Metal-enhanced S2 fluorescence from azulene

Abstract

In this paper, we report the first observation of metal-enhanced S 2 emission at room and low temperature (77K). The S 2 emission intensity of Azulene is enhanced by close proximity to Silver island films (SiFs). In this regard, a ≈ 2-fold higher S 2 fluorescence intensity of Azulene was observed from SiFs as compared to a glass control sample. This suggests that S 2 excited states can couple to surface plasmons and enhance S 2 fluorescence yields, a helpful observation in our understanding the interactions between plasmons and lumophores, and our continued efforts to develop a unified plasmon-lumophore/fluorophore theory. KeywordsS 2 emission; Metal-Enhanced S 2 Fluorescence; Radiative Decay Engineering; Surface Enhanced Fluorescence; Plasmon Controlled Fluorescence; Radiating Plasmons; Azulene; Metal-Enhanced Fluorescence IntroductionThe location of electronic energy levels in molecules and the probability and polarization of radiative transitions between ground and excited state can be accurately predicted by quantum mechanical theory [1]. For most molecules, no excited singlet states (S 2 , S 3 …) have been observed to emit light upon excitation, the same being true for triplet states (T 2 , T 3 …). This is summarized in Kasha's rule: The emitting electronic level of a given multiplicity is the lowest excited level of that multiplicity [2].As the first authentic example of a molecule which violates Kasha's rule, azulene has been the subject of numerous spectroscopic and photophysical investigations [3][4][5][6]. Beer and LonguetHiggins discovered the second excited level emits light upon excitation, producing the anomalous (S 2 to S 0 ) fluorescence of azulene in 1955 [6], whereas that normal S 1 to S 0 fluorescence and T 1 to T 0 phosphorescence were much too weak to be observed. The absence of S 1 to S 0 fluorescence suggests that S 1 to S 0 internal conversion is a very rapid progress in Azulene, this supposition being confirmed by the lifetime of the S 1 state being determined as 1.9 psec [7]. Compared with the analogous spacing of the isomer Naphthalene, Azulene has much wider spacing between its zero -point levels of the first two excited singlet states (S 1 , *Corresponding Author E-mail: geddes@umbi.umd.edu Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. [19], and the application of metallic surfaces to amplified (ultra fast and sensitive) assay detection [20]. Our laboratorys' current interpretation of MEF has been explained by a model whereby non-radiative energy transfer occurs from excited distal fluorophores, to the surface plasmon electrons in n...

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Metal-enhanced superoxide generation: A consequence of plasmon-enhanced triplet yields

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