Synthesis of indole-based chromophores with a tricyanofuranyl acceptor and the study of the effect of the quinoxalinone core in the  -electron bridge on the linear and nonlinear optical properties

D —π— A chromophores with indole donor and tricyanofuranyl acceptor moieties connected by vinylene and divinyl quinoxalinon e π -conjugated bridges have been sythesized and their linear and nonlinear optical properties were studied. Chromophores exibit intramolecular charge-transfer (ICT) absorption band in the visible region, positive solvatochromism, and are transparent at 850 nm. The indole-based chromophore with divinylquinoxalinone π -conjugation bridge shows large µ  value (~13000  10 -48 esu); the elongation of the bridge by the vinyl quinoxalinone unit leads to a significant increase in the values of the first hyperpolarizability.

Photophysical properties of dyes were studied in solvents of different polarity (ε = 239, Fig. 1, Table 1).ICT absorption band of both dyes is manifested in the visible region.Elongation of π-bridge by quinoxalinone-vinyl unit leads to bathochromic shift of absorption maximum of Ind-VQonV-TCF in all solvents by 61-86 nm.This is slightly less than the bathochromic shift caused by the introduction of an additional vinyl thiophene moiety in the π-bridge (up to 94 nm in chloroform solution). 35Comparing the chromophores of the dialkylaniline-vinylthiophenvinyl-tricyanofuran composition and Ind-VQonV-TCF, one can note that the replacement of the divinylthiophene bridge by divinylquinoxalinone one and the replacement of the dialkylaniline donor by indole one leads to a noticeable hypsochromic shift (up to 73 nm in chloroform solution) of the absorption maximum of the chromophore Ind-VQonV-TCF, 36 making it transparent in the shortwavelength infrared region (at 850 nm).Both chromophores are characterized by solvatochromism; they demonstrate close positive solvatochromic shifts of 31 and 39 nm, while chromophore Ind-VQonV-TCF exhibits significantly greater negative solvatochromic shift (Table 1).The structure of both chromophores was optimized at the B3LYP//6-31G* level, as a result of which conformers Ind-V-TCF-I and Ind-VQonV-TCF-I were obtained (Fig. 2).The conformational search in the OPLS4 force field resulted in a number of conformers, and subsequent refinement of the geometry of the minimum energy conformer by optimization, provided conformers Ind-V-TCF-II and Ind-VQonV-TCF-II (Fig. 3).The angles between the structural units of the chromophores are given in Table 2, for conformers I the dodecyl substituents are elongated and arranged almost perpendicular to the chromophore skeleton (Fig. 2), and for conformers II, the dodecyl substituent is extended along the backbone of the molecules (Fig. 3).
Table 2.The angles between the units of the chromophores Ind-V-TCF-I,II an d Ind-VQonV-TCF-I,II, determined from the conformational search and as a result of optimization at the B3LYP/6-31G(d) level (D-donor, Bbridge, A -acceptor moieties; Ind denotes the angles between cycles in indole moiety, B denotes the angles between cycles in quinoxalinone moiety).As can be seen from Table 2, the Ind-V-TCF-I conformer is quite flat, while in the Ind-V-TCF-II conformer the angle between the donor and acceptor is about 20, i.e. the dodecyl substituent, being located along the chromophore backbone, somewhat distorts the chromophore (Fig. 3a).In the case of the Ind-VQonV-TCF-I chromophore, the dodecyl substituent distorts the chromophore, leading to a noticeable twisting of the donor fragment relative to the bridge (the angle between the donor and the bridge and the donor and acceptor is 45.5 and 51.8, respectively).The rest of the chromophore remains flat, the deviation does not exceed 6.The conformer Ind-VQonV-TCF-II is characterized by a flatter structure than that of Ind-VQonV-TCF-I: the angle between the donor and the bridge is 17.9.In the optimized structure, this angle is 15.6 (Table 2).The Ind-V-TCF-II and Ind-VQonV-TCF-II conformers have fairly flat backbone structures.

Angle,  Ind-V-TCF-I Ind-V-TCF-II Ind-VQonV-TCF-I Ind-VQonV
The electric molecular characteristics of chromophores, calculated at M06-2X//aug-cc-pVDZ level for optimized structures of Ind-V-TCF-I,II and Ind-VQonV-TCF-I,II conformers, are summarized in Table 3.The dipole moments and polarizabilities of one chromophore differ little depending on the conformer; as for βtot, its value is somewhat larger (by ~10%) for Ind-V-TCF-I compared to Ind-V-TCF-II, which is consistent with the structure of conformers: Ind-V-TCF-I is flatter than Ind-V-TCF-II; for the Ind-VQonV-TCF-II conformer, βtot is slightly higher (by ~8%) than for Ind-VQonV-TCF-I, what is also in agreement with the flatter structure of Ind-VQonV-TCF-II.Comparison of the properties of the Ind-V-TCF-I,II and Ind-VQonV-TCF-I,II chromophores shows that chromophores with a divinylquinoxalinone bridge have larger dipole moment values (by ~25%), approximately doubled polarizability values, and strongly different βtot values -in the case of conformers Ind-V-TCF-I and Ind-VQonV-TCF-I the difference is ~ 6 times, and for Ind-V-TCF-II and Ind-VQonV-TCF-II it reaches 8 times, demonstrating the efficiency of divinylquinoxalinone bridge compared to vinylene one.Even greater differences were found in the values of chromophore figure-of-merit µ -in the case of the Ind-VQonV-TCF-I chromophore, it reaches ~1300010 -48 esu, that, in combination with optical transparency at a wavelength of 850 nm, makes it possible to consider such a chromophore as a promising candidate to be used in EO modulators with operating wavelength 850 nm. 37For previously explored chromophores, which were transparent at 850 nm, the μβ value was up to 730010 -48 esu.

Conclusions
Two new indole-based D-π-A chromophores with tricyanofuranyl acceptor and short vinylene (Ind-V-TCF) and divinylquinoxalinone (Ind-VQonV-TCF) π-conjugated bridges have been sythesized by a multistep procedure.Both chromophores exibit intramolecular charge-transfer (ICT) absorption band in the visible region and positive solvatochromism.The lengthening of π-bridge by vinylquinoxalinone unit leads to a significant increase in the values of the first hyperpolarizability (by 6-8 times) and an even greater increase in the µ value (by 8-10 times).The large µ value (~1306810 -48 esu) in combination with optical transparency at a wavelength of 850 nm makes the Ind-VQonV-TCF chromophore a promising candidate for further research aimed at creating electro-optic devices operating in the shortwave infrared window.

Experimental Section
General.The IR, NMR, UV-vis spectra and elemental analysis were registered on the equipment of Assigned Spectral-Analytical Center of FRC Kazan Scientific Center of RAS.Infrared (IR) spectra were recorded on the Bruker Vector-22 FT-IR spectrometer.NMR experiments were performed with AVANCE-400 (400 MHz for 1 H NMR, 100 MHz for 13 C NMR) spectrometer.Chemical shifts (δ in ppm) are referenced to the solvents.UV-vis spectra were recorded at room temperature on a UV-6100 Ultraviolet/Visible Spectrophotometer using 10 mm quartz cells.Spectra were registered with a scan speed of 480 nm/min, using a spectral width of 1 nm.All samples were prepared in solution with the concentrations of ca ~1.4-2.510-5 mol/L.The melting points, mp, of chromophores were determined by Melting Point Meter MF-MP-4.Organic solvents used were purified and dried according to standard methods.The reaction progress and the purity of the obtained compounds were controlled by TLC on Sorbfil UV-254 plates with visualization under UV light.The elemental analysis was carried out on a CHNS analyzer Vario Macro cube (Elementar Analysensysteme GmbH, Germany).The samples were weighed on Sartorius Cubis II (Germany) microbalance in tin capsules.VarioMacro Software V4.0.11 was used to evaluate the data received.DFT calculations.Structure of the studied chromophores and their electric characteristics (dipole moments, , molecular polarizability, α, components of first hyperpolarizability tensor, ijk) were calculated in the framework of Density Functional Theory (DFT).Chromophores geometrical parameters were optimized in gas phase at B3LYP//6-31G(d) level.To reveal the variety of chromophore conformers, conformational search was performed with OLPS4 force field, 38 the structure of the most stable conformer was then refined by DFT.Electric characteristics of two pairs of the conformers, differing by mutual arrangement of dodecyl substituent in donor and the chromophore skeleton, were calculated at the M06-2X//aug-cc-pVDZ level; the use of M06-2X density functional [39,40] and Dunning basis sets [41,42] are recognized as an adequate choice for this purpose. [43,44]The value of tot is calculated as The conformational search is performed with Macromodel program, 45