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We show that unsymmetric BODIPY compounds with one, two, and three methyl groups can be synthesized easily and efficiently by the unsymmetric reaction method. Their steady state and time-resolved fluorescence properties are examined in solvents of different polarity. These compounds show high fluorescence quantum yields (0.87 to 1.0), long fluorescence lifetimes (5.89 to 7.40 ns), and small Stokes shift (199 to 443 cm−1). The methyl substitution exhibits influence on the UV-Vis absorption and fluorescence properties, such as the blue shift in emission and absorption spectra. It is the number rather than the position of methyls that play major roles. Except for 3 M-BDP, the increase in the number of methyls on BODIPY core leads to the increase in both fluorescence quantum yield and radiative rate constant, but causes the decrease in fluorescence lifetime. H-bonding solvents increase both the fluorescence lifetime and quantum yields. The methylated BODIPYs show the ability to generate singlet oxygen (1Δg) which is evidenced by near-IR luminescence and DPBF chemical trapping techniques. The formation quantum yield of singlet oxygen (1Δg) for the compounds is up to 0.15 ± 0.05.

The fluorescence spectra, fluorescence quantum yield, and fluorescence lifetime of Acridine Red (AR), Pyronin Y (PYY), and Pyronin B (PYB) in aqueous and organic solvents were measured by steady state fluorescence, time-correlated single photon counting, and electronic absorption methods. The rate constants of radiation and non radiation process (kf and kic) were calculated to elucidate the structural effect on the fluorescence mechanism. The data for each compound are compared with that of the corresponding rhodamine dye. AR showed significant longer lifetime and higher quantum yield than PYY and PYB, because the alkyls on N atom enhance the internal conversion (IC), the longer the alkyl the faster the IC. However, the structural variation does not alter the rate constant of radiation process (kf) but does change kic significantly. The phenyl in Rhodamine B or Rhodamine 6G shows only a slight effect on the fluorescence properties. Ethanol is the solvent in which all five compounds exhibit longest lifetime and highest fluorescence quantum yield.

The fluorescence lifetime (τf), emission quantum yield (Φf), absorption and emission spectral data of 20 fluorescein derivatives were measured under the same conditions by using time-correlated single photon counting, steady state fluorescence and absorption methods to get comparable data. Based on the results, the factors and mechanism that control the fluorescence properties of the fluorescein dyes are discussed. Both Φf and τf are remarkably dependent on the substitution on either xanthene or phenyl rings, but their ratio (Φf/τf), i.e. rate constant of radiation process, is a constant value (0.20 × 10(9) s(-1)). The rate constant of nonradiation process, on the other hand, is varied with both the structure and the solvent used.

The absorption, fluorescence, and transient absorption spectra of Tetra(alpha-amino) zinc phthalocyanine, ZnPc(alpha-NH2)4, have been measured in polar solvents and compared with that of ZnPc(alpha-R)4 (R = H, NO2, OCH(CH3)2). While the latter three showed the typical photophysics of phthalocyanines, ZnPc(alpha-NH2)4 exhibits distinct spectral properties, a very low fluorescence quantum yield and a relatively long fluorescence lifetime. These observations are explained by the substantial charge transfer characters in the absorption and fluorescence spectra of ZnPc(alpha-NH2)4. NMR indicates that intramolecular H-bonding makes atoms in NH2 actually coplanar with other elements of ZnPc(alpha-NH2)4. The local excited state is non emissive and the weak emission is assigned to its charge transfer state. The transient absorption bands from laser flash photolysis located at 630 nm, 645 nm is assigned to the mono-charge transfer state, while that at 545 nm is assigned to the di-charge transfer state.

Tetra[α-(4-hydroxyphenoxy)] zinc phthalocyanine, ZnPc(α-OPhOH)(4), was synthesized and its photophysics was found to be sharply pH dependent. Dual fluorescence emission around 700 nm was observed when it is dissolved in basic solution. The fluorescence of the phthalocyanine can be sharply switched off at pH 9.1 due to the intramolecular photoinduced electron transfer (PET) in ZnPc(α-OPhONa)(4), formed by the deprotonation of ZnPc(α-OPhOH)(4). The photophysics of both ZnPc(α-OPhOH)(4) and ZnPc(α-OPhONa)(4) were studied in detail by UV-vis absorption, steady state and time-resolved fluorescence and transient absorption (TA) to reveal the fluorescence quenching mechanism. Intra-molecular PET in ZnPc(α-OPhONa)(4) from the donor, PhONa subunits, to the acceptor, ZnPc moiety, was characterized by the much smaller fluorescence quantum yield (0.003) and lifetime (0.20 ns). PET was further evidenced by the occurrence of charge separation state (CSS) in TA spectra, i.e. the bands due to anion radical of ZnPc and phenol radical. The lifetime of the charge separation state is ca. 3 ns, the efficiency of PET is ca. 99% and the rate constant of PET is 2.3 × 10(10) s(-1).

The deprotonation of dibenzofluorescein (DBFL), a long wavelength fluorescence probe, results in the simultaneous occurrence of neutral form, monoanion and dianion under physiological conditions. The fluorescence properties of the former two cannot be measured directly because they are always coexistent with some other species. By measuring the fluorescence under various pHs we computed the fluorescence parameters for each species involved in the prototropic equilibria of DBFL, including each species’ emission spectrum, excitation spectrum, emission and excitation maximum, fluorescence quantum yield and lifetime. It was found that the monoanion is the most fluorescent chromospheres (Φ f = 0.66, compared to Φ f = 0.25 for dianion, 0.18 for cation and 0.0 for the neutral form). Together with the computed pK as, we are able to suggest that the monoanion plays a major role under physiological conditions when DBFL is used as a fluorescence probe, contrary to the assumption in literature.