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.     

Rational design of small indolic squaraine dyes with large two-photon absorption cross section

Small organic dyes with large two-photon absorption (TPA) cross sections (δ) are more desirable in many applications compared with large molecules. Herein, we proposed a facile theoretical method for the fast screening of small organic molecules as potential TPA dyes. This method is based on a theoretical analysis to the natural transition orbitals (NTOs) directly associated with the TPA transition. Experimental results on the small indolic squaraine dyes (ISD) confirmed that their TPA cross sections is strongly correlated to the delocalization degree of the NTOs of the S₂ excited states. Aided by this simple and intuitive method, we have successfully designed and synthesized a small indolic squaraine dye (ISD) with a remarkable δ value above 8000 GM at 780 nm. The ISD dye also exhibits a high singlet oxygen generation quantum yield about 0.90. The rationally designed TPA dye was successfully applied in both two-photon excited fluorescence cell imaging and in vivo cerebrovascular blood fluid tracing.     

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.     

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.     

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.     

Electronic and vibronic contributions to two-photon absorption in donor-acceptor-donor squaraine chromophores

Many squaraines have been observed to exhibit two-photon absorption at transition energies close to those of the lowest energy one-photon electronic transitions. Here, the electronic and vibronic contributions to these low-energy two-photon absorptions are elucidated by performing correlated quantum-chemical calculations on model chromophores that differ in their terminal donor groups (diarylaminothienyl, indolenylidenemethyl, dimethylaminopolyenyl, or 4-(dimethylamino)phenylpolyenyl). For squaraines with diarylaminothienyl and dimethylaminopolyenyl donors and for the longer examples of 4-(dimethylamino)phenylpolyenyl donors, the calculated energies of the lowest two-photon active states approach those of the lowest energy one-photon active (1B(u)) states. This is consistent with the existence of purely electronic channels for low-energy two-photon absorption (TPA) in these types of chromophores. On the other hand, for all squaraines containing indolinylidenemethyl donors, the calculations indicate that there are no low-lying electronic states of appropriate symmetry for TPA. Actually, we find that the lowest energy TPA transitions can be explained through coupling of the one-photon absorption (OPA) active 1B(u) state with b(u) vibrational modes. Through implementation of Herzberg-Teller theory, we are able to identify the vibrational modes responsible for the low-energy TPA peak and to reproduce, at least qualitatively, the experimental TPA spectra of several squaraines of this type.     

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.     

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.     

Novel triphenylamine-based two-photon absorption dyes including benzophenone parts

New two-photon absorption dyes including benzophenone parts,p-N,N-diphenylamino-p'-phenacylstilbene(C1),4,4-di-(4- benzoylstyrene)yltriphenylamine(C2) and 4,4',4-tri(4-benzoylstyrene)yltriphenylamine(C3),were synthesized and characterized by ~1H NMR and elemental analysis.The ratio of the molar extinction coefficients of compounds was approximately equal to the ratio of the branch amount of the compounds,namely,C1:C2:C3 was 1:2:3.Extremely high fluorescence quantum yields were detected for these compound...     

Triphenylamine derivatized phenylacetylene macrocycle with large two-photon absorption cross-section

A phenylacetylene macrocycle (PAM) derivative containing triphenylamine as the framework was synthesized in one-step Sonogashira coupling. The photophysical and electrochemical properties were investigated in details. This hexamer shows significant enhancement in two-photon absorption cross-section relative to reported PAM derivatives.     

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.     

Strongly red-fluorescent novel donor-pi-bridge-acceptor-pi-bridge-donor (D-pi-A-pi-D) type 2,1,3-benzothiadiazoles with enhanced two-photon absorption cross-sections

Novel donor-pi-bridge-acceptor-pi-bridge-donor (D-pi-A-pi-D) type 2,1,3-benzothiadiazole fluorescent dyes connected to the N,N-diarylamino terminus via various type pi-conjugate spacers exhibits large two-photon absorption cross-sections and high fluorescent quantum yields in orange-red color.     

New cyano-substituted organic dyes containing different electrophilic groups: aggregation-induced emission and large two-photon absorption cross section

Three aggregation-induced emission active dyes (3a–c) were synthesized and their one- and two-photon absorption properties have been investigated. They were all found to be weakly fluorescent in THF solution, while they exhibited dramatic fluorescence enhancement in water/THF mixtures. The solid fluorescence of 3a–c was recorded and their fluorescence quantum efficiency (Φ_F) values were determined to be 8.0%, 8.1%, and 16.4%, respectively. Moreover, the two-photon absorption (2PA) cross-sections (σ) of 3a–c were measured and 3a showed the highest value of 702 GM. The excellent aggregation-induced emission and 2PA properties provide a promising alternative for biophotonic materials.     

60]Fullerene metal complexes with large effective two-photon absorption cross-section

Fourteen novel complexes of fullerene C(60) with metal dialkyldithiophosphate, {[(RO)(2)PS(2)](2)M}·2C(60) (R = Me and Et; M = Mn, Fe, Co, Ni, Cu, Zn and Pd), can be obtained in high yield by the reaction of metal(II) dialkyldithiophosphate complexes with C(60) fullerene. Their structures are determined by (1)H, (13)C, and (31)P NMR spectroscopy, elemental analysis, FT-IR and UV-vis, and are supplied by single crystal X-ray data. The compound {[(EtO)(2)PS(2)](2)Pd}·2C(60) was also studied by the HRTEM image and SAED patterns. Studies of the optical properties of the complexes of fullerene C(60) with metal dialkyldithiophosphate show that these compounds all have very strong nonlinear optical absorption effects. The nonlinear absorption of fourteen complexes of fullerene C(60) with metal dialkyldithiophosphate (in o-dichlorobenzene and in the solid state) was measured by the open-aperture Z-scan technique at a wavelength of 532 nm. DFT calculations are also used to discuss the stability of these complexes and to confirm the structural assignments.     

Aggregates of quadrupolar dyes: giant two-photon absorption from biexciton states

A model for aggregates of quadrupolar (DAD or ADA) molecules is presented that relaxes the dipolar approximation for intermolecular electrostatic interactions. New effects, including the appearance of bound biexcitons in clusters of nonpolar molecules, are predicted with interesting and unforeseen consequences on the material properties. Specifically, we show that the large two-photon absorption cross-section, typical of quadrupolar chromophores, can be further amplified by orders of magnitude as a result of aggregation.     

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.     

Synthesis of C60-diphenylaminofluorene dyad with large 2PA cross-sections and efficient intramolecular two-photon energy transfer

The first, highly two-photon active C60 derivative comprised of a A-sp3-D conjugate structure was synthesized showing effective two-photon absorption cross-sections (sigma 2' = 196 x 10(-48) cm4 sec-1 molecule-1) in the nanosecond regime among the best values for diphenylaminofluorene-based AFX chromophores.