Abstract:
Fluorescent probes have been extensively used for understanding the environment around the probe molecule at nanoscopic/microscopic levels. Donor-acceptor based multiparametric fluorescent probes are highly efficient to address the minimum alteration of the physical
properties of different organized media[1]. These molecules are highly environment sensitive, and they efficiently report the changes in the immediate environment by multiple fluorescence parameters: alteration of steady-state parameters like fluorescence intensity, emission wavelength, fluorescence anisotropy and the corresponding three time dependent parameters. BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes) dyes are an emerging class of fluorophores that are strongly absorbing and show high emission quantum yield[2,3].
Generally, BODIPYs have long-wavelength emissions due to a smaller HOMO-LUMO gap. Thus, when conjugated with other organic chromophores/fluorophores, fluorescence is often
observed from the charge transfer state. Electron-donating groups like carbazole alter the absorption and emission bands of the BODIPY core depending on the substitution position [4,5]. These molecules generally form a donor-acceptor system where the carbazole or TPA units act as donors and BODIPYs as acceptors.
Although there are several reports on donor-acceptor-based carbazole-BODIPY systems, the photophysics of these molecules has not been investigated in greater detail. The linking of
efficient donor-acceptor moieties at appropriate orientations may make these molecules active to sense the alteration of the micro-environment. A donor-acceptor system is expected to behave differently than a typical BODIPY system. They may exhibit high environment
sensitivity due to strong intramolecular charge transfer characteristics. It has been reported that
the bis-BODIPY molecules show higher absorbance, more intense emission, and efficient
charge transfer behavior than their corresponding mono-BODIPYs[6,7].
Thus, our objective is to understand better, the impact of the conjugation of a carbazole
derivative to a BODIPY core at the meso-position. Further, we wish to compare the changes in
properties of the system when the same carbazole moiety is conjugated to a second BODIPY
unit at its meso site. In the current study, we have carried out a comparative study of the
photophysics of two molecules, N-butylcarbazole BODIPY (CBZ-BDP) and N-butylcarbazole
bis-BODIPY (CBZ-bisBDP). Studies include the evaluation of various photophysical
parameters such as steady-state absorption, fluorescence, and time-dependent fluorescence
decay. We have also conducted some preliminary studies on the application potential of the
two dyes.

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