A study of a group of compounds based on the 1,4-bis(phenylethynyl)benzene (1) architecture was undertaken to
improve our understanding of their photophysics and the factors which control their geometry and hence the π-
conjugation pathway in the ground and excited state of these compounds. 1 exists as a range of molecular rotamers in
the ground state, resulting from the low barrier to rotation around their C(sp)-C(sp2) bonds. These compounds are
highly conjugated systems with good electron conducting properties, due to delocalisation of the HOMO and LUMO
over the molecule. In the electronic excited state they are capable changing their molecular conformation and will adopt
a planar, or near planar, low energy conformation prior to fluorescence emission in solution. In a glassy matrix at 77 K
with sterically hindering substituents on the benzene rings of 1, emission form high and low energy conformations are
observed. 1 is highly emissive owing to the high oscillator strength of the S1→S0 transition. All the compounds studied
maintained their C≡C character in the excited singlet and triplet states. The substitution of the central benzene ring in 1
with a thiophene moiety increases the singlet oxygen generation quantum yield, which is consistent with greater
intersystem crossing to the triplet excited state.
Molecular two-photon absorption (TPA) has attracted a lot of interest over recent years due to the many applications it offers both in biological imaging and in material science, constantly needing new optimized molecules with large TPA cross-sections. Various structures and functional groups have been studied; however, the use of electron-withdrawing boron groups has not been fully examined yet. As such compounds are known to lead to interesting photoluminescence and nonlinear optical (NLO) properties, we have investigated the TPA properties of a novel series of A-π-A quadrupoles, based on dimesitylborons as acceptor end-groups. Our experimental study reveals that intramolecular charge transfer is a crucial point in these TPA fluorophores, and can be modulated via changing the planarity of the molecule. We have obtained such planar molecules using vinylene spacers, which can release the steric hindrance close to the dimesitylboron end-group. The series of NLO-phores described here is promising for optical power limiting, with excellent TPA/transparency trade-off, and the work has highlighted that perfluorophenylene could be a key component for the future of TPA.
A series of structurally-related multipolar chromophores of different symmetry (dipolar, quadrupolar, octupolar, dendritic...), and shape (rod-like, Y-shaped...) propeller-shaped, were investigated for optical power limiting based on multiphoton absorption processes. Their design is based on the functionalization of nanoscale linear or branched conjugated backbones with electro-active (i.e. electron-releasing or electron-withdrawing) peripheral and core/node groups. Their two-photon absorption (TPA) spectra were determined by investigating their two-photon-excited fluorescence properties in the NIR region using pulsed excitation in the femtosecond regime. These studies provide evidence that the charge symmetry plays an important role, the quadrupolar chromophores leading to giant TPA cross-sections in the visible red. Furthermore, modulation of the nonlinear absorptivity/transparency/photostability trade-off can be achieved by playing on the nature of the electroactive groups and of the spacers. Interestingly, higher-order charge symmetries and branched structures provide an innovative route for TPA amplification and/or spectral broadening in the NIR region.
Two-photon fluorescence polarisation and stimulated emission depletion dynamics are investigated in three high two-photon cross-section push-push polyenes: OM62, LP79 and OM77 and compared to the behaviour of a standard fluorophore (rhodamine 6G). Two-photon fluorescence anisotropy measurements (R(0) and Omega) were undertaken using picosecond time-correlated single photon counting (TCSPC). For OM62 and LP79 these are consistent with a diagonal two-dimensional transition tensor with SXX>SYY. For OM77 the contribution of off-diagonal elements (SXY & SYX) appears significant. Two-photon fluorescence anisotropy decay data is combined with streak camera measurements of excited state population depletion to determine stimulated emission cross-sections and ground state vibrational relaxation times. Cross-sections for STED in all three polyenes were found to be significantly higher than those for rhodamine 6G. The efficiency of STED is however dependent on the degree to which the S1→S0 transition is saturated by the DUMP pulse; this is mediated by fast ground state vibrational relaxation. Of the three polyenes, LP79 is seen to combine a large stimulated emission cross-section (c.a. 13σ(r6G)) with rapid ground state relaxation (τR=240fs).
Nowadays, it seems evident that a unique nonlinear optical (NLO)material cannot offer simultaneously linear transparency,colour neutrality and broadband optical limiting efficiency at the performance levels required for sensor and eye protection against all laser threats.Several combinations of NLO materials were investigated last few years, including multicell or multilayer geometries.
The approach presented here combines multiphoton absorption with nonlinear scattering. For that purpose, singlewall carbon nanotubes are suspended in various solutions of multiphoton absorbing chromophores. Such combinations allow us to obtain optical limiters of high linear transmittance and excellent colour neutrality. Broadband optical limiting is expected from the association of these two broadband materials,and enhanced optical limiting efficiency is expected from cumulative effects in the nanosecond regime.
We report here on the optical limiting studies performed with nanosecond laser pulses on several families of multiphoton absorbers in chloroform,with carbon nanotubes suspended in the solutions. The performances of these samples are compared with those of simple multiphoton absorber solutions and carbon nanotube suspensions, and the differences observed are interpreted in terms of cumulative NLO effects and adverse aggregation phenomenon. Ways to optimise stability of the suspensions are also experimented and discussed.
Novel conjugated chromophores were designed and investigated for optical power limitation based on multiphoton absorption processes. Their design is based on the push-push functionalization of a semi-rigid elongated system derived from the extension of biphenyl cores. Biphenyl moieties with tunable twist angle were examined. Phenylene-vinylene rods were selected as connecting spacers between the core and the electroactive end groups to ensure effective electronic conjugation while maintaining suitable transparency. These derivatives combine wide linear transparency and enhanced nonlinear absorptivities in the visible range. Pump-probe Kerr ellipsometry indicates large excited-state absorption cross-sections (with typical σe values of 5 10-16 cm2) while nanosecond nonlinear transmission measurements and optical limitation experiments reveal very strong nonlinear absorption that can be fitted by a three-photon absorption process (leading to α3 values up to 18000 cm3 GW-2). Such behavior results from a sequential multiphoton process involving excited-state absorption subsequent to two-photon excitation (with typical σ2 values of 5 10-20 cm4 GW-1). Both the linear transparency, the photostability and the nonlinear absorption spectral characteristics of these derivatives can be tuned by playing on the biphenyl twist angle. As a result, chromophores combining good linear transparency and enhanced nonlinear absorptivities in the visible range have been obtained.
Our aim has been the design of optimized NLO-phores with very high two-photon absorption (TPA) cross-sections (s2) in the red-NIR region, while maintaining high linear transparency and high fluorescence quantum yield. Our molecular engineering strategy is based on the push-push or pull-pull functionalization of semi-rigid nanoscale conjugated systems. The central building blocks were selected as rigid units that may assist quadrupolar intramolecular charge transfer by acting either as a (weak) donor or acceptor core. Quadrupolar molecules derived either from a phenyl unit, a rigidified biphenyl moiety or a fused bithiophene unit have been considered. Conjugated oligomers made of phenylene-vinylene and/or phenylene-ethynylene units were selected as connecting spacers between the core and the electroactive end groups to ensure effective electronic conjugation while maintaining suitable transparency/fluorescence. The TPA cross-sections were determined by investigating the two-photon-excited fluorescence properties using a Ti:sapphire laser delivering fs pulses. Both the nature of the end groups and of the core moiety play an important role in determining the TPA spectra. In addition, by adjusting the length and nature of the conjugated extensor, both amplification and spectral tuning of TPA cross-sections can be achieved. As a result, push-push fluorophores which demonstrate giant TPA cross-sections (up to 3000 GM) in the visible red, high fluorescence quantum yields and good transparency in the visible range have been obtained.
Novel microscopies based on nonlinear optical (NLO) phenomena are attracting increasing interest in the biology community owing to their potentialities in the area of real-time, non-damaging imaging of biological systems. In particular, second-harmonic generation (SHG) and two-photon excited fluorescence (TPEF) are NLO phenomena that scale with excitation intensity squared, and thus give rise to an intrinsic 3-dimensional resolution when used in microscopic imaging. In this perspective, we have implemented a molecular engineering approach toward NLO-probes specifically designed for SHG and/or TPEF imaging of cellular membranes. We have designed nanoscale rod-like fluorophores showing very large TPEF cross-sections in the visible red, outperforming standard fluorophores such as fluorescein by up to two orders of magnitude. Bolaamphiphilic derivatives combining high TPEF cross-sections and affinity for cellular membranes were prepared. Their incorporation into model or cell membranes can be monitored by TPEF microscopy. Amphiphilic push-pull chromophores showing both high TPA and SHG cross-sections in the near-IR region were designed as NLO-probes for imaging of biological membranes by simultaneous SHG and TPEF microscopy. These NLO-phores offer intriguing potentialities for imaging of fundamental biological processes such as adhesion, fusion or for reporting of membrane electrical potentials.
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