One- and two-photon absorption of highly conjugated multiporphyrin systems in the two-photon Soret transition region

Abstract

This study presents a detailed investigation of near-infrared one- and two-photon absorption (TPA) in a series of highly conjugated (porphinato)zinc(II) compounds. The chromophores interrogated include meso-to-meso ethyne-bridged (porphinato)zinc(II) oligomers (PZn(n) species), (porphinato)zinc(II)-spacer-(porphinato)zinc(II) (PZn-Sp-PZn) complexes, PZn(n) structures featuring terminal electron-releasing and -withdrawing substituents, related conjugated arrays in which electron-rich and -poor PZn units alternate, and benchmark PZn monomers. Broadband TPA cross-section measurements were performed ratiometrically using fluorescein as a reference. Superficially, the measurements indicate very large TPA cross-sections (up to approximately 10(4) GM; 1 GM = 1x10(-50) cm(4) s photon(-1)) in the two-photon Soret (or B-band) resonance region. However, a more careful analysis of fluorescence as a function of incident photon flux suggests that significant one-photon absorption is present in the same spectral region for all compounds in the series. TPA cross-sections are extracted for the first time for some of these compounds using a model that includes both one-photon absorption and TPA contributions. Resultant TPA cross-sections are approximately 10 GM. The findings suggest that large TPA cross-sections reported in the Soret resonance region of similar compounds might contain significant contributions from one-photon absorption processes.

Figure 1

Structures of [(porphinato)zinc(II)]-based monomers, dimers, and trimers used in this study.

Figure 2

“Apparent” TPA, Σ₂, and TPE, q₂Σ₂, cross-sections measured for all 15 porphyrin compounds. Labels on y-axes indicate either Σ₂ (TPA) or q₂Σ₂ (TPE). Solid lines: one-photon absorptivity (a.u., linear y-axis scale not shown) as a function of 2×the one-photon excitation wavelength (nm). Dashed lines connecting data points: “Apparent” TPA or TPE cross-sections (GM) as calculated using a ratiometric emission-based measurement technique. Error bars (±30%) represent the estimated systematic error.

Figure 3

Fluorescence intensity as a function of incident excitation laser beam intensity and wavelength. The four plots correspond to four different compounds. Within each plot, power-law tests at more than one excitation wavelength are shown. The straight dotted lines show linear regression fits of the measured data points on a log-log scale plot. The values of excitation laser wavelength (nm) are indicated in the chart legend. The slope fit values (m) are indicated above each dotted line. Line segments with slopes of 1 and 2 are shown for reference. Inset: Data points on the dashed line show apparent TPE cross-section spectra (GM) as a function of wavelength (nm); the solid line shows one-photon absorption spectra (a.u., y-axis scale not shown) as a function of 2×the incident laser excitation wavelength (nm).

Figure 4

Fluorescence intensity as a function of incident excitation laser beam intensity and sample concentration. The straight dotted lines show linear regression fits of the measured data points on a log-log scale plot. Values of sample concentration are indicated in the chart legend. Slope fit values (m) are indicated above each dotted line. Line segments with slopes of 1 and 2 are shown for reference. Inset: compound structural diagrams.