TPA-active D–π–D fluorophores with rigid, planar cores from phenylene to indenofluorene and indolocarbazole

Abstract

A series of D–π–D quadrupolar fluorophores with systematically varied π-conjugated cores were prepared using the Heck reaction and their single- and two-photon photophysical properties were examined. Introduction of extended π-conjugated cores such as indenofluorene/indolocarbazole was found to induce only small shifts in the absorption and emission maxima versus the phenylene linked fluorophore but greatly enhance their single- and two-photon absorption cross-sections. The effects of positional substitution on the properties of the indolocarbazole derivatives were also investigated.

Introduction

Since the pioneering work of Parthenopoulos and Rentzepis [1] and Webb and co-workers [2] on three-dimensional optical data storage and two-photon laser scanning fluorescence microscopy, molecules with strong two-photon absorption (TPA) cross-sections (δ₂) have attracted growing interest [3], [3](a), [3](b), [3](c), [3](d), [3](e). Many chromophores with a large variety of structures have been synthesized and had their TPA cross-sections measured, and fundamental efforts have been made to elucidate structure-property relationships in these systems. Among them, D–π–D and A–π–A one-dimensional quadrupolar systems have been successfully employed to enhance δ₂ values. The quadrupolar intramolecular charge transfer taking place between either the electron-donating (D) or electron-accepting (A) end-groups and the π-conjugated cores is believed to be an important factor in this enhancement [4], [4](a), [4](b). Further efforts to introduce stronger acceptors or donors as well as elongated cores to enhance this charge transfer have confirmed this assumption [5]. Very large δ₂ values have also been obtained with D–A–D, A–D–A, D–A–D–A–D, and A–D–A–D–A systems [3], [3](a), [3](b), [3](c), [3](d), [3](e), [6], [6](a), [7], [7], [8], [8](a). However, this success is often at the cost of reduced fluorescence quantum yield and/or pronounced red-shift of the absorption and emission bands. Blanchard-Desce and co-workers recently reported a systematic study on the structure-property relationships of various quadrupolar fluorophores with different types of π-conjugated cores. The results of this work indicated that increasing the connector length between D or A groups induces systematic and pronounced red-shifts and hyperchromic effects on the absorption bands [9].

Most applications of TPA and two-photon excited fluorescence (TPEF) require excitation in the near infrared region (700–900 nm), meaning that a molecular engineering approach to enhance the TPA cross section of the materials without inducing large shifts in their absorption bands would be highly desirable. Although D–π–D quadrupolar compound 1 and its fluorene analogue 2 have been exclusively examined as TPA-active compounds [10], [10](a), [10](b), [10](c), [11], little attention has been paid to their spectral similarities, particularly the linear absorption and single-photon excited fluorescence emission spectra in dilute solutions, which span nearly the same regions despite their molecular structural differences. Thus, in the current investigation, we have examined the structure-function relationship of D–π–D systems with respect to lengthening of the π-conjugated core, in particular focusing on extended fluorene and carbazole-based systems.

These systems, compounds 1–6 in Fig. 1, contain highly rigid and planar π-conjugated cores, with solubilizing alkyl groups. Diphenylamino moieties were chosen as the electron-donating end-groups with styryl bridging units to the varying π-conjugated cores. The linking units employed in this study were: 1,4-phenylene (1); 9,9-diethyl-9H-fluoren-2,7-yl (2); 9-n-pentyl-9H-carbazole-2,7-yl (3); 6,6,12,12-tetra-n-butyl-6,12-dihydroindeno[1,2-b]fluoren-2,8-yl (4); 6,12-di-n-octyl-6,12-dihydro-6,12-diazaindeno[1,2-b]fluoren-2,8-yl (5) and 6,12-di-n-octyl-6,12-dihydro-6,12-diazaindeno[1,2-b]fluoren-3,9-yl (6). Indenofluorene and indolocarbazole moieties have been widely employed in organic light emitting diode (OLED) and organic transistor materials, although there are few reports of their use in TPA chromophores [12], [13]. A compound similar to 4 with methyl rather than n-butyl substituents has recently been described in the patent literature and shown to exhibit high electro-luminescent efficiency [14]. Lin and co-workers also reported recently the synthesis and two-photon properties of a multipolar chromophore derived from triphenylamine with three diphenylaminoindenofluorenyl branches [15]. To understand the effects of positional substitution, if any, isomers 5 and 6 were also prepared.