Two-photon absorption cross section determination for fluorene derivatives: analysis of the methodology and elucidation of the origin of the absorption processes

A comprehensive analysis of the well-known open aperture Z-scan method, using a modified equation for the change in transmittance, is presented and accounts for discrepancies in two-photon absorption (2PA) cross sections between picosecond and femtosecond excitation. This new approach takes into account excited-state absorption and stimulated emission of the molecules studied. The two-photon absorption cross-section spectra of a series of six fluorene-based derivatives, determined using picosecond pulses, over a broad spectral range (500-900 nm), and this approach using a modified fitting procedure in the open aperture Z-scan is reported. We demonstrate that the fluorene derivatives exhibit two-photon absorption cross-section values between 700 and 5000 GM, when excited into the two-photon allowed electronic state. Excitation anisotropy spectra, measured to investigate the nature of the observed linear and nonlinear absorption bands, are presented and provide insight into the 2PA process.     

Strong two-photon absorption in new asymmetrically substituted porphyrins: interference between charge-transfer and intermediate-resonance pathways

We study two-photon absorption (2PA) in two series of new free-base porphyrins with 4-(diphenylamino)stilbene or 4,4'-bis-(diphenylamino)stilbene (BDPAS) attached via pi-conjugating linkers at the porphyrin meso-position. We show that this new substitution modality increases the 2PA cross section in the Soret band region (excitation wavelength 750-900 nm) of the core porphyrin by nearly 2 orders of magnitude, from sigma(2) approximately 10 GM for the meso-phenyl-substituted analogue to sigma(2) approximately 10(3) GM for the ethynyl-linked BDPAS-porphyrin dyad. The 2PA properties are quantitatively described by considering two different and interfering 2PA quantum transition pathways. The first path involves virtual transition via intermediate one-photon resonance. The second path bypasses the intermediate resonance and occurs due to a large permanent dipole moment difference between the ground and the final electronic states. To our best knowledge, this is the first experimental observation of the combined effect of these two pathways on one particular two-photon transition, resulting in quantum-interference-modulated 2PA strength.     

Nonlinear optical transmission properties of C60 dyads consisting of a light-harvesting diphenylaminofluorene antenna

Highly enhanced nonlinear absorption cross section values of C60(>DPAF-C2M), C60(>DPAF-C9), and C60(>DPAF-C10) dyads were detected up to 5400, 9700, and 14000 GM, respectively, in the 2.0 ps region in toluene at the concentration of 1.5 x 10(-3) M. They were correlated to a trend showing higher efficiency in light transmittance attenuation down to 39-46% for the dyads C60(>DPAF-C10) and C60(>DPAF-C9) with the increase of irradiance intensity up only to 140 GW/cm(2). The phenomena were attributed to additional enhancement on the excited-state absorption of (1)C60*(>DPAF-Cn ) in the subpicosecond to picosecond region over the two-photon absorption of C60(>DPAF-Cn ) in the femtosecond region. Its accumulative 2.0 ps absorption cross sections were estimated to be 8900 GM for (1)C60*(>DPAF-C9), roughly one order of magnitude higher than its intrinsic femtosecond 2PA cross sections.     

Enhancement of two-photon absorption cross section and singlet-oxygen generation in porphyrin-cored star polymers

We report a newly synthesized polymer of a star-shaped porphyrin compound(TPA-FxP) with four oligofluorene arms at its meso positions with the pronounced enhancement of the two-photon properties and the generation of singlet oxygen by utilizing the two-photon excited fluorescence resonance energy transfer.The steady-state spectra and transient triplet-triplet absorption spectra give evidence that the enhanced two-photon absorption cross section results from not only the through-space energy transfer(Frster...     

Synthesis, two-photon absorption and AIE properties of multibranched thiophene-based triphenylamine derivatives with triazine core

Two new multibranched thiophene-based triarylamine derivatives with 1,3,5-triazine core are synthesized and characterized. Their one-and two-photon absorption properties and aggregation-induced emission effect have been investigated. Both the STAPA-based compounds are AIE active. The two-photon absorption (2PA) cross sections measured by the open aperture Z-scan technique are determined to be 620 and 1610 GM for STAPA-a and STAPA-b in chloroform,respectively, which dramatically increase with the introduction of alkyl chains. The relationship between their structures and properties on one-and two-photon absorption and aggregation-induced emission is discussed, which allows us to examine the effect of introducing alkyl chains. In addition, solvent effects also show a significant influence on the 2PA cross section. The two compounds with excellent AIE and 2PA properties provide attractive alternatives for the biophotonic materials.     

Extremely strong near-IR two-photon absorption in conjugated porphyrin dimers: quantitative description with three-essential-states model

Two-photon absorption spectra (2PA) of a series of conjugated dimers and the corresponding monomer were studied in the near-IR region. All of the dimers show very large peak cross section values, sigma(2) = (3-10) x 10(3) GM (1 GM = 1 x 10(-50) cm(4) s photon(-1)), which is several hundred times larger than that obtained for the corresponding monomer in the same region. We explain such dramatic cooperative enhancement by a combination of several factors, such as strong enhancement of the lowest one-photon Q-transition, better resonance conditions in the three-level system, dramatic enhancement of the excited-state singlet-singlet transition, and parallel arrangement of consecutive transitions in dimers, as compared to perpendicular arrangement in the monomer. We show that the absolute values of the 2PA cross section in these molecules are quantitatively described by a quantum-mechanical expression, derived for the three-level model. We also demonstrate the possibility of singlet oxygen generation upon one- and two-photon excitation of these dimers, which makes them particularly attractive for photodynamic therapy.     

Two-photon absorption properties of iron(II) and ruthenium(II) trischelate complexes of 2,2':4,4' ':4',4' '-quaterpyridinium ligands

A series of RuII or FeII trischelate complex salts containing N-methyl/aryl-2,2':4,4' ':4',4' '-quaterpyridinium ligands that has previously been subjected to quadratic nonlinear optical studies (Coe, B. J. et al. J. Am. Chem. Soc. 2005, 127, 13399) has now been investigated for two-photon absorbing behavior. Z-scan measurements using a 750 nm laser afford reasonably large two-photon absorption (2PA) cross-sections sigma2 of ca. 62-180 GM for the RuII complexes, but only very weak 2PA is observed for the FeII compounds. The excited-state and 2PA properties of the representative chromophore [RuII(Me2Qpy2+)3]8+ (Me2Qpy2+=N' ',N' '-dimethyl-2,2':4,4' ':4',4' '-quaterpyridinium) have also been investigated by using semiempirical intermediate neglect of differential overlap/multireference-determinant single and double configuration interaction computations with the optimized geometry obtained via density functional theory. The calculated sigma2 value of ca. 624 GM at 1.70 eV for this metal-to-ligand charge-transfer chromophore is about 10 times larger than that obtained from the Z-scan studies.     

Strong two-photon absorption of self-assembled butadiyne-linked bisporphyrin

Two-photon absorption (2PA) properties of self-assembled porphyrins were investigated. The butadiyne-linked porphyrin array exhibited a 20 times larger 2PA cross section than the meso-meso-linked self-assembled array due to the expansion of pi-conjugation. Higher-order nonlinear absorption was also observed in the former porphyrin.     

Resonance enhancement of two-photon absorption in fluorescent proteins

We measure two-photon absorption (2PA) spectra of wild-type green fluorescent protein, cyan fluorescent protein, and monomeric red fluorescent protein in absolute cross section values in a wide spectral range (lambda2PA = 550 - 1300 nm), and find, for the first time to our knowledge, a new S0 --> Sn 2PA transition in all three proteins in the short-wavelength region. This transition is strongly resonantly enhanced, showing 2PA cross section values of approximately 20-160 GM, which are at least 2-4 times higher than those measured in the lowest energy (S0 --> S1) transition of corresponding proteins. We also show that the change of permanent dipole moment upon S0 --> S1 excitation (|Deltamu10|) can be deduced from 2PA cross section, providing a new tool for fast evaluation of |Deltamu10| in physiological conditions.     

Transient excited-state absorption and gain spectroscopy of a two-photon absorbing probe with efficient superfluorescent properties

The synthesis, linear photophysical properties, two-photon absorption (2PA), excited-state transient absorption, and gain spectroscopy of a new fluorene derivative tert-butyl 4,4'-(4,4' (1E,1'E)-2,2'-(9,9-bis(2- (2-ethoxyethoxy)ethyl)-9H-fluorene-2,7-diyl)bis(ethene-2,1-diyl)bis(4,1 phenylene)]dipiperazine-1-carboxylate (1) are reported. The steady-state linear absorption and fluorescence spectra, along with excitation anisotropy, fluorescence lifetimes, and photochemical stability of 1 were investigated in a number of organic solvents at room temperature. The 2PA spectra of 1 with a maximum cross-section of ~ 300 GM were obtained with a 1 kHz femtosecond laser system using open-aperture Z-scan and two-photon-induced fluorescence methods. The transient excited-state absorption (ESA) and gain kinetics of 1 were investigated by a femtosecond pump-probe methodology. Fast relaxation processes (~1-2 ps) in the gain and ESA spectra of 1 were revealed in ACN solution, attributable to symmetry-breaking effects in the first excited state. Efficient superfluorescence properties of 1 were observed in a nonpolar solvent under femtosecond excitation. One- and two-photon fluorescence microscopy imaging of HCT 116 cells incubated with probe 1 was accomplished, suggesting the potential of this new probe in two-photon fluorescence microscopy bioimaging.     

Generation of DNA photolesions by two-photon absorption of a frequency-doubled Ti:sapphire laser

The formation of spatially localized regions of DNA damage by multiphoton absorption of light is an attractive tool for investigating DNA repair. Although this method has been applied in cells, little information is available about the formation of lesions by multiphoton absorption in the absence of exogenous or endogenous sensitizing agents. Therefore, we have investigated DNA damage induced in vitro by direct two-photon absorption of frequency-doubled femtosecond pulses from a Ti:sapphire laser. We first developed a quantitative polymerase chain reaction assay to measure DNA damage, and determined that the quantum yield of lesions formed by one-photon absorption of 254 nm light is 7.86×10(-4). We then measured the yield of lesions resulting from exposure to the visible femtosecond laser pulses, which exhibited a quadratic intensity dependence. The two-photon absorption cross section of DNA has a value (per nucleotide) of 2.6 GM at 425 nm, 2.4 GM at 450 nm, and 1.9 GM at 475 nm. A comparison of these in vitro results to several in vivo studies of multiphoton photodamage indicates that the onset of DNA damage occurs at lower intensities in vivo; we suggest possible explanations for this discrepancy.     

Two-dimensionally extended porphyrin tapes: synthesis and shape-dependent two-photon absorption properties

We report the synthesis and characterization of L- and T-shaped porphyrin tapes as extensible structural motifs of two-dimensionally extended porphyrin tapes. The two-photon absorption (TPA) cross-section values (sigma((2))) for L- and T-shaped porphyrin tapes as well as those for linear trimeric and tetrameric porphyrin tapes were measured by an open-aperture Z-scan method at 2300 nm, a wavelength at which the one-photon absorption contribution is either zero or almost negligible. Under these conditions, the sigma((2)) values for the linear porphyrin tape trimer and tetramer were determined to be 18 500 and 41 200 GM, respectively. The sigma((2)) value for the L-shaped trimer was determined to be 8700 GM, which is only half that of the linear trimer, whereas the sigma((2)) value for the T-shaped tetramer was measured to be 35 700 GM. These results clearly indicate the dependence of the TPA cross-section on the molecular shape, which underscores the importance of directionality in the pi-conjugation pathway for the enhancement of TPA cross- section.     

Strong cooperative enhancement of two-photon absorption in double-strand conjugated porphyrin ladder arrays

We present the two-photon absorption (2PA) spectra of a series of conjugated porphyrin oligomers containing N = 2, 4, 8, and ca. 13 monomer units, meso-meso connected with butadiyne linkers. We demonstrate that, in the coplanar double-strand arrays, self-assembled upon addition of 4,4'-bipyridyl, the conjugation length increases dramatically, leading to very strong cooperative enhancement of 2PA. We analyze the scaling of 2PA in both the double-strand and rotationally free single-strand arrays and show how the effective conjugation length in both cases is linked to the observed 2PA properties. By introducing a "conjugation signature" for the 2PA strength, we show that, in double-strand arrangement, the conjugation embraces the whole molecule up to the tetramer level, whereas in single-strand arrangement, it is always less than N, except for N = 2, but keeps increasing until N = 8. Our finding of extremely strong 2PA cross section, sigma2 approximately 105 GM, in double-strand oligomers peaking at 1.3 mum can find use for signal processing in fiber-optic devices.     

Strong enhancement of the two-photon absorption of tetrakis(4-sulfonatophenyl)porphyrin diacid in water upon aggregation

A strong enhancement of the two-photon absorption (TPA) cross section of tetrakis(4-sulfonatophenyl)porphyrin diacid (H(4)TPPS(2-)) at various wavelengths when a J-type aggregate is formed in water with respect to the one observed for the H(4)TPPS(2-) monomer in a mixture of water, dimethyl sulfoxide (DMSO), and urea is presented. The TPA properties are characterized by the open aperture Z-scan technique and the ultrafast two-photon absorption spectroscopy with white light continuum probe (TPA-WLCP) technique. The observed enhancement is discussed in terms of possible electronic cooperative effects in the aggregate.     

Relation between two-photon absorption and dipolar properties in a series of fluorenyl-based chromophores with electron donating or electron withdrawing substituents

We investigate two-photon absorption (2PA) in a series of fluorenyl-based 9,9-diethyl-2-ethynyl-7-((4-R-phenyl)ethynyl)-9,9a-dihydro-4aH-fluorene chromophores with R being various electron donating (ED) and electron withdrawing (EW) groups. We use wavelength-tunable femtosecond laser pulses to measure the 2PA cross sections in the lowest dipole-allowed transition and show that the substituents with stronger ED or EW character enhance the peak 2PA cross section (up to σ(2) ～ 60-80 GM) while the neutral substituents lead to smaller cross sections, σ(2) < 10 GM. We apply two-level approximation to establish a quantitative relation between the 2PA in the pure electronic transition (0-0) and the corresponding change of the permanent electric dipole moment upon the excitation (Δμ). This relation is elucidated by comparing Δμ values obtained from the 2PA measurements with quantum-chemical calculations and with measurements of solvatochromic shifts in a series of solvents. We show that the calculated Δμ correlate well with the values obtained from the 2PA spectroscopy. The Δμ values obtained from the solvatochromic shifts agree well with the above two methods for the chromophores with neutral or weak EW or ED substituents. On the other hand, stronger EW or ED end groups give much larger Stokes shifts, which lead to an overestimation of the Δμ values. We tentatively attribute this effect to the excitation-induced electronic density change occurring predominantly at the substituent side of the molecule, which causes the effective point dipole associated with the Δμ to interact more strongly with the surrounding solvent.     

A superfluorescent fluorenyl probe with efficient two-photon absorption

The linear photophysical, excited state absorption (ESA), superfluorescence, and two-photon absorption (2PA) properties of 4,4'-(1E,1'E)-2,2'-(7,7'(1E,1'E)2,2'(4,4'-sulfonylbis(4,1-phenylene))bis(ethane-2,1-diyl)bis(9,9-didecy-9H-fluorene7,2-diyl))bis(ethane-2,1-diyl)bis(N,N-diphenylaniline) (1) were investigated in organic and aqueous media with respect to its potential application in biological imaging. The analysis of linear photophysical properties revealed a rather complex nature of the main one-photon absorption band, strong solvatochromic effects in the steady-state fluorescence spectra, single-exponential fluorescence decay, and high fluorescence quantum yields in organic solvents (≈1.0). The ESA spectra of 1 suggested potential for light amplification in nonpolar media while efficient superfluorescence in cyclohexane was demonstrated. The degenerate 2PA spectra of 1 were obtained over a broad spectral range (640-900 nm), using a standard two-photon induced fluorescence method under 1 kHz femtosecond excitation. Two well defined 2PA bands with maximum 2PA cross sections up to 1700 GM in the higher energy, short wavelength band and ≈1200 GM in the lower energy, long wavelength band of 1 were shown. The potential use of 1 in bioimaging was demonstrated via one- and two-photon in vitro fluorescence imaging of HCT 116 cells.     

Single- and two-photon properties of a dye-derivatized Roussin's red salt ester (Fe2(mu-RS)2(NO)4) with a large TPA cross section

The synthesis, characterization, photochemistry, and two-photon photophysical properties of a new dye-derivatized iron sulfur nitrosyl cluster Fe2(mu-RS)2(NO)4 (AFX-RSE, RS = 2-thioethyl ester of N-phenyl-N-(3-(2-ethoxy)phenyl)-7-(benzothiazol-2-yl)-9,9-diethyl-fluoren-2-yl-amine) were investigated. Under continuous photolysis, AFX-RSE decomposes with modest quantum yields (Phi(diss) = (4.9 +/- 0.9) x 10(-3) at lambda(irr) = 436 nm) as measured from the loss of the nitrosyl bands in the IR absorbance spectrum. Nitric oxide (NO) was qualitatively demonstrated to be photochemically produced via single-photon excitation through the use of an NO-specific electrode. Steady-state luminescence measurements have shown that AFX-RSE fluorescence is about 88% quenched relative to the model compound AF-tosyl. This is attributed to a relatively efficient energy transfer from the excited states of the antenna chromophores to the dinuclear metal center, with the subsequent production of NO. In addition, the two-photon absorption (TPA) cross sections (delta) were measured for the AF-chromophores via the two-photon excitation (TPE) photoluminescence technique using a femtosecond excitation source. The TPA cross section of AFX-RSE was found with this technique to be delta = 246 +/- 8 GM (1 GM = 10(-50) cm4 s photon(-1) molecule(-1)).     

A two-photon antenna for photochemical delivery of nitric oxide from a water-soluble, dye-derivatized iron nitrosyl complex using NIR light

The experiments described here demonstrate the use of two-photon excitation (TPE) to sensitize nitric oxide (NO) release from a dye-derivatized iron/sulfur/nitrosyl cluster Fe2(mu-RS)2(NO)4 (Fluor-RSE, RS = 2-thioethyl ester of fluorescein) with near-infrared (NIR) light in the form of femtosecond pulses from a Ti:sapphire laser. TPE at 800 nm leads both to weak fluorescence from the organic chromophore at lambda(max) = 532 nm and to NO labilization from the cluster. Since the emission from the reference compound Fluor-Et (the ethyl ester of fluorescein) under identical conditions (50/50 CH3CN/phosphate buffer (1 mM) at pH 7.4) is considerably more intense, the weaker emission from Fluor-RSE and the NO generation indicate that the fluorescein excited states initially formed by TPE are largely quenched by energy transfer to the cluster core. The two-photon absorption (TPA) cross section of Fluor-RSE at 800 nm was determined to be delta = 63 +/- 7 GM via the TPA photoluminescence technique. This can be compared to the TPA cross section of 36 GM reported for fluorescein dye in pH 11 aqueous solution and of 32 +/- 3 GM for Fluor-Et measured under conditions comparable to those used for Fluor-RSE. Pulse intensity dependence studies showed that the quantity of NO released from the latter as the result of NIR photoexcitation follows a quadratic relationship to excitation intensity, consistent with the expectation for a TPE process. These studies demonstrate the potential utility of a two-photon antenna for sensitization of the photochemical release of an active agent (in this case, NO) from a photoactive pro-drug.     

Charge transfer induced enhancement of near-IR two-photon absorption of 5,15-bis(azulenylethynyl) zinc(II) porphyrins

Intramolecular charge transfer in 5,15-bis(azulenylethynyl) substituted zinc(ii) porphyrin leads to a significant enhancement of two-photon absorption at near-IR region, which has been investigated by femtosecond Z-scan method.