Water-Soluble Paracyclophane Fluorophores with Large Two-Photon Action Cross Sections
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I9.42.1
Water-Soluble Paracyclophane Fluorophores with Large Two-Photon Action Cross Sections Han Young Woo, Dmitry Korystov, Alexander Mikhailovsky and Guillermo C. Bazan Mitsubishi Chemical Center for Advanced Materials, Department of Materials, Institute for Polymers and Organic Solids, University of California, Santa Barbara, California 93106, USA ABSTRACT A series of tetradonor substituted [2.2]paracyclophane-based two-photon absorption (TPA) fluorophores were designed, synthesized and characterized. Different substituents were chosen to modulate the conjugation and the donor strength and to allow the molecules to be neutral (N series, organic-soluble) or charged (C series, water-soluble). The overall set of compounds series allows us to examine the correlation among the solvent polarity, donor ability of the substituents, and TPA action cross section (ηδ, where δ is the TPA cross section and η is the quantum yield). In water, a significant drop of both δ and η is observed, approximately one-third relative to those in toluene. Weaker donors and/or weaker acceptors with enhanced conjugation are a better choice due to a minimized quenching related to strong charge transfer (CT) upon excitation. One thus needs to fine-tune the magnitude of CT and find an optimal balance among CT, δ and η. INTRODUCTION Recently, two-photon fluorescence microscopy (TPM) is attracting much attention as a powerful 3D imaging tools for biological systems [1]. The intrinsic characteristics of nonlinear excitation provide three-dimensional spatial resolution, deep penetration, and ability to image turbid samples and avoid out-of focus background and photodamage. For the best TPM performance, the two-photon action cross section, defined as ηδ, should be as large as possible. The term δ is the TPA cross section (expressed in GM = 1 x 10-50 ⋅ cm4 ⋅ s ⋅ photon-1 ⋅ molecule-1), which provides the probability of TPA at a particular frequency. A large fluorescence quantum yield (η) is also desired for better signal to noise. Currently, most of the fluorophores being used for TPM are those that have been developed for one-photon excitation, and they have relatively small values of ηδ, in the range of 1 ~ 50 GM [2]. TPM-specific fluorophores are required for better resolution and reduced photodamage. We note that recent molecular guidelines for the design of molecules with large TPA is to induce strong intramolecular charge transfer (CT) of the excited states upon excitation [3]. However, in polar aqueous media required for biological samples, the strong CT leads to a reduced η and lower overall ηδ [4]. In this contribution, we report a synthetic entry into water-soluble TPA chromophores based on [2.2]paracyclophane (pCp) which show exceptionally large ηδ values. We also provide the synthesis of the organic-soluble counterparts and the overall set of compounds series allows the examination of TPA behavior in solvents with different polarities. Finally, we discuss the molecular design strategy to obtain large ηδ values in water.
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