Fluorescent small-molecule probes are useful tools for many different fields of biological and biomedical research. The structure, physiochemical and photophysical properties of the fluorescent dye determine the types of studies and assays that are possible using probe conjugates. There is continuing need for the development of new fluorescent dyes, particularly for live cell imaging studies. Desirable features include bright, efficient emission properties, biological and photostability, lack of toxicity, favorable solubility and membrane permeability. We have recently developed a new fluorescent dye containing the triazaborolopyridinium core, which was first reported by our group in 2011. This new scaffold was derived from hydrazones of pyridine-2-yl hydrazine, and dyes containing this core structure are commonly referred to as "HPY" dyes. In this study, the photophysical properties of a series of new monomeric and dimeric HPY probes derived from 9-fluorenone or cyclopentanone were characterized as part of ongoing efforts to expand the HPY dye palette. In general, important recognized photophysical characteristics of the HPY dyes include increased brightness in nonpolar solvents, maximum brightness and fluorescence intensity was observed in 1-octanol. The presence of extensive conjugation in the hydrazone, as represented by the 9-fluorenone and lack thereof in the cyclopentanone derivatives are described in this study, results in the shift of absorption maxima to longer wavelengths and lower energy excitation. The identity of substituents at position 5 of the pyridine ring also effects the absorption/emission features, and enables further fine-tuning of the photophysical properties, and physicochemical properties such as solubility provided. The brightest monomeric fluorenylidene dye series was found for derivatives with direct attachment of a carboxyamide group at position 5, exhibiting excitation maxima at ~490 nm. The presence of aliphatic substituents at position 5 resulted in lower energy excitaion (530 nm), with trends for brightness depending on the hybridization of the substituent: sp3 > sp > sp2. The presence of sulfur directly attached to the 5-position of the pyridine ring diminished the brightness significantly ( up to 10 fold) in the fluorenylidene series. This effect was further influenced by the oxidation state of the sulfur atom; the sulfide derivative exhibits a relatively large 87 nm Stoke's shift with [Lambda][subscript ems]=618 nm, upon excitation at 530 nm. The electron poor sulfonyl exhibited a hypsochromic shift in excitation maxima to 490 nm, similar to the carboxylic analogs, and the sulfone was twice as bright as the sulfide analog, with a narrow 24 nm Stoke's shift and [Lambda][subscript ems]=531 nm. Within the fluorenylidene series, homo-dimers containing a flexible linkage between the dyes displayed reduced quantum efficiency and brightness relative to the monomeric counterpart, mimicking the effect of aggregation causing self-quenching in the solid state. The direct connection of two HPY (530) dyes through a short, rigid, conjugated ethynyl linkage in homodimer (8c) resulted in an interesting dimer with significantly increased quantum efficiency and overall brightness in comparison with monomeric dyes possessing an sp-hybridized substituent at position 5. A series of three hetero-dimers possessing flexible non-conjugated linkages of varying length were evaluated. The spectra obtained by excitation of the sp3-linked heterodimer (8f) 490 nm displayed the highest absorptivity constant (77,400) and greatest overall brightness (62,000) relative to the entire series of HPY dyes that have been characterized to date. These attributes are expected to be well suited for application in chemical biology and live cell imaging studies.