9034 Macromolecules 2000, 33, 9034-9039 Photophysical Properties of Pyrene-Functionalized Poly(propylene imine) Dendrimers Lane A. Baker and Richard M. Crooks* Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842-3012 Received August 7, 2000; Revised Manuscript Received October 3, 2000 ABSTRACT: Four generations of poly(propylene imine) dendrimers have been covalently modified with pyrene moieties and examined by fluorescence spectroscopy. Emission and excitation spectra were obtained so that the generational dependence of the dendrimers, especially the extent of steric crowding, could be correlated to the photophysical properties of pyrene. In particular, the effect of dendrimer concentration and generation on pyrene-pyrene interactions was evaluated. More excimer emission was observed for higher generation dendrimers, while little or no evidence for interdendrimer interactions was observed over more than 2 orders of magnitude in dendrimer concentration. Excitation spectra revealed the presence of preassociated pyrene moieties. Protonation of the tertiary amine units was shown to increase monomer fluorescence significantly, while only slightly increasing the observed excimer fluorescence. Introduction Recent studies by Paleos et al. have focused on the In this paper, we report the photophysical properties properties of pyrene noncovalently trapped within poly- 42 43,44 of poly(propylene imine) (PPI) dendrimers1 modified on (amido amine) (PAMAM) and PPI dendrimers. their periphery with fluorescent pyrene moieties. Specif- These reports have shown that pyrene can be seques- ically, derivatization of the primary amine end groups tered in the interior cavity of dendrimers and selectively of generation 2, 3, 4, and 5 PPI dendrimers has been released by protonating interior tertiary amines at the achieved via amidation with an activated pyrene de- branch points. This is a consequence of the hydrophobic rivative, yielding polymers peripherally modified with nature of pyrene and the enhanced hydrophilicity of the 8, 16, 32, and 64 pyrene moieties, respectively. The dendrimer interior upon protonation. Fox and co-work- results indicate that the extent of interaction between ers have modified polyether dendrons with pyrene and terminal pyrene groups, which correlates to excimer naphthalene and examined the energy transfer proper- 45 fluorescence, is related to the generation-dependent ties between the core and these terminal groups. 46 47,48 dendrimer structure. Specifically, more excimer fluo- Dendrimers having aryl and carbosilane backbones rescence arises from higher generation dendrimers that covalently modified with pyrene have been studied by are more sterically crowded on their surfaces. Adams and Vitukhnovsky, respectively. Adams et al. Dendrimers2-8 are a class of highly symmetric (in selectively modified each dendron of several different some cases), monodisperse polymers that, relative to generation dendrimers with one pyrene moiety (three their linear analogues, possess a well-defined structure. pyrene moieties per dendrimer independent of genera- A number of technological uses have been proposed for tion) and studied these molecules with time-resolved dendrimers,3,5 and therefore a significant effort has been fluorescence techniques. They observed increased mobil- directed toward understanding their physical and chemi- ity in the end groups from generation one to three by cal properties. Chromophore-labeled dendrimers, in monitoring the magnitude of the excimer and monomer particular, have proven to be an informative means for fluorescence bands. Vitukhnovsky modified the core of studying the properties of dendrimers.9-23 Recent re- the dendrimer with a single pyrene moiety and used ports on the peripheral modification of dendrimers with time-resolved and polarized fluorescence techniques to moieties such as dansyl sulfonate,24,25 porphyrins,26 correlate the reorientation times of pyrene to the azobenzenes,27 oligo(p-phenylene vinylenes),28 and cou- dendrimer structure. Moore and Kopelman have studied marin29,30 moieties have shed new light on the structure excimer formation and energy migration in pheny- 49 and possible applications of dendrimers. lacetylene dendrimers having perylene traps. The fluorescent properties of pyrene are characterized In this paper, we describe the functionalization and by long lifetimes,31 sensitive solvatochromic shifts,32,33 spectroscopic properties of four generations of PPI and easily distinguished excimer species that arise when dendrimers fully substituted with pyrene. The results multiple pyrene molecules are π-stacked or in close indicate that the excimer-to-monomer fluorescence ratio proximity.31 For these reasons, pyrene is an obvious increases as dendrimer generation increases. Addition- candidate to exploit in the study of polymer structure ally, fluorescence excitation spectra confirm the pres- and function. For example, previous studies have shown ence of preaggregated pyrene moieties. Spectra obtained that pyrene can be used to probe structure and both at low pH indicate that monomer fluorescence is intermolecular34 and intramolecular interactions for a quenched by protonated tertiary amines present in the variety of polymers, notably (hydroxypropyl)cellulose35-38 dendrimer backbone more strongly than excimer fluo- and poly(ethylene imine),39-41 a polymer closely related rescence resulting from aggregated pyrene. Supporting to the PPI dendrimers in this study. evidence for the generational dependence of dendrimer generation on the fluorescent properties is provided by * To whom correspondence should be addressed. e-mail crooks@ molecular dynamics (MD) simulations. On the basis of tamu.edu; fax 979-845-1399; phone 979-845-5629. the results described in this paper, we propose that 10.1021/ma001379c CCC: $19.00 © 2000 American Chemical Society Published on Web 11/03/2000 Macromolecules, Vol. 33, No. 24, 2000 Poly(propylene imine) Dendrimers 9035 1 functional groups attached to dendrimer surfaces at PPI-Py32 (48.6 mg, 16% yield). H NMR (200 MHz, CD2- high density can interact with one another and thereby Cl2): δ 1.35 (124H br, NCH2CH2CH2N, NCH2CH2CH2NH, exhibit new functions. Such “cooperative effects” are NCH2CH2CH2CH2N), 2.12 (308H br, N(CH2)3, COCH2, COCH2- closely related to the “proximity effect” demonstrated CH2), 3.05 (128H br, CH2NH, PyCH2), 7.20 (32H br, NH), - 13 here and might have practical applications for sensing 7.35 8.30 (576H m, Py). C NMR (500 MHz, CDCl3): δ 24.6 (NCH CH CH CH N, NCH CH CH N), 27.2 (NCH CH CH - and catalysis. Similar observations have been made 2 2 2 2 2 2 2 2 2 2 NHCO), 27.8 (COCH2CH2CH2), 33.0 (CH2Py), 36.2 (CH2CH2- for other multifunctional molecules, such as cyclo- - - - CO), 38.0 (CH2CH2NHCO), 51 53 (N(CH2)3), 123 136 (Py), dextrins,50 52 cryptands,53 and linear and hyperbranched 173.3 (CO). IR (NaCl plates): amide I 1641 cm-1, amide II - polymers.54 56 Dendrimers are an especially intriguing 1541 cm-1. 1 scaffold for controlling cooperativity between functional PPI-Py64 (161.7 mg, 32% yield). H NMR (200 MHz, CD2- groups because the average spacing between moieties Cl2): δ 1.35 (252H br, NCH2CH2CH2N, NCH2CH2CH2NH, can be controlled by varying the dendrimer generation NCH2CH2CH2CH2N), 2.10 (628H br, N(CH2)3, COCH2, COCH2- CH2), 3.02 (256H br, CH2NH, PyCH2), 7.10-8.30 (640H br, and the length of spacer groups separating the active 13 functional group from the dendrimer itself. Moreover, NH, Py). C NMR (500 MHz, CDCl3): δ 24.7 (NCH2CH2CH2- CH2N, NCH2CH2CH2N), 27.2 (NCH2CH2CH2NHCO), 27.8 the number of active functional groups can be controlled (COCH CH CH ), 32.9 (CH Py), 36.1 (CH CH CO), 37.9 over a very wide range, which is limited only by the 2 2 2 2 2 2 (CH2CH2NHCO), 51-53 (N(CH2)3), 123-136 (Py), 173.4 (CO). maximum dendrimer generation that can be synthe- IR (NaCl plates): amide I 1643 cm-1, amide II 1538 cm-1. sized. Molecular Modeling. Modeling was performed in a man- ner analogous to previous reports in the literature.57-60 Mo- Experimental Section lecular models were created using the Cerius2 (version 4.0) Chemicals. Second through fifth generation amine-termi- software package (Molecular Simulations, Inc.; San Diego, CA). 61 nated poly(propylene imine) dendrimers (PPI-(NH2)X)(X ) The DREIDING force field version 2.21 was used for opti- 8, 16, 32, and 64 for generations 2, 3, 4, and 5, respectively) mization and molecular dynamics simulations. Specifically, were used as received (DSM Fine Chemicals, The Nether- each generation of PPI dendrimers was constructed from a lands). 1-Pyrenebutyric acid N-hydroxysuccinimide ester (Al- model of the previous generation by adding the appropriate drich 97%), anhydrous dichloromethane (Aldrich 99.8%), and number of propylamine branches. The SMART algorithm and sulfuric acid (EM Scientific) were also used as received. standard convergence settings were used to energy minimize Triethylamine (Aldrich 99%) was stored over molecular sieves each generation. Molecular dynamics (MD) simulations were (Aldrich, 3 Å) prior to use. Spectroscopic grade N,N-dimeth- then performed by the NVT method (constant volume and ylformamide (Acros 99+%) was used to prepare solutions for temperature) using the Nose´ temperature thermostat (0.01 ps all spectroscopic measurements. relaxation) for 10 ps at 1000 K, followed by 250 ps at 300 K Procedures. Dendrimers were functionalized by dissolving with a step size of 1 fs. The structures were again energy (separately) PPI-(NH2)8 (84.5 mg ) 0.109 mmol), PPI-(NH2)16 minimized. After obtaining models of each generation, all (84.9 mg ) 0.0503 mmol), PPI-(NH2)32 (89.4 mg ) 0.0254 primary amines were functionalized with pyrenebutyric acid mmol), or PPI-(NH2)64 (147.0 mg ) 0.0205 mmol) in 10 mL groups, thereby creating models of the dendrimers of interest. of dry dichloromethane containing 0.2 mL of dry triethylamine. Minimization and dynamics were performed again, exactly as Sufficient pyrenebutyric acid N-hydroxysuccinimide ester dis- described for the unmodified dendrimers. Static properties solved in 2 mL of dry dichloromethane was added to each reported are calculated based on the final minimized structure.