Two-Photon Photosensitized Production of Singlet Oxygen: Sensitizers with Phenylene-Ethynylene-Based Chromophores Sean P. McIlroy, † Emiliano Clo ´, † Lars Nikolajsen, ‡ Peter K. Frederiksen, † Christian B. Nielsen, † Kurt V. Mikkelsen,* ,‡ Kurt V. Gothelf,* ,† and Peter R. Ogilby* ,† Department of Chemistry, University of Aarhus, DK-8000 A ° rhus, Denmark, and Department of Chemistry, University of Copenhagen, DK-2100 Copenhagen, Denmark kmi@theory.ki.ku.dk; kvg@chem.au.dk; progilby@chem.au.dk Received October 11, 2004 Singlet molecular oxygen (a 1 Δ g ) has been produced and optically monitored in time-resolved experiments upon nonlinear two-photon excitation of photosensitizers that contain triple bonds as an integral part of the chromophore. Both experiments and ab initio computations indicate that the photophysical properties of alkyne-containing sensitizers are similar to those in the alkene- containing analogues. Most importantly, however, in comparison to the analogue that contains double bonds, the sensitizer containing alkyne moieties is more stable against singlet-oxygen- mediated photooxygenation reactions. This increased stability can be advantageous, particularly with respect to two-photon singlet oxygen imaging experiments in which data are collected over comparatively long time periods. Introduction The lowest excited electronic state of molecular oxygen, singlet molecular oxygen (a 1 Δ g ), is an intermediate in many oxidation reactions. 1,2 In many systems of practical importance, these reactions occur in samples that contain heterogeneous, phase-separated domains. 3,4 Thus, for work in such systems, it could be of great use to both produce and detect singlet oxygen in time- and space- resolved experiments. Singlet oxygen can be efficiently produced upon ir- radiation of a sensitizer which, in turn, transfers its energy of excitation to ground-state oxygen (X 3 Σ g - ). 5 Sensitizers can be a chromophore inherent to a given system, as is the case with many functional polymers. 6 Alternatively, a sensitizer can be specifically added to a system, as in the case of photodynamic therapy where light is used as a tool to selectively kill biological cells (e.g., cancer cells). 7 Until recently, sensitizer excitation has always been achieved via a linear one-photon transition between the sensitizer ground state, S 0 , and a singlet excited state of the sensitizer, S n . 5 After excitation, singlet oxygen is most efficiently produced upon oxygen quenching of the sen- sitizer triplet state, T 1 , formed upon intersystem crossing, S 1 f T 1 . 5 It has been demonstrated, however, that singlet oxygen can also be produced upon nonlinear two-photon excita- tion of a sensitizer. 8-11 In this case, an excited-state S m * To whom correspondence should be addressed. † University of Aarhus. ‡ University of Copenhagen. (1) Foote, C. S. Acc. Chem. Res. 1968, 1, 104-110. (2) Frimer, A. A., Ed. Singlet Oxygen; CRC Press: Boca Raton, 1985; Vol. I-IV. (3) Oleinick, N. L.; Morris, R. L.; Belichenko, I. Photochem. Photo- biol. Sci. 2002, 1,1-21. (4) Lissi, E. A.; Encinas, M. V.; Lemp, E.; Rubio, M. A. Chem. Rev. 1993, 93, 699-723. (5) Schweitzer, C.; Schmidt, R. Chem. Rev. 2003, 103, 1685-1757. (6) Dam, N.; Scurlock, R. D.; Wang, B.; Ma, L.; Sundahl, M.; Ogilby, P. R. Chem. Mater. 1999, 11, 1302-1305. (7) Dougherty, T. J.; Gomer, C. J.; Henderson, B. W.; Jori, G.; Kessel, D.; Korbelik, M.; Moan, J.; Peng, Q. J. Natl. Cancer Inst. 1998, 90, 889-905. (8) Frederiksen, P. K.; Jørgensen, M.; Ogilby, P. R. J. Am. Chem. Soc. 2001, 123, 1215-1221. (9) Poulsen, T. D.; Frederiksen, P. K.; Jørgensen, M.; Mikkelsen, K. V.; Ogilby, P. R. J. Phys. Chem. A 2001, 105, 11488-11495. (10) Frederiksen, P. K.; McIlroy, S. P.; Nielsen, C. B.; Nikolajsen, L.; Skovsen, E.; Jørgensen, M.; Mikkelsen, K. V.; Ogilby, P. R. J. Am. Chem. Soc. 2005, 127, 255-269. 1134 J. Org. Chem. 2005, 70, 1134-1146 10.1021/jo0482099 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/28/2005