Resonance Energy Transfer in the Solution Phase Photophysics of -Re(CO) 3 L + Pendants Bonded to Poly(4-vinylpyridine) L. L. B. Bracco, †,‡ M. P. Juliarena, †,‡ G. T. Ruiz, †,‡ M. R. Fe ´liz, §,‡ G. J. Ferraudi, ⊥ and E. Wolcan* ,†,‡ INIFTA, Facultad de Ciencias Exactas, UniVersidad Nacional de La Plata, Casilla de Correo 16, Sucursal 4, (B1906ZAA) La Plata, Argentina, Notre Dame Radiation Laboratory, Notre Dame, Indiana 46556-0579 ReceiVed: March 14, 2008; ReVised Manuscript ReceiVed: June 19, 2008 Polymers with general formula {[(vpy) 2 vpyRe(CO) 3 (tmphen) + ]} n {[(vpy) 2 vpyRe(CO) 3 (NO 2 -phen) + ]} m (NO 2 - phen ) 5-nitro-1,10-phenanthroline; tmphen ) 3,4,7,8-tetramethyl-1,10-phenanthroline); vpy ) 4-vinylpyridine) were prepared and their morphologies were studied by transmission electron microscopy (TEM). Multiple morphologies of aggregates from these Re I polymers were obtained by using different solvents. Energy transfer between MLCT Reftmphen and MLCT RefNO 2 -phen excited states inside the polymers was evidenced by steady state and time-resolved spectroscopy. Current Fo ¨rster resonance energy transfer theory was successfully applied to energy transfer processes in these polymers. Introduction Numerous studies have been concerned with thermal and photochemical reactions of inorganic polymers in the solid-state and solution phase. Interest in their photochemical and photo- physical properties is driven by their potential applications in catalysis and optical devices. 1–12 The properties in the solution phase of the polymers I and II (see Scheme 1) were investigated in previous works. 1,9,12 Marked differences were found between the photochemical and photophysical properties of polymers I and II and those of the related monomeric complexes, pyRe I (CO) 3 L + (L ) phen, 2,2′-bpy). The main cause of these differences is the photoge- neration of MLCT excited sates in concentrations that are much larger when -Re I (CO) 3 L + chromophores are bound to poly-4- vinylpyridine, (vpy) 600 . This is the photophysical result of Re I chromophores being crowded in strands of a polymer instead of being homogeneously distributed through solutions of a pyRe I (CO) 3 L + complex. The recently communicated association of several hundred strands of II in nearly spherical aggregates also contributes to the crowding of chromophores in small spaces in the solution, where the interaction between excited states becomes appreciable. 12 The photogeneration of MLCT excited states in close vicinity within a polymer strand makes possible the study of energy transfer processes if donor and acceptor pendants are distributed along the strand. Resonance energy transfer (RET) is a widely prevalent photophysical process through which an electronically excited “donor” molecule transfers its excitation energy to an “acceptor” molecule such that the excited-state lifetime of the donor decreases. If the donor happens to be a fluorescent molecule, RET is referred to as fluorescence resonance energy transfer, FRET. The importance of FRET is ubiquitous. In polymer science, Fo ¨rster 13 theory is used to study the interface thickness in polymer blends, phase separation and conformational dynam- ics of polymers. 14–17 In biological sciences, the technique of FRET is being exploited to design supramolecular systems that can be used to harvest light in artificial photosynthesis as these light-harvesting systems of plants and bacteria involve unidi- rectional transfer of absorbed radiation energy to the reaction center via a multistep FRET mechanism. Recent advances in fluorescence resonance energy transfer have led to qualitative and quantitative improvements in the technique, including increased spatial resolution, distance range, and sensitivity. These advances, due largely to new fluorescent dyes, but also to new optical methods and instrumentation, have opened up new biological applications. 18 Besides these, FRET is commonly used in scintillators and chemical sensors. 19–23 We have applied ligand substitution reactions of the Re I complexes to the derivatization of polymers III, IV, V, VI, and VII (Schemes 2 and 3). Polymers III-VII consist of a poly-4-vinilpyridine backbone with pendant chromophores -Re I (CO) 3 (NO 2 -phen) and -Re I (CO) 3 (tmphen), where NO 2 -phen and tmphen stand for 5-nitro-1,10-phenanthroline and 3,4,7,8-tetramethyl-1,10- phenanthroline, respectively. Morphologies of these polymers were studied using TEM. In polymers V-VII, we observed intramolecular RET between the luminescent MLCT Reftmphen excited states of -Re I (CO) 3 (tmphen) chromophores and the MLCT RefNO 2 -phen excited states of -Re I (CO) 3 (NO 2 -phen). In this paper, luminescence quantum yields and lifetimes of polymers V-VII are discussed in terms of the current RET theories applicable to energy transfer between acceptors and donors randomly distributed in a polymer. Experimental Part Flash-Photochemical Procedures. Optical density changes occurring on a time scale longer than 10 ns were investigated with a flash photolysis apparatus described elsewhere. 24–26 In these experiments, 25 ns flashes of 351 nm (ca. 25-30 mJ/ pulse) light were generated with a Lambda Physik SLL-200 excimer laser. The energy of the laser flash was attenuated to values equal to or less than 20 mJ/pulse by absorbing some of * To whom correspondence should be adressed. Telephone: 54-221 425 7430/425 7291. Fax: +54 221 425 4642. E-mail: ewolcan@inifta.unlp.edu.ar. † Member of CONICET staff. ‡ INIFTA, Facultad de Ciencias Exactas, Universidad Nacional de La Plata. § Member of CICPBA staff. ⊥ Notre Dame Radiation Laboratory. J. Phys. Chem. B 2008, 112, 11506–11516 11506 10.1021/jp802241k CCC: $40.75  2008 American Chemical Society Published on Web 08/20/2008