Multi-colour and red emissions from a small donor–acceptor molecule by breaching Kasha's rule

Abstract

Reported herein is a small (MW 260 Da) organic molecule that emits multicolour and red lights, capitalising on simultaneous anti-Kasha and donor–acceptor effects. The molecule emits from two excited states, namely S₁ and S₃. Introduction of donor–acceptor substituents resulted in the large Stokes shifts (>5000 cm⁻¹) with negligible spectral overlap.

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Multi-colour organic emitters are of great importance in real-life applications including multi-colour bioimaging, multi-colour display technologies, white light emissions, anti-counterfeiting, etc.¹ In particular, small organic fluorophores are highly desirable because of their improved cell permeability, reduced perturbation of biological function, and simplicity in solid-state device fabrication.² However, achieving broad colour tunability from blue to red within a single, low-molecular-weight molecule remains a formidable challenge. Most red-emissive organic systems rely on extensive π-conjugation or multiple donor–acceptor substitutions, frequently leading to high molecular weights (often ≥300 Da), diminished quantum yields, and predominantly single-colour emission.³ Herein we describe a small organic molecule with a molecular weight of 260 Da that (i) emits red light in both solid and solution; (ii) produces multiple colours as a function of excitation wavelengths by breaching Kasha's rule and (iii) displays large Stokes shifts with little spectral overlap between absorption and emission bands. Such multifaceted properties from a single yet small molecule are unique and represent a noteworthy advance in molecular photophysics.

The design of this molecule stemmed from two basic concepts: first, the presence of an acenaphthylene core, which exhibits anti-Kasha emission⁴ and second, the strategic installation of donor–acceptor substituents on opposite sides of the acenaphthylene core to obtain red-shifted emission⁵ and large Stokes shifts,⁶ which would lead to poor overlap between absorption and emission bands.⁷ Furthermore, realising anti-Kasha emissions from a donor–π–acceptor (D–π–A) architecture is particularly appealing, as such systems hold promise for advanced applications, including ratiometric bioimaging, detection of cellular microenvironments, photodynamic therapy, etc.⁸

Recently, we reported vicinal diones like, acenaphthylene-1,2-dione, phenanthrene-9,10-dione, and pyrene-4,5-dione and their derivatives that can disobey Kasha's rule and emit from multiple excited states,⁹ leading to multi-colour and even white light emissions.¹⁰ However, these emissions were largely confined to the green-yellow region (∼550 nm). To extend the emission into the red region, we designed compound 3 comprising an electron donor substituent (OMe) at one end of the acenaphthylene-1,2-dione and to make the other side a better acceptor, one of the carbonyl groups was reacted with malononitrile (Fig. 1a). For comparison, analogues bearing an electron-withdrawing nitro group (2) and no substituent (1) were also synthesised. These molecules display two emission bands, one in the blue region and another in the yellow to red (based on the substituents). The effect of substituents is clearly visible in their longer wavelength emissions; compound 3 emits in the most red-shifted region, while that of 2 in the most blue-shifted region. Compound 1 exhibited emission that fell in between 2 and 3. Interestingly, the high-energy emission bands did not display any substituents effects, implying that the emission may solely originate from the acenaphthylene core. This clear separation of emissive underpins the excitation-dependent multicolour behaviour observed in these compact molecules.

Fig. 1 (a) Chemical structure of compounds 1–3 and (b) normalized absorption (dotted line) and PL (solid line, λ_ex = λ_max, Table 1) in ACN at 10⁻⁵ M.

The synthesis of these derivatives and characterisation are presented in the SI, and the purity was confirmed using high-performance liquid chromatography (HPLC) (SI, Fig. S1–S10) and elemental analysis.¹¹ Single crystal structures of compounds 1 and 3, and HRMS further confirm the integrity of these molecules (SI, Fig. S32 and S33).

Their absorption spectra revealed that compounds 1–3 possess three absorption bands (Fig. 1b). While compound 3 absorbs in the most red-shifted region (λ_ab = 482 nm), compound 2 exhibited the most blue-shifted absorption bands (λ_ab = 414 nm), and the same for 1 appeared to be in between (λ_ab = 435 nm), clearly indicating a strong substituents effect. Interestingly, all the compounds showed minimal solvatochromism in solvents of different polarities (Fig. S17–S19); the absorption maxima slightly red-shifted for compounds 1 and 3, indicating that the excited state is more polar compared to the ground state. This, however, is not true for 2; the absorption band at 414 nm undergoes hypsochromic shifts, implying that the ground state is more polar compared to the excited state.¹² Next, we recorded their emission spectra in acetonitrile (c = 10⁻⁵ M, Fig. 1b, Table 1). Excitation (λ_ex = 435 nm) of 1 resulted in an emission band at 587 nm (ϕ_PL = 5.56 ± 0.31%, τ_PL = 5.37 ± 0.10 ns). However, for 2 (λ_ex = 414 nm), the emission band shifted towards blue and appeared at 567 nm (ϕ_PL = 1.14 ± 0.29%, τ_PL = 1.67 ± 0.04 ns). As expected, 3 exhibited the most red-shifted emission band at 642 nm (ϕ_PL = 2.84 ± 0.39%, τ_PL = 1.41 ± 0.12 ns, λ_ex = 482 nm).¹³ These data indicate the effect of the donor–acceptor system in a small molecule to shift the emission wavelength into the red region. Interestingly, all three molecules exhibit large Stokes shifts of over 5000 cm⁻¹ with the largest value of 6520 cm⁻¹ for compound 2, resulting in a negligible spectral overlap (SI Table S1).

Table 1 Photophysical properties of compounds 1–3 in solution

Compound^a	λabs.^b (nm)	λem^c (nm)	τFL^d (ns)	ΦFL^e (%)	λem^f (nm)	τFL^g (ns)	ΦFL^h (%)
a Photophysical studies were measured in ACN at c = 10^−5 M.b Absorption maxima of all three compounds.c Emission data recorded for all three compounds at λex = 360 nm.d Lifetimes (λex = 340 nm) of compounds 1–3 for high energy emission bands.e Relative PLQYs of compounds 1–3 for high energy emission bands.f Emission maxima recorded at λex(1) = 435 nm, λex(2) = 414 nm and λex(3) = 482 nm.g Lifetimes (λex = 450 nm) of compounds 1–3 for low energy emission bands.h Relative PLQYs of compounds 1–3 for low energy emission bands.
1	435	425	1.61 ± 0.01	7.45 ± 0.21	587	5.37 ± 0.10	5.56 ± 0.31
2	414	425	1.27 ± 0.23	2.64 ± 0.39	567	1.67 ± 0.04	1.14 ± 0.29
3	482	426	1.57 ± 0.03	5.15 ± 0.19	642	1.41 ± 0.12	2.84 ± 0.30

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As the molecules exhibit multiple absorption bands, we decided to excite them at different wavelengths to investigate their possible anti-Kasha effects. Excitation (λ_ex = 310 nm) of 1 resulted a in a strong emission band at 425 nm along with a band at 587 nm (Fig. 2a and d). However, the latter became the sole emission band at λ_ex = 440 nm. This resulted in the emission colours that ranging from blue to yellow. Similarly, compound 2 exhibited two emission bands at 425 nm and 567 nm when the excitation wavelengths were tuned: yielding colours that changing from blue to orangish-yellow (Fig. 2b and e). Not surprisingly, compound 3 was found to display excitation wavelength-dependent emissions; the shorter wavelength emission was observed at 426 nm while the longer wavelength emission was at 642 nm, illuminating colours from blue to red (Fig. 2c and f). These data clearly indicated that the high energy emission is independent of substituents (Δλ_em = 1 nm) and primarily involved core-to-core electronic transition, while the red-shifted emission band is highly substituent dependent (Δλ_em = 75 nm). The excitation spectra recorded for each of these emission bands clearly indicated that compounds 1–3 may emit from multiple excited states by breaching Kasha's rule (SI, Fig. S12–S14). Furthermore, the lifetimes and quantum yields for these emission bands are different, confirming that they originated from different excited states (Table 1, SI, Fig. S23–S25). To further prove that the two emission bands were not originating from impurities, 2D excitation–emission correlation diagrams were plotted for all the compounds. A strong correlation between the emission bands indeed supports that the emissions originate from the same compounds (SI, Fig. S12–S14).¹⁴

Fig. 2 Excitation wavelength-dependent emission studies of compounds (a) 1, (b) 2 and (c) 3 and their corresponding CIE plots (d) 1, (e) 2 and (f) 3 in ACN at 10⁻⁵ M.

Interestingly, all three molecules exhibited strong solvatochromism, resulting in a dramatic change in their emission behaviour (Fig. 3, SI, Fig. S17–S19). For example, for compound 1, the λ_em = 425 nm band did not show any shifts but changed its intensity upon changing the polarity (Fig. 3a and b). On the contrary, the red-shifted emission band (λ_em = 587 nm) showed significant shifts and increased in intensity, indicating a strong charge-transfer character and the stabilisation of the excited state in polar solvents, which is in accordance with the absorption studies. Consequently, a wide range of colour tunability from blue to orangish-yellow, including near-white light (DCM, CIE coordinates 0.27, 0.28) was observed (SI, Fig. S17). By contrast, for compound 3, the intensity of the λ_em = 642 nm band dramatically reduced accompanied by the red shifts in more polar solvents (Fig. 3e and f). This could be due to the stabilization of the excited state along with the opening of a non-radiative channel, possibly by the rotation of the methoxy group.¹⁵ Nevertheless, it showed a nice colour tunability in various solvents from blue to orange (SI, Fig. S19). Solvatochromism was also demonstrated by compound 2. As the solvent polarity increased, the emission peak at 567 nm shifted towards blue, suggesting that the ground state is more polar and stabilised by the polar solvents (Fig. 3c and d). It is unclear, nevertheless, why the emission intensity significantly decreased at λ_em = 425 nm. Overall, all three derivatives exhibited a charge transfer character of their low energy emission band, and for compounds 1 and 3 the excited state is more polar, while for 2, it is the ground state.

Fig. 3 Emission studies of compounds 1–3 in solvents of different polarities. (a), (c) and (d) Are normalized emission spectra, and (b), (d) and (f) are the corresponding CIE plots of 1, 2 and 3, respectively. λ_ex = 365 nm for all compounds.

Finally, the solid-state emissions of these new molecules were recorded. Interestingly, the changing of excitation energy did not result in the multiple emission bands, a common feature of molecules that obey Kasha's rule. This can be explained from the crystal structure of compounds 1 and 3 (Fig. 4a). The very short intermolecular distance, through π–π stacking, speeds up the rate of IC from the higher excited state(s), yielding emission only from the lowest excited state (i.e. S₁). Nevertheless, compounds 1 and 3 exhibited significant red shifts (595 nm and 673 nm, respectively) in their solid state compared to solution state emission (Fig. 4b). On the contrary, compound 2 displayed blue shifts (535 nm) in its solid-state emission. Although the aggregation-induced emission¹⁶ (i.e. in the solid state) often resulted in bathochromic shifts, the hypsochromic shifts for 2 can be possibly attributed to the smaller reorganisation energy in the solid state compared to its solution state.¹⁷ Nevertheless, 3 yielded red light in its solid state.¹⁸

Fig. 4 Solid state packing of compounds (a) 1 and (b) 3 displaying their intermolecular distances. (c) Solid state fluorescence spectra of compounds 1–3 (λ_ex = 365 nm). (d) Calculated natural transition orbitals (NTOs) of compounds 3 using PBE functional and 6-311+g(d,p) basis set for S₀ → S₁ absorption and (e) calculated emissive states of compound 3. The energy levels are reported from the optimised S₁ state.

To understand the photophysical behaviour of these compounds, we performed geometry optimisation and calculated their absorption and emission spectra using PBE functional and 6-311+g(d,p) basis set.¹⁹ The highest occupied molecular orbitals (HOMO) of all compounds reside on the aromatic core and partly on the substituents, while that of the lowest unoccupied molecular orbitals (LUMO) is on the carbonyl and malononitrile segments, which are largely separated, and thus, indicates a charge transfer nature of electronic transitions between the HOMO and LUMO (Fig. 4d, SI, Fig. S34). The calculated absorption spectra revealed that the electronic transitions occurred from S₀ to S₁, S₃, S₄ and S₅ (S₂ is a dark state), resulting in the multiple bands in the electronic absorption spectra. However, the emission spectra, on the other hand, showed two relaxation processes, S₃ → S₀ (blue-shifted band) and S₁ → S₀ (red-shifted band), which involve HOMO−1 → LUMO and HOMO → LUMO electronic transitions, respectively (SI Tables S8–S13). As depicted in Fig. 4d, all the transitions are π* → π types. Furthermore, although the substituents' contribution to the HOMO orbitals is noticeable in all compounds, the HOMO−1 orbitals lack it entirely. Consequently, there will be strong substituent effects in the low-energy emission band. On the other hand, the high-energy emission band may not exhibit a substituent effect, consistent with the experimental results. Additionally, the dipole moments in the ground state of the optimised structures are relatively high for compounds 1 and 3 (11.13 D and 12.94 D, respectively), and they are increased in the first excited state to 12.41 D and 15.09 D, respectively. On the contrary, the dipole moment in the ground state of 2 is low (only 5.61 D), while the same is marginally reduced to 5.17 in the first excited state. These data also support the experimental data, suggesting that the first excited state is more polar for 1 and 3 and possesses strong charge transfer character, while the ground state is more polar for 2 and may not display significant charge transfer character.

The calculated emission spectra of 3 revealed that the S₁ and S₃ are the bright state that emit in the visible range (Fig. 4e). The energy differences between S₁ and S₂ and S₂ and S₃ are 0.91 eV and 0.42 eV, respectively. Fluorescence from higher excited states is made possible by a relatively large ΔE_S3–S2, which essentially lowers the rate of IC. Remarkably, S₂ was found to be a dark state with a zero oscillator strength. This makes us believe that the blue-shifted emission possibly occurs from S₃, which perfectly matches with that of the experimental data (λ_em(calc.) = 388 nm, λ_em(expt.) = 426 nm). This is also true for the low energy emission band (λ_em(calc.) = 665 nm, λ_em(expt.) = 642 nm) that originates from S₁ (Fig. 4e). The calculated emission spectra also agreed well for the compounds 1 and 2. Thus, it is evident from both experimental and theoretical data that the malononitrile-substituted acenaphthylene diones violate Kasha's rule by emitting from two distinct excited states.

In summary, we report three donor–acceptor small organic molecules that emit in the red region with large Stokes shifts yet without affecting the anti-Kasha emissions. TDDFT calculations support the presence of two bright states, namely S₁ and S₃ resulting in a nice colour tunability from blue to red as a function of excitation wavelength. Additionally, the introduction of donor–acceptor substituents yielded red-emitting species , like 3, as evidenced by the molecular orbital distribution on the acenaphthylene core. Consequently, this study will pave the way towards the development of multi-coloured and red emissive small molecules that can be useful in various fields, including multi-colour OLEDs, anticounterfeiting, bioimaging and photodynamic therapy.²⁰

Conflicts of interest

There are no conflicts to declare.

Data availability

CCDC 2528322 and 2528323 contain the supplementary crystallographic data for this paper.^21a,b

The data supporting this paper have been included as part of the supplementary information (SI). Supplementary information: detailed synthetic procedure, characterization, necessary spectra, and theoretical data. See DOI: https://doi.org/10.1039/d6cc00738d.

Acknowledgements

S. P. acknowledges the SRM Institute of Science and Technology for providing instrumental facilities and Nanotechnology Research Center (NRC), SRMIST for single crystal X-ray diffractometer facility. This work is financially supported by DST-SERB, India (no. SRG/2021/001033). P. G. is grateful to SRMIST for fellowships. S. D. and A. S. acknowledge IISER Thiruvananthapuram for facilities.

References

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