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[structures: see text] Seven new 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dyes with phenolic or naphtholic subunits on position 8 and with substituents having different electron driving forces on positions 3 and 5 were synthesized. Their absorption and steady-state fluorescence properties were investigated as a function of solvent. The novel compounds, with the exception of 4,4-difluoro-8-(4-hydroxyphenyl)-3,5-bis-(4-methoxyphenyl)-4-bora-3a,4a-diaza-s-indacene, are characterized by absorption maxima in the range 493-515 nm and small (400-600 cm(-1)) Stokes shifts. The exceptional dye has absorption maxima in the 570-580 nm region and fluorescence emission maxima around 610-620 nm, depending on the solvent. In aqueous solution, the dyes show a large fluorescent enhancement upon increasing the acidity of the solution. They can be used in aqueous solution as fluorescent pH probes excitable with visible light, with pKa values ranging from 7.5 to 9.3, depending on the substitution pattern on positions 3, 5, and 8.

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Derivatives of p-cresol 1-4 were synthesized, and their photochemical reactivity, acid-base, and photophysical properties were investigated. The photoreactivity of amines 1 and 3 is different from that for the corresponding ammonium salts 2 and 4. All compounds have low fluorescence quantum yields because the excited states undergo deamination reactions, and for all cresols the formation of quinone methides (QMs) was observed by laser flash photolysis. The reactivity observed is a consequence of the higher acidity of the S1 states of these p-cresols and the ability for excited-state intramolecular proton transfer (ESIPT) to occur in the case of 1 and 3, but not for salts 2 and 4. In aqueous solvent, deamination depends largely on the prototropic form of the molecule. The most efficient deamination takes place when monoamine is in the zwitterionic form (pH 9-11) or diamine is in the monocationic form (pH 7-9). QM1, QM3, and QM4 react with nucleophiles, and QM1 exhibits a shorter lifetime when formed from 1 (τ in CH3CN = 5 ms) than from 2 (τ in CH3CN = 200 ms) due to the reaction with eliminated dimethylamine, which acts as a nucleophile in the case of QM1. Bifunctional QM4 undergoes two types of reactions with nucleophiles, giving adducts or new QM species. The mechanistic diversity uncovered is of significance to biological systems, such as for the use of bifunctional QMs to achieve DNA cross-linking.

The photophysical behavior of BCECF [2′,7′-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein]scurrently the most widely used fluorescent pH indicator for near-neutral intracellular pH measurementsshas been explored by using absorption and steady-state and time-resolved fluorescence measurements. The influence of ionic strength as well as total buffer concentration on the absorbance and steady-state fluorescence has been investigated. The apparent acidity constant of the pH indicator determined by absorbance and fluorescence titration is dependent on the added buffer and salt concentrations. A semiempirical model is proposed to rationalize the variations in the apparent pK a values. The excited-state proton exchange of BCECF at physiological pH becomes reversible upon addition of phosphate buffer, inducing a pH-dependent change of the fluorescence decay times. Fluorescence decay traces collected as a function of total buffer concentration and pH were analyzed by global compartmental analysis yielding the following values of the rate constants describing excited-state dynamics of BCECF: k 01 ) 3.4 × 10 8 s -1 , k 02 ) 2.6 × 10 8 s -1 , k 21 ≈ 1 × 10 6 M -1 s -1 , k 12 B ) 1.4 × 10 8 M -1 s -1 , and k 21 B ) 4.3 × 10 7 M -1 s -1 .

In this report, we describe the fluorescence kinetics and the deterministic identifiability of the intermolecular excited-state proton dissociation reaction and how the addition of pH buffer affects both. In the absence of buffer, the time-resolved fluorescence decays as a biexponential function with decay times that are invariant with pH. The information that the proton association rate in the excited state is negligible in combination with fluorescence decay traces measured at different pH, excitation, or emission wavelengths does not provide enough useful information for the unique determination of the rate constants and the spectral parameters related to absorption and emission. Hence, the model of intermolecular excited-state proton dissociation in the absence of pH buffer is not identifiable. When a pH buffer is added to this photophysical system, the proton exchange becomes reversible and the decay times now are a function of pH and buffer concentration. The deterministic identifiability analysis shows that for the unique determination of all rate constants one should collect a minimum of three fluorescence decays characterized by at least two different pH and at least two different nonzero buffer concentrations. In addition to these three traces, minimally one biexponential fluorescence trace corresponding to the pH probe in the absence of buffer has to be recorded. The requirement that at least two of these traces should be collected at the same pH, excitation, and emission wavelengths leads to unique identifiability.

Hydroxymethylphenols strategically substituted with the 2-hydroxy-2-adamantyl moiety, AdPh 8-10, were synthesized, and their photochemical reactivity was investigated. On excitation to the singlet excited state, AdPh 8 undergoes intramolecular proton transfer coupled with a loss of H(2)O giving quinone methide 8QM. The presence of 8QM has been detected by laser flash photolysis (CH(3)CN-H(2)O 1:1, tau = 0.55 s) and UV-vis spectroscopy. Singlet excited states of AdPh 9 and 10 in the presence of H(2)O dehydrate giving 9QM and 10QM. Photochemically formed QMs are trapped by nucleophiles giving the addition products (e.g., Phi for methanolysis of 8 is 0.55). In addition, the zwitterionic 9QM undergoes an unexpected rearrangement involving transformation of the 2-phenyl-2-adamantyl cation into the 4-phenyl-2-adamantyl cation (Phi approximately 0.03). An analogous rearrangement was observed with methoxy derivatives 9a and 10a. Zwitterionic 9QM was characterized by LFP in 2,2,2-trifluoroethanol (tau = 1 mus). In TFE, in the ground state, AdPh 10 is in equilibrium with 10QM, which allowed for recording the (1)H and (13)C NMR spectra of the QM. Introduction of the adamantyl substituent into the o-hydroxymethylphenol moiety increased the quantum yield of the associated QM formation by up to 3-fold and significantly prolonged their lifetimes. Furthermore, adamantyl substituent made the study of the alkyl-substituted quinone methides easier by LFP by prolonging their lifetimes and increasing the quantum yields of formation.

Excitation of 2-hydroxy-3-(diphenylhydroxymethyl)anthracene (7) to S1 initiates photodehydration giving the corresponding quinone methide (QM) that was detected by laser flash photolysis (LFP) in 2,2,2-trifluoroethanol, TFE (λ = 580 nm, τ = 690 ± 10 ns). The QM decays by protonation, giving cation (λ = 520 nm, τ = 84 ± 3 μs) which subsequently reacts with nucleophiles. The rate constants in the reactions with nucleophiles were determined by LFP, whereas the adducts were isolated via preparative photolyses. The photogeneration of QMs in the anthrol series is important for potential use in biological systems since the chromophore absorbs at wavelengths > 400 nm. Antiproliferative investigations conducted with 2-anthrol derivative 7 on three human cancer cell lines showed higher activity for irradiated cells.