Phosphahelicenes: From Chiroptical and Photophysical Properties to OLED Applications

Abstract

Herein, the experimental physicochemical and chiroptical properties of a series of phosphahelicenes are reported, focusing on their UV/Vis absorption, luminescence, electronic circular dichroism, optical rotations, and circularly polarized luminescence. Furthermore, detailed analysis of absorption and ECD spectra performed with the help of quantum-chemical calculations allowed us to highlight general features of these helicenic phosphines. Finally, due to well-suited electrochemical properties and thermal stability, the systems were successfully used as emitters in organic light-emitting diodes.

Introduction

Helicenes are non-planar polycyclic aromatic hydrocarbons (PAHs) formed of ortho-condensed aromatic rings. Due to their peculiar helical and π-conjugated structure, they display unique chiroptical properties such as huge values of optical rotation (OR) parameters, intense electronic circular dichroism (ECD), and substantial circularly polarized luminescence (CPL).^[1] Accordingly, they have revealed appealing applications in many domains, particularly in optoelectronics.^[2] However, carbohelicenes generally display low fluorescence quantum yields (Ф_FL) because their non-planar π-electron system triggers spin-orbit coupling (SOC) that accelerates intersystem crossing (ISC) pathways (Ф_ISC: ca. 0.9).^[3] An appealing strategy to modify emission properties of helicenes consists in introducing heteroatom(s) or heterocycle(s) in the helical structure. For example, significant improvements of helicenes’ fluorescence properties have been recently achieved via introduction of such heterocycles as carbazole,^[2b,4a] maleimide,^[2e,4b] siloles,^[4c] oxaborine,^[4d] or azaborole.^[4e] Among other possible heterocycles, the phosphole ring is an interesting subunit for tuning emission properties of helicenes since phosphole derivatives are known to be efficient fluorescent emitters for organic light-emitting diodes (OLEDs).^[5,6] Recently, the introduction of a phosphole ring within a helicenic structure has enabled to construct a new class of helical systems, that is, phosphahelicenes.^[7,8] Some of these helically chiral compounds, typified in Figure 1, appeared highly promising in enantioselective catalysis,^[9] both as chiral ligands for gold-catalyzed enantioselective enyne cycloisomerizations,^[7d–f] and as organocatalysts in asymmetric [3+2] cyclizations of γ-substituted allenes with electron-poor olefins.^[7g,h] However, the photophysical properties of these heterohelicenes, determining their potential application in OLEDs, have not been reported to date.

Figure 1.

Chemical structures of the phosphahelicenes 1–5 studied in this work. Men*=l-menthyl.

In this work, we describe the photophysical (i.e. UV/Vis absorption and fluorescence) and chiroptical properties (i.e. ECD and OR), and examine the CPL activity^[4,8c,10] of a series of phosphahelicenes including phosphahelicene oxides 1–4,^[7b,d] and phosphahelicene sulfide 5 shown in Figure 1. The experimental data are supported and analyzed with the help of density functional calculations. The physical properties recorded in solution showed that the P-containing helicenes exhibit features expected for a material that can be inserted as active emitter in OLEDs. The results for a series of built OLED devices demonstrate the potential of these compounds for optoelectronic applications.

Results and Discussion

UV/Vis absorption and emission of 1–5

Compounds 1–4 have been prepared as described in previous papers.^[7b,d] The phosphahelicene sulfide 5 is a new compound. It has been synthesized from the corresponding oxide 1b, as reported in the Supporting Information.

Compounds 1a and 1b display carbo[6]helicene units meta-fused with one terminal oxophosphole ring containing a pendant phenyl ring at position 5.^[7d] In both systems the menthyl group is directed toward the inner groove of the helix (endo isomer). These epimeric compounds can be considered as pseudo-enantiomers since they demonstrate reverse stereochemistry of the helix (P/M) and of the P atom (R_P/S_P) but unchanged stereochemistry of the chiral l-menthyl group. Consequently, as can be seen from Figure 2a, 1a,b display almost identical UV/Vis spectra consisting of sets of very strong absorption bands (ε>20×10³M⁻¹cm⁻¹) between 250–350 nm (i.e. at 276, 323, 345 nm, with the one around 276 nm reaching ε ≈50×10³M⁻¹ cm⁻¹), and several bands of medium and low intensity (2.7×10³M⁻¹cm⁻¹<ε<20×10³M⁻¹cm⁻¹) between 350–450 nm. It is also worth noting that the phosphahelicene sulfide 5, which has the same structure and stereochemistry as 1b, displays only slightly modified UV/Vis spectrum compared to 1a,b (see Figure S25). This shows that the nature of the P-function (P=O in 1b vs. P=S in 5, Figure S25) does not influence the corresponding absorption properties and implies that these properties are predominantly determined by the helical π-electron system of such compounds.

Figure 2.

Comparison of the experimental UV/Vis (a) and ECD (b) spectra of phosphahelicenes 1–4 (in CH₂Cl₂, r.t., c ≈ 10⁻⁵ M) and simulated UV/Vis (c) and ECD (d) spectra of their corresponding truncated model systems 1′–4′ (see Figure 3). No spectral shift has been applied. Calculated excitation energies along with oscillator and rotatory strengths indicated as “stick” spectra. Numbered excitations correspond to those analyzed in detail. See also Supporting Information.

Compound 2 possesses a similar structure as carbo[6]helicenes 1a,b but displays a fused benzophosphole terminal unit;^[7b] accordingly it belongs to a family of phosphafluorenes oxides (dibenzophosphole oxides). Its UV/Vis spectrum shows very intense absorption bands (ε>20×10³M⁻¹cm⁻¹) between 250–350 nm (i.e. at 281, 309, 343 nm, with the one at 281 nm reaching ε≈60×10³M⁻¹cm⁻¹), one band of medium intensity at 370 nm (ε≈17×10³M⁻¹cm⁻¹) followed by very weak bands at 414 and 438 nm (ε<10³M⁻¹cm⁻¹). Overall the low-energy tail in 2 is ipsochromically (blue) shifted as compared to 1a,b and much less intense.

Compound 3 is a phospha[7]helicene oxide with an additional pendant phenyl ring at position 5 of the phosphole unit,^[7d] while 4 corresponds to an oxidized phosphafluorene-type molecule with eight ortho-condensed rings;^[7b] it can be therefore structurally classified as a phospha[8]helicene. It can be clearly seen from Figure 2 that the phosphahelicene oxides 3 and 4 display very similar UV/Vis spectra. Indeed, both 3 and 4 mainly show a strong band at ca. 283 nm (ε≈45–52×10³M⁻¹cm⁻¹) and a set of three other intense bands at% ca. 320, 335 and 355 nm (ε≈17–25×10³M⁻¹cm⁻¹), and one of moderate intensity at 395 nm (ε≈4–7×10³M⁻¹cm⁻¹); compound 3 displays an additional low-energy tail above 400 nm (ε<5×10³M⁻¹cm⁻¹). Overall, adding an ortho-fused phenyl ring, as in 4, does not influence much the UV/Vis absorption when going from a phospha[7]- (3) to a phospha[8]helicene (4) and π-conjugation appears to be more extended when a phenyl group is grafted at position 5 of the phosphole ring (i.e. in 3).

In order to provide insight into experimentally observed features of the oxo-phosphahelicene derivatives 1–4, (time-dependent) DFT calculations^[11] were then performed on truncated systems with the l-menthyl and the n-propyl substituents replaced by methyls, referred to as 1′−4′ in the following and presented in Figure 3. The same set of calculations were also carried out for the full molecule 2 using available X-ray crystal-lographic data^[7b] as a starting point in DFT geometry optimization. Results obtained for the two systems, that is, 2′ and 2, appear similar, which supports the adopted models. A full account of computational details and results of additional calculations are provided in the Supporting Information.

Figure 3.

BP/SV(P)-optimized molecular structures of the systems 1′–4′ studied as models for the phosphahelicene derivatives 1–4.

As can be seen in Figures 2a and 2c, the calculated (LCBPE0*/SV(P))^[12] UV/Vis absorption spectra agree well with the experimental ones. In particular, the increase in intensity of the low-energy tail of 1 and 3 vs. 2 and 4 is correctly reproduced. Computational analysis shows that this increase is linked with a spatially more extended π-conjugation in the “pendant-phenyl” compounds compared to their “fused-phenyl” analogues, which is clearly visible in the corresponding frontier molecular orbitals (see Figure 4 and Supporting Information). Note that the position of the phosphole ring relative to the helicene core (meta vs. ortho) seems not to affect a qualitative picture of frontier MOs in the examined systems, unlike the nature of their terminal ring (pendant vs. fused). Indeed, for the 5-phenyl-substituted phosphole derivatives 1′ and 3′, polarization of the electron density towards the phenyl moiety can be noticed in the isosurfaces of HOMO and LUMO, while for their benzo-fused analogues 2′ and 4′ those orbitals are almost evenly spread out over the whole molecule. Accordingly, the lowest-energy UV/Vis band in 1′ and 3′ can be assigned predominantly as π–π* transitions within helical π-electron system extended over terminal phosphole ring and the dangling phenyl substituent (Table 1 and Figure 4). The corresponding excitation of 3′ (no. 1 calculated at 384 nm) involves mainly the HOMO/LUMO pair. In the case of 1′, two excitations with significant oscillator strengths were computed in this spectral region, that is nos. 1 (376 nm) and 2 (367 nm) that correspond, respectively to HOMO/LUMO +1, HOMO−1, HOMO/LUMO and HOMO, HOMO−1/LUMO transitions. The contributions from the HOMO/LUMO+1 and HOMO−1/LUMO pairs in the lowest-energy excitations of 1′ afford a partial phospholephenyl→helicene and helicene→phosphole-phenyl charge-transfer (CT) character. The lowest-energy excitation with sizeable oscillator strength in 2′ and 4′, no. 2 calculated at, respectively 352 and 363 nm, is noticeably blueshifted and generally less intense as compared to those aforementioned for 1′ and 3′. For both 2′ and 4′ the excitation has a large contribution from HOMO/LUMO pair that represents π–π* transition within π-electron system delocalized across the whole molecule, and a smaller contribution from HOMO−1/LUMO+1 (for 2′)/HOMO−1/LUMO (for 4′) π– π * transition mainly within helicene core. The observed blueshift of the low-energy UV/Vis absorption correlates well with the increased HOMO–LUMO gap for 2′, 4′ vs. 1′, 3′ originating mostly from an increase (destabilization) of the LUMO energy level that is seen for the systems with fused terminal phenyl ring. Note that no noticeable involvement of P(O)R group was revealed in the low-energy excitations of 1′−4′ implying that its main role is to rigidify the molecule, increase the size of the helical structure and extend its π-electron conjugation up to the terminal phosphole-phenyl rings. Its effect is thus very similar to that of BR₂ moiety in our recently reported azaborahelicene derivatives.^[4e]

Figure 4.

Isosurfaces (0.03 au) of frontier MOs of 1′–4′. Values listed in the parentheses are the corresponding orbital energies, in eV.

Table 1.

Selected dominant excitations and occupied (occ)–unoccupied (unocc) MO pair contributions (greater than 10%) of the model phosphahelicene derivatives 1′–4′.

Excitation	E [eV]	λ [nm]	f	R [10−40 cgs]	Occ no.	Unocc no.	%
P-1′
1	3.29	376	0.089	51.06	136	138	30.8
					135	137	28.2
					136	137	19.9
2	3.38	367	0.366	424.46	136	137	65.1
					135	137	16.8
3	3.70	335	0.219	452.64	135	138	43.8
					136	138	20.0
					134	137	11.7
M-2′
2	3.53	352	0.102	−116.97	129	130	76.5
					128	131	12.2
3	3.79	327	0.424	−992.61	129	131	36.8
					128	131	25.3
					128	130	21.6
P-3′
1	3.23	384	0.182	27.78	136	137	83.4
3	3.57	347	0.081	415.27	136	139	55.2
					134	137	18.1
4	3.68	337	0.179	483.04	135	137	40.8
					136	138	14.4
					135	139	12.5
					135	138	10.7
P-4′
2	3.41	363	0.029	4.54	129	130	68.3
					128	130	12.3
3	3.64	341	0.213	1026.96	129	131	37.3
					128	130	22.8
					127	130	21.6

The emission properties of phosphahelicene oxides 1–4 and the phosphahelicene sulfide 5 were then examined in CH₂Cl₂ solutions at room temperature and in 2-methyltetrahydrofuran (MeTHF) at 77 K at concentration of 10⁻⁵ M. At room temperature, all compounds behave as blue fluorescent emitters, with emission wavelengths around 450 nm (Table 2). Beside compounds 3 and 4, systems 1, 2 and 5 show highly structured emission coming from the vibronic C=C stretching modes, typical for helicoidal π-conjugated structures. The main effect of the helicenic core on the emission properties of 1–5 is however seen in the room temperature quantum yields (Ф_f) and their low temperature emission spectra (see Supporting Information). Indeed, while helicenes with terminal phosphole oxide units, that is, 1a,b and 3, display reasonable Ф_f (7–11%), a drastic decrease in its value is observed for the benzo-fused phospha[8]helicene oxide 4 (Ф_f=2%) and the phosphahelicene sulfide 5 (Ф_f=1%). The highest quantum yield is obtained for the phosphafluorene-type phospha[6]helicene oxide 2 (Ф_f=32%). Furthermore, at 77 K, the emission spectra of the rigid phosphafluorene-based compounds 2 and 4 demonstrate fluorescence emission accompanied by phosphorescence which is typical for helicenic structures.^[4e] For compounds 1, 3 and 5, which display pendant phenyl rings at position 5 of the phosphole ring, only fluorescent emission is observed indicating that such freely rotating group promotes non-radiative deactivation processes. This observation shows the importance of a choice of a phosphole substructure, phosphafluorene vs. benzophosphole, since different deactivation pathways of the excited state are noticed.

Table 2.

UV/Vis absorption and emission data for phosphahelicenes 1–5 along with their electrochemical properties and decomposition temperature values. See also Supporting Information.

Cmpd	Absorption λmax [nm] (ε [m−1 cm−1])[a]	Emission r.t.[a] λmax [nm]	Φf r.t.[b]	Emission 77 K[c] λmax [nm]	Eox [V][d]	Ered [V][d]	Tdecomp [8C][f]
1a	278 (55700), 317 (37200), 347 (29100), 375 (22800), 384 (17300), 415 (7500), 436 (3700)	449, 476, 509	0.10	F: 432, 443, 465, 501, 533	+ 0.96[e]	−2.30[e]	332
1b	278 (55100), 320 (37000), 334 (30500), 360 (23300), 376 (24000),395 (17200), 414 (7800), 438 (4100)	455, 481, 514	0.07	F: 439, 446, 471, 504, 541	+ 0.95[e]	−2.28[e]	334
2	281 (60100), 309 (29400), 343 (29100), 370 (16700), 382 (8500), 407 (1180), 430 (770)	439, 465, 493	0.32	F: 430,458,488 P: 548, 594, 645	+ 0.96[e]	–	352
3	284 (52600), 322 (26900), 338 (25000), 356 (18700), 435 (1900)	456, 472 (broad)	0.11	F: 441, 471, 498	+ 0.94	−2.20	335
4	282 (45000), 317 (21700), 334 (19800), 357 (17500), 383 (8400), 408 (1100), 435 (800)	448, 465,	0.02	F: 437,465,495 P: 583, 635	+ 0.94[e]	−	344
5	278 (53500), 320 (42800), 345 (35400), 380 (26600), 440 (4100)	456, 479 516	0.01	F: 443, 472, 505	+ 0.93[e]	−2.33[e]

^[a]Measured in CH₂Cl₂ at c=10⁻⁵M at room temperature.

^[b]Measured relative to quinine sulfate (H₂SO₄, 0.1 M), Ф_{f ref}=0.546±15%.

^[c]Measured in MeTHF at c= 10⁻⁵M at 77 K. ‘F’-fluorescence. ‘P’-phosphorescence.

^[d]Obtained during cyclic voltammetric investigations in 0.1 m Bu₄NPF₆ in CH₂Cl₂. Platinum electrode diameter: 1 mm, sweep rate: 200 mVs⁻¹. All values referenced to the reversible formal potential of the ferrocene/ferrocenium couple.

^[e]Irreversible process.

^[f]Measured by thermogravimetric analysis at 10% decomposition.

Electrochemical properties of 1–5

The electrochemical behavior of 1–5 was investigated by cyclic voltammetry (CV). The CV curves were recorded in degassed CH₂Cl₂ solutions that contained 0.2M of n-tetrabutylammonium hexafluorophosphate (Bu₄NPF₆) as the supporting electrolyte and 10⁻³m of phosphahelicene. The corresponding redox properties are summarized in Table 2. The compounds 1, 3 and 5 demonstrate similar behavior. They are oxidized and reduced at around +0.95 V and −2.30 V (vs. Fc/Fc⁺), respectively, however only 3 presents reversible redox processes. The nature of the P-function seems to play rather a minor role. The phosphahelicene sulfide 5 is slightly easier to oxidize and slightly more difficult to reduce which implies an energetic destabilization of the HOMO and the LUMO levels. The electrochemical HOMO–LUMO gap remains however similar for all three compounds bearing the pendant phenyl substituent. The phosphafluorene-type compounds 2 and 4 exhibit only an irreversible oxidation process at around +0.95 V (vs. Fc/Fc⁺); no reduction is seen in the electrochemical windows. These data show that a different redox behavior can be observed depending directly on the organophosphorus substructure.

Chiroptical properties of 1–5

The electronic circular dichroism spectra (ECD, Figure 2b and Supporting Information) and optical rotation parameters (ORs, Table 3) of compounds 1–5 revealed features considered as classical for helicene derivatives. The two epimeric M and P phosphahelicene oxides 1a,b display mirror-image ECD spectra with [6]-P-endo-1a, used as an example, showing several strong negative bands at 260, 274, 304, and 315 nm (Δε=−200, −235, −116, and −110M⁻¹cm⁻¹, respectively), several intense positive bands at 346, 355, 372, and 392 nm (Δε=+180, +140, +150, and +103M⁻¹cm⁻¹, respectively), and a very weak one at 418 nm (Δε=+17M⁻¹cm⁻¹). The benzo-fused helicene [6]-M-endo-2 demonstrates a generally similar ECD spectrum to its phenyl-substituted helicene analogue [6]-M-endo-1b but overall more intense and with less low-energy bands: Δε=+82, +317, −155, −285, and 177M⁻¹cm⁻¹ at 258, 276, 329, 343, and 366 nm, respectively. Helicenes [7]-P-endo-3 and [8]-P-endo-4 display almost identical ECD spectra, with negative intensity centered at around 260, 280, and 305 (Δε≈−50, −240, and −95M⁻¹cm⁻¹, respectively) and a positive band around 355 nm (Δε≈+215 to +240M⁻¹cm⁻¹) with unresolved vibronic structure and higher intensity than the corresponding band in [6]-P-endo-1a. Finally, note that the phosphahelicene sulfide [6]-M-endo-5 displays ECD spectrum very similar to that of its oxide analogue [6]-M-endo-1b, especially in the low- and medium-energy (>330 nm) spectral regions (see Figure S26), confirming minor influence of the P-function on the (chir)optical absorption properties of such compounds (see above).

Table 3.

Experimental and calculated optical rotation parameters of the phosphahelicene derivatives 1–5 studied in this work. Specific OR [α]_D in deg cm³ dm⁻¹ g⁻¹ and molar OR [ϕ]_D in deg cm²dmol⁻¹.

System	Exptl[a]		Calcd[b]
	${\left[\alpha \right]}_{\text{D}}^{23}$	${\left[\varphi \right]}_{\text{D}}^{23}$	BHLYP [ϕ]D	LC-PBE0* [ϕ]D
P-1a	2340	16350	23551	25380
M-1b	−2486	−17380
M-2	−2450	−16520	−18319 (−13605)	−19977 (−14595)
P-3	2145	15000	18199	20349
P-4	2670	17950	19198	21107
M-5	−2280	−16280	–	–

^[a]Measured in CH₂Cl₂ at c ≈10⁻⁵ m.

^[b]Values for P-1′, M-2′ and M-2 (in parentheses), P-3′, and P-4′. See also Supporting Information.

The experimental ECD envelopes for 1–4 are well reproduced by the TD-DFT calculations performed for the corresponding truncated model systems 1′−4′ (see Figure 2b,d). Analysis of MO-pair contributions to dominant excitations in the low- and medium-energy regions of the simulated ECD spectra (Table 1, Figure 4, and Supporting Information) demonstrates predominant π– π * character of the main positive/ negative band in P-1, P-3, P-4/ M-2 with, indeed, negligible involvement of P(O)R group MOs likely due to their lower orbital energies. For the isoelectronic benzo-fused phospholes 2′ and 4′ the band originates mainly from excitation no. 3 calculated at 327 and 341 nm, respectively. The excitation involves HOMO, HOMO−1/LUMO+1, HOMO−1/LUMO pairs for 2′ and HOMO/LUMO+1, HOMO−1,HOMO−2/LUMO for 4′ that all correspond to π–π* transitions within helicene π-electron core with benzophosphole→helicene and helicene→benzophos-phole CT signature due to the fact that the engaged MOs are centered in different parts of the compound’s π-system. For the phenyl-substituted phosphahelicenes 1′ and 3′ two excitations with sizeable rotatory strength values were found to be responsible for the experimentally observed main positive ECD intensity: nos. 2 and 3 for 1′ and nos. 3 and 4 for 3′. The higher-energy (shorter-wavelength) excitations, no. 3 for 1′ and no. 4 for 3′, calculated for both systems at around 336 nm, have assignment similar to that described above for 2′ and 4′ (see Table 1, Figure 4, and Supporting Information). The lower-energy excitations, no. 2 for 1′ and no. 3 for 3′, with contributions from HOMO, HOMO−1/LUMO and HOMO/LUMO +2, HOMO−2/LUMO pairs, respectively, correspond to π– π * transitions involving π -system delocalized generally across the whole molecule with non-negligible CT component. Due to its predominant HOMO/LUMO character the excitation no. 2 of 1′ has lower energy (367 nm) than the excitation no. 3 of 3′ (347 nm) and in consequence it appears responsible for additional low-energy ECD band observed in the experimental spectrum of 1. The above assignment of the simulated ECD spectra of 1′−4′ confirms that the primary effect of 1-oxophosphole unit in 1–5 is to extend helicoidal π-electron system up to the phenyl substituent of the phosphole ring that can be involved then in electronic π–π*-type transitions.^[4e]

The experimental specific (resp. molar) optical rotations of phosphahelicenes 1–5 are gathered in Table 3. Their magnitudes all fall within the same range, that is, from +2145°cm³dm⁻¹g⁻¹ (resp. +15000°cm²dmol⁻¹) for [7]-P-endo-3 to +2670 (resp. +17950) for [8]-P-endo-4. Although it is interesting to note that while 3 and 4 display almost identical ECD spectra they show the lowest and highest optical rotation values of the whole series, no visible relation between structures of the examined phosphahelicenes and their ORs is observed. This can be however due to experimental errors in the measurements that are estimated to be around ±5–10%. Calculations performed for the truncated model systems 1′−4′ predict qualitatively consistent OR parameters for all the molecules but they give noticeably larger values than the corresponding experimental ones for 1–4 (Table 3 and Supporting Information). The overestimation is slightly higher for values obtained using the LC-PBE0* functional employing an “optimally tuned” range-separation parameter for a helicene^[12] than for those calculated with BHLYP and it seems to be predominantly linked with the replacement of the chiral menthyl group in 1–4 by the achiral methyl in 1′−4′. Indeed, computations for the full system 2 leads to visibly lower OR values. This indicates that although the menthyl substituent seems not to determine to any significant degree ECD spectra of phosphahelicenes 1–5 (see Supporting Information), it clearly affects their optical rotations. Note that n-propyl groups present in 1–5 can also indirectly–by affecting molecular structures of these systems–influence the corresponding OR values.^[13]

Circularly polarized luminescence (CPL) activity, which combines emission and chirality, is a particularly appealing chiroptical property of organic chiral chromophores such as helicenic derivatives.^[10] There is indeed a growing interest in CPL,^[14] and CPL-emitters have recently been incorporated into OLEDs to obtain circularly polarized (CP) OLEDs.^[2h] The CPL activity of phosphahelicenes 1–5 was therefore also examined via measurements in degassed CH₂Cl₂ solutions at concentrations around 1×10⁻³ M. The epimeric helicenes [6]-P-endo-1a and [6]-M-endo-1b display mirror-image CPL spectra (Figure 5) with luminescence anisotropy factor g_lum=+8×10⁻⁴ and −7×10⁻⁴ at 452 nm (excitation at 404–416 nm) for P-1a and M-1b, respectively. These values are in the same range as for organic helicene derivatives,^[1] and very similar to g_lum determined for triphenylene-based phospha[7]helicene and benzopicene-based phospha[9]helicene whose CPL activity has been very recently reported.^[8d] As for other phosphahelicenes 2–5, no measurable CPL was detected. These compounds may either undergo decomposition when subjected to the xenon lamp of the CPL experimental set-up or display too low CPL signal to be measured.

Figure 5.

CPL (top, solid lines added to show luminescence spectral line shape) and total luminescence (bottom) spectra of P-1a (top red) and M-1b (top black) in degassed CH₂Cl₂ solution (c=10⁻³M) at 295 K, upon excitation at 404 and 416 nm, respectively.

OLED application

All the compounds 1–5 present decomposition temperature above 330°C (Table 2) indicating good thermal stability which is a critical issue for OLED device stability and lifetime. However, taking into account their (photo)physical properties, only 2 could be of potential use as emitting material (EM) in OLEDs. To investigate its performance in OLEDs, we built a series of OLED devices with structures of indium tin oxide (ITO)/ copper phthalocyanine (CuPc) (10 nm)/ N,N′-di(1-naphtyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′ diamine (α-NPB) (40 nm)/ tris(4-carbazoyl-9-ylphenyl)amine (TCTA) (10 nm)/ 1,3-bis(N-carbazolyl)benzene (m-CP) : 2 (4 or 3%) (20 nm)/ 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) (50 nm)/ LiF(1.2 nm)/ Al (100 nm) where CuPc is the hole injection layer, α-NPB and TCTA are the hole transporting layers, and TPBi is the electron transporting layer (see Supporting Information for details).

Since phosphahelicene 2 presents irreversible redox processes, it was used as a dopant in an m-CP matrix (doping rate of 3% and 4% w for device A and B, respectively; Table 4). As evidenced by the CIE coordinates, a blue fluorescent electroluminescence was obtained, spectrum of which is not affected by the increase of the current density and matches the emission of 2 in the solid state (Figure S36). The emission of m-CP is only slightly observed at 408 nm, which indicates an almost quantitative energy transfer from the m-CP layer to the emitter 2. For devices A and B, luminance and external quantum efficiency are moderate likely due to a low charge transport. The current efficiency (CE) reaches a constant plateau for current densities above 10 mA cm⁻² (Figures S37–38). The maximum CE value is 0.94 and 0.75 cd A⁻¹ for device A and B, respectively. By increasing the dopant concentration in the m-CP matrix from 3% in device A to 4% in B the efficiency of the OLED drops dramatically from 94% to 52%. Finally, with the aim of preparing CP-OLEDs, that is, OLEDs emitting a circularly polarized light, the polarization of the light was measured by using a quater-wave polarizer. However, no circular polarization could be detected with this type of device. This result is probably due to the very low g_lum of compound 2, which could not even be measured in our CPL apparatus. Further studies are currently being performed towards optimizing the OLEDs structure in order to produce CP light.

Table 4.

Electroluminescent performance of OLED devices incorporating compound 2 in emitting layer.

Device	Doping rate of 2 [%]	Von[a] [V]	EQE[b] [%]	CE[b] [cd A−1]	PE[b] [lmW−1]	CIE color coordinates
A	3	4.1	0.94	0.94	0.36	0.21; 0.22
B	4	4.1	0.52	0.75	0.24	0,20; 0,28

^[a]Threshold voltage recorded at luminance of 1 cd m⁻²,

^[b]EQE=external quantum efficiency, CE=current efficiency, and PE=power efficiency recorded at 30 mA cm⁻² corresponding to the maximum efficiency of the devices.

Conclusions

Careful analysis of experimental and theoretical photophysical and chiroptical features of phosphahelicenes 1–5 enabled us to reveal factors that influence properties of such compounds. We have highlighted, for instance, the role of the pendant phenyl substituent of the terminal phosphole unit, compared to the fused benzo-group, in the overall extended π-conjugation of phosphahelicene derivatives. The pendant phenyl group also strongly influences the emissive properties of such systems, both at room temperature and at 77 K, with the occurrence of deactivating processes that are not observed for the more rigid benzo-fused derivatives. It was also showed that stereochemistry of the menthyl substituent present at the P-atom has no strong influence on the ECD spectra of phosphahelicenes 1–5, but may affect their corresponding optical rotation values. On the other hand, the nature of the P-function (P=O vs. P=S) was found to have overall negligible influence on all photophysical and chiroptical properties of such compounds. Finally, we showed that the phosphahelicene derivative [6]-M-endo-2 which demonstrated the best performance in terms of emission, electrochemical properties and thermal stability among the systems considered here, could be used as a blue-emitting chiral dopant in an OLED device. Further studies are ongoing toward the manufacture of phosphahelicene-based OLEDs displaying circularly polarized electroluminescence.

Experimental Section

Experimental and computational details are described in the Supplementary Information.