Design and synthesis of core and peripherally functionalized with 1,8-naphthalimide units fluorescent PAMAM dendron as light harvesting antenna

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Abstract

This paper reports on the design, synthesis and spectral characteristics of a novel PAMAM dendron (7), core and peripherally functionalized with 1,8-naphthalimide fluorophores. The novel compound 7 was configured as light harvesting antenna where the system surface is functionalized with “donor” dyes (blue emitting 1,8-naphthalimides) that are capable of absorbing light and efficiently transferring the energy to a single “acceptor” dye (yellow-green emitting 1,8-naphthalimide) in the focal point of the dendron. The overlap between the emission of the donor and the absorbance of the acceptor was more than 95%. As a result of the energy transfer, the blue emission intensity of the periphery in the donor–acceptor system was decreased with 93%, while the yellow-green core fluorescence enhancement (λex = 360 nm) of the system was more than 26 times with respect to the fluorescence intensity of the comparative yellow-green emitting 1,8-naphthalimide. This indicates efficient energy transfer between the donor and acceptor dye fragments and that the novel compound 7 would be able to act as a highly efficient light harvesting antenna.

Introduction

Molecular systems capable of light harvesting and energy transfer are currently of great interest [1]. Progress in the study of natural photosynthetic systems has provided the impetus to design artificial light harvesting assemblies based on a variety of architectures. For instance, numerous reports on organic [2], organometallic [3], supramolecular [4], and polymeric [5] chromophore assemblies capable of acting as light harvesting antennae have appeared in the literature. More recently, dendritic light harvesting assemblies have also attracted much attention [6].

Dendrimers are well-defined macromolecules exhibiting a three-dimensional structure that is roughly spherical or globular. A characteristic of dendritic macromolecules is the presence of numerous peripheral chain ends that all surround a single core. The globular shape of dendrimers provides a large surface area that can be decorated with the chromophores. In this context, labeling of dendritic macromolecular architectures with fluorophore units is one of the viable routes of generating suitable luminescent dendritic macromolecules having well-defined branched and compartmentalized structures [7]. Particularly useful for this purpose are solvatochromic fluorophores such as 1,8-naphthalimide derivatives. Because of their strong fluorescence and good photostability, the 1,8-naphthalimide derivatives have found application in a number of areas including coloration of polymers [8], laser active media [9], potential photosensitive biologically units [10], fluorescent markers in biology [11], analgesics in medicine [12], light emitting diodes [13], photoinduced electron transfer sensors [14], fluorescence switchers [15], electroluminescent materials [16], liquid crystal displays [17] and ion probes [18].

Constructing fluorophore-terminated amidoamine branches around a luminescent group could profitably alter the luminescence signals in the macromolecular structure and amplify the signals for sensing purposes. Our first investigations on the synthesis and photophysical properties of polyamidoamine (PAMAM) derivatives comprising 1,8-naphthalimide units in their periphery were published before [19]. In this paper, we report the synthesis of a novel light harvesting antenna (7) by both core and peripherally fluorescent functionalization of a PAMAM dendron together with the photophysical and fluorescence signaling properties of the system (Scheme 1).

In this system (Scheme 1), the dendron surface is functionalized with “donor” dyes (blue emitting 4-allyloxy-1,8-naphthalimide fluorophores) that are capable of absorbing light and efficiently transferring the energy to a single “acceptor” dye (yellow-green emitting 1,8-naphthalimide) in the focal point of the dendron, that is, dendron branches act as the “molecular lens”.

In order to receive a more complete comparative picture for the influence of both the branched (core) and peripheral fluorophores in the molecule of the light harvesting antenna 7 on its photophysical properties, the previously synthesized yellow-green emitting amidoamine-dendronized 1,8-naphthalimide 5 [20] and the blue emitting 4-allyloxy-N-n-butyl-1,8-naphthalimide 8 [21] were involved in the present study as reference compounds (Scheme 2).

Section snippets

Materials

The starting 4-bromo-1,8-naphthalic anhydride 1 was prepared according to the reported procedure [22]. Amidoamine-functionalized 1,8-naphthalimide 5 [20] and 4-allyloxy-N-n-butyl-1,8-naphthalimide 8 [21] were prepared as described before. Butylamine, ethylenediamine, allyl alcohol and methyl acrylate (Fluka, Merck), p.a. grade, were used without purification. All solvents (Fluka, Merck) were pure or of spectroscopy grade.

Methods

FT-IR spectra were recorded on a Varian Scimitar 1000 spectrometer at 2 cm−1

Design and synthesis of light harvesting antenna

The core and peripherally functionalized PAMAM dendron (7) was designed as fluorescent light harvesting antenna. It is well known that absorption and fluorescence characteristics of the 1,8-naphthalimide derivatives depend on the nature of the substituent at C-4 position of the 1,8-naphthalimide ring. 4-Alkylamino-1,8-naphthalimides are yellow-green fluorophores with maximal absorption in the blue region at about λA = 430–440 nm [12], [17], [20], [24], [24]. In contrast,

Conclusions

A novel amidoamine dendron 7, core and peripherally functionalized with 1,8-naphthalimide fluorophores, was synthesized for the first time based on a convergent approach. The novel compound 7 was designed as light harvesting antenna capable of absorbing light by its periphery and efficiently transferring the energy to a single acceptor dye in the focal point of the system. Absorption and fluorescence characteristics of the donor–acceptor system were determined and discussed. The overlap between

Acknowledgements

This work was supported by the National Science Foundation of Bulgaria (project VU-X-201/06). Vladimir Bojinov and Nikolai Georgiev also acknowledge the Science Foundation at the University of Chemical Technology and Metallurgy (Sofia, Bulgaria).

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