Novel laser-induced luminescence resulting from benzophenone/O-propylated p-tert-butylcalix[4]arene complexes. A diffuse reflectance study Luis F. Vieira Ferreira,* a Margarida R. Vieira Ferreira, b José P. Da Silva, a,c Isabel Ferreira Machado, a Anabela S. Oliveira a and José V. Prata b a Centro de Química-Física Molecular – Complexo Interdisciplinar, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal. E-mail: LuisFilipeVF@ist.utl.pt b Secção de Química Orgânica, Departamento de Engenharia Química, Instituto Superior de Engenharia de Lisboa, R. Conselheiro Emídio Navarro, 1950-062 Lisboa, Portugal c FCT, Universidade do Algarve, Campus de Gambelas, 8005-139 Faro, Portugal Received 10th June 2003, Accepted 23rd July 2003 First published as an Advance Article on the web 6th August 2003 Laser-induced room temperature luminescence of air-equilibrated benzophenone/O-propylated p-tert-butylcalix- [4]arene solid powdered samples revealed the existence of a novel emission, in contrast with benzophenone/p-tert- butylcalix[4]arene complexes, where only benzophenone emits. This novel emission was identified as phosphorescence of 1-phenyl-1,2-propanedione, which is formed as the result of an hydrogen atom abstraction reaction of the triplet excited benzophenone from the propoxy substituents of the calixarene. Room temperature phosphorescence was obtained in air-equilibrated samples in all propylated hosts. The decay times of the benzophenone emission vary greatly with the degree of propylation, the shortest lifetimes being obtained in the tri- and tetrapropylated calixarenes. Triplet–triplet absorption of benzophenone was detected in all cases, and is the predominant absorption in the p-tert-butylcalix[4]arene case, where an endo-calix complex is formed. Benzophenone ketyl radical formation occurs with the O-propylated p-tert-butylcalix[4]arenes hosts, suggesting a different type of host/guest molecular arrangement. Diffuse reflectance laser flash photolysis and gas chromatography–mass spectrometry techniques provided complementary information, the former about transient species and the latter regarding the final products formed after light absorption. Product analysis and identification clearly show that the two main degradation photoproducts following laser excitation in the propylated substrates are 1-phenyl- 1,2-propanedione and 2-hydroxybenzophenone, although several other minor photodegradation products were identified. A detailed mechanistic analysis is proposed. While the solution photochemistry of benzophenone is dominated by the hydrogen abstraction reaction from suitable hydrogen donors, in these solid powdered samples, the α-cleavage reaction also plays an important role. This finding occurs even with one single laser pulse which lasts only a few nanoseconds, and is apparently related to the fact that scattered radiation exists, due to multiple internal reflections possibly trapping light within non-absorbing microcrystals in the sample, and is detected until at least 20 μs after the laser pulse. This could explain how photoproducts thus formed could also be excited with only one laser pulse. 1 Introduction Time-resolved laser-induced luminescence, diffuse reflectance laser flash photolysis and ground-state diffuse reflectance absorption spectroscopy are relatively new techniques that can be applied to study opaque and crystalline systems. 1–3 These solid-state photochemical methods have been recently applied by our group to study several organic compounds adsorbed onto different hosts, including p-tert-butylcalix[4]-, -[6]- and -[8]arenes, 4 microcrystalline cellulose, 5 silicalite, cyclodextrins 6 and silica, 7 amongst others. Benzophenone (BZP) is an extremely useful molecule for probing new hosts. The n π* absorption transition is known to be very sensitive to the nature of the environment and also exhibits a photochemistry which depends on the properties of the host. 4b,c,5a,8 Calixarenes are important macrocyclic phenol–formaldehyde polycondensates with hydrophobic bowl-shaped cavities. 9,10 The ability of calixarenes and calixarene derivatives to form inclu- sion complexes, accommodating guest molecules in their intra- molecular cavities, greatly depends on the size and geometry of guest molecule, but also on the host cavity. 4,9,10 Very few photochemical studies of neutral organic com- pounds within calixarenes have been presented until now (ref. 4 and references therein). As an example, we have recently used β-phenylpropiophenone and benzophenone to study the form- ation of inclusion complexes of these neutral organic com- pounds with p-tert-butylcalix[4]arene. 4c Time-resolved diffuse reflectance absorption and emission techniques were applied to powdered solid samples of these compounds, establishing beyond doubt that the ketones are included within the cavity of the calixarene through the observation of room temperature phosphorescence in air-equilibrated samples for both guests, following laser excitation. Calixarenes can be extremely useful hosts for use in environ- mental chemistry studies due to their ability to selectively bind, separate and sense neutral organic (and ionic) contaminants, depending not only on the size of the calixarene nanocavity, but also on the substituents attached to the upper and lower rims. 9,10 Calixarene derivatives can also be used for stabilisation of reaction intermediates, for catalysis through encapsulation and also for molecular transport and delivery studies. 10 In this paper, a new family of calixarene hosts is studied using a well-known probe as a guest molecule: we present a DOI: 10.1039/b306582k Photochem. Photobiol. Sci., 2003, 2, 1002–1010 1002 This journal is © The Royal Society of Chemistry and Owner Societies 2003 Published on 06 August 2003. Downloaded by Universidade do Algarve (UALG) on 13/05/2016 15:13:34. View Article Online / Journal Homepage / Table of Contents for this issue