A laser flash photolysis set-up is described for nanosecond time resolved study of transient species. The system described uses 30 nsec, 355-nmuv laser pulse as the photolysing source. Laser flash photolysis of acridine in various organic solvents (polar, non-polar and hydroxylic) has been carried out. The quantum yields of intersystem crossing and C-radicals (in hydroxylic solvents) have been calculated from the transient absorption measurements. The quenching of the triplet state by chloranil is efficient in all the solvents and free radical ions are formed as products only in the polar solvents. Biphotonic ionisation of acridine occurs at high laser intensities and is more efficient in hydroxylic solvents compared to benzene or acetonitrile.

Maximum entropy method (MEM) was used for resolving multiple decay components in synthetic and experimental time-resolved fluorescence decay. The distribution of lifetimes is unimodal or multimodal and there is a one-to-one correspondence between the results of MEM and discrete exponential analysis. The distribution is symmetric in log(τ) space and approximately Gaussian. The width of the distribution is sensitive to several factors related to the experimental or analysis conditions, such as the peak count, discretization in τ space, completeness of decay and theχ² stopping criterion. Therefore, the width of the distribution cannot be a useful indicator of the extent of heterogeneity of lifetimes in a sample of a complex biological system.

Time-resolved area normalized emission spectroscopy (TRANES) is a new method for the analysis of fluorescence of dyes in complex chemical and biological systems (A S R Koti, M M G Krishna and N Periasamy, 2001,J. Phys. Chem.105, 1767). The model-free method extends the power of time-resolved emission spectroscopy (TRES) analysis and removes the ambiguity in the interpretation when the emission spectrum is time-dependent. Observation of an isoemissive point in TRANES analysis of fluorescence is an unambiguous indication for the presence of two emissive species in the sample. The isoemissive point occurs at a wavelength where the ratio of the radiative rates of the two species is equal to the ratio of their total radiative rates. The polarity-sensitive nile red dye shows time-dependent emission spectra in the organized bilayer assemblies of TX micelle and bilayer egg-phosphotidylcholine (egg-PC) membrane. Time-dependent spectra in complex systems support many important models (solvation model and heterogeneity in the ground and/or excited state). TRANES analysis shows that the fluorescence emission of nile red in TX micelle and egg-PC membrane is due to two emissive species solubilized in different sites.

Five heteroleptic, cyclometalated (C^∧N) Iridium(III) complexes of acetylacetone (acac) and 1-phenyl-isoquinoline (piq) derivatives, Ir(acac)(piq) ₂, Ir(acac)(2,4-difluoro-piq) ₂, Ir(acac)(4-trifluoromethylpiq) ₂, Ir(acac)(4-N,N-dimethyl-piq) ₂, Ir(acac)(4-acetyl-piq) ₂, were synthesized and characterized. The ((C^∧N) ₂ Ir(acac) complexes in toluene showed phosphorescence (λ_max = 598 nm to 658 nm) with quantum yields (0.1 to 0.32) and microsecond lifetimes (0.43 to 1.9 μs). The complexes were non-luminescent in thin films due to self-quenching but luminescent when lightly doped (5%) in a host organic material, 4,4' -Bis(Ncarbazolyl)- 1,1' -biphenyl (CBP). The HOMO levels determined using cyclic voltammetric oxidation potentials were in the range−5.48 to−5.80 eV. Electroluminescence properties and performance of the Ir complexes dopedin CBP (active layer) were studied in a multilayer (ITO/F4TCNQ/TPD/doped CBP/BCP/LiF/Al) organic light emitting device (OLED). The electroluminescense (EL) spectra of the device matched with the phosphorescent spectra of the Ir complexes. The turn-on voltage at ∼4.5 V, maximum brightness of 7600 cd/m² and current efficiency of ∼7.0 cd/A at a brightness of ∼100 cd/m² indicate that these are promising OLED materials.