Enhancement of the intrinsic fluorescence of acridine and its induced circularly polarized luminescence (CPL) in ionic two-coordinate coinage metal complexes

Abstract

The intrinsic π–π* fluorescence of acridine is weak due to the deactivation via coupling with a close-lying non-emissive ¹n–π* state. By employing the lone pair of acridine to establish coordination bonds in a series of two-coordinate Cu(I) and Ag(I) complexes, significantly enhanced intrinsic fluorescence of acridinyl is observed with a photoluminescent quantum yield (Φ_PL) approaching 50%. Using chiral camphorsulfonate counterions, the cationic complex was induced to emit circularly polarized fluorescence with a reasonable g_lum of 5 × 10⁻³.

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Tian-Yi Li

Dr Tian-Yi Li obtained his BSc and MSc in chemistry and inorganic chemistry, respectively, from Nanjing University under the supervision of Professor You-Xuan Zheng. In 2014, he joined the group of Professor Koen Vandewal and Professor Karl Leo in Dresden Integrated Center for Applied Physics and Photonic Materials, TU Dresden. He received his PhD in physics in early 2018 with final grade of Summa Cum Laude. In the same year, he joined the department of chemistry at University of Southern California as a postdoctoral fellow in the group of Prof. Mark E. Thompson. In 2021, he moved to University of Science and Technology Beijing and was appointed full professor in chemistry as an individual researcher. His research interests include design of novel luminescent materials, photophysical dynamics, electroluminescent devices, as well as chemical sensors, circularly polarized luminescence and optoelectronic devices.

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Introduction

Since its first isolation in 1870,¹ the acridine molecule, a simple N-heterocyclic analog of anthracene, has exhibited elusive and uncertain intrinsic fluorescent behavior.² Despite its large conjugated structure, acridine is nearly non-emissive in crystalline state and hydrocarbon solvents. To explain this phenomenon, a long-standing debate had persisted for decades regarding whether the nature of the S₁ state is ¹π–π* or ¹n–π*. Nevertheless, it is concrete that the radiative decay rate (k_r) of S₁, on the order of 10⁶ s⁻¹,³ is much lower than its non-radiative decay rate (k_nr). This can be attributed to the strong vibronic coupling with a close-lying ¹n–π* state.^4,5 Recently, acridinyl has been widely employed as an electron-donating functional moiety to construct luminescent molecules.^6–10 However, as far as the authors are aware, none of them emit intrinsic fluorescence from acridinyl-dominated π–π* transitions.

It has been observed that acridine can be emissive, with a photoluminescent (PL) quantum yield (Φ_PL) around 0.40, in protonic solvents such as water, owing to the formation of acridinium cation through protonation (Fig. 1).^11,12 The lone pair on the N atom is stabilized by the N–H hydrogen bond, leading to a destabilized ¹n–π* state. Consequently, the coupling of this state with the ¹π–π* state becomes negligible, leading to an enhanced intrinsic fluorescence of acridine from its ¹π–π* state. Similarly, the establishment of a coordination bond can also occupy the lone pair. It is inferred that the intrinsic blue fluorescence from acridine can also be observed in organometallic compound with acridinyl directly bonded to the metal center.

Fig. 1 Enhancement of intrinsic fluorescence of acridine in two-coordinate ionic CMAcr complexes.

The great success of luminescent noble metal phosphorescent complexes for electroluminescence has been witnessed in the past decade.¹³ Observed in both organic and organometallic materials, thermally activated delayed fluorescence (TADF) has been regarded as another method to achieve high electroluminescent efficiency.^14–16 Luminescent complexes of coinage metal (Cu, Ag, and Au) exhibit a rich variety of molecular design strategies and PL properties.^17–21 Most of these complexes possess four- or three-coordination geometries and feature phosphorescent or TADF.^22–25 Recently, a family of neutral two-coordinate d¹⁰ coinage metal complexes with the carbene–metal–amine (CMA) structure has emerged as very promising TADF emitters.^26–31 When the negative monovalent secondary amine in CMA is replaced by N-heterocyclic aromatic ligands, such as pyridinyl and quinolinyl, a new family of ionic organometallic compounds with the two-coordinate coinage metal cationic complex and a counter anoin can be realized. Steffen and Marian et al. have demonstrated their decent stability and detailed excited state arrangement.^32–34 However, due to the high-lying ¹π–π* states in pyridinyl and quinolinyl with limited conjugation, no fluorescence in the visible light region was observed.

Herein, we prepared a series of ionic carbene–metal–acridine (CMAcr) complexes with Cu(I) and Ag(I) (Fig. 2a). Both theoretical simulations and PL investigations suggested intrinsic fluorescence dominated by the ¹π–π* transition of the acridine ligand. A high Φ_PL exceeding 0.45 was recorded in the crystalline state, which was more than fifteen times higher than that of free acridine. Furthermore, using chiral L-/D-camphorsulfonate counter anions (L-/D-CS), the chiral anion strategy which was proposed by Zhong et al. to realize circularly polarized luminescence (CPL) from achiral luminescent cations was applied to these CMAcr complexes.^35–37 Blue fluorescence of acridinyl can be induced to generate CPL properties with a reasonable CPL dissymmetry factor (g_lum) around 5 × 10⁻³.

Fig. 2 (a) Synthetic scheme of CMAcr; (b) molecular structures of 1, 2, and 3 in single crystals (ORTEP was drawn with 50% probability ellipsoids); (c) superposition of molecular structures and (d) crystal packing diagram of 1 and 4.

Results and discussion

The ionic CMAcr Cu(I) and Ag(I) complexes were synthesized by reacting the NHC–M(I)–Cl intermediate complex, where monoamido-aminocarbene (MAC) and 1,3-bis-(2,6-diisopropylphenyl)imidazolinium (SIPr) carbenes were utilized, with acridine in the presence of Ag(I) salt under aerobic conditions. The reaction was proceeded via the formation of AgCl precipitate, and the counter anion in the final complexes was determined by the employed Ag(I) salt. Complexes 1–3 were prepared using AgOTf (OTf = trifluoromethanesulfonate) in commercially available THF, while chiral camphor sulfonic (CS) anions in 4 and 5 were introduced by using AgCS enantiomers (prepared according previous reports, see ESI† for details). The syntheses of 4/5 were conducted in a mixed solution of CH₃OH and MeCN (1 : 1 v/v). All desired ionic complexes were obtained in high yields exceeding 85% within 30 min.

Single crystals of complexes 1–4 were obtained by diffusing diethyl either into their CH₂Cl₂ solutions. In crystals (Fig. 2b), they exhibited linear coplanar geometries with C_NHC–M(I)–N bond angles from 177° to 179° and dihedral angles between ligands from 0.5° to 4.1°, exhibiting a linear coplanar geometry which is similar as most of CMA complexes. The coordinate bonds in Cu(I) complexes were almost identical in length around 1.90 Å. The Ag(I) complex 2 presented longer C_NHC–Ag(I) bond of 2.08 Å and Ag(I)–N of 2.13 Å. The M(I)–N bond lengths in CMAcr complexes were obviously longer than those in CMA complexes, suggesting weaker coordination of the metal with the lone pair of the aromatic N atom. No significant structural variation was observed for the cationic complex when OTf in 1 was replaced by the chiral L-CS in 4 (Fig. 2c). However, according to the packing diagrams (Fig. 2d), 1 crystalized into an achiral monoclinic P2₁/c space group, while a four-order-helix orthorhombic P2₁2₁2₁ chiral space group was obtained for 4. This suggested that the chiral anion exerted a determinant and distinctive influence on the structures of the crystalline samples of CMArc.

Absorption spectra of 1 and acridine were recorded in CH₂Cl₂, and that of acridinium was obtained in the pH 2.5 acidic water (Fig. 3a). All the compounds exhibited intense sharp bands from 320 to 370 nm, peaking around 355 nm, which can be attributed to transitions from S₀ to high-lying singlet excited states (Table S7, ESI†). The S₀ → S₁ transition band of acridine had an onset around 390 nm with an extinction coefficient of approximately 4000 L mol cm⁻¹. In the region from 370 to 450 nm, similar broad and structured S₀ → S₁ transitions were observed for 1 and acridinium. These bands presented significant red shifts compared to that of acridine, showing the onsets at 440 and 450 nm, respectively. The extinction coefficient of this band was over 4000 L mol cm⁻¹ for 1, but was lower than 3000 L mol cm⁻¹ for acridinium. According to the NTO analyses (Fig. 3b), the S₁ state of acridine, lying 3.75 eV above the ground state, was characterized as a ¹π–π* transition with an oscillator strength of 0.0791, while a ¹n–π* characterized S₂ state was only 0.36 eV higher with a sixty-times smaller oscillator strength of 0.0013. The S₁ state was stabilized to 3.50 and 3.37 eV for 1 and acridinium, agreeing with the sequence of their red-shifted absorption onsets. The S₁ state maintained the same ¹π–π* nature in acridinium with a lower oscillator strength of 0.0534 compared to acridine. It became ¹dπ–π* in 1 due to the contribution of the Cu(I) d orbital in its HOMO, and the oscillator strength increased to 0.1159. This explained the variations on the molar extinction coefficients of these compounds. Please note that the contribution of the metal d orbital to the S₀ → S₁ transition of 1 was limited. Thus, it can still be regarded as an intrinsic transition of the acridinyl ligand.

Fig. 3 (a) Absorption spectra of CMAcr complexes in solution; (b) NTO analyses of acridine, acridinium and 1.

Similar intense S₀ → S₁ transition bands were observed for 2 with Ag(I) ion and 3 with SIPr. The onset of the low-energy band in 2 was slightly blue-shifted, again demonstrating the ¹dπ–π* character of the S₁ state. Since the π* orbitals on the carbene ligands had negligible contribution to either HOMO or LUMO in CMAcr complexes, the S₀ → S₁ transition of 3 was almost identical to that of 1. Despite differences in the chirality of counter ions, the absorption spectra of 4 and 5 were almost the same. The absorption onsets of 4 and 5 remained unchanged compared to 1, but their extinction coefficients decreased significantly.

The PL properties of CMAcr complexes were investigated in CH₂Cl₂ (Fig. 4a and ESI†). The representative complex 1 exhibited slightly structured emission band peaking around 455 nm. The emission energy of 1 was between those of acridine and acridinium. Compared to a low Φ_PL of 7.0% for acridine, it increased more than four-folds for 1, reaching 29%. With a τ of 8.2 ns, k_r was calculated to be 3.5 × 10⁷ s⁻¹, similar to that of 2.3 × 10⁷ s⁻¹ in acridine, suggesting fluorescence emission dominated by the ¹π–π* transition on the acridinyl ligand. Owing to the occupation of the lone pair by the coordination bond, the enhanced intrinsic fluorescence of acridinyl in 1 can be attributed to the elimination of coupling with the dark ¹n–π* state, as reflected by the significantly depressed k_nr from 31 × 10⁷ s⁻¹ in acridine to 8.6 × 10⁷ s⁻¹ in 1. The PL properties of 2 and 3 in solution were also characterized as fluorescence with almost identical emission bands and k_r values. The Φ_PL was 43% for 2 and 18% for 3. However, the Φ_PL values of 4 and 5 with chiral CS anions were 8.0% and 8.8% in CH₂Cl₂, which can be blamed to their weak S₀ → S₁ transition revealed by the absorption spectra.

Fig. 4 (a) Emission spectra of 1, acridine and acridinium in solution; (b) emission spectra of CMAcr complexes in crystalline state.

In crystalline powder, unlike the broad emission spectrum of acridine, all the CMAcr complexes exhibited structured emissive bands with negligible shifts in comparison to those in solution (Fig. 4b and Table 1). All their Φ_PL values exceeded 18%, and those of 1, 4 and 5 were as high as ∼45%, which were about fifteen-times higher than that of 3.0% for the crystalline acridine. These emissions also can be characterized as fluorescence from the acridinyl dominated ¹π–π* transition, with τ from 8.3 to 13 ns and k_r from 2.2 × 10⁷ to 4.4 × 10⁷ s⁻¹.

Table 1 PL properties of CMAcr complexes in crystalline powder

Compound	λ em (nm)	τ (ns)	Φ PL	k r (10^7 s^−1)	k nr (10^7 s^−1)
Acridine	452	1.3	0.03	2.3	75
1	462, 489	13	0.46	3.5	4.2
2	446, 471	8.3	0.18	2.2	10
3	460, 482	10	0.30	3.0	7.0
4	465, 487	11	0.46	4.2	4.9
5	462, 490	10	0.44	4.4	5.6

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To evaluate the optical activities with circularly polarized light, circular dichroism (CD) spectra were recorded for CMAcr complexes 1, 4, and 5 in CH₂Cl₂ solution (Fig. 5a). Since no chiral source was involved in 1, it was optically inactive for the absorption of circularly polarized light. The CD spectra of 4 and 5 showed no signals from 350 to 440 nm, where intense π–π* transitions were observed in their UV-vis absorption spectra. However, mirror-imaged CD signals began to emerge as the wavelength decreased to less than 350 nm, resembling the CD spectra of the corresponding chiral CS acid enantiomers. This suggested that the chiroptical properties of 4 and 5 were solely determined by the chiral CS anions in solution. This hypothesis was further supported by the CPL spectra in solution (Fig. S24, ESI†), as no CPL signal was observed for neither of them in the visible light region. In contrast, mirror-imaged CPL signals closely resembling the PL spectra of 4 and 5 were detected in their crystalline powders, meanwhile 1 with achiral counter ion exhibited no CPL signal in the same condition (Fig. 5b). The chiral space group of 4 and 5 in crystals, induced by the chiral CS anions, appeared to be the key factor for the generation of the CPL signals. Moreover, reasonable g_lum values around 5 × 10⁻³ were recorded, which were comparable to many reported CPL molecules.^38–42

Fig. 5 (a) CD spectra in solution and (b) CPL spectra in crystalline of 1, 4, and 5.

Conclusions

In summary, we have developed a series of two-coordinate ionic Cu(I) and Ag(I) CMAcr complexes featuring N-heterocyclic carbene and acridine ligand. Compared to free acridine, the attachment of the NHC–M(I) moiety significantly boosted the Φ_PL of the intrinsic fluorescence of acridine by over fifteen times in the crystalline state, reaching 46%. Since the blue fluorescence of a CMAcr was dominated by the acridinyl, PL properties was insensitive to the different carbene ligands and metal ions. Chiral CS anions were introduced to prepare a pair of CMAcr enantiomers, which preferred to adopt chiral space group in the crystalline state. Their crystalline powder presented CPL signal with reasonable g_lum values around ±5 × 10⁻³. The ionic CMAcr complex reported in this work not only demonstrated a practical way to enhance the intrinsic fluorescence of acridine, but also endowed it with CPL property via the chiral anion strategy. Moreover, chiral ligands which are widely used to construct organometallic CPL materials can be integrated into CMAcr complexes to introduce a second chiral source.^39,43,44 This allows for an in-depth investigation of the influence of both chiral ligands and chiral anions on the CPL properties.

Data availability

The data supporting this article have been included as part of the ESI.† Crystallographic data for 1, 2, 3 and 4 have been deposited at the CCDC under 2344086, 2344088, 2344089, 2344090.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the fundings from the Beijing Natural Science Foundation (2232036) and National Natural Science Foundation of China (22205014).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed synthetic procedures and chemical modifications, crystallographic data, theoretical simulations and PL measurements. CCDC 2344086 and 2344088–2344090. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4tc02531h

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