Introduction of Electron Donor Groups into the Azulene Structure: The Appearance of Intense Absorption and Emission in the Visible Region

In this work, through the Suzuki–Miyaura cross-coupling reaction with high yields, new π-conjugated azulene compounds containing diphenylaniline groups at positions 2 and 6 of azulene were synthesized. The obtained diphenylaniline–azulenes have intensely visible-light absorbing and emitting (in the wavelength range from 400 to 600 nm) properties. It has been shown that such unique optical properties, in particular fluorescent emission in the region of blue and green photoluminescence (λem at 495 and 525 nm), which were absent in the original azulene, are the result of the electron donor effect of diphenylaniline groups, which significantly changes the electronic structure of azulene and leads to the allowed HOMO → LUMO electron transition.


Introduction
The growing interest in aromatic compounds with an extended π-electron conjugation system is due to their importance as functional materials for organic optoelectronics.
Currently, much attention is paid to arylated, as well as aromatic and heteroaromatic, compounds substituted with electron acceptor and/or electron donor groups.
One advantage of such aromatic systems is that they can fine-tune the electronic structure of materials in order to optimize performance and morphology.
The structure of azulene can be considered a tropylium cation condensed with a cyclopentadienyl anion (Figure 1a).This non-alternating and polarized azulene structure results in high energy levels of HOMO frontal orbitals and low energy levels of LUMO orbitals compared to other conventional aromatic hydrocarbons [11][12][13][14][15].In azulene, positions 1 and 3 have large HOMO coefficients, and positions 2 and 6 have large HOMO-1 and LUMO coefficients (Figure 1b) [11][12][13][14][15]. Besides, azulene, unlike its colorless isomer naphthalene, has a blue color and shows the absorption caused by the transition of S 0 -S 1 with an λmax of about 580 nm [27].However, for the allowed π-π*-transition, the molar coefficient of this absorption is very small and is only 350 M −1 cm −1 [27].
Molecules 2024, 29, x FOR PEER REVIEW 2 of 10 sitions 1 and 3 have large HOMO coefficients, and positions 2 and 6 have large HOMO-1 and LUMO coefficients (Figure 1b) [11][12][13][14][15]. Besides, azulene, unlike its colorless isomer naphthalene, has a blue color and shows the absorption caused by the transition of S0-S1 with an λmax of about 580 nm [27].However, for the allowed π-π*-transition, the molar coefficient of this absorption is very small and is only 350 M −1 cm −1 [27].Thus, the introduction of different functional groups into the azulene structure, in particular electron donor groups at positions 2 and 6, can lead to significant changes in its electronic structure and give new functional materials with unique photophysical properties.
Here, we report the synthesis of novel π-conjugated azulene compounds 4 and 6 containing electron donor diphenylaniline groups at positions 2 and 6 via Suzuki-Miyaura cross-coupling.Also, in the study of their optical properties and redox behavior, we show that similar molecular constructs of azulene allow the electronic transition of HOMO → LUMO and lead to the appearance of new intense absorption and emission bands in the visible region of the spectrum.

Results and Discussion
Synthetic routes leading to azulene π-conjugate compounds: 2-(N,N-diphenylaniline)azulene 4 and 2,6-bis(N,N-diphenylaniline)-azulene 6 are presented in Schemes 1 and 2. Thus, the introduction of different functional groups into the azulene structure, in particular electron donor groups at positions 2 and 6, can lead to significant changes in its electronic structure and give new functional materials with unique photophysical properties.
Here, we report the synthesis of novel π-conjugated azulene compounds 4 and 6 containing electron donor diphenylaniline groups at positions 2 and 6 via Suzuki-Miyaura cross-coupling.Also, in the study of their optical properties and redox behavior, we show that similar molecular constructs of azulene allow the electronic transition of HOMO → LUMO and lead to the appearance of new intense absorption and emission bands in the visible region of the spectrum.
Molecules 2024, 29, x FOR PEER REVIEW 2 of 10 sitions 1 and 3 have large HOMO coefficients, and positions 2 and 6 have large HOMO-1 and LUMO coefficients (Figure 1b) [11][12][13][14][15]. Besides, azulene, unlike its colorless isomer naphthalene, has a blue color and shows the absorption caused by the transition of S0-S1 with an λmax of about 580 nm [27].However, for the allowed π-π*-transition, the molar coefficient of this absorption is very small and is only 350 M −1 cm −1 [27].Thus, the introduction of different functional groups into the azulene structure, in particular electron donor groups at positions 2 and 6, can lead to significant changes in its electronic structure and give new functional materials with unique photophysical properties.
Here, we report the synthesis of novel π-conjugated azulene compounds 4 and 6 containing electron donor diphenylaniline groups at positions 2 and 6 via Suzuki-Miyaura cross-coupling.Also, in the study of their optical properties and redox behavior, we show that similar molecular constructs of azulene allow the electronic transition of HOMO → LUMO and lead to the appearance of new intense absorption and emission bands in the visible region of the spectrum.
The obtained diphenylaniline-azulenes 4 and 6 are dark green and red-brown substances (unlike the blue color of the original azulene), which are completely soluble at room temperature in organic solvents such as dichloromethane, chloroform, and chlorobenzene.
The electron spectra of diphenylaniline-azulene 4 and 6 in the UV-Vis range in dichloromethane (DCM) at room temperature are presented in Figure 2, and the corresponding data are summarized in Table 1.As can be seen from the first scheme, the key molecule 2-(4,4,5,5-tetramethyl-1,3,2dioxaborolanyl)-azulene 2 was obtained by the direct C 2 -H borylation of azulene 1 with Bis (pinacolato) diboron using an iridium catalyst according to the procedure [28].Further, the Suzuki-Miyaura coupling between 4-bromotriphenylamine 3 and borylazulene 2 in THF/water mixture (4:1) in the presence of the Pd(PPh 3 ) 2 CI 2 catalyst gives the final product 4 in a high 86% yield.
The obtained diphenylaniline-azulenes 4 and 6 are dark green and red-brown substances (unlike the blue color of the original azulene), which are completely soluble at room temperature in organic solvents such as dichloromethane, chloroform, and chlorobenzene.
The electron spectra of diphenylaniline-azulene 4 and 6 in the UV-Vis range in dichloromethane (DCM) at room temperature are presented in Figure 2, and the corresponding data are summarized in Table 1.To assess the electron donor effect of the diphenylaniline groups and the efficiency of π-conjugation, the absorption spectra of compounds 4 and 6 were compared with the spectrum of the original azulene 1 (Figure 2).As shown in Figure 2, the monosubstituted diphenylaniline azulene 4 at position 2 shows a new broad absorption band in the visible spectrum with the maximum at 436 nm and molar absorption coefficient of 24,637 M −1 cm −1 (Table 1).To assess the electron donor effect of the diphenylaniline groups and the efficiency of π-conjugation, the absorption spectra of compounds 4 and 6 were compared with the spectrum of the original azulene 1 (Figure 2).As shown in Figure 2, the monosubstituted diphenylaniline azulene 4 at position 2 shows a new broad absorption band in the visible spectrum with the maximum at 436 nm and molar absorption coefficient of 24,637 M −1 cm −1 (Table 1).
Diphenylaniline-azulene 6, disubstituted at positions 2 and 6, also demonstrates a new strong absorption band in the visible range, with a maximum at 468 nm and molar coefficient of 86,888 M −1 cm −1 (Table 1).As shown in Figure 2, the visible absorption maximum 6 is shifted to the red region at 32 nm and has an intensity several times higher than the absorption maximum of the monosubstituted compound 4.
Such changes can occur as a result of a twofold expansion of the π-conjugation and an increase in electron delocalization (Figure 3), leading to a decrease in the energy gap of HOMO-LUMO molecule 6.Thus, it has been shown that by introducing electron donor diphenylaniline groups into the azulene structure, namely at positions 2 and 6, unique strong electron absorbances (ε 24,637 M -1 cm -1 and ε 86,888 M -1 cm -1 ) and fluorescent emission (257 and 264 a.u) are induced in the visible region of the spectrum.
The fluorescence spectra of diphenylaniline-azulenes 4 and 6 in DCM at room temperature are presented in Figure 4, and the corresponding data are summarized in Table 1.
To understand the electronic structure of the obtained diphenylaniline-azulenes 4 and 6, and the relationship between structure and optical property, calculations were performed using density functional theory (DFT) with the B3LYP/6-31G * functional (Figure 5, see Supplementary Materials).As can be seen from Figure 5, the frontal orbitals of HOMO π-conjugated compounds 4 and 6 are allocated both along the azulene skeleton and along the diphenylaniline group.This distribution of orbitals may result from the loosening interaction between the HOMO-1 of azulene and the HOMO of N,N-diphenylaniline [38] because the carbon atoms 2 and 6 are in the nodal plane in the HOMO of azulene, while in HOMO-1 they have large coefficients (Figure 1b).
In addition, it is shown that the HOMO of diphenylaniline-azulenes 4 (−4.85 eV) and 6 (−4.74 eV) are located higher in level than the HOMO of the original azulene 1 (−5.19 eV) and have a reduced HOMO-LUMO energy gap by 0.42 and 0.58 eV (Figure 5), As can be seen from Figure 4, the monosubstituted compound 4 shows a new broad emission band in the visible region, with the maximum at 495 nm (with excitation at the wavelength of 425 nm) (Table 1).
Disubstituted molecule 6 also shows a new intense emission band in the visible spectrum, with the maximum at 523 nm (with excitation at the wavelength of 425 nm) (Table 1).From Figure 4, it can be seen that the maximum of the visible emission of 6 is shifted to the red region at 30 nm and has a higher intensity than the fluorescence emission of compound 4 (Table 1).The red shift of the fluorescent emission band with increasing intensity can also occur as the result of expansion of π-conjugation (Figure 3), leading to a decrease in the energy gap between the HOMO and LUMO of compound 6.
The ability of π-conjugated compounds 4 and 6 to intensely emit visible light in the region of blue and green photoluminescence is unique, since the original azulene 1 does not possess this property (Table 1).
Thus, it has been shown that by introducing electron donor diphenylaniline groups into the azulene structure, namely at positions 2 and 6, unique strong electron absorbances (ε 24,637 M −1 cm −1 and ε 86,888 M −1 cm −1 ) and fluorescent emission (257 and 264 a.u) are induced in the visible region of the spectrum.
To understand the electronic structure of the obtained diphenylaniline-azulenes 4 and 6, and the relationship between structure and optical property, calculations were performed using density functional theory (DFT) with the B3LYP/6-31G * functional (Figure 5, see Supplementary Materials).
As can be seen from Figure 5, the frontal orbitals of HOMO π-conjugated compounds 4 and 6 are allocated both along the azulene skeleton and along the diphenylaniline group.This distribution of orbitals may result from the loosening interaction between the HOMO-1 of azulene and the HOMO of N,N-diphenylaniline [38] because the carbon atoms 2 and 6 are in the nodal plane in the HOMO of azulene, while in HOMO-1 they have large coefficients (Figure 1b).
In addition, it is shown that the HOMO of diphenylaniline-azulenes 4 (−4.85 eV) and 6 (−4.74 eV) are located higher in level than the HOMO of the original azulene 1 (−5.19 eV) and have a reduced HOMO-LUMO energy gap by 0.42 and 0.58 eV (Figure 5), apparently due to the inversion of the order of energy levels of molecular orbitals between the original compound 1 and diphenylaniline-azulenes 4 and 6 [38].
As a result of such changes in the electronic structure of azulene, the previously forbidden electronic transition HOMO → LUMO becomes allowed [38] and, as a consequence, leads to unique intense absorption and emission in the visible region, which we actually ob-serve from the absorption and fluorescence spectra of compounds 4 and 6 (Figures 2 and 4, Table 1).
apparently due to the inversion of the order of energy levels of molecular orbitals between the original compound 1 and diphenylaniline-azulenes 4 and 6 [38].
As a result of such changes in the electronic structure of azulene, the previously forbidden electronic transition HOMO → LUMO becomes allowed [38] and, as a consequence, leads to unique intense absorption and emission in the visible region, which we actually observe from the absorption and fluorescence spectra of compounds 4 and 6 (Figures 2 and 4, Table 1).To determine the electrochemical properties, the redox behaviors of diphenylaniline-azulenes 4 and 6 were investigated by cyclic voltammetry (CV) (Figure 6).Measurements were made with a standard three-electrode configuration (see Supplementary Materials).
As shown in Figure 6, the monosubstituted compound 4 exhibits reversible double oxidation peaks at 0.46 V and 0.75 V, respectively.In addition, molecule 4 shows an irreversible reduction peak at −1.86 V. Disubstituted molecule 6 shows an irreversible single oxidation peak at 0.65 V, as well as an irreversible reduction peak at −1.85 V.
According to the onset of the oxidation peak (0.32 V for Eox onset ) and the onset of the reduction peak (−1.65 V for Ered onset ), we can conclude that the corresponding HOMO and LUMO energy levels of molecule 4 are −4.88 eV and −2.75 eV, respectively.Also, with the onset of oxidation peak (0.38 V for Eox onset ) and the onset of reduction peak (−1.68 V for Ered onset ), the HOMO and LUMO energy levels of compound 6 are −4.82eV and −2.72 eV, respectively.Energy levels were calculated using the following formula: It should be noted that the energy levels of HOMO diphenylaniline-azulenes 4 and 6 obtained as a result of electrochemical studies are close to those calculated on the basis of theoretical modeling (DFT calculations, Figure 5).
The theoretical values of LUMO energy levels of compounds 4 and 6 are slightly different from the experimental ones, resulting in an approximate difference of 0.85 V and 0.76 V, respectively, in the band gap obtained using these two methods [39].To determine the electrochemical properties, the redox behaviors of diphenylanilineazulenes 4 and 6 were investigated by cyclic voltammetry (CV) (Figure 6).Measurements were made with a standard three-electrode configuration (see Supplementary Materials).

Materials and Methods
NMR spectra of 1 H and 13 C were recorded on a JNM-ECA 500 spectrometer in CDCl3 at room temperature using tetramethylsilane (TMS) as an internal standard.The NMR spectrometer operating frequencies were 500 MHz ( 1 H) and 126 MHz ( 13 C).IR spectra were recorded on an Avatar-360 Fourier spectrometer in KBr tablets.Mass spectra (EI) were determined with an Agilent 6530 Accurate-Mass Q-TOF LC/MS system.Elemental analysis was performed on a CHNS-O UNICUBE-elemental analyzer.The melting points were determined on a Melting Point M-560 apparatus.
Absorption spectra were recorded on a Shimadzu UV-1800 spectrophotometer.Fluorescence spectra were recorded on an Agilent Cary Eclipse fluorescence spectrophotometer.Cyclic voltammetry (CV) measurements were performed on a PalmSens electrochemical analyzer.As shown in Figure 6, the monosubstituted compound 4 exhibits reversible double oxidation peaks at 0.46 V and 0.75 V, respectively.In addition, molecule 4 shows an irreversible reduction peak at −1.86 V. Disubstituted molecule 6 shows an irreversible single oxidation peak at 0.65 V, as well as an irreversible reduction peak at −1.85 V.
According to the onset of the oxidation peak (0.32 V for Eox onset ) and the onset of the reduction peak (−1.65 V for Ered onset ), we can conclude that the corresponding HOMO and LUMO energy levels of molecule 4 are −4.88 eV and −2.75 eV, respectively.Also, with the onset of oxidation peak (0.38 V for Eox onset ) and the onset of reduction peak (−1.68 V for Ered onset ), the HOMO and LUMO energy levels of compound 6 are −4.82eV and −2.72 eV, respectively.Energy levels were calculated using the following formula: E HOMO = −4.80eV − [(Eox onset ) − E 1/2 (ferrocene)] and E LUMO = −4.80eV − [(Ered onset ) − E 1/2 (ferrocene)] [39].
It should be noted that the energy levels of HOMO diphenylaniline-azulenes 4 and 6 obtained as a result of electrochemical studies are close to those calculated on the basis of theoretical modeling (DFT calculations, Figure 5).
The theoretical values of LUMO energy levels of compounds 4 and 6 are slightly different from the experimental ones, resulting in an approximate difference of 0.85 V and 0.76 V, respectively, in the band gap obtained using these two methods [39].

Materials and Methods
NMR spectra of 1 H and 13 C were recorded on a JNM-ECA 500 spectrometer in CDCl 3 at room temperature using tetramethylsilane (TMS) as an internal standard.The NMR spectrometer operating frequencies were 500 MHz ( H) and 126 MHz ( 13 C).IR spectra were recorded on an Avatar-360 Fourier spectrometer in KBr tablets.Mass spectra (EI) were determined with an Agilent 6530 Accurate-Mass Q-TOF LC/MS system.Elemental analysis was performed on a CHNS-O UNICUBE-elemental analyzer.The melting points were determined on a Melting Point M-560 apparatus.
Absorption spectra were recorded on a Shimadzu UV-1800 spectrophotometer.Fluorescence spectra were recorded on an Agilent Cary Eclipse fluorescence spectrophotometer.Cyclic voltammetry (CV) measurements were performed on a PalmSens electrochemical analyzer.

1 4 6 Figure 5 .
Figure 5. Distribution of HOMO and LUMO frontal molecular orbitals of diphenylanilineazulenes 4 and 6 compared to the MO of the original azulene 1.

Figure 5 .
Figure 5. Distribution of HOMO and LUMO frontal molecular orbitals of diphenylaniline-azulenes 4 and 6 compared to the MO of the original azulene 1.