Three-Photon Spectroscopy of Porphyrins

Abstract

Recent advances in laser technology have made three-photon (3P) microscopy a real possibility, raising interest in the phenomenon of 3P absorption (3PA). Understanding 3PA of organic chromophores is especially important in view of those imaging applications that rely on exogenous probes, whose optical properties can be manipulated and optimized. Here, we present measurements and theoretical analysis of the degenerate 3PA spectra of several phosphorescent metalloporphyrins, which are used in the construction of biological oxygen probes. The effective 3PA cross sections (σ (3) ) of these porphyrins near 1700 nm, a new promising biological optical window, were found to be on the order of 1000 GM3 (1 GM3 = 10 −83 cm 6 s 2 ), therefore being among the highest values reported to date for organic chromophores. To interpret our data, we developed a qualitative four-state model specific for porphyrins and used it in conjunction with quantitative analysis based on the time-dependent density functional theory (TDDFT)/a posteriori Tamm−Dancoff approximation (ATDA)/sum-over-states (SOS) formalism. The analysis revealed that B (Soret) state plays a key role in the enhancement of 3PA of porphyrins in the Q band region, while the low-lying two-photon (2P)-allowed gerade states interfere negatively and diminish the 3PA strength. This study features the first systematic examination of 3PA properties of porphyrins, suggesting ways to improve their performance and optimize them for imaging and other biomedical applications.

syn-Diarylphthalimidoporphyrins: Effects of Symmetry Breaking on Two-Photon Absorption and Linear Photophysical Properties

“…Using eq it is straightforward to define the contribution of all channels encompassing a certain intermediate estate i , or simply the contribution Σ i of state i , to the total transition strength δ (2) (see SI.7). The contributions of states Q and B and of the lowest 2P-active state Δ (marked with asterisk in Figure ) for PdDAPIP are presented in Table as an example. The contributions of the analogous states in D 4 h -symmetric PdTAPIP are shown for comparison.…”

“…A similar effect of the Δ state was observed recently in the case of three-photon absorption in π-extended porphyrins. 91…”

“…To interpret the experimental findings we carried out calculations of 2PA using the TDDFT/SOS methodology, as described previously ,, (see SI for details). The calculated 2PA spectra of porphyrins Pd- 12 and Pd- 13 are shown in Figure .…”

Application of TD-DFT Theory to Studying Porphyrinoid-Based Photosensitizers for Photodynamic Therapy: A Review

“…Only recently was the possibility of three-photon absorption of porphyrinoids explored experimentally and theoretically [ 125 , 126 , 127 ]. The results based on the SOS formulas indicate fair agreement between the computational and experimental results for a range of substituted porphyrins.…”

“…The results based on the SOS formulas indicate fair agreement between the computational and experimental results for a range of substituted porphyrins. The predicted cross-sections were overestimated by a factor of 2, but all trends were reproduced correctly [ 125 ].…”