Resonance enhancement of two-photon absorption in porphyrins

Exceptional three- to six-photon absorption at organometallic dendrimers

The light-intensity dependence of multi-photon absorption (MPA) affords outstanding spatial control. Furthermore, compared to the higher-energy photons needed for analogous linear absorption, the lower-energy photons involved in MPA often correspond to important wavelengths, such as those of the biological and telecommunications "windows". It is therefore of crucial importance to develop molecules that exhibit outstanding MPA cross-sections. However, although progress has been made with two-photon absorption, there is currently a dearth of efficient instantaneous n-photon absorbers (n > 2), a key reason being the scarcity of structure-property studies required to understand higher-order MPA. We herein report systematically-varied metallodendrimers up to third-generation in size, together with their nonlinear absorptive responses over the spectral range 600-2520 nm. We show that the dendrimers exhibit exceptional instantaneous three- to six-photon absorption cross-sections, with maximal values increasing with dendrimer generation and installation of solubilizing group, and we report that changing the groups at the dendrimer periphery can shift the wavelengths of the nPA maxima. We also describe time-dependent DFT studies that have facilitated assignment of the key linear and nonlinear transitions and disclosed the crucial role of the metal in the outstanding MPA performance.

Experimental upper bounds for resonance-enhanced entangled two-photon absorption cross section of indocyanine green

Resonant intermediate states have been proposed to increase the efficiency of entangled two-photon absorption (ETPA). Although resonance-enhanced ETPA (r-ETPA) has been demonstrated in atomic systems using bright squeezed vacuum, it has not been studied in organic molecules. We investigate for the first time r-ETPA in an organic molecular dye, indocyanine green (ICG), when excited by broadband entangled photons in near-IR. Similar to many reported virtual state mediated ETPA (v-ETPA) measurements, no r-ETPA signals are measured, with an experimental upper bound for the cross section placed at 6(±2) × 10-23 cm2. In addition, the classical resonance-enhanced two-photon absorption (r-TPA) cross section of ICG at 800 nm is measured for the first time to be 20(±13) GM, where 1 GM equals 10-50 cm4 s, suggesting that having a resonant intermediate state does not significantly enhance two-photon processes in ICG. The spectrotemporally resolved emission signatures of ICG excited by entangled photons are also presented to support this conclusion.

Blue-shift photoconversion of near-infrared fluorescent proteins for labeling and tracking in living cells and organisms

Photolabeling of intracellular molecules is an invaluable approach to studying various dynamic processes in living cells with high spatiotemporal precision. Among fluorescent proteins, photoconvertible mechanisms and their products are in the visible spectrum (400-650 nm), limiting their in vivo and multiplexed applications. Here we report the phenomenon of near-infrared to far-red photoconversion in the miRFP family of near infrared fluorescent proteins engineered from bacterial phytochromes. This photoconversion is induced by near-infrared light through a non-linear process, further allowing optical sectioning. Photoconverted miRFP species emit fluorescence at 650 nm enabling photolabeling entirely performed in the near-infrared range. We use miRFPs as photoconvertible fluorescent probes to track organelles in live cells and in vivo, both with conventional and super-resolution microscopy. The spectral properties of miRFPs complement those of GFP-like photoconvertible proteins, allowing strategies for photoconversion and spectral multiplexed applications.

Entangled Photon Spectroscopy

The enhanced interest in quantum-related phenomena has provided new opportunities for chemists to push the limits of detection and analysis of chemical processes. As some have called this the second quantum revolution, a time has come to apply the rules learned from previous research in quantum phenomena toward new methods and technologies important to chemists. While there has been great interest recently in quantum information science (QIS), the quest to understand how nonclassical states of light interact with matter has been ongoing for more than two decades. Our entry into this field started around this time with the use of materials to produce nonclassical states of light. Here, the process of multiphoton absorption led to photon-number squeezed states of light, where the photon statistics are sub-Poissonian. In addition to the great interest in generating squeezed states of light, there was also interest in the formation of entangled states of light. While much of the effort is still in foundational physics, there are numerous new avenues as to how quantum entanglement can be applied to spectroscopy, imaging, and sensing. These opportunities could have a large impact on the chemical community for a broad spectrum of applications.In this Account, we discuss the use of entangled (or quantum) light for spectroscopy as well as applications in microscopy and interferometry. The potential benefits of the use of quantum light are discussed in detail. From the first experiments in porphyrin dendrimer systems by Dr. Dong-Ik Lee in our group to the measurements of the entangled two photon absorption cross sections of biological systems such as flavoproteins, the usefulness of entangled light for spectroscopy has been illustrated. These early measurements led the way to more advanced measurements of the unique characteristics of both entangled light and the entangled photon absorption cross-section, which provides new control knobs for manipulating excited states in molecules.The first reports of fluorescence-induced entangled processes were in organic chromophores where the entangled photon cross-section was measured. These results would later have widespread impact in applications such as entangled two-photon microscopy. From our design, construction and implementation of a quantum entangled photon excited microscope, important imaging capabilities were achieved at an unprecedented low excitation intensity of 10⁷ photons/s, which is 6 orders of magnitude lower than the excitation level for the classical two-photon image. New reports have also illustrated an advantage of nonclassical light in Raman imaging as well.From a standpoint of more precise measurements, the use of entangled photons in quantum interferometry may offer new opportunities for chemistry research. Experiments that combine molecular spectroscopy and quantum interferometry, by utilizing the correlations of entangled photons in a Hong-Ou-Mandel (HOM) interferometer, have been carried out. The initial experiment showed that the HOM signal is sensitive to the presence of a resonant organic sample placed in one arm of the interferometer. In addition, parameters such as the dephasing time have been obtained with the opportunity for even more advanced phenomenology in the future.

Surface-enhanced hyper-Raman scattering of Rhodamine 6G isotopologues: Assignment of lower vibrational frequencies

We report a comprehensive experimental and theoretical study of the lower-wavenumber vibrational modes in the surface-enhanced hyper-Raman scattering (SEHRS) of Rhodamine 6G (R6G) and its isotopologue R6G-d4. Measurements acquired on-resonance with two different electronic states, S₁ and S₂, are compared to the time-dependent density functional theory computations of the resonance hyper-Raman spectra and electrodynamics-quantum mechanical computations of the SEHRS spectra on-resonance with S₁ and S₂. After accounting for surface orientation, we find excellent agreement between experiment and theory for both R6G and its isotopologue. We then present a detailed analysis of the complex vibronic coupling effects in R6G and the importance of surface orientation for characterizing the system. This combination of theory and experiment allows, for the first time, an unambiguous assignment of lower-wavenumber vibrational modes of R6G and its isotopologue R6G-d4.

Functional titanium dioxide nanoparticle conjugated with phthalocyanine and folic acid as a promising photosensitizer for targeted photodynamic therapy in vitro and in vivo

Photodynamic therapy (PDT) is a promising cancer treatment that can be implemented using various agents. The conventional photosensitizer Al (III) phthalocyanine chloride tetrasulfonic acid (Pc) has limitations of selectivity in tumor targeting, low affinity to cancer cells, and low two-photon absorption. This study presents a novel photosensitizer FA-TiO₂-Pc, which has the TiO₂ nanoparticle conjugated with a tumor targeting agent of folic acid (FA), and Pc. FA-TiO₂-Pc possessed high targeted photodynamic therapeutic activity and excellent biocompatibility. This promising photosensitizer showed high therapeutic drug efficiency in vitro at a low concentration dose and short incubation time under one-photon excitation (OPE). In vivo, when treated with a low dose of FA-TiO₂-Pc and low light irradiation, the tumor growth was depressed in mice bearing HeLa xenograft tumors with minimal side effects. In addition, the two-photon absorption of FA-TiO₂-Pc was also enhanced compared to Pc, proving that FA-TiO₂-Pc system has a great potential to be used for the therapy of the folate receptor positive cancer cells in both OPE-PDT and two-photon excitation (TPE)-PDT agents.

Two-photon absorption property of Cl-substituted diphenylacetylenes by optical-probing photoacoustic spectroscopy

Two-photon absorption spectra and two-photon absorption cross sections of Cl-substituted diphenylacetylenes (ClDPAs) were investigated by optical-probing photoacoustic spectroscopy and quantum chemical calculations for the first time. The two-photon absorption spectra of ClDPAs exhibited intense two-photon absorption bands at around 480 nm, which are forbidden by one-photon absorption. The two-photon absorption cross sections σ⁽²⁾ of o-, m-, and p-ClDPAs at 476 nm were determined to be 22 ± 1, 23 ± 1, and 38 ± 2 GM, respectively. Compared with diphenylacetylene (DPA) (27 GM at 472 nm), the σ⁽²⁾ values of o- and m-ClDPAs were lower, while that of p-ClDPA was higher. Simulated two-photon absorption spectra using the TD-B3LYP/6-311+G(d,p) level of calculations within the Tamm-Dancoff approximation, based on the three-state model, well agreed with the experimental results. The difference in the σ⁽²⁾ values of DPA and ClDPAs was responsible for those in the transition dipole moments between the intermediate and the final states.

[Morphological evaluation of singlet phototherapy in the treatment of periodontal diseases in an experimental study]

The aim of the study was to compare the effectiveness of laser singlet phototherapy and traditional photodynamic therapy the treatment of periodontal diseases in an animal model. The experimental model involved 70 male rats Wistar in which periodontitis was modeled and treated: in group I (30 animals) a nanosecond laser device for medical use with a wavelength of 1270 nm was used for 7 sessions in a 400 ns pulse mode, an average radiation power of 2 W, and a radiation density of 200 J/cm², group II (30 animals) received photodynamic therapy with the administration of a photosensitizer, followed by irradiation with a laser wavelength of 660 nm for 7 sessions 2 Wt average radiation power, group III (controls, 10 animals) - traditional drug therapy. Morphological studies were performed on 7, 14 and 21 day after treatment. On day 7th and 14th the study revealed In group I the presence of full blood vessels and diffuse expressed leukocyte infiltration with an admixture of macrophages, in group II - pronounced edema of the tissue and vasoconstriction. On day 21 the picture included in group I regenerated periodontal ligament with dilated full blood vessels on the border with the bone beams of the alveolar bone, in group II a moderately pronounced edema of the periodontal ligament with single dilated vessels, in controls significantly destroyed periodontal ligament substituted with granulation tissue and periodontal ligament. Thus, the treatment of periodontitis with the methods of singlet phototherapy leads to the development of reactive inflammation and significant vascularization of periodontal tissues which contributes to the rapid regeneration and stability of remission.

Ultrafast dynamics of hemin aggregates

The effects of solvents on the conformation of hemin and their implications on the dynamics of the complex have been studied using the time-resolved optical Kerr effect (OKE) with 35 fs laser pulses (at a central wavelength of 800 nm). The OKE enabled estimation to be made of the third-order nonlinear electronic susceptibility (χ⁽³⁾) of hemin solutions: it was found to be significantly smaller than that in hemin thin films. The real and imaginary components of χ⁽³⁾ were negative in both the solvents, suggesting that one-photon as well as two-photon absorption processes contribute to the nonlinear electronic susceptibility of hemin. Our study of the ultrafast heme dynamics not only unveils the instantaneous electronic response related to electronic susceptibility but also brings to the fore a novel libration process that has hitherto remained undetected. The hindered rotation in the femtosecond domain that may be responsible for this libration process possibly stems from π-π hemin oligomers formed in aqueous solution. The present results provide new insights into the conformational dynamics in the self-assembly of heme oligomers that may also be significant in certain pathogenic conditions where free heme is formed in biological systems.

Two-Photon Spectroscopy of a Series of Platinum Acetylides: Conformation-Induced Ground-State Symmetry Breaking

With the goal of elucidating electronic and conformational effects on structure-spectroscopic property relationships in platinum acetylides, we synthesized a series of nominally centrosymmetric chromophores trans-Pt(PBu₃)₂(C≡C-Phenyl-X)₂, where X = diphenylamino (DPA), NH₂, OCH₃, t-Bu, CH₃, H, F, benzothiazole (BTH), CF₃, CN, and NO₂. We collected one- and two-photon absorption spectra and also performed density functional theory (DFT) and time-dependent (TD) DFT calculations on the ground- and excited-state properties of these compounds. The DFT calculations revealed facile rotation between the two ligands, suggesting that the compounds exhibit nonplanar ground-state conformations in solution. TDDFT calculation of the S₁ state energy and transition dipole moment for a nonplanar conformation gave good agreement with experiment. Two-photon absorption spectra obtained from these compounds allowed estimation of the change of permanent electric dipole moment upon vertical excitation from ground state to S₁ state. The values are small Δμ < 1.0 D for neutral substituents such as CH₃, H, and F but increase sharply to Δμ ≈ 11 D for electron-accepting NO₂. When in a nonplanar conformation, the corresponding calculated Δμ values showed good agreement with the experimental data indicating that the two-photon spectra result from nonplanar ground-state conformations. Previously studied related chromophores having extended conjugation ( Rebane, A.; Drobizhev, M.; Makarov, N. S.; Wicks, G.; Wnuk, P.; Stepanenko, Y.; Haley, J. E.; Krein, D. M.; Fore, J. L.; Burke, A. R.; Slagle, J. E.; McLean, D. G.; Cooper, T. M. J. Phys. Chem. A 2014 , 118 , 3749 - 3759 ) show similar dependence of Δμ on the substituents, which allows us to conclude that the excited-state properties of these floppy chromophores are a function of the electronic properties of the substituents, ligand size, and nonplanar molecular conformation.

Cooperative porphyrin-quadrupolar based triad for combined two-photon induced fluorescence and singlet oxygen generation

The design and synthesis of a cooperative multichromophoric triad system which combines the large two-photon absorption properties of fluorene-cored bis-donor quadropolar dyes and the remarkable sensitizing properties of the porphyrin subunit (i.e. high intersystem crossing and ability to produce singlet oxygen by energy transfer to oxygen from its triplet excited state) is described. After irradiation the energy can be transferred from the quadrupolar chromophores to the porphyrin with an estimated 80% efficiency via a FRET process. Moreover both the two-photon absorption properties of the quadrupolar subunits and the sensitizing and fluorescence properties of the porphyrin are retained indicating that deleterious competing processes (such as photo-induced electron transfer) are prevented in such molecular architectures thanks to the implemented design. As a result, the two-photon absorption induced singlet oxygen generation efficiency of the triad in the NIR region is found to be enhanced by an order of magnitude as compared to the porphyin subunit. Potential applications of these porphyrin-based multichromophoric systems for photodynamic therapy based upon two-photon excitation in the NIR region might be possible since it overcomes the low two-photon absorption response of porphyrin while fully retaining their remarkable photosensitizing properties.

Cooperative enhancement of the nonlinear optical response in conjugated energetic materials: A TD-DFT study

Conjugated energetic molecules (CEMs) are a class of explosives with high nitrogen content that posses both enhanced safety and energetic performance properties and are ideal for direct optical initiation. As isolated molecules, they absorb within the range of conventional lasers. Crystalline CEMs are used in practice, however, and their properties can differ due to intermolecular interaction. Herein, time-dependent density functional theory was used to investigate one-photon absorption (OPA) and two-photon absorption (TPA) of monomers and dimers obtained from experimentally determined crystal structures of CEMs. OPA scales linearly with the number of chromophore units, while TPA scales nonlinearly, where a more than 3-fold enhancement in peak intensity, per chromophore unit, is calculated. Cooperative enhancement depends on electronic delocalization spanning both chromophore units. An increase in sensitivity to nonlinear laser initiation makes these materials suitable for practical use. This is the first study predicting a cooperative enhancement of the nonlinear optical response in energetic materials composed of relatively small molecules. The proposed model quantum chemistry is validated by comparison to crystal structure geometries and the optical absorption of these materials dissolved in solution.

Two-Photon Absorbing Phosphorescent Metalloporphyrins: Effects of π-Extension and Peripheral Substitution

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)8 near 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.

Nonlinear Optical Materials for the Smart Filtering of Optical Radiation

The control of luminous radiation has extremely important implications for modern and future technologies as well as in medicine. In this Review, we detail chemical structures and their relevant photophysical features for various groups of materials, including organic dyes such as metalloporphyrins and metallophthalocyanines (and derivatives), other common organic materials, mixed metal complexes and clusters, fullerenes, dendrimeric nanocomposites, polymeric materials (organic and/or inorganic), inorganic semiconductors, and other nanoscopic materials, utilized or potentially useful for the realization of devices able to filter in a smart way an external radiation. The concept of smart is referred to the characteristic of those materials that are capable to filter the radiation in a dynamic way without the need of an ancillary system for the activation of the required transmission change. In particular, this Review gives emphasis to the nonlinear optical properties of photoactive materials for the function of optical power limiting. All known mechanisms of optical limiting have been analyzed and discussed for the different types of materials.

Regioselective phenylene-fusion reactions of Ni(ii)-porphyrins controlled by an electron-withdrawing meso-substituent

Oxidation of 10,15,20-triaryl Ni(ii)-porphyrins bearing an electron-withdrawing substituent at the 5-position with DDQ and FeCl3 gave 10,12- and 18,20-doubly phenylene-fused Ni(ii)-porphyrins regioselectively. A doubly phenylene-fused meso-chloro porphyrin thus prepared was reductively coupled to give a meso-meso linked dimer, which was further converted to a quadruply phenylene-fused meso-meso, β-β, β-β triply linked Zn(ii)-diporphyrin via inner-metal exchange followed by oxidation with DDQ and Sc(OTf)3. As compared to the usual meso-meso, β-β, β-β triply linked Zn(ii)-diporphyrin, this π-extended porphyrin dyad exhibits a smaller HOMO-LUMO gap and a larger two-photon absorption cross-section.

Cooperative enhancement versus additivity of two-photon-absorption cross sections in linear and branched squaraine superchromophores

Our investigation of the nonlinear optical properties of a series of oligomeric squaraine dyes showed enhanced cross sections for linear oligomers but only additivity for branched systems.

Quantum chemical insights into the dependence of porphyrin basicity on the meso-aryl substituents: thermodynamics, buckling, reaction sites and molecular flexibility

The chemical and sensing properties of porphyrins are frequently tunedviathe introduction of peripheral substituents. Their interaction with the porphyrin core is investigated.

Two-photon antenna-core oxygen probe with enhanced performance

Recent development of two-photon phosphorescence lifetime microscopy (2PLM) of oxygen enabled first noninvasive high-resolution measurements of tissue oxygenation in vivo in 3D, providing valuable physiological information. The so far developed two-photon-enhanced phosphorescent probes comprise antenna-core constructs, in which two-photon absorbing chromophores (antenna) capture and channel excitation energy to a phosphorescent core (metalloporphyrin) via intramolecular excitation energy transfer (EET). These probes allowed demonstration of the methods' potential; however, they suffer from a number of limitations, such as partial loss of emissivity to competing triplet state deactivation pathways (e.g., electron transfer) and suboptimal sensitivity to oxygen, thereby limiting spatial and temporal resolution of the method. Here we present a new probe, PtTCHP-C307, designed to overcome these limitations. The key improvements include significant increase in the phosphorescence quantum yield, higher efficiency of the antenna-core energy transfer, minimized quenching of the phosphorescence by electron transfer and increased signal dynamic range. For the same excitation flux, the new probe is able to produce up to 6-fold higher signal output than previously reported molecules. Performance of PtTCHP-C307 was demonstrated in vivo in pO2 measurements through the intact mouse skull into the bone marrow, where all blood cells are made from hematopoietic stem cells.

Photophysics of glycosylated derivatives of a chlorin, isobacteriochlorin and bacteriochlorin for photodynamic theragnostics: discovery of a two-photon-absorbing photosensitizer

The photophysical properties of a chlorin, isobacteriochlorin and bacteriochlorin built on a core tetrapentafluorophenylporphyrin (TPPF20 ) and the nonhydrolyzable para thioglycosylated conjugates of these chromophores are presented. The photophysical characterization of these compounds was done in three different solvents to correlate with different environments in cells and tissues. Compared with TPPF20 other dyes have greater absorption in the red region of the visible spectrum and greater fluorescence quantum yields. The excited state lifetimes are from 3 to 11 ns. The radiative and nonradiative rate constants for deactivation of the excited state were estimated from the fluorescence quantum yield and excited state lifetime. The data indicate that the bacteriochlorin has strong absorption bands near 730 nm and efficiently enters the triplet manifold. The isobacteriochlorin has a 40-70% fluorescence quantum yield depending on solvent, so it may be a good fluorescent tag. The isobacteriochlorins also display enhanced two-photon absorption, thereby allowing the use of 860 nm light to excite the compound. While the two-photon cross section of 25 GM units is not large, excitation of low chromophore concentrations can induce apoptosis. The glycosylated compounds accumulate in cancer cells and a head and neck squamous carcinoma xenograft tumor model in mice. These compounds are robust to photobleaching.

Porphyrins fused with strongly electron-donating 1,3-dithiol-2-ylidene moieties: redox control by metal cation complexation and anion binding

A new class of redox-active free base and metalloporphyrins fused with the 1,3-dithiol-2-ylidene subunits present in tetrathiafulvalene, termed MTTFP (M = H2, Cu, Ni, Zn), have been prepared and characterized. The strong electron-donating properties of MTTFP were probed by electrochemical measurement and demonstrated that oxidation potentials can be tuned by metalation of the free base form, H2TTFP. X-ray crystal structures of H2TTFP, ZnTTFP, and CuTTFP revealed that a severe saddle-shape distortion was observed with the dithiole rings bent out of the plane toward one another in the neutral form. In contrast, the structure of the two-electron oxidized species (CuTFFP(2+)) is planar, corresponding to a change from a nonaromatic to aromatic structure upon oxidation. A relatively large two-photon absorption (TPA) cross-section value of H2TTFP(2+) (1200 GM) was obtained for the free base compound, a value that is much higher than those typically seen for porphyrins (<100 GM). Augmented TPA values for the metal complexes were also seen. The strong electron-donating ability of ZnTTFP was further enhanced by binding of Cl(-) and Br(-) as revealed by thermal electron-transfer between ZnTTFP and Li(+)-encapsulated C60 (Li(+)@C60) in benzonitrile, which was "switched on" by the addition of either Cl(-) or Br(-) (as the tetrabutylammonium salts). The X-ray crystal structure of Cl(-)-bound ZnTTFP was determined and provided support for the strong binding between the Cl(-) anion and the Zn(2+) cation present in ZnTTFP.

Amplified two-photon absorption in trans-A2B2-porphyrins bearing nitrophenylethynyl substituents

We show that peripheral nitro groups enhance the maximum two-photon absorption cross-section of trans-A(2)B(2)-porphyrins bearing two phenylethynyl substituents by more than one order of magnitude. Maximum values as high as 1000 GM result from realization of suitable conditions for effective resonance enhancement along with a lowering of the energy and intensification of the two-photon allowed transitions in the Soret region.

Anti-Stokes fluorescence from endogenously formed protoporphyrin IX--implications for clinical multiphoton diagnostics

Multiphoton imaging based on two-photon excitation is making its way into the clinics, particularly for skin cancer diagnostics. It has been suggested that endogenously formed protoporphyrin IX (PpIX) induced by aminolevulinic acid or methylaminolevulinate can be applied to improve tumor contrast, in connection to imaging of tissue autofluorescence. However, previous reports are limited to cell studies and data from tissue are scarce. No report shows conclusive evidence that endogenously formed PpIX increases tumor contrast when performing multiphoton imaging in the clinical situation. We here demonstrate by spectral analysis that two-photon excitation of endogenously formed PpIX does not provide additional contrast in superficial basal cell carcinomas. In fact, the PpIX signal is overshadowed by the autofluorescent background. The results show that PpIX should be excited at a wavelength giving rise to one-photon anti-Stokes fluorescence, to overcome the autofluorescent background. Thus, this study reports on a plausible method, which can be implemented for clinical investigations on endogenously formed PpIX using multiphoton microscopy.

Synthesis and self-organization of fluorene-conjugated bisimidazolylporphyrin and its optical properties

A conjugated-bisimidazolylporphyrin bridged by bis(ethynylfluorene) was synthesized and organized into linear polymer through self-coordination having mean molecular weights, M_w and M_n, of ~2.1 × 10⁵ Da and ~1.6 × 10⁵ Da, respectively. A large two-photon absorption cross section value of 3.4 × 10⁵ GM (per dimer unit) was observed. This value was comparable to that of the previously reported self-assembled linear polymer consisting of butadiyne-bridged imidazolylporphyrins. The two-photon absorption properties could be controlled by tuning the wavelength and absorption intensity of the one-photon absorption.

Carbohydrate-conjugated porphyrin dimers: synthesis and photobiological evaluation for a potential application in one-photon and two-photon photodynamic therapy

We report the synthesis of bioconjugated zinc porphyrin dimers 1a-e designed as photosensitizers for one-photon and two-photon excited photodynamic therapy. These macrocycles are substituted with carbohydrate units (glucose, mannose, lactose) in order to target tumor cells over-expressing lectin membrane receptors. Polarity, singlet oxygen production and in vitro photocytotoxicity are studied to determine their photodynamic therapy potentiality.

Substituent effect on two-photon absorption properties of conjugated porphyrins

Two-photon absorption (2PA) cross section values of bisacetylene-connected bisporphyrins were measured by using a nanosecond open aperture Z-scan method. The maximum effective cross section values of alkyl and aryl meso-substituted bisporphyrins were 14,000 GM at 890 nm and 7,700 GM at 860 nm, respectively, and the value of alkyl-substituted bisporphyrin was almost twice of that of aryl-substituted one. In one-photon absorption spectra, Q-bands of alkyl-substituted bisporphyrin were red-shifted at 635 nm and 685 nm, and intensified compared with those of aryl-substituted one at 625 nm and 678 nm, respectively, indicating higher π-electron density of the former compound compared to that of the later. The large enhancement observed for alkyl-substituted bisporphyrin is attributed to the increase in π- electron density compared with aryl-substituted one. It was found that the alkyl group is better to obtain large 2PA cross section values than the aryl group as the meso-substituent of the porphyrin.

Bisporphyrin connected by tetrathiafulvalene

A new porphyrintetrathiafulvalene composite, where two porphyrins are bridged by te trathiafulvalene (TTF) using acetylene bonds was synthesized. The Q-band of the monomeric porphyrin appears at 590 nm whereas that of the composite is red-shifted to 620 nm and intensified. The Soret band is also red-shifted from 427 nm to 435 nm and much broadened, indicating the expansion of π-conjugation over the porphyrin and tetrathiafulvalene units. The HOMOs and LUMOs were calculated using time-dependent density functional theory. Voltammetric experiments revealed that the first oxidation potential of the TTF moiety in the composite was shifted by +155 mV compa red with TTF in the absence of composite. The effective two-photon absorption (2PA) cross section values were measured by using a nanosecond open aperture Z-scan method. The maximum effective 2PA cross section values were obtained at 760 nm, as 7300 GM in benzonitrile and 5900 GM in toluene. The values obtained in the polar solvent were 1.2 to 1.5 times larger than those in the nonpolar solvent.