Two-Photon Absorption in Tetraphenylporphycenes:  Are Porphycenes Better Candidates than Porphyrins for Providing Optimal Optical Properties for Two-Photon Photodynamic Therapy?

Abstract

Porphycenes are structural isomers of porphyrins that have many unique properties and features. In the present work, the resonant two-photon absorption of 2,7,12,17-tetraphenylporphycene (TPPo) and its palladium(II) complex (PdTPPo) has been investigated. The data obtained are compared to those from the isomeric compound, meso-tetraphenylporphyrin (TPP). Detection of phosphorescence from singlet molecular oxygen, O2(a(1)Delta(g)), produced upon irradiation of these compounds, was used to obtain two-photon excitation spectra and to quantify two-photon absorption cross sections, delta. In the spectral region of 750-850 nm, the two-photon absorption cross sections at the band maxima for both TPPo and PdTPPo, delta = 2280 and 1750 GM, respectively, are significantly larger than that for TPP. This difference is attributed to the phenomenon of so-called resonance enhancement; for the porphycenes, the two-photon transition is nearly resonant with a comparatively intense one-photon Q-band transition. The results of quantum mechanical calculations using density functional quadratic response theory are in excellent agreement with the experimental data and, as such, demonstrate that comparatively high-level quantum chemical methods can be used to interpret and predict nonlinear optical properties from such large molecular systems. One important point realized through these experiments and calculations is that one must exercise caution when using qualitative molecular-symmetry-derived arguments to predict the expected spectral relationship between allowed one- and two-photon transitions. From a practical perspective, this study establishes that, in comparison to porphyrins and other tetrapyrrolic macrocyclic systems, porphycenes exhibit many desirable attributes for use as sensitizers in two-photon initiated photodynamic therapy.

Nonlinear Optical Properties of Porphyrins

“…[155] Not only does expansion or contraction of the tetrapyrrole system affect the photophysical properties and nonlinear be- . [156] Other optical limiting studies have targeted sol-gel doped materials, [41] the incorporation of symmetric porphyrins into boric acid doped glass, [157] and J-aggregates.…”

Blood-vessel closure using photosensitizers engineered for two-photon excitation

“…The majority of chromophores possess low two-photon cross-sections, of the order of 1-100 Goeppert-Mayer units (1 GM = 10 -50 cm 4 s photon -1 ). For example, the two FDA-approved PDT photosensitisers, verteporfin and Photofrin (cross sections 50 GM and 10 GM respectively) 17 , are unlikely to be suitable for TPE-PDT, as the high light intensities needed to achieve a therapeutic effect are close to the thresholds for photothermal or photomechanical damage 18 .Several design strategies for TPE-PDT photosensitisers have been reported recently [11][12][13][14][15][16] , but few of these compounds have yet been studied in vitro 15 , and, to date, none have progressed to in vivo testing. Porphyrin derivatives are often effective PDT agents, as exemplified by verteporfin and Photofrin 4 .…”

“…Several design strategies for TPE-PDT photosensitisers have been reported recently [11][12][13][14][15][16] , but few of these compounds have yet been studied in vitro 15 , and, to date, none have progressed to in vivo testing. Porphyrin derivatives are often effective PDT agents, as exemplified by verteporfin and Photofrin 4 .…”

Alternative selection rules for one- and two-photon transitions in tribenzotetraazachlorin: Quasi-centrosymmetrical π-conjugation pathway of formally non-centrosymmetrical molecule

“…3(a)), which in turn is still less than the asymmetry of non-symmetrical isomers of H 2 TPP (where the 60 • twisting of phenyl rings causes a perturbation of otherwise centrosymmetric wavefunction of the tetrapyrrole ring). 40,41 Therefore, both H 2 TtBuPc and H 2 TBTAC can be considered as quasicentrosymmetrical, compared to stronger perturbed H 2 TPP and even non-centrosymetrical porphyrin no. 7 of Ref.…”