1,10-Pyrene-monoimides: a new class of electron acceptors and excimer-forming fluorescent dyes

Abstract

We present the first synthesis of highly fluorescent 1,10-pyreneimide. The peri-fused imide group grants the compound a reversible single-electron reduction, while the pyrene core causes distinct red-shifted excimer emission at higher concentrations. Pyrene monoimides display valuable photophysical properties for a wide variety of applications from fluorescent tags to electron accepting moieties in organic semiconductors. We explore the impact of four different R-groups on solubility, optical and electronic properties, including the propensity to form excimers.

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Imide-functionalized rylenes, such as perylene diimide and naphthalene diimide, are ubiquitous in organic semiconductors due to their high electron affinities, charge mobilities, and overall stability.¹ The asymmetric mono-functionalized perylene monoimide (PMI) and 1,8-naphthalimide have also been widely studied, especially as fluorescent dyes.^2–4 These versatile compounds have been used as emitters in OLEDs,^5–8 acceptors or interfacial layers in solar cells,^9,10 redox active species in flow batteries,¹¹ and G-quadruplex binding ligands in anticancer drugs,^12,13 to name only a few. With these many exciting applications in mind, we aimed to address some of the limitations of these materials. PMIs, for example, suffer from low solubilities and 1,8-naphthalimides require functionalization at the 4-position to be emissive, adding synthetic complexity.

Herein we report on the synthesis, optical properties, and electronic properties of 1,10-pyreneimide (PyI) 4a–d. Pyrene monoimide has a fused, four-ring pyrene core which can be functionalized in multiple positions yet is more compact and therefore more soluble than PMI. Additionally, we demonstrate that PyIs share pyrene's propensity to form excimers, in which a long-lived excited state species forms an emissive complex with another ground state molecule. This concentration-dependent red-shift in emission makes pyrene and its derivatives useful as spatial fluorescence probes to study macromolecular and biological assemblies, though pyrene's poor solubility and ultraviolet-only excitation are limiting.^14–16 Adding an imide substituent increases the molecules oxidative stability and shifts excitation and emission wavelengths into the visible range.

1,10-Pyreneimides were synthesized for the first time by adapting our recently reported protocol for pyrenediimides (Scheme 1).¹⁷ Starting from pyrene, a single methyl ketone is attached via Friedel–Crafts acylation, then transformed to a vinyl chloride which, through flash vacuum pyrolysis, undergoes cyclization to cyclopenta[cd]pyrene 1.¹⁸ Oxidation to the anhydride proceeds in two steps: first benezeneseleninic acid anhydride forms the dione 2 and then NaOH and peroxide complete the oxidation to form 1,10-pyreneanhydride 3. From 3 a dehydration in the presence of a primary amine forms the pyreneimide. The chosen substituents in this communication demonstrate the effect of different solubilizing groups on the physical and optical properties of these dyes. 4a, with a symmetrically branched –CH(C₅H₁₁)₂ “swallow tail,” is readily soluble in organic solvents (dichloromethane, ethyl acetate, acetone, acetonitrile), but not in water. Introducing a polyethylene glycol chain in 4b using m-PEG4-amine yields a viscous oil, as opposed to powdery solids 4a,c,d. While more amphiphilic than 4a, 4b is not water soluble. Water solubility over 2 mg mL⁻¹ was achieved for 4c, due to the charged quaternary ammonium of the methylated piperazine moiety. Finally, the condensation with gamma-aminobutyric acid forms the carboxylic acid derivative 4d. The carboxylic handle would allow 4d to be attached to larger biological molecules, for example to study peptide binding.¹⁹

Scheme 1 Synthesis of 1,10-pyreneimide derivatives 4a–d. (i =acetyl chloride, AlCl₃, DCM; ii = PCl₅, DCM; iii = FVP, 1000 °C; iv = benzeneseleninic anhydride, PhCl; v = NaOH(aq), H₂O₂; and vi = RNH₂, DMF.).

The optical properties of PyI 4a–d were investigated and are shown in Fig. 1 and Table S1. In dichloromethane, all the compounds exhibit clear vibronic splitting and a small Stokes shift (<20 nm), as expected of their rigid aromatic cores. Broader, red-shifted emission is observed in all the solid samples. In samples 4b–d, solid emission occurred from approximately 500 to 800 nm, giving the samples an orange colour under UV illumination (Fig. 1 insets). In solid 4a, however, the solid-state packing is less uniform, as evidenced by a shoulder band at 450 nm, giving the solid a yellow appearance under UV illumination. The optical properties of PyI 4c and 4d were also measured in water and 10 mM aqueous triethyl amine, respectively (Fig. S16 and S17). In both cases, a broadening of the absorption bands was observed, and the Stokes shift increased to 56 nm due to stabilizing hydrogen-bonding with the polar solvent.

Fig. 1 Normalized UV-vis absorption (solid blue) and emission spectra (dashed red) of 4a–d (a–d, respectively) in dichloromethane as well as emission spectra of solid powder samples or spin-cast film (dashed yellow). Photographs of the corresponding solid samples under UV illumination are shown as an inset.

Quantum yields of 4a,b,d in dichloromethane solution were all above 0.90 (Table S1), much higher than pyrene in dichloromethane (Φ = 0.28).²⁰ For comparison, N–CH₃-1,8-napthalimide has a quantum yield of only 0.03, due to a high rate of intersystem crossing to the triplet state.²¹ The quantum yield of PyI 4a (Φ = 0.93) is also much higher than the analogous pyrenediimide with the same side chain (N–C(C₅H₁₁)₂-1,5,6,10-pyrenediimde, Φ = 0.29).¹⁷ Derivative 4c was insufficiently soluble in dichloromethane, so its quantum yield was measured in water and found to be 0.06.

Similarly to pyrene and pyrenediimides, 1,10-pyreneimides form excimers at higher concentrations and/or through solvent induced aggregation. In all instances, a large red shift of over 100 nm was observed from monomer emission to excimer emission, making the transition easy to see with the naked eye (Fig. 2 and Table S1). The nature of the side chain in compounds 4a–d led to changes in the solvents and concentrations at which excimer emission was observed. Excimer formation is a diffusion-controlled process, and the polarity of the side chains impacted the compounds’ solubility and excited state dynamics.

Fig. 2 Excimer formation in solutions of 4a in (a) increasing concentrations in DCM and (b) in increasing v/v fractions of water in acetonitrile (normalized). (c) Fluorescence decay profiles (λ_ex = 280 nm) of 4a in dichloromethane (blue trace: monomer emission, λ_em = 460 nm)(orange trace: excimer emission, λ_em = 570 nm). (d and e) Excimer formation in increasing concentrations of aqueous 4c.

For example, in dichloromethane solutions of 4a and 4b, excimer emission increases relative to monomer emission as the solute's concentration goes up. In 4a, excimer emission reached a similar intensity as monomer emission at 2.86 mM (Fig. 2a), while in 4b, this was observed at only 0.91 mM (Fig. S18a), indicating that the excited species of more polar 4b formed the excimer complex more readily in non-polar dichloromethane. Excimer emission in dilute solutions of 4a (5.00 µM) in acetonitrile could also be induced by increasing the v/v% of water, in which 4a is not soluble (Fig. 2b). As the v/v% of water increased, the observed emission transitioned from monomeric (max λ_em = 430 nm) in 0% H₂O to exclusively excimer emission (max λ_em = 562 nm) in 90% H₂O. The emission profiles are excitation independent (Fig. S20) and excitation spectra measured for λ_em = 428, 450 nm and λ_em = 570, 600 nm both match the absorption spectra of 4a, indicating that both monomer and excimer emission derive from excitation of the same species (Fig. S21).

Compounds 4b–d were too amphiphilic to form excimers in acetonitrile/water solutions, but some excimer emission was observed in solutions of 4d in 90% H₂O/DMSO (Fig. S18b). Compound 4c was highly soluble in water and in these solutions, excimer formation was observed in concentrations as low as 0.05 mM (Fig. 2d). At these lower concentrations, both the monomer and excimer stayed highly fluorescent, resulting in a visually distinct transition from teal-coloured monomer emission to orange-coloured excimer emission (Fig. 2e).

The presence of an excited-state process leading to a dynamic excimer was further confirmed through time-resolved fluorescence. Time-correlated single photon counting (TCSPC) measurements were run on a 3.0 mM solution of 4a in dichloromethane. At this concentration, both monomer and excimer emission are observed, and we were able to detect two different decay profiles in the same solution (Fig. 2c). When monitoring monomer emission at 460 nm, the single exponential decay corresponded to a lifetime of 7.18 ns (Fig. S19a). When monitoring excimer emission at 570 nm, a rise in the intensity as a function of time was observed. This observed rise component is indicative of an excited state process, such as excimer formation, in which the emissive species is produced only after the initially excited species forms the exciplex. Since this process is diffusion controlled, the lifetimes of excimers are non-exponential, and while a bi-exponential fit with a negative amplitude for the rise component returns a good merit of fit, it does not provide a meaningful lifetime value (Fig. S19b).^22,23 To approximate the lifetime of the excimer species, we therefore used a single exponential fit on the decay profile after reaching peak intensity. Since any contribution from monomer emission at 570 nm would be negligible, the resulting lifetime of 12.06 ns can be attributed to the excimer species. An increased lifetime in excimer emission versus monomeric is typical and has been reported in 1,8-napthalimides²⁴ and pyrene derivatives.²⁵

Cyclic voltammetry measurements of 4a in DCM show a single reversible reduction with a half-wave potential of −1.73 V versus the ferrocene/ferrocenium (Fc/Fc+) internal reference (Fig. 3). Assuming a Fc/Fc+ redox couple at 4.8 eV to vacuum, we estimate a lowest unoccupied molecular orbital (LUMO) level of −3.07 eV. Though ΔE_p was large (220 mV) for a reversible single-electron process, it was similar to the ΔE_p observed for ferrocene (153 mV) and the measured i_pa/i_pc ratio was close to one at 1.22 (Fig. S22). No oxidation was observed within the solvent window. As expected, N–C(C₅H₁₁)₂-1,10-pyreneimide 4a has a lower reduction potential than N–C(C₅H₁₁)₂-1,5,6,10-pyrenediimide (first E_1/2 = −1.19 V), but displays nearly identical electrochemical behavior as N-phenyl-1,8-naphthalimide which also has a reversible reduction wave at E_1/2 at −1.72 V versus Fc/Fc+.²⁶

Fig. 3 Cyclic voltammogram of 4a in 0.20 M TBAPF6 in DCM at a scan rate of 100 mV s⁻¹.

Density functional theory (DFT) modelling was carried out on gas-phase models of 4a–d, using truncated side chains for 4a and 4b (See SI). The LUMO energy levels for 4a,b,d were all calculated to be approximately 2.7 eV with band-gaps of ∼3.4 eV (Table S1). In general, the molecular orbitals were delocalized over the pyrene monoimide, with the HOMO orbitals more concentrated on the pyrene moiety and the LUMO orbitals on the imide groups. The R-groups attached at the imide nitrogen had little impact on HOMO/LUMO energy levels due to the lack of electron density at the node on nitrogen. In 4c, however, the LUMO orbitals are entirely concentrated on the methylated piperazine moiety bearing the charged ammonium. The theoretical band gap for 4c was calculated to be 2.20 eV; however, this contradicts the onset of the UV-vis absorption spectrum. The calculated reorganization energy for N–CH₃-1,10-pyreneimide was found to be 0.2416 eV and the electron affinity 1.32 eV.

In summary, we have synthesized 1,10-pyrene monoimides for the first time, introducing a new class of highly fluorescent electron acceptors. These molecules display intense emission in both solution and the solid state, along with distinctive red-shifted excimer fluorescence with increased excited-state lifetimes—beneficial features for emissive materials and spatial fluorescent probes. Their reversible, stable reductions occur at potentials comparable to 1,8-naphthalimides, underscoring their promise as robust acceptor moieties in organic semiconductors. The influence of R-groups on solubility and optical behaviour highlights the potential of further functionalization to fine-tune the optoelectronic properties of 1,10-pyrene monoimides.

S. DF. & A. S.: formal analysis, investigation, validation. J. S.: conceptualization, formal analysis, investigation, methodology, resources, supervision, visualization, writing – original draft, writing – review and editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: (1) synthetic procedures & compounds characterization: full experimental details for compounds 2, 3, and 4a–d, NMR spectra (¹H, ¹³C), and HR-MS data. (2) Spectroscopic data: infrared spectra, UV-vis absorption spectra, fluorescence emission spectra and time-resolved spectroscopy (raw trace of counts, exponential fit curves, weighted residuals). (3) Computational data: geometry optimization results with cartesian coordinates, reorganization energy calculations, DFT approximations of HOMO & LUMO densities and values. See DOI: https://doi.org/10.1039/d5nj04374c.

Acknowledgements

Thank you to Viktoriia Rutckaia at the Photonics Facility in the Advanced Science Research Center of the City University of New York for lifetime measurements. We gratefully acknowledge the University of Delaware Mass Spec. Core Facility (supported by grants from NIGMS 5 P30 GM110758-2 and P20GM104316) for high resolution mass spectrometry analysis. We also gratefully acknowledge the contributions of Daniel McIlhenny of Fordham University for his synthetic work on starting materials. S. DF. and A. S. gratefully acknowledge the financial support of Fordham University Rose Hill Undergraduate Summer Research Grants.

Notes and references

1. X. Zhan, A. Facchetti, S. Barlow, T. J. Marks, M. A. Ratner, M. R. Wasielewski and S. R. Marder, Adv. Mater., 2011, 23, 268–284.
2. J. L. Segura, H. Herrera and P. Bäuerle, J. Mater. Chem., 2012, 22, 8717–8733.
3. S. Banerjee, E. B. Veale, C. M. Phelan, S. A. Murphy, G. M. Tocci, L. J. Gillespie, D. O. Frimannsson, J. M. Kelly and T. Gunnlaugsson, Chem. Soc. Rev., 2013, 42, 1601–1618.
4. R. Roy, A. Khan, O. Chatterjee, S. Bhunia and A. L. Koner, Org. Mater., 2021, 3, 417–454.
5. D. Kolosov, V. Adamovich, P. Djurovich, M. E. Thompson and C. Adachi, J. Am. Chem. Soc., 2002, 124, 9945–9954.
6. O. Bezvikonnyi, D. Gudeika, D. Volyniuk, J. V. Grazulevicius and G. Bagdziunas, New J. Chem., 2018, 42, 12492–12502.
7. W. Zeng, H.-Y. Lai, W.-K. Lee, M. Jiao, Y.-J. Shiu, C. Zhong, S. Gong, T. Zhou, G. Xie, M. Sarma, K.-T. Wong, C.-C. Wu and C. Yang, Adv. Mater., 2018, 30, 1704961.
8. X. Chen, C. Xu, T. Wang, C. Zhou, J. Du, Z. Wang, H. Xu, T. Xie, G. Bi, J. Jiang, X. Zhang, J. N. Demas, C. O. Trindle, Y. Luo and G. Zhang, Angew. Chem., Int. Ed., 2016, 55, 9872–9876.
9. B. Schweda, M. Reinfelds, J. Hofinger, G. Bäumel, T. Rath, P. Kaschnitz, R. C. Fischer, M. Flock, H. Amenitsch, M. C. Scharber and G. Trimmel, Chem. – Eur. J., 2022, 28, e202200276.
10. M. Chen, Y. Tang, R. Qin, Z. Su, F. Yang, C. Qin, J. Yang, X. Tang, M. Li and H. Liu, ACS Appl. Mater. Interfaces, 2023, 15, 5556–5565.
11. M. Shahsavan, C. Wiberg and P. Peljo, Chem. Commun., 2022, 58, 12692–12695.
12. S. A. Ohnmacht, C. Marchetti, M. Gunaratnam, R. J. Besser, S. M. Haider, G. Di Vita, H. L. Lowe, M. Mellinas-Gomez, S. Diocou, M. Robson, J. Šponer, B. Islam, R. Barbara Pedley, J. A. Hartley and S. Neidle, Sci. Rep., 2015, 5, 11385.
13. M. S. Malik, S. Farooq Adil, Z. Moussa, H. M. Altass, I. I. Althagafi, M. Morad, M. A. Ansari, Q. M. Sajid Jamal, R. J. Obaid, A. A. Al-Warthan, T. B. Shaik and S. A. Ahmed, Front. Chem., 2021, 9, 630357.
14. J. Duhamel, Langmuir, 2012, 28, 6527–6538.
15. H. Maeda, T. Maeda, K. Mizuno, K. Fujimoto, H. Shimizu and M. Inouye, Chem. – Eur. J., 2006, 12, 824–831.
16. K. Ayyavoo and P. Velusamy, New J. Chem., 2021, 45, 10997–11017.
17. K. Johnston, A. McCostis, E. Mikita, E. Jaffal and J. A. Schneider, Org. Lett., 2025, 27, 3107–3110.
18. M. Sarobe, J. W. Zwikker, J. D. Snoeijer, U. E. Wiersum and L. W. Jenneskens, J. Chem. Soc., Chem. Commun., 1994, 89–90.
19. G. Loving and B. Imperiali, J. Am. Chem. Soc., 2008, 130, 13630–13638.
20. Y. Niko, S. Kawauchi, S. Otsu, K. Tokumaru and G. Konishi, J. Org. Chem., 2013, 78, 3196–3207.
21. V. Wintgens, P. Valat, J. Kossanyi, L. Biczok, A. Demeter and T. Bérces, J. Chem. Soc., Faraday Trans., 1994, 90, 411–421.
22. J. R. Lakowicz, H. Cherek, I. Gryczynski, N. Joshi and M. L. Johnson, Biophys. Chem., 1987, 28, 35–50.
23. A. S. R. Koti, M. M. G. Krishna and N. Periasamy, J. Phys. Chem. A, 2001, 105, 1767–1771.
24. D. W. Cho and D. W. Cho, New J. Chem., 2014, 38, 2233–2236.
25. V. Kumar, B. Sk, S. Kundu and A. Patra, J. Mater. Chem. C, 2018, 6, 12086–12094.
26. L. A. Hall, H. J. Windsor, B. Chan, D. M. D’Alessandro and G. Lakhwani, J. Phys. Chem. Lett., 2025, 16, 1529–1534.

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