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The main aim of work presented here is to understand photophysical processes of organic dyes and to design better organic molecules/systems which can be applied in many applications such as solar cells, photodynamic therapy, and fluorescence sensors. Developments of novel multichromophore organic materials for the above mentioned applications were made using computational tools. A brief description of the history of computational chemistry was given based on the photochemistry of organic dyes in the introductory chapters and also the importance of basis sets and functionals was discussed in order to produce accurate computational results. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were performed to understand the photophysical processes in the porphyrin-perylene bisimide (HTPP-PDI) dyad that exhibited long-lived triplet states. The DFT results show that breaking the rigidity of PDI in HTPP-PDI was responsible for the generation of long-lived triplet states. Furthermore, six porphyrin derivatives were designed by introducing a 4,4’-dicarboxybutadienyl functional group to the porphyrin moiety and studied to investigate the substituent effects on the non-coplanarity, molecular orbitals, and excitation wavelength of the porphyrin donor. Five of the six proposed porphyrin derivatives are promising donors in the HTPP-PDI dyad to replace HTPP for its potential use in photodynamic therapy. Six donor- accepter(s) systems were designed for their potential application in solar cells. Four D-A1-A2 architectural triads, MTPA-TRC-AEAQ, MTPA-TRC-HTPP, MTPA-TRC-PDI, and MTPA-TRC-PBI were designed. The cascade electronic energy levels were obtained and experimentally observed, which lead to sequential electron transfers from 1MTPA* to TRC and then to AEAQ (HTPP/PDI/PBI) module as well as a hole transfer from 1AEAQ*(1HTPP*/1PDI*/1PBI*) to MTPA module. Therefore, all the D-A1-A2 systems we have designed are ambipolar. Interestingly, the lifetime of charge separated states of the newly designed MTPA*+-TRC-AEAQ*- was elongated to 650 ns, an eightfold of that of the donor-acceptor MTPA-TRC parent molecule (80 ns). However, different charge separated state lifetimes were obtained for MTPA*+-TRC-PDI*-(22ns) and MTPA*+-TRC-PBI*-(75ns). The photophysical results suggested that the charge separated state may decay to the triplet state when the charge separated state exhibits a higher energy level than the triplet state. Further, the photovoltaic tests indicated potential applications of MTPA-TRC-AEAQ in solar cells. DFT and TDDFT calculations were performed together with experimental studies to explore the nature of fluorescence enhancement in the anthracene-based sensor after the addition of Zn2+. A 23-fold fluorescence emission was quenched via non-radiative decay pathway in the absence of Zn2+. However, when the Zn2+ chelated to the sensor fluorescence intensity was increased remarkably. A 32-fold fluorescence increase was overserved and calculation results suggested this could be due to the inhibition of the electron-transfer pathway and enhanced rigidity of sensor-Zn2+ complex. The response selectivity of Zn2+ over Ca2+, Mg2+, Cu2+, and Hg2+ ions was also studied using DFT calculations and it was found that Zn2+ has a strong binding affinity to the sensor, which could be a potential application in the detection of Zn2+.

Fluorescence lifetime determination and allied studies find application in spectroscopy in general and fiber optic biosensors in particular. Instruments and sensors cited in literature however use open loop, intensity based techniques with sophisticated detectors and components. We propose phase sensitive signal processing schemes to estimate the fluorescence lifetime using simple detectors and components, without compromising on accuracy. The performance of the schemes proposed is analysed and contrasted from a communications (signals and systems) point of view. The resolution and sensitivity limits imposed in processing the signal, by systematic errors and additive noise, are derived for the schemes suggested. It is found that systematic errors impose a phase resolution limit of about 2°. We then study the suitability of different detectors and channels for application in phase sensitive fluorescence biosensors we analyse the effect of systematic limitations as well as additive noise, in the detection/transmission process, from the point of view of the components used. Certain fundamental limits of operation in terms of excitation intensities are derived for different detector-channel combinations, with a view to obtain a given resolution. A photodiode used with a fiber bundle is found to be sufficient for accurate phase read outs with 10"4 radians resolution. A PMT used in conjunction with a multimode fiber serves as a very good device for microsensing applications Lastly, the biosensor for oxygen sensing, the ruthenium complex, is studied for standardisation of the sensor. We examine the quenching of fluorescence, the repeatability and reusability of the sensor, the stability of the instrument and such.

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