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J. Phys. Chem. B 2007, 111, 6620-6627
Mechanisms for Fluorescence Depolarization in Dendrimers† Veronica Vicinelli,‡ Giacomo Bergamini,‡ Paola Ceroni,‡ Vincenzo Balzani,*,‡ Fritz Vo1 gtle,*,§ and Oleg Lukin*,⊥ Dipartimento di Chimica “G. Ciamician”, UniVersita` di Bologna, Via Selmi 2, I-40126 Bologna, Italy, Kekule´ -Institut fu¨r Organische Chemie und Biochemie der UniVersita¨t Bonn, Gerhard-Domagk Strasse 1, D-53121 Bonn, Germany, Department of Materials, Institute of Polymers, ETH Zurich, 8093 Zurich, Switzerland ReceiVed: January 19, 2007; In Final Form: February 23, 2007
We have investigated the fluorescence properties of dendrimers (Gn is the dendrimer generation number) containing four different luminophores, namely terphenyl (T), dansyl (D), stilbenyl (S), and eosin (E). In the case of T, the dendrimers contain a single p-terphenyl fluorescent unit as a core with appended sulfonimide branches of different size and n-octyl chains. In the cases of D and S, multiple fluorescent units are appended in the periphery of poly(propylene amine) dendritic structures. In the case of E, the investigated luminophore is noncovalently linked to the dendritic scaffold, but is encapsulated in cavities of a low luminescent dendrimer. Depending on the photophysical properties of the fluorescent units and the structures of the dendrimers, different mechanisms of fluorescence depolarization have been observed: (i) global rotation for GnT dendrimers; (ii) global rotation and local motions of the dansyl units at the periphery of GnD dendrimers; (iii) energy migration among stylbenyl units in G2S; and (iv) restricted motion when E is encapsulated inside a dendrimer, coupled to energy migration if the dendrimer hosts more than one eosin molecule.
Introduction Dendrimers1,2
constitute a class of repeatedly branched molecules combining considerable molar masses with symmetrical structure and monodispersity. They are currently attracting the attention of a great number of scientists because of their unusual chemical and physical properties differing from those of nonbranched analogues (e.g., linear polymers) and the wide range of potential applications. Possibilities to construct dendrimers containing selected, covalently linked chemical units in predetermined sites of their structure and to complex guests in internal cavities of dendrimers are particularly interesting to obtain the desired functions. Dendrimers containing luminescent units have been extensively investigated in the past few years, both from a fundamental viewpoint (e.g., theoretical studies on energy transfer processes,3 fluorescence at the single molecule level4) and for a variety of applications (e.g., light harvesting,5 changing the “color” of light,6 sensing with signal amplification,7 quenching and sensitization processes8). Luminescent dendrimers are also suitable for fluorescence anisotropy measurements9 that can give information on specific properties, such as (a) rotational reorientation times and hydrodynamic properties,10 (b) delocalization of the excitation energy,11 (c) local motions of subunits,12 (d) individual molecule rotational isomerism,13 and (e) aggregate formation.14 We have selected four types of dendrimers to illustrate the different fluorescence anisotropy mechanisms that can be †
Part of the special issue “Norman Sutin Festschrift”. * To whom correspondence should be addressed. (Balzani) Phone: +39-051-2099560.Fax: +39-051-2099456.E-mail:
[email protected]. (Vo¨gtle) Fax: +49-228-735662. E-mail:
[email protected]. (Lukin) Fax: +41 44 633 13 97. E-mail:
[email protected]. ‡ Universita ` di Bologna. § Universita ¨ t Bonn. ⊥ ETH Zurich.
observed, depending on the nature and position of the luminescent unit(s) in the dendritic structure (Gn is the dendrimer generation number). As shown in Schemes 1 and 2, the four selected systems are based on four different luminophores, namely, terphenyl (T), dansyl (D), stilbenyl (S), and eosin (E). In the case of T (Scheme 1), the dendrimers have a single fluorescent moiety, that is, the T core. In the cases of D and S (Scheme 2a), multiple fluorescent units are appended in the periphery of poly(propylene amine) dendritic structures. In the case of E (Scheme 2b), the investigated luminophore does not belong to the dendritic structure, but is encapsulated in cavities of the low luminescent fourth generation poly(propylene amine) dendrimer G4B containing 1,2-dimethoxybenzene (B) units at the periphery. Experimental Part The syntheses, characterizations, absorption and emission spectra of the p-terphenyl cored dendrimers,15 and poly(propylene amine) dendrimers functionalized with D,16 S,17 and B18 units have been previously reported. Eosin Y (E), both as tetrabutyl ammonium and disodium salt, and the solvents used were purchased from Aldrich. All the experiments were carried out at 298 K on airequilibrated solutions using the solvents specified below. UVvis absorption spectra were recorded with a Perkin-Elmer λ40 spectrophotometer using quartz cells with path length of 1.0 cm. Fluorescence spectra were performed with a Perkin-Elmer LS-50 spectrofluorimeter equipped with a Hamamatsu R928 phototube. Fluorescence lifetimes and steady-state and timeresolved anisotropy measurements were performed by an Edinburgh FLS920 spectrofluorometer equipped with a TCC900 card for data acquisition in time-correlated, single-photon counting experiments (0.5 ns time resolution) with a D2 lamp and a LDH-P-C-405 pulsed diode laser. The fluorescence decay
10.1021/jp070468p CCC: $37.00 © 2007 American Chemical Society Published on Web 04/05/2007
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J. Phys. Chem. B, Vol. 111, No. 24, 2007 6621
SCHEME 1: Dendrimers with a Terphenyl T Core
curves were collected under 0° and 90° polarization angles. Global analysis on polarized fluorescence decay curves was performed by FAST software (version 1.8.1) produced by Edinburgh Instruments Ltd. The estimated experimental errors are (2 nm on the band maximum, 5% on the molar absorption coefficient, 5% on the fluorescence lifetime and steady-state anisotropy, and 15% on the anisotropy decay time.
transition moments are not collinear and when the molecule undergoes a change in orientation during the excited-state lifetime. The fluorescence anisotropy decay can be fitted by a monoexponential model (eq 2)
Results and Discussion
where θ is the rotational relaxation time. The rotational relaxation time, θ, can be related to the hydrodynamic volume, Vh, by the Stokes-Einstein-Debye equation. In particular, for compounds having a van der Waals volume much bigger than the volume of the solvent molecules, “sticking” boundary conditions are applicable; i.e., the form of the rotor has no influence, since it moves together with solvent molecules, and θ can be expressed as
Fluorescence Anisotropy. For a simple molecular species, fluorescence anisotropy depends on the different orientation of the absorption and emission transition moments. The fluorescence anisotropy is defined as
r ) (I|| - I⊥)/(I|| + 2I⊥)
(1)
where I|| and I⊥ are the emission intensities registered when the emission and excitation polarizers are oriented parallelly or perpendicularly, respectively. For randomly oriented molecules, directly after excitation, the anisotropy (r0) will be 0.4 if the absorption and emission transition dipole moments have the same orientation. The value of the anisotropy decreases if the
r(t) ) r0 e-t/θ
θ ) Vhη/kBT
(2)
(3)
where η is the viscosity of the solvent; kB, the Boltzmann constant; and T, the absolute temperature. If a supramolecular system contains only one fluorescent unit, fluorescence depolarization takes place, as for simple molecular
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Vicinelli et al.
SCHEME 2: Poly(propylene amine) Dendrimers with (a) Dansyl (D) and Stylbenyl (S) Peripheral Units; (b) Eosin (E) Encapsulated in G4 Carrying 1,2-Dimethoxybenezene (B) Peripheral Units
species, if the absorption and emission transition moments are intrinsically non collinear and as a result of change in orientation of the fluorophore during the excited-state lifetime. Such a dynamic change in orientation may be caused by (i) rotation of the supramolecular system as a whole and (ii) local motions of the fluorophore in the case of a flexible supramolecular architecture. For a supramolecular system that contains more than one identical fluorescent unit with different orientations, fluorescence depolarization can also occur by (iii) energy migration from the originally excited unit to a differently oriented one. Of course, the rates of global rotation, local movements, and energy migration must compete with the rate of deactivation of the fluorophore excited state. At constant temperature, the rate of global rotation depends on the size of the species and the viscosity of the solvent, the rate of local movements depends on the specific position of the fluorophore
in the supramolecular structure and on the nature of the bonds that link the fluorophore to the supramolecular scaffold, and the rate of energy migration depends on the overlapping between the absorption and emission spectra of the fluorophores and their distance. As mentioned in the introduction, it is possible to construct dendrimers of different sizes that contain one or more selected fluorescent units in predetermined sites of the dendritic architecture. Furthermore, it is possible to incorporate fluorescent molecules in the dendritic cavities. We have exploited these possibilities to make systems that exhibit different fluorescence depolarization properties controlled by the three abovementioned mechanisms (global rotation, local motion, and energy migration). In dendritic structure containing more than one fluorophore, multiple excitation can lead to singlet-singlet annihilation and,
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TABLE 1: Steady-State and Time-Resolved Fluorescence Anisotropy Ta G1Ta G2Ta G3Ta Dd G2Dd G3Dd G4Dd G5Dd Sf G2Sf Eg E ⊂ G4B (0.1:1)g E ⊂ G4B (7:1)g
solvent
rss
θ (ns)
DCMb DCM/PGly 1:1 DCMb DCM/PGly 1:1 DCMb DCM/PGly 1:1 DCMb DCM/PGly 1:1 DCM DCM/PGly 1:30 DCM DCM/PGly 1:30 DCM DCM/PGly 1:30 DCM DCM/PGly 1:30 DCM DCM/PGly 1:30 AN AN/PGly 1:30 AN AN/PGly 1:30 DCM DCM DCM
