Simultaneous control of emission localization and two-photon absorption efficiency in dissymmetrical chromophores

C. K's secondary address for this work: CNRS UMR6082 FOTON, INSA de Rennes, 20 avenue des Buttes de Coësmes, CS 70839, 35708 RENNES cedex 7, FranceWe thank E. Leroux for technical assistance in the synthesis, S. Soualmi in electrochemical mesaurements and M. H. V. Werts for help in the TPEF measurements.International audienceThe aim of the present work is to demonstrate that combined spatial tuning of fluorescence and two-photon absorption (TPA) properties of multipolar chromophores can be achieved by introduction of slight electronic chemical dissymmetry. In that perspective, two model series of structurally related chromophores have been designed and investigated. One is based on rod-like quadrupolar chromophores bearing either two identical or different electron-donating (D) end groups and the other on three-branched octupolar chromophores built from a trigonal donating moiety bearing identical or different acceptor (A) peripheral groups. The influence of the electronic dissymmetry is investigated by combined experimental and theoretical studies of the linear and nonlinear optical properties of dissymmetrical chromophores compared to their symmetrical counterparts. In both types of systems (i.e., quadrupoles and octupoles), experiments and theory reveal that excitation is essentially delocalized and that excitation involves synchronized charge redistribution (i.e., concerted intramolecular charge transfer) between the different D and A moieties within the multipolar structure. In contrast, the emission stems only from a particular dipolar subunit bearing the strongest D or A moiety due to fast excitation localization after excitation, prior to emission. Hence, control of emission characteristics (polarization and emission spectrum), can be achieved, in addition to localization, by controlled introduction of electronic dissymmetry (i.e., replacement of one of the D or A end-groups by a slightly stronger D′ or A′ unit). Interestingly, slight dissymmetrical functionalization of both quadrupolar and octupolar compounds does not lead to significant loss in TPA responses and can even be beneficial due to the spectral broadening and peak position tuning that it allows. This study thus reveals an original molecular engineering route allowing TPA enhancement in multipolar structures, due to concerted core-to-periphery or periphery-to-core intramolecular charge redistribution upon excitation, while providing for control of emission localization. Such a route could be extended to more intricate (dendritic) and multipolar (3D) systems

Ingénierie de fluorophores pour l'absorption à deux photons et applications

L absorption à deux photons (ADP) connaît aujourd hui un engouement grandissant dû aux nombreuses applications que présente ce phénomène dans des domaines variés (imagerie biologique, limitation optique, microfabrication...). Après une description de l ADP, des avantages de ce processus et des différents absorbeurs à deux photons déjà existants, ce travail décrit la mise en place d une ingénierie d octupôles permettant d accéder à des fluorophores dont l émission de fluorescence peut être modulée du bleu au rouge et présentant une forte excitabilité à deux photons dans la gamme 700 1000 nm. Des relations structure-propriétés ont été établies ce qui a permis une optimisation de ces systèmes octupolaires. L utilisation des fluorophores synthétisés en imagerie biologique et dans des matériaux de type sol-gel sera aussi discutée. Enfin, la dernière partie de ce travail a permis d accéder à des fluorophores optimisés pour l'absorption à deux photons dérivés du motif dipyrrométhène de bore.Two-photon absorption (TPA) has attracted growing interest over recent years owing to its applications in various fields such as biological imaging, optical limitation, as well as 3D microfabrication. After introducing the field of TPA, the advantages of this process and the main two-photon absorbing fluorophores, this work describes the engineering of octupolar compounds, leading to a modular emission from the blue to the red region and to an enhancement of the two-photon fluorescence in the 700-1000 nm range. Relationships between structures and properties have been established and allowed to optimize these octupolar systems. The use of these fluorophores in biological imaging and in sol-gel materials is discussed. The last part of the work deals with a new concept of two-photon probes based on the association of a two-photon absorbing core with peripheral boron dipyrromethene emitting moieties.RENNES1-BU Sciences Philo (352382102) / SudocSudocFranceF

Towards “smart” multiphoton fluorophores: strongly solvatochromic probes for two-photon sensing of micropolarity

International audienceNew fluorophores, combining broad, very high two-photon absorption in the near-infrared region with a marked dependence of their emission spectra on solvent polarity, have been designed as model probes for two-photon sensing of the chemical environment

Branching of dipolar chromophores: effects on linear and nonlinear optical properties

C. Katan present address: CNRS UMR6082 FOTON, INSA de Rennes, 20 avenue des Buttes de Coësmes, CS 70839, 35708 RENNES cedex 7, FranceInternational audienceStructurally related chromophores of different symmetry (dipolar, V-shaped, octupolar) are investigated and compared for elucidation of the combined role of branching and charge symmetry on absorption, photoluminescence and two-photon absorption (TPA). Their design is based on the assembly of one, two or three -conjugated dipolar branches on a central core. Two series of branched structures obtained from a central triphenylamine core and dipolar branches having different charge-transfer characters are investigated: photophysical properties are studied and TPA spectra are determined through two-photon excited fluorescence experiments using fs pulses in the 700-1000 nm range. Calculations based on time-dependent quantum-chemical approaches, as well as the Frenkel exciton model, complement experimental findings. Experiments and theory reveal that a multidimensional intramolecular charge transfer takes place from the central electron-donating moiety to the periphery of the branched molecules upon excitation, whereas fluorescence stems from a dipolar branch. Symmetry and inter-branch electronic coupling are found to be responsible for amplification of the TPA response of branched compounds with respect to their monomeric analogues. In particular, an enhancement is observed in regions where the TPA bands overlap, and TPA activation is obtained in spectral regions where the dipolar analogue is almost two-photon transparent. Thus, appropriate tuning of the number of branches, of the coupling between them, and modulation of intramolecular charge transfer from core to periphery open the way for substantial improvement of TPA efficiency or TPA induction in desired spectral regions