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. 2016 Jul 1;7(7):4685-4693.
doi: 10.1039/c6sc00614k. Epub 2016 Apr 8.

Functionalized cationic [4]helicenes with unique tuning of absorption, fluorescence and chiroptical properties up to the far-red range

I Hernández Delgado  1 , S Pascal  1 , A Wallabregue  1 , R Duwald  1 , C Besnard  2 , L Guénée  2 , C Nançoz  3 , E Vauthey  3 , R C Tovar  4 , J L Lunkley  4 , G Muller  4 , J Lacour  1
Affiliations

Functionalized cationic [4]helicenes with unique tuning of absorption, fluorescence and chiroptical properties up to the far-red range

I Hernández Delgado et al. Chem Sci. .

Abstract

Unprecedented regioselective post-functionalization of racemic and enantiopure cationic diaza [4]helicenes is afforded. The peripheral auxochrome substituents allow a general tuning of the electrochemical, photophysical and chiroptical properties of the helical dyes (26 examples). For instance, electronic absorption and circular dichroism are modulated from the orange to near-infrared spectral range (575-750 nm), fluorescence quantum efficiency is enhanced up to 0.55 (631 nm) and circularly polarized luminescence is recorded in the red (|glum| ∼ 10-3).

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Figures

Fig. 1
Fig. 1. Cationic aza helical chromophores and the scope of this work.
Scheme 1
Scheme 1. Synthesis of functionalized [4]helicenes. Reagents and conditions: (a) HNO3 (60% aq.), CH2Cl2, 25 °C, 15 min. (b) POCl3, DMF, 90 °C, 8 h. (c) H2, Pd/C (0.2 equiv.), CH2Cl2/MeOH, 25 °C, 1 h. (d) (i) HCHO (12 equiv.), NaBH3CN (7 equiv.), THF, 25 °C, 30 min; (ii) AcOH, 3 h; (iii) NaOH; (iv) 1 M aq. HBF4. (e) tBuONO (1.5 equiv.), TMSN3 (1.2 equiv.), MeCN, 0 to 25 °C, 3 h. (f) ArCCH, CuSO4·5H2O (0.2 equiv.), ascorbic acid (0.3 equiv.), NaHCO3 (0.3 equiv.), MeCN/H2O, 25 °C, 12 h. (g) NaN3 (1.5 equiv.), TfOH (3 equiv.), MeCN, 25 °C, 10 min. (h) NCCH2CN (3 equiv.), Ph3P (20 mol%), MeCN, 130 °C (MW), 25 °C, 1 h. (i) (i) ArCH2X (1.2 equiv.), Ph3P (1.3 equiv.), CH2Cl2; (ii) NaH (2 equiv.), DCM, 25 °C, 30 min; (iii) 1 M aq. HBF4. (j) AcOH (15 equiv.), Zn (5 equiv.), CH2Cl2, 25 °C, 3 h. (k) mCPBA (5 equiv.), CH2Cl2, 25 °C, 2 h. (l) MeI (3 equiv.), K2CO3 (5 equiv.), MeCN, reflux, 1 h. (m) NaH2PO4 (1 equiv.), NaClO2 (2 equiv.), H2O2, MeCN, 60 °C, 1 h. (n) (i) SOCl2 (6 equiv.), CH2Cl2, 25 °C, 10 min; (ii) for 17a,b: MeMgI or PhMgI (1.5 equiv.), –5 °C, 30 min, then 1 M aq. HBF4; for 17c,e: EtOH, PhOH or EtSH (5 or 15 equiv.), 25 °C, 10 min or 2 h; for 17f,g: PrNH2 or Et2NH (15 equiv.), 0 °C then 25 °C, 15 min.
Fig. 2
Fig. 2. Anisotropic displacement ellipsoids plot at 50 percent probability level of the crystal structure of 6 (only the P enantiomer is shown).
Fig. 3
Fig. 3. Voltammetric curves of acetonitrile ([TBA][PF6] 10–1 M) solutions of 5 (red), 17c (orange), 1 (black), 15 (blue) and 8 (green) (10–3 M) recorded at a Pt working electrode (ν = 0.1 V s–1).
Fig. 4
Fig. 4. Selected absorption spectra in acetonitrile (10–5 M) at 293 K as a function of substituent Y. Top and bottom diagrams depict spectra presenting hypsochromic and bathochromic shifts compared to ref. 1 respectively. Insets: typical coloration of acetonitrile solutions (10–4 M).
Fig. 5
Fig. 5. Normalized fluorescence spectra of selected [4]helicene dyes in acetonitrile.
Fig. 6
Fig. 6. M and P enantiomers of 1, 5, 6 and 7 (YH, NO2, CHO, NH2).
Fig. 7
Fig. 7. (Top) UV-Vis-NIR ECD spectra of the M-helices for compounds 1 (green), 5 (red), 6 (orange) and 7 (blue) in acetonitrile (10–5 M) at 293 K. (Bottom) UV-Vis-NIR electronic absorption spectra in acetonitrile (10–5 M). Inset: ECD in the vis-NIR range.
Fig. 8
Fig. 8. Circularly polarized luminescence (upper curves) and total luminescence (lower curves) spectra of P-(+) and M-(–)-1 (left), 5 (middle), and 6 (right) in 2 mM degassed dichloromethane solutions at 295 K, upon excitation at 497/473, 495/473, and 455/472 nm, respectively (black for P-(+) and red for M-(–)).
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