Water-Soluble Naphthalene Diimides as Singlet Oxygen Sensitizers

Abstract

Bromo- and/or alkylamino-substituted and hydrosoluble naphthalene diimides (NDIs) were synthesized to study their multimodal photophysical and photochemical properties. Bromine-containing NDIs (i.e., 11) behaved as both singlet oxygen ((1)O2) photosensitizers and fluorescent molecules upon irradiation at 532 nm. Among the NDIs not containing Br, only 12 exhibited photophysical properties similar to those of Br-NDIs, by irradiation above 610 nm, suggesting that for these NDIs both singlet and triplet excited-state properties are strongly affected by length, structure of the solubilizing moieties, and pH of the solution. Laser flash photolysis confirmed that the NDI lowest triplet excited state was efficiently populated, upon excitation at both 355 and 532 nm, and that free amine moieties quenched both the singlet and triplet excited states by intramolecular electron transfer, with generation of detectable radical anions. Time-resolved experiments, monitoring the 1270 nm (1)O2 phosphorescence decay generated upon laser irradiation at 532 nm, allowed a ranking of the NDIs as sensitizers, based on their (1)O2 quantum yields (ΦΔ). The tetrafunctionalized 12, exhibiting a long-lived triplet state (τ ~ 32 μs) and the most promising absorptivity for photodynamic therapy application, was tested as efficient photosensitizers in the photo-oxidations of 1,5-dihydroxynaphthalene and 9,10-anthracenedipropionic acid in acetonitrile and water.

Big, Strong, Neutral, Twisted, and Chiral π Acids

“…With two distinct, reversible reduction steps, the CVs did not differ in appearance from those of other NDIs. [11,12] The energy of the LUMO with respect to vacuum was calculated by assuming À5.1 eV for Fc + /Fc. The obtained values are summarized in Figure 1.…”

“…[3] Naphthalenediimides (NDIs) [11] are privileged motifs because their intrinsic Q zz value of + 19 B is very high, whereas their LUMO is very low (À4.00 eV), and both can be easily varied by means of core substituents. Popular choices to maximize p acidity are cyano groups [6,7] and sulfones.…”

“…Expansion of their core usually reduces p acidity, because the influence of electronwithdrawing substituents decreases. [11] Moreover, core-expanded NDIs usually do not have twisted and chiral topologies. However, the availability of large, strong, neutral, twisted, and chiral p acids will be the key to further elaborate on the functional relevance of anion-p interactions.…”

“…However, the availability of large, strong, neutral, twisted, and chiral p acids will be the key to further elaborate on the functional relevance of anion-p interactions. In this work, we combine two central p acids, namely, NDIs [11] and perfluoroarenes, [8] through sulfur bridges [12,14] to secure facile access to expanded surfaces of precisely variable topology (chair, boat), chirality (central, axial, planar), p acidity (600 mV), and oriented p-acidity gradients (Figure 1). Sulfur bridges [12,14] were particularly attractive to vary p acidity through simple and robust redox chemistry, to minimize reductive radical formation at high p acidity, [1] and, most importantly, to introduce chiral twisting on the sulfoxide level.…”

Strategies for the Synthesis of Functional Naphthalene Diimides

“…[24] Moreover, such systems were applied in zipper-type aggregates for solar cells with inherent transport gradients. [25,39] In addition, core-substituted NDI chromophores found applications as pH or metal-ion sensors, [40] for the sensitization of singlet oxygen, [41] and for interaction with G-quadruplex DNA. [42] In addition to heteroatom nucleophiles, numerous carbon nucleophiles were introduced in the form of organometallic compounds in transition-metal-catalyzed C À C cross-coupling reactions with NDI derivatives.…”