Spectral properties of probes based on pyrene and piperazine: the singlet and triplet route of deactivation

Abstract

Probes, composed of pyrene (Py) and piperazine (7,15-diazadispiro[5.1.5.3]hexadecane or 7,15-diazadispiro[5.1.5.3]hexadecane-14,16-dione) (PA) linked directly or through methylene group (Me) were prepared. Probe with direct link is 15-(1-pyrenyl)-7,15-diazadispiro[5.1.5.3]hexadecane (PyPA). In 15-(1-pyrenylmethyl)-7,15-diazadispiro[5.1.5.3]hexadecane-14,16-dione (PyMePAH) pyrene is connected with piperazine through methylene bridge. The parent amine of PyMePAH was oxidized to the form of stable nitroxyl radical (PyMePAO) in position 7. Oxidation of the amine PyPA did not yield any aminooxide. The deactivation processes of the probes were monitored by fluorescence (singlet excited state route) as well as by nanosecond laser flash photolysis techniques (triplet excited state route). The spectral features of the multifunctional probes were compared. Spectral measurements were performed in methanol and polymer matrices as polymethyl methacrylate (PMMA) and polystyrene (PS). The absorption and the fluorescence spectra of the probe PyPA do not exhibit vibrationally resolved bands while the probes with the methylene link PyMePAH (or PyMAPAO) have well resolved vibrational structure similar to that obtained with pyrene. Absorption spectra of the probes have got similar shape in solvent as well as in polymer matrix. The quantum yield of fluorescence of the PyPA relative to anthracene is rather high as compared with PyMePAH or PyMePAO in aerated methanol. The quantum yield of the PyMePAH or PyMePAO relative to anthracene is higher in deaerated methanol and polymer matrices except polystyrene. The lifetime of fluorescence of PyPA is rather short about 5 ns and those of PyMePAH or PyMePAO lies in the range above 10–300 ns. It is strongly dependent on the experimental conditions. The extent of intramolecular quenching expressed as the ratio of relative quantum yields of PyMePAH and PyMePAO and respective lifetimes is rather low indicating that the intramolecular quenching is not very effective. Nanosecond laser flash photolysis was also used to examine the triplet excited state route of deactivation of these novel probes. The formation of this excited state which exhibits a T–T absorption band in the range 360–550 nm, occurred at 355 nm laser excitation in these probes as well as for parent pyrene. Triplet states of pyrene chromophore of all probes were efficiently quenched by oxygen with rate constant about 2 × 10 9 dm 3 mol −1 s −1. Intermolecular quenching of triplet states of the all probes by N-oxyl radical (1-oxo-2,2,6,6-tetramethylpiperidine, TEMPO) was rather low. The rate constant was evaluated to about 5 × 10 7 dm 3 mol −1 s −1. The intramolecular triplet quenching expressed as the ratio of the rate constants of triplet decay of the oxidized probe PyMePAO to PyMePAH was 5 in methanol and 7.7 in cyclohexane.