Characterization of Carotenoid Aggregates by Steady-State Optical Spectroscopy Chen Wang, † Christopher J. Berg, † Cheng-Chih Hsu, Brittany A. Merrill, and Michael J. Tauber* Department of Chemistry and Biochemistry, University of California at San Diego, 9500 Gilman Drive MC 0314, La Jolla, California 92093, United States * S Supporting Information ABSTRACT: The carotenoids have low-lying triplet excited states and can self-assemble in some solvents to form weakly or strongly coupled aggregates. These qualities make carotenoid aggregates useful for studies of singlet fission, where an outstanding goal is the correlation of interchromophoric coupling to the dynamics and yield of triplet excited states from a parent singlet excited state. Three aggregates of zeaxanthin, two weakly coupled and one strongly coupled, are characterized by steady-state spectroscopic methods including temperature-dependent absorption, fluorescence, and resonance Raman spectroscopy. The absorption spectra for each type of aggregate are distinct; however, an analysis of band positions reveals some important shared characteristics and suggests that the strongly coupled H-aggregate contains a subpopulation of weakly coupled constituents. Temperature-dependent absorption spectroscopy indicates that one of the weakly coupled aggregates can be converted to the other upon heating. The emission spectra of the three aggregates have similar profiles that are overall red-shifted by more than 1000 cm −1 relative to the monomer. The emission quantum yields of the aggregates are 5 to 30 times less than that of the monomer, with the lowest yield for the strongly coupled aggregate. The vibrational spectra of the chromophores support only slight perturbations from the structure of solvated monomers. Our findings support the conclusion that all three aggregates are best characterized as H-aggregates, in agreement with a prior theoretical study of lutein aggregates. I. INTRODUCTION The self-assembly of organic molecular chromophores can lead to new photophysical properties that are not available to the monomer. The photophysics that emerge upon coupling of various chromophores have an impact on applications ranging from photographic science 1,2 to solar energy conversion. 3 In extended systems such as aggregates and thin films, it is well known that subtle changes in preparation conditions can lead to significantly different packing arrangements, and therefore intermolecular interactions, between the molecular constitu- ents. Changes in absorption, emission, and other properties caused by exciton coupling within aggregates have been investigated for numerous chromophores, including cyanines or merocyanines, 4−6 conjugated oligomers, 7−9 xanthenes, 10 acenes, 11 rylene diimides, 12−16 porphyrins/chlorins, 3,17 and carotenoids. 18 Reviews that encompass several classes of dyes or self-assembled systems with mixed chromophores are available. 19,20 One photophysical mechanism that generally requires the proximity of two chromophores is singlet fission. In this mechanism, a chromophore is photoexcited to its singlet excited state and subsequently partitions its energy over two neighboring chromophores that are both left in triplet excited states. The field of singlet fission has undergone a revival in recent years, in large part because of potential benefit to solar energy conversion. 21,22 An important requirement for efficient fission is that the combined electronic energy of the two triplet excited states be less than that of the parent singlet excited state. 23 As expected, the orientation and coupling strength between chromophores is also important. Theoretical studies have provided some guidance and predictions on the optimal coupling for various classes of chromophores. 22,24 However, there is a shortage of experimental studies that probe the link between fission dynamics and coupling. Ideally, the coupling strength and orientation among a group of chromophores would be varied and correlated with the yield or rate of triplet excited-state formation. Such an approach has been successfully pursued with covalent dimers of tetracene, where low yields of singlet fission have been found for some configurations. 25,26 Our aim has been to correlate the coupling within assemblies of carotenoids with the dynamics of singlet fission in these systems. 27,28 The chromophore zeaxanthin was selected because its electronic energy levels are favorable for the production of triplet excited states via fission as well as its propensity to aggregate. A high yield of triplet excited states via singlet fission was found for an aggregate of zeaxanthin denoted as J1. 28 The J1-aggregate has a steady-state absorption spectrum that is not strongly red- or blue-shifted relative to the monomer, therefore, the coupling between chromophores is Special Issue: Richard A. Mathies Festschrift Received: July 13, 2012 Revised: July 28, 2012 Published: August 9, 2012 Article pubs.acs.org/JPCB © 2012 American Chemical Society 10617 dx.doi.org/10.1021/jp3069514 | J. Phys. Chem. B 2012, 116, 10617−10630