1. Introduction

Phenanthrenes are representative π-conjugated polyaromatic compounds consisting of three ortho-fused benzenoid units. They are considered interesting materials for photoluminescent applications [1, 2]. They are known to exhibit good electrical and optical properties due to their unique photoluminescence (PL) and electroluminescence properties [3, 4, 5, 6, 7] and their ease of use in the fabrication of new sensors or nanostructured materials. Among other distinctive properties of phenanthrenes are their higher thermal stability, lower toxicity than other polyaromatic systems, and their ready functionalization, which make them very desirable for applications in materials science [8, 9, 10].

Indeed, the synthesis of phenanthrene derivatives with important properties has been attracting attention in the manufacture of new materials, opening up new fields of potential applications. Inspired by the literature, many phenanthrene derivatives have been synthesized, and their photophysical and thermal properties including their electrochemistry have been intensively studied. These derivatives offer access to various organic devices such as organic light-emitting diodes (OLEDs) [11, 12, 13, 14, 15, 16, 17, 18], solar cells [19, 20, 21], superconductors [22, 23], etc. In an independent study, Zhang and co-workers reported the synthesis of phenanthro-imidazole 1 (Figure 1) and used it for OLEDs [24]. More recently, Akula and co-workers have reported the synthesis of ruthenium complex 2, revealing good optical, electrochemical, and thermal properties, which has been used as a promising electron-transport material for organic solar cells [25].

Phenanthrene derivatives are important molecules exhibiting interesting properties and have been attracting interest not only for their applications as organic materials but also for their versatile synthetic route. Up until now, various strategies have been applied for producing these π-conjugated systems. One of the most widely employed methods is the photocyclization approach, which is considered fast, economical, and practical. In addition, using this method, it is easy to obtain new electron-rich structured phenanthrenes with a wide range of characterizations and applications.

2. Experimental section

3. Results and discussion

In this work, the synthetic approach followed for the construction of tricyclic architectures was based on the oxidative photocyclization reaction of suitably functionalized olefins. First, p-fluorophenylacetonitrile (1) was reacted with different commercially available aromatic aldehydes according to a Knoevenagel reaction, producing α,β-unsaturated nitriles 2a–f with (Z)-configuration in 65–94% yields (Scheme 1 and Table 1). The resulting nitriles are well soluble in different organic solvents including toluene, acetone, dichloromethane, chloroform, and ethyl acetate.

p-Fluorophenylacetonitrile (1) was also reacted with 2-thiophene carbaldehyde under the same conditions. This provided 2f in 75% yield as a yellow solid (Scheme 2).

Fluorinated α,β-unsaturated nitriles 2a–f were then irradiated with a 150 W mercury lamp, through a Pyrex glass jacket, in toluene for approximately 3 h in the presence of a catalytic amount of iodine as an oxidizing agent. Unfortunately, this reaction conducted on a 500 mg scale per run provided only the desired fluorinated phenanthrene derivatives P1–5 in 70–97% yields (Scheme 3 and Table 2). It is worth noting that the photo-irradiation conditions are compatible with the different functionalities grafted on the target tricyclic compounds. No photodimer was observed during the photocyclization step. The whole photocyclodehydrogenation reaction proceeded via isomerization of (Z)-2 to (E)-2 followed by intramolecular electrocyclization leading to trans-dihydrophenanthrenes, which were oxidized to phenanthrene derivatives.

The phenanthrene derivatives were characterized by NMR spectroscopy. For example, the proton NMR spectrum of compound P5 (Figure 2) displayed three characteristic singlets at 7.24, 7.78, and 8.09 ppm, which were attributed to protons H₁, H₄, and H₁₀. A triplet of doublets (J₁ = 1.2 Hz, J₂ = 9 Hz) was observed at δ = 7.44 ppm, which is typical of proton H₇.

We investigated the ¹H–¹H COSY NMR spectrum of P5 (Figure 3) to provide more structural information. Two doublets of doublets at 8.10 ppm (J₁ = 1.2 Hz, J₂ = 11.4 Hz) and 8.23 ppm (J₁ = 6 Hz, J₂ = 8.7 Hz) are attributed to H₅ and H₈, respectively, which couple with H₇.

The photo-irradiation of the α,β-unsaturated nitrile 2f, conducted under the same conditions, provided 5-fluoronaphto[2,1-b]thiophene-8-carbonitrile P6 as a white solid in 65% yield (Scheme 4).

The proton NMR spectrum (Figure 4) of compound P6 shows a triplet of doublets at 7.36 ppm (J₁ = 2.4 Hz, J₂ = 9 Hz) attributed to proton H₆. Two doublets at 7.78 ppm (J = 5.4 Hz) and 7.85 ppm (J = 5.4 Hz) relative to H₂ and H₃ and a singlet at 8.23 ppm, which is typical of proton H₉, are observed.

By using ¹H-¹H COSY NMR spectroscopy, it is possible to differentiate the peaks for protons H₄ and H₇. In Figure 5, the H₄ signal is located at 7.88 ppm as a doublet of doublets (J₁ = 2.4 Hz, J₂ = 9.6 Hz), while the H₇ signal is overlapped with the H₉ proton signal and is found at 8.23 ppm (J₁ = 5.4 Hz, J₂ = 9 Hz).

UV–Vis spectra of the synthesized α,β-unsaturated nitriles 2a–f were recorded in chloroform solutions (C ≈ 3 × 10⁻⁵ mol⋅L⁻¹) at room temperature. As seen in Figure 6, the UV–Vis spectra of 2a–f display several absorption bands below 275 nm (256, 263, and 270 nm) with slightly different absorption coefficients (64000–82000 M⁻¹⋅cm⁻¹) as indicated in Table 3. They are accompanied by one broad absorption band in the low-energy region (Δ𝜆 > 65 nm). These absorption bands are characteristic of π–π^∗ and n–π^∗ transitions. By examining the low-energy region, it is clear that 2d shows the strongest absorption band at 315 nm (𝜀 = 3.89 × 10⁴ M⁻¹⋅cm⁻¹), 2c displays a weaker band at 322 nm (𝜀 = 1.21 × 10⁴ M⁻¹⋅cm⁻¹), and 2e exhibits the longest band at 349 nm (the molar extinction coefficient is more than twice as high as that for 2c). Moving from 2a to 2e, the addition of a methoxy group induces a bathochromic shift of 12 nm for the low-energy band (centered at 337 nm) with a slight decrease in the absorption intensity (Δ𝜀 ≈ 2400 M⁻¹⋅cm⁻¹). From 2d to 2b, replacing the fluorine atom by a bromine atom results in a significant decrease in the absorption intensity for the band located between 277 and 367 nm. It appears that the α,β-unsaturated nitriles with electron donor substituents (e.g., methoxy group) on the benzene ring (2a and 2e) or with thiophene nuclei (2f) shift the low-energy ( >300 nm) absorption maxima to higher wavelengths. The α,β-unsaturated nitriles with electron-withdrawing substituents (e.g. cyano, fluoro, and bromo groups) (2b, 2c, and 2d) shift the low-energy ( >300 nm) absorption maxima to lower wavelengths. The electron-donating substituents lead to a bathochromic shift of the absorption maxima at longer wavelengths between 377 and 400 nm. Electron-withdrawing substituents lead to hypsochromic shifts. The optical band gap energy (E_g‐op) of each α,β-unsaturated nitrile has been estimated according to the empirical formula

The photocyclization of the α,β-unsaturated nitriles into the corresponding phenanthrene derivatives leads to significant modifications in their absorption spectra by the appearance of new bands between 275 and 400 nm. The photophysical properties of compounds P1–6 are then examined in chloroform solutions (C ≈ 3 × 10⁻⁵ mol⋅L⁻¹) at room temperature. These are displayed in Figure 7. The absorption spectra of these compounds show similar features including three intense bands located between 250 and 275 nm (𝜀 ∼ 69200–93900 M⁻¹⋅cm⁻¹) and a shoulder peak around 275 nm followed by several less intense bands up to 380 nm (Table 4). The weaker absorption bands are observed in the low-energy region with intensities in the range 5100–18000 M⁻¹⋅cm⁻¹. Moving from P1 to P5, the presence of a second methoxy group induces a bathochromic shift of approximately 2–5 nm for the absorption maxima at 275–375 nm with a significant decrease in intensities for the bands at 316 and 327 nm in the range 14300–16100 M⁻¹⋅cm⁻¹. Compared to P4, the bromine atom in P2 induces a bathochromic shift of approximately 3–8 nm for the bands at 299, 311, 338, and 354 nm. The absorption bands of P2 below 316 nm are less intense than those of P4, but above 316 nm, they become slightly more intense. The optical band gap energy (E_g‐op) of each α,β-unsaturated nitrile is calculated, and the results are summarized in Table 4.

The PL spectra of phenanthrene derivatives P1–6 are recorded in chloroform solutions (C ∼ 10⁻⁵ mol⋅L⁻¹) at room temperature (Figure 8). Compounds P1–5 exhibit structured emissions displaying several bands at different intensities (Table 5). The difluorinated compound P4 shows an maximum emission at 353 nm followed by three other bands (369, 375, and 392 nm) that are less intense and a shoulder peak at 415 nm. The phenanthrene-like derivative P6 exhibits an emission with three bands at 348, 362, and 381 nm and one shoulder peak at 398 nm. The maximum emission of P6 (𝜆_ems = 362 nm) is shifted to the highest wavelengths with 7 nm, in comparison to that of P4 (𝜆_ems = 353 nm). The emission spectrum of P2 shows the same signature as that of P4, including three emission bands (359, 376, and 397 nm) and a shoulder peak (415 nm) with enhanced intensities and are shifted to the highest wavelengths with 11–14 nm. Unlike the previous compounds, P5 shows two emission bands at 366 and 383 nm followed by a shoulder peak located at 400 nm, the first of which is the most intense. P1 exhibits two emission bands at 361 and 399 nm with almost the same intensities and a shoulder peak at 417 nm. Finally, the emission spectrum of compound P3 is totally different. It is shifted more to the highest wavelengths (red-shifted) than the others since it displays an emission at 437 nm and a shoulder peak at 450 nm. The destructured profile as well as the bathochromic shift of this emission illustrates the intense character of the charge transfer within the molecule P3 in the excited state.

The electrochemical behaviors of the phenanthrene derivatives P1–6 are evaluated using cyclic voltammetry (CV) in dichloromethane with nBu₄NPF₆ (0.1 M) as the supporting electrolyte versus the SCE. As shown in Figure 9, the cyclic voltammograms, recorded at a scanning rate of 50 mV/s, indicate that compounds P1–6 exhibit irreversible anodic and cathodic peaks. Analysis of responses in CV allowed the determination of the oxidation potentials, which were found to be 1.42, 1.48, 1.46, 1.55, 1.28, and 1.64 V, versus the SCE. In addition, the reduction potentials were −1.67, −1.60, −1.59, −1.63, −1.75, and −1.67 V for P1–6, respectively.

An empirical method [26, 27] was then utilized to examine the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels as well as the electrochemical band gap (E_g‐el) as follows:

• E_HOMO (IP, ionization potential) = −(V_onset‐ox − V_FOC + 4.8)
• E_LUMO (EA, electron affinity) = −(V_onset‐red − V_FOC + 4.8)
• E_g‐el = (E_LUMO − E_HOMO) eV.

Here the value 4.8 represents the energy level of the ferrocene/ferrocenium couple, V_FOC corresponds to the ferrocene half-wave potential (0.50 V), V_onset‐ox is the oxidation onset, and V_onset‐red is the material reduction onset, all measured versus Ag/AgCl. As shown in Table 6, the calculated electrochemical band gap (E_g‐el) varies from 3.03 to 3.31 eV.

The difference between the electrochemical and optical band gaps is attributed to the interface barrier between the electrode and each phenanthrene derivative [28, 29] and the exciton binding energy. Interestingly, the energy band gap values of the tricyclic units P1–6 appear to be less than the electrochemical band gap (E_g‐el = 3.46 eV) of a benzo[ghi]perylene derivative bearing two electron-withdrawing methoxy carbonyl groups, which has been elaborated and used recently for the construction of field-effect transistors [30].

The LUMO levels estimated for systems P1–6 are as good as that of bathocuproine (BCP), which is extensively used as an electron-injection, hole-blocking layer in OLEDs [31]. It prohibits excitons diffusing toward the Al electrode where they would otherwise be quenched. However, the quick crystallization of BCP reduces the device performance [32, 33]. For this reason, it is better to replace BCP by another material in such devices. In fact, P1–6 might be suitable candidates to play the role of BCP by serving as hole-transport as well as electron-transport layers (Figure 10).

4. Conclusion

In this work, we prepared new functional fluorophenanthrene derivatives through a short synthetic approach under mild conditions by using commercially available and very low cost starting materials. The absorption and the PL properties of the target phenanthrenes were experimentally evaluated in solutions, and significant results were noted. Phenanthrenes P1, 2, and 4-6 reveal a blue emission. P3 shows a red-shifted emission due to an impressive intramolecular charge transfer. The electrochemical behavior of these tricyclic systems was also evaluated, demonstrating a notable charge transfer interaction owing to their π-conjugated electronic system. These properties suggest the application of these compounds as candidates for OLED-like and electroluminescent devices.

Acknowledgment

The authors extend their appreciation to the deanship of scientific research at University of Tabuk for funding this work through research group number RGP S-1440-0315.