Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Nature of Excimer Formation in Crystalline Pyrene Nanoparticles Ryan D. Pensack,*,† Rachel J. Ashmore,‡ Angela L. Paoletta,† and Gregory D. Scholes*,† †
Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States Department of Chemistry, Millersville University, Millersville, Pennsylvania 17551, United States
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ABSTRACT: Pyrene is an exemplary conjugated organic chromophore with a strong propensity for self-association through an excited-state process known as excimer formation. Pyrene “excimers” and molecular “excimers” more generally are strongly avoided in some applications, such as in light harvesting, yet have found widespread use in others, such as in sensing and structure determination. Despite this disparate view and despite their widespread use, a fundamental understanding of the structure and dynamics of these collective excitations remains outstanding. In this work, we shed key insights into the nature of excimer formation in crystalline pyrene. We developed a flash precipitation procedure incorporating a polymer additive that enabled us to prepare aqueous suspensions of crystalline pyrene nanoparticles. We provide evidence that the molecular-level packing in the nanoparticles is equivalent to the equilibrium packing of the single crystal and show that excimer formation is the primary excited-state decay pathway. We find that excimer formation in the crystalline pyrene nanoparticles occurs in two stages on a picosecond time scale and suggest that intermolecular structural dynamics are largely responsible for the observed two-stage dynamics. We discuss an exciton theory description of molecular “excimers” and provide insights into their mechanism of formation, which we argue is best viewed simply as the relaxation of a singlet exciton into an excimer geometry.
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applications, pyrene excimers have a long fluorescence lifetime and exhibit a high fluorescence yield,55 making them an efficient blue-light emitter.24 Despite the use of pyrene excimers in a wide variety of applications, answering questions such as what exactly constitutes an excimer and how excimers form have remained elusive. Are excimers good or bad, for example? In the context of light harvesting applications, excimers have been labeled as badthey have been described as trap states31,36,38,56,57 that strongly limit the extent of energy migration,58−65 making them undesirable for energy conversion.66−70 As described above, however, excimers seem to have many desirable attributes, which also includes luminescence efficiencies substantially higher than that of the respective monomers.71 Addressing the question of whether excimers are good or bad necessitates a detailed understanding of what exactly excimers are. The majority of this understanding derives from classical studies of pyrene in concentrated solutions by Birks and coworkers72−75 and others.76 A critical finding of this body of work was that the diffusive encounter between one molecule in an electronic excited state and another nearby molecule in its ground electronic state results in a molecular “excimer”; that is,
INTRODUCTION Pyrene is an exemplary aromatic hydrocarbon used in a diverse range of applications including sensing of metal ions1−3 and biomolecules,4−6 evaluating the structure and dynamics of polymers,7−16 proteins,17−21 and nucleic acids,22,23 and has even found use in energy conversion applicatons.24,25 Central to pyrene’s utility is its strong propensity to form excimers, a term originally used to describe the continuum emission observed in concentrated atomic vapors.26,27 In addition to pyrene, other notable excimer-forming aromatic hydrocarbons include perylene and derivatives,28−40 fluorene and derivatives,41−45 benzene and derivatives,46,47 and other lower (n ≤ 3) acenes,45,47−50 carbazole derivatives,51,52 and DNA base pairs.53 Pyrene excimers, in particular, have found widespread use in applications for a variety of reasons. In sensing applications, pyrene monomer and excimer exhibit clearly differentiable fluorescence emission bands that can be detected at low levels with highly sensitive spectroscopic methods such as fluorescence spectroscopy.54 The strong propensity for pyrene excimer formation means that pyrene molecules have a strong tendency to self-associate. Because excimer formation is a local effecti.e., excimer emission is only observed when two pyrene molecules are in close proximity (
