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.     

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.     

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.     

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.     

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.     

Aromatic core-modified twisted heptaphyrins[1.1.1.1.1.1.0]: syntheses and structural characterization

[reaction: see text] First examples of aromatic core-modified twisted heptapyrrolic systems with six meso links have been obtained via a general [3 + 4] acid-catalyzed condensation method. Despite the twisted conformation, these heptaphyrins exhibit aromaticity in solution, unlike most of the all-aza analogues.     

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.     

Enhanced two-photon absorption and ultrafast dynamics of a new multibranched chromophore with a dibenzothiophene core

A series of novel compounds with dibenzothiophene core branched structures have been synthesized, and their two-photon absorption (TPA) properties were investigated. Two-photon fluorescence (TPF) and z-scan techniques were carried out, and a significant enhancement in the TPA cross section was observed for ST-G2, which possesses the largest generation number among the studied samples. By using different solvents, the largest nonlinear optical (NLO) response was observed in the most polar solvent. Ultrafast pump-probe experiments were performed to probe the excited state dynamics in the branched molecules, and the obtained results further confirmed the TPA enhancement mechanism. Time-resolved fluorescence (TRFL) and TRFL anisotropy measurements reveal that there is an ultrafast charge localization to the intramolecular charge transfer (ICT) state followed by relaxation with a lifetime longer than 1 ns.     

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.     

Meso-meso linked core modified 22pi smaragdyrins with unusual absorption properties

The synthesis of the first meso-meso linked core modified smaragdyrins with unusual single photon properties is reported.     

Y-shaped two-photon absorbing molecules with an imidazole-thiazole core

Two new classes of two-photon absorbing Y-shaped molecules have been developed to possess an imidazole-thiazole core and a stilbene-type conjugation pathway with either nitro or sulfonyl as terminal electron-accepting group.     

Two novel terpyridine-based chromophores with donor-acceptor structural model containing modified triphenylamine moiety: Synthesis, crystal structures and two-photon absorption properties

Two novel terpyridine-based chromophores with D-A (D = donor, A = acceptor) structural model containing modified triphenylamine moiety (L1 and L2 ) have been conveniently synthesized via formylation and reduction in satisfactory yields, and fully characterized. The single crystals of them were obtained and determined by X-ray diffraction analysis. The relationships between structure and photophysical properties of the two chromophores were investigated both experimentally and theoretically. The measured maximum TPA cross-sections per molecular weight (δmax /MW) of the chromophores are 0.63 GM/(g mol) (L1) and 0.72 GM/(g mol) (L2), respectively, in DMF as a high polar solvent. The results indicate that the value of δmax/MW could be well tuned by the intramolecular charge transfer (ICT), which could be realized by introducing additional elecron-donor/acceptor groups.     

Two-photon absorption strength: a new tool for the quantification of two-photon absorption

In this paper, we define the two-photon absorption strength, a new characterization tool, similar to the oscillator strength, but for two-photon absorption. It allows the quantification of the two-photon absorption properties of molecular systems which are one-photon transparent. Its definition is such that the corresponding numerical values are around 100 for small molecules. We also show that this new theoretical tool allows the direct comparison of experimental and theoretical data without requiring the introduction of any arbitrary band width. As an example, the experimental and theoretical (AM1+CNDOS and HF+CIS3-21G) two-photon absorption properties of the 2,2'-bi(9,9-dihexylfluorene) molecule are compared.     

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.     

Design of dipicolinic acid ligands for the two-photon sensitized luminescence of europium complexes with optimized cross-sections

The multistep synthesis of an extensive series of push-pull donor-pi-conjugated dipicolinic acid ligands is described. The charge transfer character of the ligand can be tuned by changing the donor group (CH 2R, OR, SR, or NR 2) or the nature of the conjugated backbone (phenyl, phenylethynyl, naphtylethynyl, bis(phenylethynyl), or chalcone). The photophysical properties of related D 3 symmetric europium complexes (absorption and luminescence) were measured. Experiments using two-photon sensitized luminescence of a Eu (III) complex reveal large two-photon absorption (TPA) cross-section values (775 GM at 740 nm) in dichloromethane. Furthermore, some structure-property relationships can be derived from this systematic study, allowing an optimization of TPA properties of lanthanide complexes.     

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.     

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.     

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.