Novel 2,1,3-benzothiadiazole-based red-fluorescent dyes with enhanced two-photon absorption cross-sections

This paper reports the two-photon absorbing and orange-red fluorescence emitting properties of a series of new 2,1,3-benzothiadiazole (BTD)-based D-pi-A-pi-D-type and star-burst-type fluorescent dyes. In the D-pi-A-pi-D-type dyes 1-6, a central BTD core was connected with two terminal N,N-disubstituted amino groups via various pi-conjugated spacers. The star-burst-type dyes 8 and 10 have a three-branched structure composed of a central core (benzene core in 8 and triphenylamine core in 10) and three triphenylamine-containing BTD branches. All the BTD-based dyes displayed intense orange-red color fluorescence in a region of 550-689 nm, which was obtained by single-photon excitation with good fluorescent quantum yield up to 0.98 as well as by two-photon excitation. Large two-photon absorption (TPA) cross-sections (110-800 GM) of these BTD dyes were evaluated by open aperture Z-scan technique with a femtosecond Ti/sapphire laser. The TPA cross-sections of D-pi-A-pi-D-type dyes 2-6 with a benzene, thiophene, ethene, ethyne, and styrene moiety, respectively, as an additional pi-conjugated spacer are about 1.5-2.5 times larger than that of 1c with only a benzene spacer. The TPA cross-sections significantly increased in three-branched star-burst-type BTDs 8 (780 GM) with a benzene core and 10 (800 GM) with a triphenylamine core, which are about 3-5 times larger than those of the corresponding one-dimensional sub-units 9 (170 GM) and 11 (230 GM), respectively. The ratios of sigma/e(pi) between three-branched and one-dimensional dyes were 6.5:3.8 (for 8 and 9) and 6.0:4.0 (for 10 and 11), which are larger than those predicted simply on the basis of the chromophore number density (1:1), according to a cooperative enhancement of the two-photon absorbing nature in the three-branched system.     

Extended squaraine dyes with large two-photon absorption cross-sections

Extended bis(donor)-substituted squaraine chromophores exhibit very high two-photon cross-sections (as high as 33 000 GM) in the near-IR; these can be attributed to the combination of large transition dipoles with small detuning energies. The modulus of the third-order nonlinear optical susceptibility at 1.3 mum has been found to be 7.0 x 10-11 esu for one of these chromophores.     

Bis-1,4-(p-diarylaminostryl)-2,5-dicyanobenzene derivatives with large two-photon absorption cross-sections

Synthesis and physical properties of novel multibranched two-photon materials are reported. The compound with three units of 4-(p-diphenylaminostyryl)-2,5-dicyanostyryl moieties attached to the central triphenylamine core exhibits a very large two-photon absorption cross-section.     

2,6-Bis(styryl)anthracene derivatives with large two-photon cross-sections

The first synthesis of 2,6-bis(styryl)anthrance derivatives with very large two-photon cross sections is reported.     

Multipolar symmetric squaraines with large two-photon absorption cross-sections in the NIR region

The two-photon absorption (TPA) properties of two extended symmetric squaraine dyes are thoroughly characterized from the experimental and quantum-chemical point of view. The two molecules are specially engineered to have a multipolar structure with increasing complexity, D-π-A-π-D and A'-π-D-π-A-π-D-π-A', respectively. The experimental TPA spectra, measured by means of the Z-scan technique in the femtoseconds regime, display considerably high values of TPA cross sections (σ(TPA)) for both molecules. In particular, the squaraine with the more extended structure shows the highest value of σ(TPA) ever reported for this class of molecules. CIS and TDDFT calculations of the one and two-photon absorption properties are carried out to clarify the origin of the observed TPA properties and fully characterize the electronic properties of these compounds. The calculations, in good agreement with the experimental data, suggest that the origin of this exceptionally high σ(TPA) can be ascribed to the presence of a peripheral A' group, that increases the density of excited states involved in the TPA process.     

Novel heteroaromatic-based multi-branched dyes with enhanced two-photon absorption activity

The first examples of heterocycle-based multi-branched dyes with efficient two-photon absorption (TPA) activity are reported; the novel chromophores exhibit large TPA cross sections (as high as 1600 x 10(-50) cm4 s photon(-1) molecule(-1), measured with 150 fs laser pulses at 800 nm); a strong cooperative enhancement in the branched systems with respect to the one-dimensional sub-units is found.     

Ultrafast dynamics in multibranched structures with enhanced two-photon absorption

The understanding of the mechanism of the enhanced two-photon absorption (TPA) in multibranched chromophore systems is of importance to the design of materials with the large TPA cross-sections and for future applications. In this communication, the mechanism of enhanced TPA properties is investigated. For a dendritic model system, the excited-state dynamics for both population (T1-process) and phase relaxation (T2-process) processes involved are investigated by a combination of time-resolved spectroscopic techniques. The results of time-resolved fluorescence anisotropy are compared with previous results obtained from other branched chromophore systems. It is found that the PRL-701 trimer system, which possesses the large enhancement of two-photon absorption cross-section, gives a faster anisotropy decay (fluorescence upconversion and transient absorption), a longer population relaxation time (fluorescence lifetime), and a weaker coupling to the solvent (a larger photon echo peak shift initial value). New strategies for rational design of large TPA materials can be achieved based on a better understanding of the mechanism of the enhancement.     

Triphenylamine derivatives with large two-photon cross-sections

[structure: see text] Triphenylamine derivatives have been synthesized and shown to exhibit large two-photon cross-sections at a wide range of wavelengths and the largest value measured by fs Z-scan experiment. Moreover, a linear relationship is noted between the two-photon cross-sections measured by ns fluorescence and fs Z-scan methods.     

Aromatic core modified decaphyrins with the largest two-photon absorption cross-sections: syntheses and characterization

[structure: see text] Core-modified aromatic decaphyrins with 42pi conjugated electrons exhibit the highest two-photon absorption cross-section value (sigma(2) = 108,000 GM) known for any organic molecule, suggesting possible device applications in the field of nonlinear optics.     

High two-photon cross-sections in bis(diarylaminostyryl) chromophores with electron-rich heterocycle and bis(heterocycle)vinylene bridges

Chromophores in which vinylene units and either pyrrole or dialkoxythiophene groups form the bridge between two diarylaminophenyl groups exhibit two-photon cross-sections of 1000-5000 GM at 600-650 nm.     

Excited states and two-photon absorption of some novel thiophenyl Pt(II)-ethynyl derivatives

The photophysical characterization of two new compounds related to bis((4-(phenylethynyl)phenyl)ethynyl)bis(tributylphosphine)platinum(II), here abbreviated Pt1, is reported. For the first new compound ATP1, the inner phenyl rings (closer to the Pt atom) in Pt1 are replaced by thiophene rings bridging at the 2,5-positions. In compound ATP2, the outer phenyl groups are replaced by thiophene rings bonded at the 2-position. Specifically, we report on the fluorescence quantum yield, two-photon absorption, triplet decay times and two-photon absorption induced emission spectra of the molecules in THF solutions. The results were compared with those of Pt1 and Pt1 capped with an acetonide-protected 2,2-bis(methylol)propionic acid (bis-MPA) ester group (Pt1-G1). The photophysical properties of the organic dye 7-(diethylamino)coumarin (Coumarin 110) were determined and used as a reference material. The two-photon absorption cross section around 720-740 nm of ATP1 and ATP2 was found to be of the same order of magnitude as for Pt1-G1, i.e., between 5 and 10 GM, but slightly larger for ATP1 than for ATP2 (1 GM = 1 G?ppert-Mayer = 10(-50) (cm(4) s)/photon). The fluorescence decay time of all compounds was found to be very short (subnanosecond) with quantum yields 0.0045, 0.0007, 0.0011 and 0.0020 for ATP1, ATP2, Pt1-G1 and Pt1, respectively, measured at excitation wavelength 373 nm. Just as Pt1 and Pt1-G1, both thiophenyl derivatives showed large intersystem crossing capabilities and phosphorescence, characteristic for a triplet state that can enhance the nonlinear absorption and optical power limiting by triplet state absorption. The phosphorescence emission of the thiophenyl derivatives was red-shifted in comparison with Pt1 and Pt1-G1, and the phosphorescence decay times were on the order of 200-500 ns, as for the Pt1 compound.     

o-nitrobenzyl photolabile protecting groups with red-shifted absorption: syntheses and uncaging cross-sections for one- and two-photon excitation

We evaluated the o-nitrobenzyl platform for designing photolabile protecting groups with red-shifted absorption that could be photolyzed upon one- and two-photon excitation. Several synthetic pathways to build different conjugated o-nitrobenzyl backbones, as well as to vary the benzylic position, are reported. Relative to the reference 4,5-dimethoxy-2-nitrobenzyl group, several o-nitrobenzyl derivatives exhibit a large and red-shifted one-photon absorption within the near-UV range. Uncaging after one-photon excitation was studied by measuring UV-visible absorption and steady-state fluorescence emission on model caged ethers and esters. In the whole series investigated, the caged substrates were released cleanly upon photolysis. Quantum yields of uncaging after one-photon absorption lie within the 0.1-1 % range. We observed that these drop as the maximum wavelength absorption of the o-nitrobenzyl protecting group is increased. A new method based on fluorescence correlation spectroscopy (FCS) after two-photon excitation was used to measure the action uncaging cross section for two-photon excitation. The series of o-nitrobenzyl caged fluorescent coumarins investigated exhibit values within the 0.1-0.01 Goeppert-Mayer (GM) range. Such results are in line with the low quantum yields of uncaging associated with cross-sections of 1-50 GM for two-photon absorption. Although the cross-sections for one- and two-photon absorption of o-nitrobenzyl photolabile protecting groups can be readily improved, we emphasize the difficulty in enlarging the corresponding action uncaging cross-sections in view of the observed trend of their quantum yield of uncaging.     

Enhanced two-photon absorption with novel octupolar propeller-shaped fluorophores derived from triphenylamine

[structure: see text] Novel octupolar fluorophores derived from the symmetrical functionalization of a triphenylamine core with strong acceptor peripheral groups via phenylene-ethynylene linkers have been synthesized and shown to exhibit high fluorescence quantum yields, very large TPA cross-sections in the red-NIR region, and suitable photostability.     

Calculations on the octupolar molecules with enhanced two-photon absorption cross sections based on the Zn (II) and Cu (I) as centers

The equilibrium geometries, electronic structures, as well as one- and two-photon absorption cross sections of a series of octupolar chromophores with Zn(2+) or Cu(+) as coordinate centers and 4,4'-bis(dibutylaminostyryl)-[2,2']-bis(bipyridyl) as ligands have been determined by using B3LYP/6-31G and ZINDO methods. These molecules are designed by controlled combination of two or three bipyridyl ligands with the metal centers. The results show that Zn(2+) is an effective template for the design of octupolar structures which enable it to form tetrahedral and octahedral coordinated complexes; while Cu(+) only exists in a tetrahedral coordinated complex, comparing the tetrahedral complex with Zn(2+) as the center with that of Cu(+) as the center, it is found that the complex with the Cu(+) center is a better two-photon absorption material than the former as far as the transparency/nonlinearity is concerned. Furthermore, for the same metal center of Zn(2+), both one- and two-photon absorptions of the tetrahedral complex are redshifted relative to those of the octahedral complex, is attributed to the spiroconjugation effect in the tetrahedral complex. Our theoretical findings are consistent with recent experimental observations and provide an important foundation for the design of improved transparency-nonlinearity two-photon absorption materials.     

Predictions of novel two-photon absorption bands in fluorescent proteins

By means of time-dependent density functional theory, we calculate the two-photon cross-sections for the lowest relevant excitations in some model chromophores of intrinsically fluorescent proteins. The two-photon strength of the first, one-photon active transition varies among the various chromophores, in line with experimental findings. Interestingly, additional transitions with large two-photon cross-sections are found in the 500-700 nm region arising from near-resonant enhancement, as revealed by few-state model analysis. Multiphoton excitation of fluorescent proteins in this spectral region can lead to relevant application for bioimaging.     

不同类型二苯乙烯衍生物双光子吸收截面的理论研究

用AM1和INDO/CI理论方法,系统研究不同类型二苯乙烯衍生物(D-π-D,A-π-A,D-π-A)的结构和电子光谱。在正确的UV-vis光谱基础上,预测了双光子吸收峰的位置,用SOS公式计算了三阶非线性光学系数及双光子吸收截面,并从微观上探讨了不同取代基对双光子吸收截面的影响。     

Theory of two-photon absorption in poly(diphenyl) polyacetylenes

In this paper, we present a theoretical study of the nonlinear optical response of the newly discovered conjugated polymer poly(diphenyl)polyacetylene (PDPA). In particular, we compute the third-order nonlinear susceptibility corresponding to two-photon absorption process in PDPA using: (a) independent-particle Hückel model, and (b) using the correlated-electron Pariser-Parr-Pople (P-P-P) model coupled with various configuration-interaction methodologies such as the singles-configuration-interaction (SCI), the multi-reference-singles-doubles CI (MRSDCI), and the quadruples-CI (QCI) method. At all levels of theory, the polymer is found to exhibit highly anisotropic nonlinear optical response, distributed over two distinct energy scales. The low-energy response is predominantly polarized in the conjugation direction, and can be explained in terms of chain-based orbitals. The high-energy response of the polymer is found to be polarized perpendicular to the conjugation direction, and can be explained in terms of orbitals based on the side phenylene rings. Moreover, the intensity of the nonlinear optical response is also enhanced as compared to the corresponding polyenes, and can be understood in terms of reduced optical gap.     

Synthesis and photophysical properties of novel pyrimidine-based two-photon absorption chromophores

Three new two-photon absorption chromophores based on a pyrimidine core were synthesized by Aldol condensation in the absence of any organic solvents. Their single-photon spectroscopic characterization as well as their two-photon absorption properties is reported. In addition, strong modulation of single-photon and two-photon fluorescent spectra of these molecules by (de)protonation is also discussed.     

Enhanced two-photon absorption of novel four-branched chromophore via vibronic coupling

A novel four-branched chromophore TOZ-4 with starburst linker was synthesized and showed two-photon absorption cross-section (δ) as large as 5254 GM, which was principally resulted from vibronic coupling enhancement.     

Novel 5-(oligofluorenyl)-1,10-phenanthroline type ligands: synthesis, linear and two-photon absorption properties

The synthesis and characterization of new 1,10-phenanthroline-based chromophores LT1, LT2 and LD1 featuring fluorene unit(s) are reported. Their absorption and emission as well as their two-photon absorption properties in the 450-650 nm spectral range are discussed in comparison with the parent 1,10-phenanthroline and already described ligands L1 and L2.