Dual Emissive Zn(II) Naphthalocyanines: Synthesis, Structural and Photophysical Characterization with Theory-Supported Insights towards Soluble Coordination Compounds with Visible and Near-Infrared Emission

Abstract

Metal phthalocyaninates and their higher homologues are recognized as deep-red luminophores emitting from their lowest excited singlet state. Herein, we report on the design, synthesis, and in-depth characterization of a new class of dual-emissive (visible and NIR) metal naphthalocyaninates. A 4-N,N-dimethylaminophen-4-yl-substituted naphthalocyaninato zinc(II) complex (Zn-NMe₂Nc) and the derived water-soluble coordination compound (Zn-NMe₃Nc) exhibit a near-infrared fluorescence from the lowest ligand-centered state, along with a unique push-pull-supported luminescence in the visible region of the electromagnetic spectrum. An unprecedentedly broad structural (2D-NMR spectroscopy and mass spectrometry) as well as photophysical characterization (steady-state state and time-resolved photoluminescence spectroscopy) is presented. The unique dual emission was assigned to two independent sets of singlet states related to the intrinsic Q-band of the macrocycle and to the push-pull substituents in the molecular periphery, respectively, as predicted by TD-DFT calculations. In general, the elusive chemical aspects of these macrocyclic compounds are addressed, involving both reaction conditions, thorough purification, and in-depth characterization. Besides the fundamental aspects that are investigated herein, the photoacoustic properties were exemplarily examined using phantom gels to assess their tomographic imaging capabilities. Finally, the robust luminescence in the visible range arising from the push-pull character of the peripheral moieties demonstrated a notable independence from aggregation and was exemplarily implemented for optical imaging (FLIM) through time-resolved multiphoton micro(spectro)scopy.

Keywords: (TD-)DFT; dual fluorescence; multiscale–multimodal imaging; photoacoustic/optoacoustic spectroscopy; steady-state and time-resolved multiphoton micro(spectro)scopy (FLIM).

Figure 1

Synthesis of zinc(II) naphthalocyaninates.

Figure 2

Synthesis of 3,4,12,13,21,22,30,31-octakis(4-(N,N,N-trimethylammonium)phenyl)-2,3-naphthalocyaninato zinc(II) (Zn-NMe₃Nc).

Figure 3

UV-vis absorption spectra normalized at the corresponding λ_max for Zn-OMeNc (a), Zn-NMe₂Nc (b), and Zn-NMe₃Nc (c) in DMSO, as well as for Zn-NMe₃Nc in water (d).

Figure 4

Absorption (red), excitation (black, λ_em = 920 nm), and emission (blue, λ_ex = 750 nm) spectra of Zn-OMeNc (a), Zn-NMe₂Nc (b), Zn-NMe₃Nc in DMSO (c) and Zn-NMe₃Nc in water (d).

Figure 5

Fluorescence in DMSO: (a) full-range emission spectrum of Zn-OMeNc (blue), λ_ex = 335 nm; (b) full-range spectrum of Zn-NMe₂Nc (red), λ_ex = 335 nm; (c) normalized emission spectra of Zn-OMeNc (blue) and Zn-NMe₂Nc (red) in the visible region, λ_ex = 335 nm; (d) normalized emission spectra of Zn-OMeNc (blue) and Zn-NMe₂Nc (red) in the near-infrared region, λ_ex = 750 nm. In the full-range emission spectra (a,b), the bands may appear slightly different (if compared with (c,d), both in shape and intensity), due to the different instrumental settings and filters employed (for details, see Section 3.3).

Figure 6

Fluorescence in DMSO and in water: (a) full-range emission spectrum of Zn-NMe₃Nc (blue) in DMSO, λ_ex = 335 nm; (b) full-range emission spectrum of Zn-NMe₃Nc (red) in water, λ_ex = 335 nm; (c) normalized emission spectra of Zn-NMe₃Nc in DMSO (blue) and water (red) in the visible region, λ_ex = 335 nm; (d) normalized emission spectra of Zn-NMe₃Nc in DMSO (blue) and water (red) in the near-infrared region, λ_ex = 750 nm. In the full-range emission spectra (a,b), the bands may appear slightly different (if compared with (c,d), both in shape and intensity) due to the different instrumental settings and filters employed (for details see Section 3.3).

Figure 7

Optimized ground state structures of Zn-OMeNc, Zn-NMe₂Nc, and Zn-NMe₃Nc, obtained with DFT.

Figure 8

Calculated UV-vis absorption spectra of Zn-OMeNc, Zn-NMe₂Nc, and Zn-NMe₃Nc in DMSO (red lines), together with the corresponding experimental data (black lines).

Figure 9

Isosurface plots (isovalue = 0.02) of the molecular orbitals of Zn-OMeNc, Zn-NMe₂Nc, and Zn-NMe₃Nc, with the highest contributions describing the S_1/2 → S₀ de-excitation.

Figure 10

Isosurface plots (isovalue = 0.02) of the molecular orbitals of Zn-OMeNc, Zn-NMe₂Nc, and Zn-NMe₃Nc, with the highest contributions describing the S₃ → S₀ de-excitation.

Figure 11

Calculated 0-0 emission wavelengths (dashed lines) from the S_1/2 (red) and S₃ (blue) states, obtained with the PBE0 functional, compared to experimental fluorescence spectra (black lines) of Zn-OMeNc, Zn-NMe₂Nc, and Zn-NMe₃Nc in DMSO. For Zn-NMe₂Nc, additional results using the M06-2X functional are shown (S_1/2 in orange, S₃ in purple).

Figure 12

MSOT photoacoustic images of gel phantoms at different concentrations: (a) Zn-Nme₂Nc in DMSO, (b) Zn-Nme₃Nc in DMSO, and (c) Zn-NMe₃Nc in water (λ_ex = 745 nm).

Figure 13

Photoacoustic spectra of Zn-NMe₂Nc in DMSO (red) and Zn-NMe₃Nc in DMSO (green) and water (blue) at different concentrations, obtained from reconstructed MSOT photoacoustic images. Spectra at concentration of 5 μM (a), 2.5 μM (b), and 1 μM (c).

Figure 14

Left: emission spectrum of the Zn-NMe₂Nc@PSMP, suspended in H₂O (λ_ex = 335 nm). Right: luminescence image of the Zn-NMe₂Nc@PSMP, observed under an optical microscope (scale bar = 10 μm; λ_ex = 355 ± 20 nm).

Figure 15

Upper row: Fluorescence micrograph of Zn-NMe₂Nc@PSMP (left); fluorescence lifetime map measured with single-photon excitation (SPE, λ_ex = 375 nm, center); emission spectrum of a discrete particle using SPE (λ_ex = 375 nm, right). Lower row: Fluorescence micrograph of Zn-NMe₂Nc@PSMP (left); fluorescence lifetime map measured with two-photon excitation (TPE, λ_ex = 810 nm, center); emission spectrum of a discrete particle using TPE (λ_ex = 810 nm, right). Raw photoluminescence decays and further details can be found in Figures S57 and S58.

Scheme 1

Synthesis of 6,7-disubstituted naphthalene-2,3-dicarbonitriles.