Intramolecular singlet–singlet energy transfer in antenna-substituted azoalkanes

Abstract

Two novel azoalkane bichromophores and related model compounds have been synthesised and photophysically characterised. Dimethylphenylsiloxy (DPSO) or dimethylnaphthylsiloxy (DNSO) serve as aromatic donor groups (antenna) and the azoalkane 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) as the acceptor. The UV spectral window of DBO (250-300 nm) allows selective excitation of the donor. Intramolecular singlet-singlet energy transfer to DBO is highly efficient and proceeds with quantum yields of 0.76 with DPSO and 0.99 with DNSO. The photophysical and spectral properties of the bichromophoric systems suggest that energy transfer occurs through diffusional approach of the donor and acceptor within a van der Waals contact at which the exchange mechanism is presumed to dominate. Furthermore, akin to the behaviour of electron-transfer systems in the Marcus inverted region, a rate of energy transfer 2.5 times slower was observed for the system with the more favourable energetics, i.e. singlet-singlet energy transfer from DPSO proceeded slower than from DNSO, although the process is more exergonic for DPSO (-142 kJ mol(-1) for DPSO versus-67 kJ mol(-1) for DNSO).

Substituent Effect on the Photoreduction Kinetics of Benzophenone

“…5 Numerous properties of the primary photoreduction process, hydrogen abstraction by triplet n-π * state ketones are well known and have been discussed in detail. [6][7][8][9][10][11][12] The n-π * triplet state was found to behave as a 1,2-biradical in several types of chemical reaction. 7,12 A notable number of studies 8 dealt with the nature of the primary photochemical process of n-π* excited aromatic carbonyls, mostly focusing on the dependence of the rate parameters on the chemical nature of the reaction partner.…”

“…7,12 A notable number of studies 8 dealt with the nature of the primary photochemical process of n-π* excited aromatic carbonyls, mostly focusing on the dependence of the rate parameters on the chemical nature of the reaction partner. 8,10,11 In reactions with hydrocarbons or alcohols the triplet excited ketones are considered 6,8 to behave like radicals, although the possibility of partial charge transfer character of the process has also been mentioned. 8 The remarkable difference in the room temperature photoreduction rate was explained on the basis of the bond strength of the C-H bond broken in the reaction.…”

Fluorescence Regeneration as a Signaling Principle for Choline and Carnitine Binding: A Refined Supramolecular Sensor System Based on a Fluorescent Azoalkane

“…k q = 9.1×10 8 M -1 s -1 for p-hydroxybenzene sulfonic acid vs. k q = 8.9×10 7 M -1 s -1 for p-n-propoxybenzene sulfonic acid in D 2 O, this work), and, possibly, also exciplex formation with the aryl rings. [37][38][39] The fluorescence quenching of DBO by CX4 is a composite effect, akin to the fluorescence quenching studied for CDs, [26,27] of static quenching in the preformed host-guest complex and dynamic quenching due to diffusive complexation within the very long fluorescence lifetime (505 ns in aerated D 2 O); [26] in addition, direct dynamic quenching needs to be taken into account. [26] The dissection of static and dynamic fluorescence quenching was not attempted in the present context, since the qualitative observation of efficient fluorescence quenching upon complexation alone paved the way for applications of the CX4·DBO complex as a fluorescent sensor.…”

Bridgehead carboxy-substituted 2,3-diazabicyclo[2.2.2]oct-2-enes: synthesis, fluorescent properties, and host-guest complexation

“…To by-pass the above problems, while maintaining high water-solubility, we have previously synthesized and comprehensively employed the derivatives 2 and 3. 14,[23][24][25][26]30 Several attempts to oxidize the hydroxymethyl DBO 2 directly to the bridgehead carboxy-substituted derivative 4 by several mild oxidation methods failed, such that we needed to resort to the conventional synthesis route based on cycloaddition with 4-N-methyl-1,2,4-triazolin-3,5-dione (MTAD, Scheme 1). The cycloaddition with MTAD required cyclohexa-1,3-dienecarboxylic acid (9), which had been previously obtained by a different route.…”

“…12,13 Derivatives of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO, 1) have recently become valuable structural and dynamic probes 14 as well as sensors for supramolecular [15][16][17][18][19][20][21][22][23] and biomolecular assemblies. [23][24][25][26][27][28][29][30][31][32][33][34][35] Until now, the derivatives with 1-hydroxymethyl (2) [23][24][25][26] and 1-aminomethyl groups (3) 30 have been employed as key precursors for subsequent derivatization, e.g., for the introduction into polypeptides, [30][31][32][33][34][35] polynucleotides, 24 or biomembranes. [26][27][28][29] Herein, we describe the DBO carboxylic acid 4 and the DBO acetic acid 5.…”