Synthesis and photophysical properties of novel benzophospholo[3,2-b]indole derivatives

Abstract

The parent benzophospholo[3,2-b]indole was prepared by the reaction of dichlorophenylphosphine with a dilithium intermediate, which was prepared in two steps from 2-ethynyl-N,N-dimethylaniline. Using the obtained benzophosphole-fused indole as a common starting material, simple modifications were carried out at the phosphorus center of the phosphole, synthesizing various functionalized analogs. The X-ray structure analysis of trivalent phosphole and phosphine oxide showed that the fused tetracyclic moieties are planar. The benzophosphole-fused indoles, such as phosphine oxide, phospholium salt, and borane complex, exhibited strong photoluminescence in dichloromethane (Φ = 67–75%).

Introduction

The chemistry of phospholes, fully unsaturated five-membered heterocyclic rings containing a phosphorus element, has drawn much attention in terms of the development of synthetic methods and elucidation of its spectroscopic properties for applications in organic field-effect transistors (OFETs) and luminescent materials [1–9]. The phosphorous atom of trivalent phosphorus compounds has a high chemical reactivity. Therefore, several reactions on the phosphorus atom such as oxidation, alkylation, and coordination to a Lewis acid can produce the corresponding phosphole derivatives with different electronic properties [10–17]. Phosphole-based ladder-type π-conjugated heteroacenes were shown to exhibit a high charge mobility and/or fluorescence quantum yields [18–24]. For example, dibenzo-fused phospholo[3,2-b]phosphole dioxides (Figure 1A) [25–26] and benzophosphole-fused tetracyclic heteroacenes, containing boron (B) [27–29], silicon (Si) [30], oxygen (O) [31], and sulfur (S) [32–34] (Figure 1B), were synthesized and their physical properties were studied. However, to the best of our knowledge, the synthesis of benzophosphole-fused indole derivatives as tetracyclic heteroacenes has not been reported. In 2015, Lu et al. reported the synthesis of only one phosphole and indole-fused pentacyclic heteroacene [35]. Recently, we reported simple and efficient synthetic routes to benzothiophene-fused benzoheteroles containing the group 15 and 16 elements using the ring-closing reaction of dilithium compounds with electrophiles bearing heteroatoms [36]. In continuation of our research, we were interested in the synthesis, molecular structure, and physicochemical properties of the parent benzophosphole-fused indole derivative and its various functionalized analogs such as the corresponding phosphine oxide, phosphonium salt, and borane–phosphine complex.

Figure 1.

Phosphole-based tetracyclic heteroacenes.

Results and Discussion

The synthesis of the parent tetracyclic molecule 10-phenyl-[1]benzophospholo[3,2-b]-N-methylindole (3), is shown in Scheme 1. The key precursor 2 was synthesized by I₂-mediated electrophilic cyclization of 2-ethynyl-N,N-dimethylaniline 1 [36–37]. Treatment of compound 2 with n-butyllithium in anhydrous THF at −78 °C and subsequently with PhPCl₂ resulted in ring closure, affording the desired benzophospholo[3,2-b]indole 3 in 66% yield.

Scheme 1.

Synthesis of benzophospholo[3,2-b]indole 3.

Then, the chemical modification of the phosphorus atom of 3 was carried out and the results are shown in Scheme 2. The treatment of 3 with hydrogen peroxide, elemental sulfur, and elemental selenium afforded the corresponding phosphine oxide 4, sulfide 5, and selenide 6, respectively. The reaction of 3 with methyl triflate afforded phospholium triflate 7. Phosphole 3 was treated with chloro(dimethyl sulfide)gold in CH₂Cl₂, resulting in P-complexation and thus affording the gold complex 8. The borane complex 9 was readily prepared from 3 by treating with borane in THF.

Scheme 2.

Chemical modifications of the phosphorus atom of 3.

The molecular structures of compounds 3–9 were confirmed by elemental and spectral analyses (¹H, ¹³C, and ³¹P NMR, MS, and IR). The ³¹P NMR spectra show the typical low-field shift (4: δ = 22.0, 5: δ = 27.0, 6: δ = 11.0, 7: δ = 6.0 ppm) relative to the parent compound 3 (δ = −29.1 ppm). The corresponding ³¹P NMR signals of gold and boron complexes 8 and 9 were observed at δ = 6.0 and 11.5 ppm, respectively. These results show that the electronic nature of the phosphorus atoms is similar to that of the oxidized species 4–7 (δ = 6.0–27.0 ppm). In the IR spectrum of 4 in KBr, a strong absorption for P=O stretching vibration at 1188 cm⁻¹ was observed. Figure 2 shows the X-ray crystal structures of the benzophospholo[3,2-b]indoles 3 and phosphine oxide 4. Selected bond lengths and angles are listed in Table 1. Figure 2 clearly shows that the tetracyclic skeletons are planar. The mean deviations are 0.022 Å for 3, and 0.040 Å and 0.059 Å for two independent molecules in the unit cell of compound 4, comparable to those of benzophosphole-fused tetracyclic heteroacenes (0.016–0.057 Å) [26,29–30,33]. The sums of the bond angles around the nitrogen atom were 359.98° for 3, and 360.02° and 359.99° for 4. In contrast, these angles around the phosphorus atom were 291.05° for 3, and 305.46° and 304.75° for 4. These facts indicate that the nitrogen atoms are sp²-hybridized, and the phosphorus center adopts pyramidal for 3 and tetrahedral geometry for 4.

Figure 2.

ORTEP drawing of compound 3 (left) and 4 (right) with 50% probability. All hydrogen atoms are omitted for clarity. One of two geometries in the unit cell was drawn for 4.

Table 1.

Selected bond length and angles.

	3	4
Bond length [Å]
P1–C1	1.8467(12)	1.8108(12)
P1–C2	1.8459(14)	1.8215(13)
P1–C3	1.7958(12)	1.7668(12)
P1–O1		1.4897(9)
N1–C4	1.4533(16)	1.4642(16)
N1–C5	1.3765(14)	1.3679(16)
N1–C6	1.3869(17)	1.388(2)
Bond angles [º]
C4–N–C5	127.05(11)	127.33(11)
C4–N–C6	125.16(10)	124.71(12)
C5–N–C6	107.77(10)	107.77(11)
C1–P–C2	99.13(5)	106.78(6)
C1–P–C3	102.94(5)	107.01(6)
C2–P–C3	88.97(6)	91.67(6)
O1–P1–C3		121.14(6)
O1–P1–C1		110.64(6)
O1–P1–C2		117.52(6)

The photophysical properties of the benzophospholo[3,2-b]indoles were evaluated using UV absorption and fluorescence spectroscopy in CH₂Cl₂. The spectra are shown in Figure 3, and the photophysical data are shown in Table 2. The functionalized phosphole derivatives 4–8 showed absorption maxima (λ_abs) at 299–307 nm and a broad absorption at ≈355 nm. In contrast, parent phosphole 3 showed narrow absorption peaks at 320, 343, and 357 nm. Additionally, these compounds exhibited very little solvent dependence (see Supporting Information File 1, Figure S2). Phosphine oxide 4 exhibited blue fluorescence with the maximum emission (λ_em) at 450 nm. The quantum yield (Ф = 75%) was high, comparable to that of phosphole and indole-fused pentacyclic heteroacene (Ф = 70%) [35]. On the other hand, a low fluorescence intensity was observed for phosphine sulfide 5 and selenide 6 (Ф = 1% and 0.3%, respectively). Quenching of fluorescence emission due to a soft sulfur substituent has been reported for several phosphine sulfide (P=S) compounds [10,15,17,20]. The cationic phospholium 7 exhibited green fluorescence (λ_em = 465 nm) with the largest red shift of this series of phospholes owing to the cationic nature of the phosphorus atom, providing particularly strong electron-accepting properties. This red shift related to methylation of a phosphorus atom is in line with other earlier studies [10,18,33]. The fluorescence intensity of 7 (Ф = 67%) was as strong as that of phosphine oxide 4. The gold and boron complexes (8 and 9, respectively) showed contrasting fluorescence properties with respect to the intensity. Complex 9 exhibited a high quantum yield (Ф = 75%), while complex 8 exhibited a weak emission (Ф = 11%). In these fluorescence spectra of compounds 4–7, a vibronic band was detected as a shoulder peak around 530 nm. In the case of phosphonium cation 7, the corresponding vibronic band was seen in the absorption spectrum at 330–340 nm.

Figure 3.

UV–vis absorption (left) and normalized fluorescence emission (right, excitation at 335 nm) spectra in CH₂Cl₂.

Table 2.

Absorption and fluorescence spectroscopy data.^a

	λmax [nm]		λem [nm]b	Ф [%]b
3	321	343	420	5.3%
4	306	355c	450	75%
5	299	355c	446	1.0%
6	307		450	0.3%
7	304		465	67%
8	307	355c	437	11%
9	306	350c	425	75%

^aIn CH₂Cl₂. ^bExcitation at 335 nm, and quantum yield using anthracene as standard. ^cBroad peak.

Density functional theory (DFT) calculations [38] were carried out at the B3LYP/LanL2DZ level of theory. The HOMO and LUMO energies of the selected compounds are given in Table 3. For fluorescent compounds 3, 4, 7, and 9, the HOMO and LUMO correspond to the π and π* orbitals of the benzophospholoindole skeletons, respectively (Figure 4). Both the HOMO and LUMO energy levels in the functionalized phosphole derivatives 4 and 9 are lower than the parent phosphole 3 owing to the increased electron deficiency of the phosphorus center. Because of the cationic nature of the phosphorus center, the energy levels in cationic phospholium 7 are significantly stabilized. In contrast to the fluorescent phospholes, calculations show that the HOMO and HOMO−1 of nonfluorescent phosphole sulfide 5 and selenide 6 have a large contribution from the lone-pair orbitals on the S and Se atoms, respectively, and the HOMO−2 is delocalized over the conjugated π-system (Figure 5). According to the time-dependent DFT calculations for 5 and 6, the S₀ → S₁ transitions are mainly dominated by the dipole-forbidden HOMO–LUMO (lp–π*) transition; this may be associated with the low fluorescence quantum yields.

Table 3.

Calculated HOMO and LUMO levels of phospholes.

Compound	HOMO [eV]a	LUMO [eV]a
3	−5.34	−1.25
4	−5.75	−1.74
5	−5.54	−1.78
6	−5.27	−1.79
7b	−8.82	−5.00
9	−5.70	−1.62

^aDFT calculation at the level of B3LYP/LanL2DZ. ^bCation part only.

Figure 4.

The spatial plots of the HOMO and LUMO of compounds 3, 4, 7 and 9. The calculations were performed at the level of B3LYP/LanL2DZ.

Figure 5.

The spatial plots of the selected molecular orbitals of compounds 5 and 6. The calculations were performed at the level of B3LYP/LanL2DZ.

Conclusion

A series of novel indole-fused phospholes were synthesized by simple chemical modifications at the trivalent phosphorus center. These organophosphorus compounds generated a whole series of derivatives from only one precursor. The X-ray crystal analysis of benzophospholo[3,2-b]indoles showed that the nitrogen atoms are sp² hybridized and the phosphorus atoms adopt pyramidal and tetrahedral geometry. A significant characteristic of the benzophosphole-fused indole derivatives is that the corresponding phosphine oxide, the phospholium salt, and the borane complex showed a high fluorescence emission. Further investigations are underway to develop functional materials including electronic devices and evaluate the physicochemical properties of these compounds by synthetic, theoretical, and spectroscopic studies.

Supporting Information

File 1

Experimental details, characterization data, and NMR spectra of all new compounds.