of N‐ and C‐substituted carbazoles

¹H and ¹³C nmr spectra of several N‐ and C‐substituted carbazoles (Series 1, 2, 3 and 4) were measured. Correlations between chemical shifts and substituent constants show that these parameters describe properly the substituent effect on the nmr phenomena. Atomic charge densities for carbazoles of Series 1, 2, 3 and 4 were calculated by using the semi empirical PM3 method. These values also show a linear correlation with the ¹³C chemical shifts. The synthesis of several carbazole derivatives 1a – 1g, 2a – 2g, 3a – 3j and 4a – 4g have been carried out according to literature procedures. The carbazoles 3i, 3j and 4c have been synthesized and fully characterized for the first time.     

Investigation of the structure, the optical properties, and the photophysics of some indolocarbazoles having terminal aromatic rings

Structures, optical properties, and photophysics of ladder indolo[3,2-b]carbazoles substituted symmetrically by phenylene and thiophene rings have been investigated theoretically and experimentally. The ground state optimized structures were obtained using the density functional theory (DFT) as approximated by the B3LYP functional and employing the 6-31G^* basis set. All derivatives were found nonplanar in their electronic ground states. The character and the energy of the singlet–singlet electronic transitions have been investigated by applying the time-dependent density functional theory (TDDFT) to the correspondingly optimized-ground-state geometries. The ab initio restricted configuration interaction (singles) method (RCIS/6-31G^*) was adopted to obtain the first singlet excited-state structures (S₁) of the molecule. TDDFT calculations performed on the S₁ optimized geometries was used to obtain emission energies. UV–vis and fluorescence spectroscopies were analyzed in conjunction with theoretical calculations. The computed excitation and emission energies were found in reasonable agreement with the experimental absorption and fluorescence spectra. Finally, the photophysical behavior of the indolocarbazoles have been studied by means of steady state and time resolved fluorescence. The overall data have allowed the determination of the rate constants for the radiative and nonradiative decay processes. Both theoretical and experimental data show that the replacement of phenylene rings by thiophene units induces a red shift in the absorption and fluorescence spectra. This behavior is interpreted in terms of the electron donor properties of the thiophene ring. On the other hand, the change of the substitutional pattern, from 2,8 to 3,9, causes a significant hypsochromic shift of the absorption and fluorescence bands.     

Comparative 13C NMR study of deuteriated and undeuteriated dibenzothiophenes,dibenzofurans, carbazoles,fluorenes and fluorenones

The NMR assignments of all carbons of dibenzothiophene and its S-oxides, dibenzofuran, various carbazoles, fluorene and fluorenone have been carried out using deuteriated derivatives and lanthanide shift reagents. Chemical shifts and coupling constants are discussed as a function of the nature of the central ring.     

Photochemistry of N-acetyl and N-benzoyl carbazoles: photo-Fries rearrangement and photoinduced single electron transfer

The photochemistry of two N-acyl carbazoles, N-acetyl and N-benzoyl carbazole, in different pure and mixed organic solvents is studied. Depending on the properties of the medium, photo-Fries rearrangement and photoinduced single electron transfer (PSET) processes are observed yielding the former 1-acyl and 3-acyl carbazoles and the latter 3-chloro-N-acyl carbazole. k_SV, k_Q and φ for fluorescence emission, conversion of N-acyl carbazole and product formation yields have been measured as well as the properties of the N-acyl carbazole radical cations formed during the PSET process (laser flash photolysis experiments). The Rehm-Weller equation is used in order to evaluate the ΔG°_ET of the PSET processes.     

Synthesis of novel star-shaped carbazole-functionalized triazatruxenes

The syntheses of novel well-defined star-shaped carbazole-functionalized triazatruxenes, which consist of six carbazole arms linked with a central triazatruxene core consisting of three carbazoles with one phenyl, are presented. Both 9-hexylcarbazole and 9-phenylcarbazole as arms gave high yields. Their chemical formulas were confirmed by means of ¹H NMR, MALDI-TOF mass spectroscopy, and elementary analysis. The fluorescent spectra showed that these compounds exhibit good blue fluorescence. The maximum fluorescence emission wavelengths were between 420 and 430 nm.     

Excited state proton transfer reactions of acridine studied by nanosecond fluorometry

Rate constants for excited state protonation reactions of acridine in aqueous solutions have been determined using fluorescence decay measurements. The value of the excited state pK derived from the rate constants is in full agreement with the one obtained from steady state fluorescence measurements. It is concluded that acridine follows a two states reaction scheme in its electronically excited state.     

吩噻嗪-Corrole 镓(III)配合物的合成、荧光及其光断裂DNA性质

合成了吩噻嗪(PTZ)-corrole二元体1-3及其镓(III)配合物4-6.采用稳态吸收与稳态发射及时间分辨的瞬态光谱技术研究了这几种化合物的光物理特性.结合荧光量子产率和荧光寿命计算得到它们的辐射和无辐射速率常数.稳态吸收光谱表明:几种二元体中,corrole镓(III)单元表现出更强的Soret带和Q带.化合物1-3的荧光量子产率分别是0.156、0.134和0.139,辐射速率常数分别为4.02′107、3.47′107和2.89′107s-1.化合物4-6的荧光量子产率分别是0.502、0.443和0.494,辐射速率常数分别为20.90′107、16.78′107和21.11′107s-1.可见,化合物4-6的荧光量子产率和辐射速率常数均高于化合物1-3.然而,化合物4-6的荧光寿命分别是2.40、2.64和2.34ns,低于自由corrole1-3.琼脂糖凝胶电泳实验表明:在光照的条件下,这些吩噻嗪-corrole镓(III)二元体化合物能够把超螺旋DNA(formI)切割成缺刻型DNA(formII).     

Synthesis of carbazoles from ynamides by intramolecular dehydro Diels-Alder reactions

[reaction: see text]. A new approach to carbazoles and benzannulated carbazoles by means of intramolecular dehydro Diels-Alder reactions of ynamides is reported. By using this approach, carbazoles and benzo[b]-, tetrahydrobenzo[b]-, naphtho[1,2-b]-, naphtho[2,1-b]-, and dibenzo[a,c]carbazoles have been prepared in moderate-to-good yields.     

Identifiability of models for intramolecular two-state excited-state processes with added quencher and coupled species-dependent rotational diffusion

The parameters describing the kinetics of excited-state processes can possibly be recovered by analysis of the fluorescence decay surface measured as a function of the experimental variables. The identifiability analysis of a photophysical model assuming errorless time-resolved fluorescence data can verify whether the model parameters can be determined and may lead to the minimal experimental conditions under which this is possible. In this work, we used the method of similarity transformation to investigate the identifiability of three kinetic models utilized to describe the time-resolved fluorescence of reversible intramolecular two-state excited-state processes in isotropic environments: (1) model without added quencher, (2) model with added quencher, (3) model with added quencher coupled with species-dependent rotational diffusion described by Brownian reorientation. Without a priori information, model 1 is not identifiable. For model 2, two sets of quenching rate constants and combinations of excited-state deactivation/exchange rate constants are possible, but they cannot be allocated to a specific excited-state species. For both sets, upper and lower limits on the excited-state deactivation/exchange rate constants can be obtained. For model 3, both spherically and cylindrically symmetric rotors, with no change in the principal axes of rotation in the latter, are considered. The fluorescence delta-response functions I(parallel)(t) and I(perpendicular)(t), for fluorescence polarized parallel and perpendicular, respectively, to the electric vector of linearly polarized excitation, are used to define the sum S(t) identically equal to I(parallel)(t) + 2 I(perpendicular)(t) and the difference D(t) identically equal to I(parallel)(t) - I(perpendicular)(t). The identifiability analysis is performed using the S(t) and D(t) functions. Also for model 3, two sets of kinetic parameters (i.e., quenching rate constants, combinations of deactivation/exchange rate constants, and rotational diffusion coefficients) exist, but these parameters cannot be assigned unequivocally to a specific species. For the three models, an infinite number of alternative spectroscopic parameters associated with excitation and emission are found.     

咔唑及其衍生物对 9-氰基蒽的荧光猝灭机理的研究

本文研究咔唑及其衍生物对9-氰基蒽(9CNA)的荧光猝灭机理。结果表明, 猝灭过程有以下三种方式:(1)一系列N-烷基咔唑及1,4-二咔唑丁烷、反式1,2-二咔唑环丁烷、N-苄基咔唑等对9CNA的荧光猝灭是通过形成激基复合物。(2)咔唑对9CNA的荧光猝灭是通过形成氢键。(3)1,3-二咔唑丙烷及N-痖烯基咔唑对9CNA的荧光猝灭是属于一般碰撞猝灭过程。以上所有猝灭过程主要都是来自电荷转移相互作用。另外, 还讨论了空间位阻对形成激基复合物的影响。并由稳态和动态荧光实验结果,应用Ware关于激基复合物的形成和解离的动力学公式计算出一系列光物理速率常数。     

ENERGY TRANSFER IN THE ACCESSORY PIGMENTS R-PHYCOERYTHRIN AND C-PHYCOCYANIN

Abstract— The photosynthetic accessory pigments R-phycoerythrin and C-phycocyanin were extracted from a red sea-weed and a filamentous blue-green alga. Investigation of quenching of phycoerythrin fluorescence by phycocyanin at room and liquid nitrogen temperatures yielded rates for the SternVolmer quenching constants ( K_q From these values, rate constants k_(D–A) were calculated for the transfer of energy from phycoerythrin to phycocyanin.
The transfer rate constants obtained were such that a collisional transfer mechanism was ruled out, and were of similar order of magnitude as the rate constants for transfer of electronic energy in mixed organic crystals.     

Dynamically Controllable Emission of Polymer Nanofibers: Electrofluorescence Chromism and Polarized Emission of Polycarbazole Derivatives

Electrochemical polymerization of a series of N‐alkyl‐2,7‐di(2‐thienyl)carbazoles in acetonitrile was performed to obtain conjugated polymers with fluorescence. Scanning electron and atomic force microscopies revealed that the surface morphology of the polymer films significantly depends on the alkyl chain lengths of the polymers. Particularly, a homopolymer bearing hexyl groups and copolymers with an average alkyl chain length of six carbon atoms show nanofiber morphology. The polymer nanofibers were stacked on a substrate electrode. The fluorescence of the polymer nanofiber film was tunable with application of voltage, with good repeatability. The X‐ray diffraction pattern of the fibers showed the structural order. The polymer nanofibers thus prepared showed an electrochemically driven change in polarized photoluminescence.     

Fluorescence quenching of pyrene monomer and excimer by CH3I

The fluorescence quenching rate constants of pyrene monomer and excimer by CH₃I were obtained at several temperatures in methylcyclohexane. Both quenching processes are kinetically controlled, allowing insight on the mechanism of quenching. The rate constants have both temperature-independent and temperature-dependent components. The temperature-independent component for both monomer and excimer fluorescence is consistent with quenching due to enhanced intersystem crossing to a lower energy triplet state. The monomer temperature-dependent component comes from the enhancement of the intersystem crossing to a higher energy triplet state. The thermally activated excimer quenching is associated with the excimer dissociation step to give a pyrene in a second triplet state plus a ground state pyrene molecule.     

Optical and electrochemical cations recognition and release from N-azacrown carbazoles

Two azacrown derivatives of carbazole have been prepared and their dipole moments in the excited state were estimated from solvatochromism. Their complexation constant K with calcium and sodium cations have been determined from the absorption spectra, the fluorescence spectra and, independently, from the first oxidation potential shift, as a function of the concentration in sodium and calcium perchlorates, when K is lower than 10⁵. The fluorescence spectra of the calcium and sodium complexes in acetonitrile show two bands different from the fluorescence of the free probes, one from the excited complex similar to the fluorescence of the protonated probes and the other one from the nitrogen decoordinated cation complexes. These results are typical for moderatly polar crowned merocyanines with the ionophore acting as an electron-donor substituent. The stepwise decoordination rate of the calcium and sodium cations from the nitrogen atom of the fluorophore is competitive with the rate constant of fluorescence which accounts for the multiple fluorescence but does not allow a distant diffusion of the cations. From the study of the longer lived radical-cation of the azacrown carbazoles, generated electrochemically, the metal cations are completely released from the azacrown receptor as revealed by the unsensitivity of the second oxidation potential to the perchlorate salts. From the probe-cation pair with the higher association constant (log K > 6) the oxidation peak for the complex allows to estimate the decrease of K upon the oxidation of the probe to be a value of K ?⁺/K = 8 × 10^-4.     

萘与2,6-萘二甲酸二甲酯形成激基复合物的光物理过程

本文用时间分辨荧光光谱证实了萘与2,6-萘二甲酸二甲酯形成激基复合物。并结合稳态荧光光谱算出光物理过程的基元过程常数,讨论了结构与基元常数之间的关系。     

Synthesis and pharmacologic study of pyridazino[4,5-b]carbazoles

Cyclization reaction of hydrazine with carbazole-2,3-methyl dicarboxylates gave 1,4-dioxo-1,2,3,4-tetrahydropyridazino[4,5-b]carbazoles. Chlorodehydroxylation provided 1,4-dichloropyridazino[4,5-b]carbazoles and nucleophilic substitution gave 1,4-dialkoxy pyridazino[4,5-b]carbazoles. These compounds were tested for cytotoxic activity against L1210 leukemia in mice.     

PHOTOINDUCED ELECTRON TRANSFER I.INTER-AND INTRAMOLECULAR QUENCHING OF ZINC PHTHALOCYANINE FLUORESCENCE BY ANTHRAQUINONE

Inter-and intramolecular quenching of zinc phthalocyaninefluorescence by anthraquinone has been studied.The diminutions offluorescence quantum yield and life-time are due primarily to electrontransfer.Apparent electron transfer rate constants were calculateddepending on the chain length and solvent polarity.     

A new method to study antioxidant capability: hydrogen transfer from phenols to a prefluorescent nitroxide

[reaction: see text]. Rate constants for hydrogen abstraction from phenols by a prefluorescent-TEMPO probe are reported. The nitroxide is employed as a potential model of peroxyl radicals. The probe works by nitroxide suppression of the fluorescence of the chromophore. The fluorescence is restored when the nitroxide abstracts a hydrogen atom to produce the diamagnetic hydroxylamine. The phenols studied in this project exhibited rate constants between 0.003 and 0.2 M(-1) s(-1). A deuterium isotope effect of 10 for TROLOX confirms that the mechanism is dominated by hydrogen transfer.     

Synthesis and electronic, photophysical, and electrochemical properties of a series of thienylcarbazoles

A series of thienylcarbazoles were synthesized by Suzuki-Miyaura and Ullmann coupling reactions. In these compounds, the 2-thienyl or 2,2'-bithiophen-5-yl group is connected at the N-, 1,8-, 3,6-, 2,7-, 2,7,N-, or 1,8,N-positions of the carbazole ring. The effects of structural variations on their electronic, photophysical, and electrochemical properties were explored by UV-vis and fluorescence spectroscopies, cyclic voltammetry (CV), and DFT calculations in evaluation of their potential as material components. The thienyl substituents at the 2,7-positions in 4, 5, and 10 are responsible for a high degree of π-conjugation and strong emission with fluorescence quantum yields up to 0.61. The CV on a series of thienylcarbazoles revealed a good electron-donating ability of 3,6-substituted carbazoles 3 and 9. The number of thiophene units was found to affect the extent of π-conjugation, the resulting HOMO-LUMO gaps, and fluorescence efficiency. The crystal structures of 5 and 9 were also disclosed.