Two-Photon Spectroscopy of the Q‑Bands of meso-Tetraphenyl- Porphyrin and -Chlorin Framework Derivatives Jordan A. Greco, † Sumie Shima, † Nicole L. Wagner, ‡ Jason R. McCarthy, † Karissa Atticks, † Christian Brü ckner,* ,† and Robert R. Birge* ,†,‡ † Department of Chemistry, University of Connecticut, 55 North Eagleville Road, Storrs, Connecticut 06269-3060, United States ‡ Department of Molecular and Cell Biology, University of Connecticut, 91 North Eagleville Road, Storrs, Connecticut 06269-3125, United States * S Supporting Information ABSTRACT: The two-photon absorption and excitation spectra in the Q-band region (900-1360 nm) were measured for a number of porphyrinoids, including simple porphyrins (porphin, meso-tetraphenyl-porphyrin), chlorins (meso-tetra- phenyl-2,3-dimethoxychlorin), and porphyrin and chlorin-like analogues (meso-tetraphenyl-porpholactone and meso-tetra- phenyl-porpholactol, respectively). These molecules were chosen to provide a series of compounds that differed structurally only in “single points”. Vibronic structure is observed in the two-photon spectra for all porphyrinoids investigated, but their relative intensities show distinct differences from the vibronic development observed in the corresponding one-photon spectra. A Franck-Condon analysis provides insight into the observed differences. The calculations also indicate that the two-photon absorptivities are associated primarily with Type I processes involving multiple intermediate states, and that accurate assignment requires a summation over at least 30 intermediate states. The Q-bands of the meso- tetraphenyl-2,3-dimethoxychlorin exhibit anomalously high two-photon absorptivities, which we have traced to facile conformational distortion of the chlorin chromophore. Calculations indicated that the relative Q-band absorptivities are sensitive to the phase and magnitude of the chlorin ring distortions. ■ INTRODUCTION The nonlinear optical properties of porphyrins and related oligopyrrolic compounds have been widely investigated due to their potential utility in medical and optical sensing technologies. 1-9 For instance, the ability of porphyrins and chlorins to generate the highly cytotoxic singlet oxygen ( 1 O 2 ) upon irradiation with visible light recommends them for use in the photodynamic therapy (PDT) of tumors. 1,10-12 However, a major limitation of one-photon-mediated PDT is associated with the visible wavelengths needed to activate a porphyrin. Blue and green light, the wavelengths absorbed best by regular porphyrins, do not penetrate tissue to any appreciable depths and possess low spatial selectivity due to scatter. 13,14 Two-photon absorption is a nonlinear optical process that involves the simultaneous absorption of two photons. 15,16 This optical process provides an advantageous alternative for activating porphyrin photosensitizers because it promotes the chromophore into an excited state using the combined energy of two near-infrared (NIR) photons (λ ≈ 700-1400 nm), which typically fall in a region where no one-photon absorption exists for the molecule. Importantly, the NIR region also falls within the optical window of biological tissue. 17 While significant effort has been directed toward the design of porphyrins, chlorins, and bacteriochlorins to enhance one- photon absorption in the NIR domain, 18-24 we focus here on enhancing the two-photon absorptivities of the two lowest-lying singlet states of porphyrin and chlorin analogues. The quadratic dependence of two-photon absorption on laser intensity allows for high spatial resolution of the PDT event and three- dimensional selectivity to target tumors at greater depths. Porphyrins comprise four pyrrole subunits linked by methine bridges, which yields a closed-conjugated aromatic 18 π- electron system that is cross-conjugated with two β,β′-double bonds (Figure 1). The fully unsaturated aromatic structure is highly planarized and dominates the chemical and physical properties of porphyrins. Electronic properties are highly sensitive to modifications of one (or both) of the β,β′-double bonds. 25,26 Reduction of a β,β′-double bond results in the formation of a chlorin. The addition of meso-aryl substituents generally leads to little cross-talk with the porphyrinoid chromophore because o-aryl-to-β-hydrogen steric interactions hold the aryl groups in idealized orthogonal orientation with respect to the chromophore; 27 however, distortion to the macrocycle core often permits interaction with meso-substitu- Received: November 25, 2014 Revised: January 16, 2015 Published: January 21, 2015 Article pubs.acs.org/JPCC © 2015 American Chemical Society 3711 DOI: 10.1021/jp5117883 J. Phys. Chem. C 2015, 119, 3711-3724