Photophysical Studies of Metalloporphyrins for Sensitized Noncoherent Photon Upconversion

Noncoherent photon upconversion (NCPU) realized by means of triplet-triplet annihilation (TTA) is of significant recent interest because of the possibility of using this phenomenon for increasing the efficiency of dye-sensitized solar cells (DSSCs). Efficiencies can be achieved if the near-IR parts of the solar spectrum can be absorbed and used in TTA. However, realizing this potential is not trivial. Dual absorption and TTA usually populates a higher excited singlet state (Sn, n ≥ 2) of the absorber molecule and the fate of this state can be a critical factor controlling efficiencies and hence the potential efficiency improvements in DSSCs. With a motivation to understand the photophysical processes that decides the fate of the product state of TTA, the role of the S2 state of a metalloporphyrin and a fullerene in solution-phase NCPU has been investigated using photophysical techniques. TTA in the model porphyrin, zinc(II) meso-tetraphenylporphine (ZnTPP) realized by excitation with a green laser, was found to occur through a short-range Dexter-type energy transfer mechanism. It was also found, contrary to a previous suggestion, that a Förster-type energy transfer cannot occur from the short-lived S2 state of ZnTPP to an acceptor molecule. It was hypothesized that prior aggregation of ZnTPP and the acceptor molecule should exist to enable such an energy transfer. For blue emitter (BE) molecules with triplet energies lower than that of ZnTPP, a triplet-triplet energy transfer (TTET) from ZnTPP to the BE followed by TTA in the BE populates its S1 state. However, this is not possible for BEs with triplet energies significantly greater than that of ZnTPP. In this case, it is proposed that the triplet ZnTPP forms a triplet exciplex with a ground state BE. The triplet exciplex then annihilates with a second triplet ZnTPP to form the S1 state of the BE. For the studies of the NCPU in C60, the BEs chosen were having triplet energies similar or slightly lower than that of C60. The NCPU in these systems follows the well-established mechanism of TTET from C60 to the BE followed by TTA in the BE to produce its S1 state. However, for systems in which the triplet energies of C60 and BE are similar, the NCPU process is controlled by entropic factors which in turn can be controlled by the concentration of the BE. Compared to this system, NCPU in a system in which the triplet of C60 lies slightly higher than that of the BE was found to be more efficient. The involvement of the higher excited singlet states of C60 (Sn, n ≥ 2) can be considered as insignificant because of the proximity of these states to the S1 state and the large rates of internal conversion from these states to the S1 state. Because triplets can be quenched by molecular oxygen, the rate of oxygen diffusion in devices based on NCPU needs to be evaluated. With this objective, upconverted S2 emission from ZnTPP produced by TTA was used as a tool to measure the rate of oxygen diffusion in a thin polymer film. It was found that the oxygen permeability is controlled by the characteristics of the polymer matrix, including its water content and its distribution. C60 has been evaluated as a possible electron acceptor for the TTA-produced S2 state of ZnTPP. Quenching of the S2 and S1 fluorescence of ZnTPP by added C60, with more efficient quenching for the Soret-excited ZnTPP+C60 species, was demonstrated. Significant ground state aggregation between ZnTPP and C60 was proven. However this factor could not account for the observed difference in the rates of quenching of the S2 and S1 fluorescence of the excited porphyrin. The difference in the rates of quenching was attributed to differences in the free energies of electron transfer from the S2 and S1 states of ZnTPP to C60. Conclusive evidence for this was obtained from transient absorption studies carried out at the University of Melbourne, which demonstrated the formation of a long-lived charge transfer state upon Soret-excitation of the porphyrin-C60 complex. However, the involvement of short-range Dexter type energy transfer could not be ruled out in this system