Structure-Dependent Photophysics of First-Generation Phenyl-Cored Thiophene Dendrimers William J. Mitchell, Andrew J. Ferguson,* Muhammet E. Ko ¨se, Benjamin L. Rupert, David S. Ginley, Garry Rumbles, Sean E. Shaheen, and Nikos Kopidakis National Renewable Energy Laboratory, 1617 Cole BouleVard, Golden, Colorado, 80401-3393 ReceiVed September 5, 2008. ReVised Manuscript ReceiVed October 21, 2008 We have prepared two series of first-generation thiophene-bridge dendrimers, with either three (3G1) or four (4G1) arms attached to a phenyl core, to elucidate their structure-property relationships. Optical properties were investigated with a combination of steady-state and time-resolved spectroscopic techniques. Steady-state spectroscopic data for the 3-arm dendrimers suggests that the exciton is delocalized over the R-conjugated thiophene segment and the phenyl core, but that the meta-linking of the dendrons prevents their electronic communication. In contrast, conjugation through the core to dendrons in the ortho and para positions is permitted in the 4-arm dendrimers, although the data suggest that the conjugation length does not extend over the full length of the R-conjugated sections of two coupled dendrons. This observation is due to steric interactions between neighboring arms, which forces the arms to twist and bend out of the plane of the phenyl core, and is particularly prevalent in disrupting the conjugation through the ortho positions. As expected, our results show that an increase in the bridge length results in an increase in the conjugation length for both dendrimers, and a subsequent red-shift of the absorption and emission. In addition, an increase in the dendron length results in an increase in the photoluminescence quantum yield and lifetime, suggesting that the ground and excited-state geometries are very similar and that the electronic transition is coupled to fewer vibrational modes. Introduction Conjugated dendrimers provide an excellent material set to supplement conjugated polymers in the arena of organic electronics. 1 They offer the processability of polymers for device applications, but they also provide precisely defined molecular structures that allow for systematic study of structure-property relationships such as the theoretical description of their structure-dependent optical and electronic properties. 2-4 It is through studies of conjugated dendrimers that clear and distinct structure-property relationships can be elucidated that may ultimately lead to improvement in performance of devices such as organic light-emitting diodes (OLEDs), 1,5 organic photovoltaic (OPV), 1,6-9 and other optoelectronic devices. Dendrimers are macromolecules with a precisely defined structure consisting of dendrons attached to a central core. The dendrons are typically branched, and the degree of the branching is known as the “generation” of the dendrimer. Depending on the desired function the dendrons can be aliphatic or the structure as a whole can be conjugated. 10,11 Conjugated dendrimers possess several possible advantages over their analogous polymers for use in optoelectronic applications, including a well-defined molecular weight, improved batch-to-batch reproducibility and a higher degree of purity. Such dendrimers have previously been synthesized with dendrons containing phenylacetylene, 12,13 phenylene, 14 and phenylenevinylene 15-17 units, and more recently den- drimers have been prepared that are conjugated through thiophene repeat units. 18-21 Thiophene has been extensively utilized as the foundation of many optoelectronic organic materials 18-24 including polymers, oligomers, and more * Corresponding author. E-mail: andrew_ferguson@nrel.gov. (1) Lo, S. C.; Burn, P. L. Chem. ReV. 2007, 107, 1097. (2) Lupton, J. M.; Samuel, I. D. W.; Beavington, R.; Frampton, M. J.; Burn, P. L.; Bassler, H. Phys. ReV.B 2001, 63, 155206. 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AdV. Mater. 1998, 10, 93. 287 Chem. Mater. 2009, 21, 287–297 10.1021/cm802410d CCC: $40.75  2009 American Chemical Society Published on Web 12/24/2008