title: Two-Photon Absorbing Phosphorescent Metalloporphyrins: Effects of π-Extension and Peripheral Substitution.
abstract: The ability to form triplet excited states upon two-photon excitation is important for several applications of metalloporphyrins, including two-photon phosphorescence lifetime microscopy (2PLM) and two-photon photodynamic therapy (PDT). Here we analyzed one-photon (1P) and degenerate two-photon (2P) absorption properties of several phosphorescent Pt (II) porphyrins, focusing on the effects of aromatic π-extension and peripheral substitution on triplet emissivity and two-photon absorption (2PA). Our 2PA measurements for the first time made use of direct time-resolved detection of phosphorescence, having the ability to efficiently reject laser background through microsecond time gating. π-Extension of the porphyrin macrocycle by way of syn-fusion with two external aromatic fragments, such as in syn-dibenzo- (DBP) and syn-dinaphthoporphyrins (DNP), lowers the symmetry of the porphyrin skeleton. As a result, DBPs and DNPs exhibit stronger 2PA into the one-photon-allowed B (Soret) and Q states than fully symmetric (D4h) nonextended porphyrins. However, much more 2P-active states lie above the B state and cannot be accessed due to the interfering linear absorption. Alkoxycarbonyl groups (CO2R) in the benzo-rings dramatically enhance 2PA near the B state level. Time-dependent density functional theory (TDDFT) calculations in combinations with the sum-over-states (SOS) formalism revealed that the enhancement is due to the stabilization of higher-lying 2P-active states, which are dominated by the excitations involving orbitals extending onto the carbonyl groups. Furthermore, calculations predicted even stronger stabilization of the 2P-allowed gerade-states in symmetric Pt octaalkoxycarbonyl-tetrabenzoporphyrins. Experiments confirmed that the 2PA cross-section of PtTBP(CO2Bu)8near 810 nm reaches above 500 GM in spite of its completely centrosymmetric structure. Combined with exceptionally bright phosphorescence (ϕphos= 0.45), strong 2PA makes Pt(II) complexes of π-extended porphyrins a valuable class of chromophores for 2P applications. Another important advantage of these porphyrinoids is their compact size and easily scalable synthesis.
authors.forename: Tatiana V
authors.initials: TV
authors.lastname: Esipova
authors.affiliation: NanoScience Technology Center, Department of Chemistry and Department of Physics, University of Central Florida , Orlando, Florida 32816, United States.
authors.forename: Héctor J
authors.initials: HJ
authors.lastname: Rivera-Jacquez
authors.affiliation: Institute of Pharmacology and Toxicology, University of Zurich , Zurich CH-8057, Switzerland.
authors.forename: Bruno
authors.initials: B
authors.lastname: Weber
authors.affiliation: NanoScience Technology Center, Department of Chemistry and Department of Physics, University of Central Florida , Orlando, Florida 32816, United States.|Photochemistry Center, Russian Academy of Sciences , ul. Novatorov 7a, Moscow 119421, Russia.|South Ural State University , Lenin pr. 76, Chelyabinsk 454080, Russia.|National Nuclear Research University MEPhI , Kashirskoye sh. 31, Moscow 115409, Russia.
authors.forename: Artëm E
authors.initials: AE
authors.lastname: Masunov
authors.forename: Sergei A
authors.initials: SA
authors.lastname: Vinogradov
mesh_terms: D049408:Luminescent Agents / Q000737:chemistry*; D008163:Luminescent Measurements; D008665:Metalloporphyrins / Q000737:chemistry*; D017785:Photons; D011789:Quantum Theory
chemical_list: D049408:Luminescent Agents; D008665:Metalloporphyrins