Aromaticity as a Guiding Concept for Spectroscopic Features and Nonlinear Optical Properties of Porphyrinoids

Abstract

With their versatile molecular topology and aromaticity, porphyrinoid systems combine remarkable chemistry with interesting photophysical properties and nonlinear optical properties. Hence, the field of application of porphyrinoids is very broad ranging from near-infrared dyes to opto-electronic materials. From previous experimental studies, aromaticity emerges as an important concept in determining the photophysical properties and two-photon absorption cross sections of porphyrinoids. Despite a considerable number of studies on porphyrinoids, few investigate the relationship between aromaticity, UV/vis absorption spectra and nonlinear properties. To assess such structure-property relationships, we performed a computational study focusing on a series of Hückel porphyrinoids to: (i) assess their (anti)aromatic character; (ii) determine the fingerprints of aromaticity on the UV/vis spectra; (iii) evaluate the role of aromaticity on the NLO properties. Using an extensive set of aromaticity descriptors based on energetic, magnetic, structural, reactivity and electronic criteria, the aromaticity of [4n+2] π-electron porphyrinoids was evidenced as was the antiaromaticity for [4n] π-electron systems. In agreement with previous studies, the absorption spectra of aromatic systems display more intense B and Q bands in comparison to their antiaromatic homologues. The nature of these absorption bands was analyzed in detail in terms of polarization, intensity, splitting and composition. Finally, quantities such as the average polarizability and its anisotropy were found to be larger in aromatic systems, whereas first and second hyperpolarizability are influenced by the interplay between aromaticity, planarity and molecular symmetry. To conclude, aromaticity dictates the photophysical properties in porphyrinoids, whereas it is not the only factor determining the magnitude of NLO properties.

Keywords: absorption spectra; aromaticity; nonlinear optical properties; porphyrinoids.

Figure 1

Selected Hückel porphyrinoids and their expected aromaticity according to the annulene model. The annulene-type conjugation pathway is depicted with bold colored bonds.

Figure 2

Hückel conformations computed for different porphyrinoids (top and side views). Red five-membered rings are linked to aromatic structures, whereas green ones to antiaromatic configurations. The symmetry point group together with a schematic representation of the convex conformer are also shown.

Figure 3

AICD plots of Hückel porphyrinoids. The large arrow denotes the direction of the induced ring current: clockwise for diatropic ring currents and anticlockwise for paratropic ring currents (isosurface value 0.03 a.u.).

Figure 4

AICD plots of (18)porphyrin (top) and (20)orangarin (bottom) for different isosurface values (in a.u.).

Figure 5

UV/vis absorption spectra of unsubstituted Hückel porphyrinoids simulated using Gaussian functions with a halfwidth of 2686 cm⁻¹.

Figure 6

Theoretical UV/vis absorption spectra of unsubstituted Hückel porphyrinoids together with the TDDFT/CAM-B3LYP oscillator strengths: (a) (20)orangarin, (b) (22)smaragdyrin, (c) (22)isosmaragdyrin, (d) (22)sapphyrin, (e) (16)norcorrole, (f) (16)porphyrin, (g) (18)porphyrin, (h) (20)porphyrin and (i) porphycene.

Figure 7

Schematic diagrams for the energy levels of selected molecular orbitals of aromatic and antiaromatic porphyrinoids. The HOMO-LUMO gap (in eV) is also shown [59].

Figure 8

Molecular orbitals for aromatic and antiaromatic Hückel porphyrinoids: (a) (16)norcorrole, (b) (18)porphyrin, (c) (20)orangarin, (d) (22)smaragdyrin. The values in red indicate the number of nodal planes in each orbital.

Figure 9

Absorption spectra of unsubstituted aromatic (left) and antiaromatic (right) Hückel porphyrinoids simulated using Gaussian functions with a halfwidth of 2686 cm⁻¹.

Figure 10

UV/vis absorption spectra of regular porphyrin (18P), oxidized porphyrin (16P) and reduced porphyrin (20P) (a) together with the MO energy diagram of selected molecular orbitals (b).

Figure 11

Evolution of the average polarizability in static and dynamic regime (1900 nm), both computed in gas-phase.

Figure 12

Dependence of the static β_HRS values (in a.u.) with the macrocycle size and the planarity of the macrocycle, as denoted by the torsional strain (Φ_p in °). For centrosymmetric structures, β_HRS equals 0.

Figure 13

Evolution of the depolarization ratio (DR) in static and dynamic regime in solvent (2128 nm).

Figure 14

Evolution of the longitudinal component of the second hyperpolarizability (γ_//) in static (a) and dynamic regime (1900 nm and 2128 nm) (b). The symmetry point group for each system is also shown.

Figure 15

Hückel conformations computed for the selected meso-substituted porphyrinoids. Red five-membered rings are linked to aromatic structures, whereas green ones to antiaromatic configurations.

Figure 16

Evolution of the UV/vis absorption spectra upon meso-substitution for (a) (16)norcorrole and (18)porphyrin; (b) [20]orangarin and [22]smaragdyrin.

Scheme 1

Reaction used to evaluate several aromaticity descriptors in octaphyrins. ISE_corr and Δη are given in kcal mol⁻¹ and Λ in ppm cgs.