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. 2025 Jun 6;31(32):e202501212.
doi: 10.1002/chem.202501212. Epub 2025 May 2.

Two-photon Excitation of Bright Diaza[4]Helicenes for Isotropic and Circularly Polarized Emission

Bibiana Fabri  1 , Davide F De Rosa  2 , Dominic J Black  2 , Rebecca Mucci  1 , Artemijs Krimovs  2 , Robert Pal  2 , Jérôme Lacour  1
Affiliations

Two-photon Excitation of Bright Diaza[4]Helicenes for Isotropic and Circularly Polarized Emission

Bibiana Fabri et al. Chemistry. .

Abstract

Helicenes are chiral organic dyes that are attracting growing attention due to the high tunability of their (chir)optical and electronic properties. In this work, a series of functionalized cationic diaza[4]helicenes based on dimethoxyquinacridinium (DMQA) scaffolds are presented. By merging branched N-alkyl side chains and triple para-functionalization with OMe groups, structures combine improved chiroptical responses (5x increase) and strong fluorescence quantum yields (Φf ≈ 70% in acetonitrile). An overall improved efficiency of the emission of circularly polarized light with BCPL values reaching 3.4 M-1 cm-1 is obtained. Additionally, two-photon excitation (2PE) studies were performed, showing good values of cross section (CS) at 810 nm. Interestingly, 2PE circularly polarized luminescence (CPL) spectra were acquired for the most performant derivatives (N-isopropyl and N-cyclohexyl); this type of measurement being usually challenging for organic molecular species. Finally, the viability of these compounds in single-photon (1PE) and 2PE microscopy is also shown.

Keywords: bright fluorophores; chiroptical properties; circularly polarized luminescence; microscopy; two‐photon excitation.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
A) cationic diaza[4]helicenes 1 with various N‐alkyl chains; B) previously reported triple para‐OMe derivative 2a; C) para‐OMe derivatives with linear (2a–2b) and branched (2c–2d) alkyl chains.
Scheme 1
Scheme 1
Synthesis of triple para‐OMe diaza[4]helicenes with differentiated nitrogen substituents. Conditions: a) [Ir(cod)OMe]2 (cod = 1,5‐cyclooctadiene), 3,4,7,8‐tetramethyl‐1,10‐phenanthroline, bis(pinacolato)diboron (B2pin2), dry THF, 80 °C, 16 hours; b) (1) H2O2 (aq), NaOH, THF, RT, 16 hours, (2) anion metathesis with KPF6 (aq); c) MeI, Cs2CO3, DMF, RT, 16 hours.
Figure 2
Figure 2
Comparison of ECD signatures at low energies of (P)‐2a (orange), (P)‐2b (red), (P)‐2c (light blue), and (P)‐2d (dark blue) in air‐equilibrated acetonitrile at RT between 450 and 650 nm and concentration 5 × 10−5 M.
Figure 3
Figure 3
2PE studies of 2a in acetonitrile with concentration 1 × 10−5 M. a) Emission spectra under 1PE (black line) and 2PE (red line); b) log versus log plot of the excitation‐power dependence of the emission intensity, the linear fit is displayed by a red line; c) 2PE (red dots, top x‐axis) and 1PE (black line, bottom axis) spectra followed at λ em = 580 nm (x and y axes are adjusted to emphasize the agreement of the spectral features).
Figure 4
Figure 4
2PE CPL spectra (λ exc = 810 nm) of compounds a) 2c and b) 2d in acetonitrile with concentrations of 1 × 10−5 M respectively.
Figure 5
Figure 5
Live cell LSCM of mouse embryonic fibroblasts (NIH‐3T3). Left: A) compound 2d, λ exc = 543 nm, λ em = 560–700 nm; B) ER‐Tracker™ Green, λ exc = 488 nm, λ em = 500–550 nm; C) RGB merge of A) and B) showing strong localization to the ER. Scale bar = 10 µm. Right: 2PE of live cell LSCM incubated with 2d (500 nM) for 2 hours. A) λ exc = 543 nm, λ em = 560–700 nm. B) λ exc = 810 nm, λ em = 500–550 nm. C) RGB merge of A) and B) showing overlap of single and 2PE. Scale bar = 25 µm.
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