**1. Introduction**

In the constantly expanding multi-disciplinary science world, fluorescent dyes attract the attention of researchers. 4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (abridged as BODIPY) dyes are compounds that are rapidly increasing in importance among fluorescent organic dyes [1]. BODIPY core is formed from the complexation of a dipyrromethene ligand with a disubstituted boron moiety, typically in the form of BF2 [1–4]. The first BODIPY was synthesized by Treibs and Kreuzer in 1968 by accident through the combining of 2,4-dimethylpyrrole and acetic anhydride in the presence of BF3.OEt2 [5]. Although the basic procedure for the synthesis of BODIPY core usually starts from a simple pyrrole condensation with a highly electrophilic carbonyl compound (e.g., aldehyde, acid anhydride, and acyl chloride), the three major routes of BODIPY synthesis are from pyrroles and acid chlorides or pyrroles and aldehydes or ketopyrroles [1]. BODIPY derivatives absorb strongly visible region, have relatively sharp emission peaks, possess high fluorescence quantum efficiencies (Φ ca. 0.5–0.8), high molar absorption coefficients (ε >7 x 104 M−<sup>1</sup> cm<sup>−</sup><sup>1</sup> ), and have relatively small Stokes' shift (around 10 nm) [1–4]. Besides, most BODIPY dyes indicate thermal and photostability in solid and solution phases are highly soluble in most organic solvents and are insensible to solvent polarity and pH [6, 7]. The BODIPY core (see the core structure and numbering in **Figure 1**) can be easily modified to bear desired functionalities at α-, β-, and mesopositions as well as through substitution of the fluorine. The addition of functional groups to the BODIPY core can have varying effects depending upon the placement and symmetry of the substituent [8, 9]. Symmetrical BODIPYs (substitution from 1,7- or 3,5-positions) appear to produce more red-shifted absorptions compared

**Figure 1.** *Chemical structure and numbering of BODIPY core.*

to either equally substituted asymmetric counterpart (substitution from 1,3- or 5,7-positions). However, the greater substitution of the BODIPY core does not necessarily produce a larger bathochromic shift, as depicted upon the comparison of the penta-substituted BODIPY (substitution from 1,3,5,7- and 8-positions) to the tetra-substituted BODIPY (substitution from 1,3,5,7-positions) [8, 9]. Red to near infrared (NIR) shifts are generally attained owing to the straightforward modification to the BODIPY core with the extension of the degree of π-delocalization. Also, the emissive behavior of BODIPY fluorophores is much affected by steric and electronic interactions of substituent moieties. Rotation of pendant components as well as their electron-donating or withdrawing effects on the conjugated core greatly influences both the brightness and absorptive and emissive properties of BODIPY [10].

Due to these excellent photophysical characteristics, BODIPY dyes increase their potential using in different applications such as fluorescent labels for biomolecules and cellular imaging [11–15], light-emitting devices [16–18], drug delivery agents [19–21], photosensitizers [22–24], fluorescent switches [25], chemosensors [26–29], energy transfer cassettes [30–33], and solar cells [34–37]. In this chapter, general photophysical properties of BODIPY and aza-BODIPY derivatives and recent studies on the photophysical properties of these dyes are presented.
