Electronic and Molecular Structures of Trigonal Truxene-Core Systems Conjugated to Peripheral Fluorene Branches. Spectroscopic and Theoretical Study Marı ´a Moreno Oliva, Juan Casado, and Juan T. Lo ´ pez Navarrete* Departamento de Quı ´mica Fı ´sica, UniVersidad de Ma ´ laga, Campus de Teatinos s/n, Ma ´ laga 29071, Spain Rory Berridge Department of Chemistry, UniVersity of Manchester, Manchester M1 9PL, United Kingdom Peter J. Skabara and Alexander L. Kanibolotsky WestCHEM, Department of Pure and Applied Chemistry, UniVersity of Strathclyde, Glasgow G1 1XL, United Kingdom Igor F. Perepichka L. M. LitVinenko Institute of Physical Organic and Coal Chemistry, National Academy of Sciences of Ukraine, Donetsk 83114, Ukraine ReceiVed: August 15, 2006; In Final Form: February 15, 2007 An analysis is performed on the molecular and electronic features in a series of trigonal molecules constituted by a central truxene core which is ramified with three oligofluorene moieties of different lengths. Arms and core are studied independently and upon threefold unification. Special emphasis is paid to the modulation of the conjugational properties in relation to substitution, molecular dimension, ring aromaticity, intermolecular forces, oxidation state, etc. Raman and optical absorption/emission spectroscopies in conjunction with computational theoretical results are combined for this purpose. The evolution of some key intensity ratios in the Raman spectra (i.e., I 1300 /I 1235 ) is followed as an indication of electronic interaction between the core and the branches. The changes of the electronic delocalization upon solvation, with varying temperature in the solid state, with the nature of the aromatic unit (bithiophene/fluorene) or after electrochemical oxidation are interpreted. The modulation of the optical properties on the basis of the structure and energetics of the orbital around the gap is also addressed. Density functional theory was used to assign the vibrational and electronic spectra. I. Introduction The field of well-defined π-conjugated oligomers is today recognized as a prominent prospect in materials science. In this field, linearly (i.e., one-dimensional) conjugated molecules constitute a fruitful area of research. 1 Recently, star-shaped conjugated oligomers have attracted tremendous interest, 2 particularly because they expand the potential of linear analogues in electronic, optoelectronic, and photonic applications [i.e., light-emitting diodes (LED), 3 photovoltaics, 4 field-effect transis- tors (FET), 5 third-order nonlinear optics, 6 etc.]. The possibility of extending the electronic structure in two dimensions leads to the realization of organic molecules built with peripheral antennae converging to a central core for energy conversion (i.e., light-harvesting systems). 4 Conversely, these new star- shaped or dendrimer-like molecules, if constructed with fluo- rescent blocks, can be viewed as condensates of more efficient or highly emitting systems. 3 In this context, the precise elucidation of the interplay between the energy and electronic states of the core and of the external branches is of relevance for improved designs. Polyfluorenes have emerged as leading electroluminescent materials with bright emission, high hole mobility, and ease of functionalization for tuning properties. 7,8 Their oligomeric homologues (i.e., oligofluorenes) serve as good models for photophysical studies and offer a greater degree of chemical purity, structural uniformity, or resistance to degradation, crucial factors for the performance of their LED devices. 9 Oligofluorene units have recently been used as arms in star-shaped architec- tures with various cores. 2h,3b-d,h,10 10,15-Dihydro-5H-diindeno- [1,2-a;1′,2′-c]fluorene (truxene), a polycyclic aromatic system with C 3 symmetry, has been used as a potential starting material for the construction of bowl-shaped fragments of fullerenes, C 3 tripodal materials in chiral recognition, and liquid crystalline compounds. 11 Recently truxene has been recognized as a promising building block in conjugated star-shaped and dendritic architectures and potentially interesting for optoelectronic applications. 2e,5b,10,12 Truxene can be viewed as three “over- lapped” fluorene fragments, which is an excellent choice as a core for the visualization of star-shaped oligofluorenes. In this work, we describe the molecular and electronic structure of a series of trigonal star-shaped oligofluorenes having truxene as the central core and up to quaterfluorene arms (see Figure 1), resulting in nanosized macromolecules with ca. 4 nm radius. 10 The 9-positions of the fluorene units are substituted with hexyl chains for improving solubility and processability. A combination of molecular spectroscopies (i.e., vibrational 4026 J. Phys. Chem. B 2007, 111, 4026-4035 10.1021/jp065271w CCC: $37.00 © 2007 American Chemical Society Published on Web 04/03/2007