Tricyanovinylpyrroles: synthesis, conformational structure, photosensitizing properties, and electrical conductivity

5-Alkyl-and 5-phenyl-substituted 2-methyl-1 H -pyrroles reacting with tetracyanoethylene selectively formed of 3-(1,2,2-tricyanovinyl)pyrroles in high yields. 2-Methyl-5-(2-thienyl)-1 H - pyrrole gave mixtures of 3-and 4-(1,2,2-tricyanovinyl)pyrroles (ratio 5:1). The main reaction of 2-(2-furyl)-5-methyl-1 H -pyrrole with tetracyanoethylene involves attack of the 2 - position of the furan ring. Ab initio calculations (HF/6-31G*) were performed to clarify the equilibrium conformation of the synthesized compounds. The photosensitizing properties and electrical conductivity were studied.


Introduction
Tricyanovinyl derivatives of heterocyclic compounds are of great interest both as molecular systems having high charge-transfer ability in the electronic ground state, and as promising conductive organic materials.
In interpreting these results it seems reasonable to take into account the considerably greater nucleophilicity of the furan ring as compared with that of the thiophene ring and the possible steric influence of the 5-methyl group impeding the substitution at the β-positions of the pyrrole ring.The reactivity of the thiophene ring is low compared with that of the furan ring and even lower than that of the pyrrole ring; therefore, this factor does not seem strong enough to affect the direction of the reaction with 5-(2-thienyl)pyrrole.The absence of products of double tricyanovinylation points to a very strong deactivating effect of the tricyanovinyl group that is transmitted from the pyrrole ring to the furan and thiophene rings.
The introduction of an N-vinyl or N-isopropenyl group in 7f and 7g, respectively, gives rise only to 3-tricyanovinyl substitution products 8f,g (yields 92 and 91%, respectively; Scheme 3).A characteristic reaction of N-vinylheterocycles is the [2+2] cycloaddition reaction leading to the formation of cyclobutane derivatives of type 98,9 (a product of this type has been obtained in the reaction of N-vinyl-4,5,6,7-tetrahydroindole with TCNE in benzene10), but this was not observed in the reaction of 7f and 7g.

Structures
The synthesized products are colored: bright dark-cherry (2a-c, 8f,g) or black-blue (4d,e, 5d,e, 6e) crystals with metal luster.The structure and composition have been proved by IR and NMR spectroscopy and by elemental analysis.
The IR spectra of all the compounds show an intense band in the 2213-2229 cm -1 region, corresponding to the nitrile group vibration.The C=C bond vibrations of the C-olefinic substituents are of low intensity and can be identified only with difficulty.The intense bands in the 3266-3351 cm -1 region of the pyrroles 2a-c, 4d,e, 5d,e, 6e correspond to the NH group vibrations.The C=C stretching vibrations of N-vinyl and N-isopropenyl groups of 8f and 8g are observed at 1644 and 1666 cm -1 , respectively.
The assignment of the 1 H NMR signals is based on the analysis of 2D spectra of correlation spectroscopy (COSY) and 2D Nuclear Overhauser Effect Spectroscopy (NOESY).
The analysis of 2D NOESY spectra made it possible to reliably distinguish between the isomers of compounds 4d and 5d, 4e and 5e, and to confirm the structure of compound 2c.Thus, in the spectrum of compound 4d a nuclear Overhauser effect can be observed for the pyrrole ring H-4 proton with the thiophene ring H-3 proton; this effect is absent for the pyrrole ring proton with the 2-methyl group.This allows the unambiguous determination of the position of the tricyanovinyl group in this compound.On the contrary, the 2D NOESY spectrum of compound 5d shows a cross peak of the pyrrole ring H-3 with the 2-methyl group but there is no cross-peak of the pyrrole ring proton with the thiophene ring proton; this is taken as evidence for the attachment of the tricyanovinyl group at ring position 4. The 2D spectrum for compound 6e exhibits cross peaks of the 5-methyl group with pyrrole H-4, H-3 with H-4, furan ring proton H-4 with proton NH and furane ring H-3 with H-4 enabling the determination of the tricyanovinyl group position.
In order to clarify the equilibrium conformation of the synthesized products ab initio calculations (HF/6-31G*) were performed with the pyrrole 2a as an example.According to the data obtained, the β-C atom of the tricyanovinyl group is significantly out of the pyrrole ring plane.The rotational isomers of compound 2a are labeled as syn-clinal (sc) and anti-clinal (ac) in accordance with the position of the β-C atom of the exocyclic double bond relatively the endocyclic bond (Figure 1).
The syn-clinal rotamer of compound 2a in the gas phase is energetically favored by 1.6 kcal/mol over the ac-form.Since the scand ac-conformers have practically the same dipole moments (µ6-31G* = 8.28 and 8.34 D), in a non-hydrogen bonding solvent the sc-rotamer of the pyrrole 2a will be also most populated one.the tricyanovinyl group is coplanar with the pyrrole ring adopting the ap-conformation.Possibly, this is due to the absence of steric hindrance but may also be inferred by the physical state (like 2-phenylpyrrole12 and biphenyls13).Since the equilibrium geometry of pyrrole 10 in the gas phase is unknown an ab initio calculation (HF/6-31G*) of the ap-form of 1-methyl-3tricyanovinylpyrrole has been obtained.The result suggests a planar structure, the calculated pyrrole ring bond lengths and valence angles are close to the values as measured in the crystal (deviations do not exceed 0.01 Å and 0.8°) (Table 1).
Table 1.Differences are mainly observed for N-1,C-6 (0.018 Å), C-3,C-7 (0.017 Å) bonds and C-3,С-7,C-9 (1.4°), C-8,С-7,C-9 (1.3°), C-10,С-8,C-11 (1.6°) valence angles.Thus, the planar structure of 10 in the crystalline state does not result from any solid phase effects.The coplanarity between the pyrrole ring and the tricyanovinyl substituent enables optimal overlapping of the π-bonds.Deviation from coplanarity in pyrrole 2a resembles a compromise of π-electron interaction and steric hindrance with the methyl group.The non-planar structure of the pyrrole 2a results in a significant lengthening of the C-3,C-7 bond compared to compound 10 (from 1.447 to 1.466 Å), some shortening (by 0.007 Å) of the C-7,C-8 bond and a 1-3° change of the C-3,C-7,C-9, C-8,C-7,C-9 and C-3,C-7,C-8 valence angles.The C-3,C-7 bond elongating in going from the pyrrole 10 to compound 2a provides direct evidence for steric inhibition of πelectron interaction.This should affect not only the equilibrium geometry of the molecule, but also some characteristics of the electronic structure such as UV spectrum, charge distribution, dipole moment, single-electron ionization energy.
It was also of interest to compare the geometries of 2-and 3-tricyanovinylpyrroles 10 and 11, which are coplanar, antiperiplanar conformers with respect to the endoand exocyclic double bonds (Figure 1).The tricyanovinyl group geometry is practically the same in both isomers 10 and 11 (Table 1).On the other hand, the ap-conformers of compounds 10 and 11 differ in pyrrole ring geometry and interfragmental bond length, which is shorter in 2-(1,2,2-tricyanovinyl)pyrrole 11.This suggests some contribution of the zwitterionic mesomeric structures to the electronic ground states of compounds 10 and 11 (Figure 2).
In line with this simplified approach, the excess of electron density (0.25 e for both compounds) is indeed concentrated on the tricyanovinyl group atoms as suggested by ab initio calculations of charge distribution in the ground state.Furthermore, since in the charge separated resonance structure of 10 the length of dipole is significantly longer than in the corresponding one of 11, it may be expected that the dipole moment of the ap-conformer of compound 10 will be larger than in the ap-form of molecule 11.The contribution of zwitterionic resonance structures should be reflected by large dipole moments, and this was confirmed by calculations (9.76 and 7.00 D for the ap-forms of 10 and 11, respectively).12) possess sensitizing properties with respect to polymeric photoconductors.This is illustrated by some data for poly-9-vinylcarbazole (PVCz) sensitized by compounds (2c), (4d), (12) (Figure 3).Their good solubility and compatibility with PVCz makes it possible to vary the concentration in the photoconductor layer over a range of 0.5-40%, and this gives a chance to create photosensitive layers with different luminous transmission.Reasonably high concentrations of compounds 2c, 4d and 12 do not affect the electric parameters of layers, as is the case with most dyes used as sensitizers.The induction surface potential (U 0 = +300 V) and its dark half-drop time (τ 0.5 = 330-300 sec) are in agreement with the corresponding values for individual layers of the polymeric photoconductor (PVCz).The dependence of electrophotographic sensitivity (S 0.5 , m2•J-1) on concentration (C, mas.%), for sensitizers 2c, 4d, 12 in a PVCz layer is presented in Table 2 for a maximum of the photoconductivity spectral distribution of these compounds.The results show the optimal concentration to be 10-15 mas.%.The introduction of 40 mas.% and more of sensitizer into the polymer leads to a fast decrease in the induced surface potential U 0 and, as a consequence, reduces electrophotographic sensitivity.The spectral distribution of photoconductivity (Figure 3) in the visible region gives rise to the sensitizer electron absorption band related to intramolecular charge transfer from the pyrrole ring
nitrogen atom onto the terminal CN group of the tricyanovinyl group with the formation of polar structures (Scheme 4).