Article pubs.acs.org/JPCB
Photophysical Investigation of Cyano-Substituted Terrylenediimide Derivatives Koen Kennes,† Yannick Baeten,† Tom Vosch,‡ Wouter Sempels,† Stoyan Yordanov,† Sebastian Stappert,§ Long Chen,§ Klaus Müllen,§ Johan Hofkens,† Mark Van der Auweraer,† and Eduard Fron*,† †
Molecular Imaging and Photonics, Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, 3001 Leuven, Belgium ‡ Nano-Science Center/Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark § Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany S Supporting Information *
ABSTRACT: Two new terrylenediimide (TDI) chromophores with cyano substituents in the bay and core area (BCN-TDI and OCN-TDI, respectively) have been characterized by a wide range of techniques, and their applicability for stimulated emission depletion (STED) microscopy has been tested. By cyano substitution an increase of the fluorescence quantum yield and a decrease of the nonradiative rate constant is achieved and attributed to a reduced charge-transfer character of the excited state due to a lower electron density of the TDI core. For BCN-TDI, the substitution in the bay area induces a strong torsional twist in the molecule which, similar to phenoxy bay-perylenediimide (PDI), has a strong effect on the fluorescence lifetime but appears to prevent the aggregation that is observed for OCN-TDI. The single-molecule photobleaching stability of BCN- and OCN-TDI is lower than that of a reference TDI without cyano substitution (C7-TDI), although less so for OCN-TDI. The photophysical properties of the excited singlet state are only slightly influenced by the cyano groups. The observed intense stimulated emission, the pump−dump−probe experiments, and STED single-molecule imaging indicate that STED experiments with the cyano-substituted TDIs are possible. However, because of aggregation and more efficient photobleaching, the performance of BCN- and OCN-TDI is worse than that of the reference compound without cyano groups (C7-TDI). Bay-substituted TDIs are less suitable for STED microscopy.
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used extensively because of their photo stability,13−15 high extinction coefficient,16 and fluorescence emission spectra that can be tuned over the entire visible range with a fluorescence quantum yield often close to unity.17 Together with their good thermal and chemical (when applied to material sciences18−20) stability, rylene dyes with suitable spectroscopic parameters can be ideal candidates for STED microscopy.21 Both the lower homologues (naphthalene-1,8-dicarboximides (NMI) and naphthalene-1,4:5,8-bis(dicarboximides) (NI)22,23) and the higher homologues (perylene-3,4- dicarboximides (PMIs),13,24 perylene-3,4:9,10-bis(dicarboximides) (PDIs),13,23,25 and terrylene-3,4:11,12-bis(dicarboximides) (TDIs)5,26) have been used extensively in artificial energy-transfer structures to mimic the processes occurring in photosynthetic systems.27,28 Moreover, the higher homologues proved to be attractive materials for various solid-state devices like organic field effect transistors (OFET)23 and photovoltaic devices (OPVs).23,29−33 In this study, we present the photophysical characterization of two cyano-substituted terrylenediimide dyes, the bay substituted BCN-TDI and the core substituted OCN-TDI
INTRODUCTION Attempts to overcome the diffraction limit in fluorescence microscopy resulted in a broad spectrum of fascinating methods that are continuously being refined.1−3 The efforts revealed the inner world of cells and have been recently rewarded with a Nobel prize. By simply modulating or switching the ability of a dye to emit fluorescence, it became possible to spatially localize adjacent objects in a sequential approach. For instance, stimulated-emission depletion (STED) microscopy1,4 and its derivatives modulate the fluorescence of dye molecules in space using defined spatial patterns of light, whereas the techniques known as PALM,5,6 STORM,7,8 and related concepts9−12 involve switching individual fluorophores stochastically in time followed by mathematical localization of their position with a resolution of tens of nanometers. All these concepts greatly rely on molecular systems with a series of well-defined and outstanding photophysical and photochemical properties such as large absorption cross section, a high fluorescent quantum yield, and stability under repetitive exposure to intense excitation light. To push forward the field of high-resolution fluorescence microscopy, the synthesis of new dyes with excellent photostability, sufficient brightness, narrow absorption and emission bands, and easy functionalization is of great importance. Rylene dyes have been © XXXX American Chemical Society
Received: October 17, 2014 Revised: November 21, 2014
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dx.doi.org/10.1021/jp5104577 | J. Phys. Chem. B XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry B
Article
Picosecond Fluorescence Time-Resolved Experiments. The fluorescence decay times have been determined by time correlated-single photon counting (TC-SPC) measurements described in detail previously.35 A time-correlated singlephoton-counting PC module (SPC 830, Becker & Hickl) was used to obtain the fluorescence decay histogram in 4096 channels. The decays were recorded with 10 000 counts in the peak channel in two time windows of 20 ns corresponding to 4.9 ps per channel and analyzed globally with a time-resolved fluorescence analysis (TRFA) software.36 The full width at halfmaximum (fwhm) of the instrumental response function (IRF) was typically in the order of 42 ps. The quality of the fits has been judged by the fit parameters χ2 (
