A comprehensive photophysical study has been carried out on a series of platinum(II)-containing phenylethynyl oligomers. The compounds are composed of a platinum center attached to two tributylphosphine ligands and two ligands that vary the number of repeat phenyl-ethynyl units (-PhsCtC-). The objective of this work is to understand the effects of increased conjugation and the influence of the platinum on the overall electronic structure of the molecule. This was done by utilizing steady-state absorption, steady-state emission, picosecond pump-probe, and nanosecond laser flash photolysis techniques. The effect of increased conjugation is a red shift of S 0 -S 1 and T 1 -T n and an increase in both the S 0 -S 1 and T 1 -T n molar extinction coefficients. The spin-orbit coupling effect of platinum on the ground and excited-state properties is reduced with increased conjugation length because the S 0 -S 1 transition is more localized on the ligand. As the ligand becomes larger, it takes on more π-π* character and therefore is spatially further away from the platinum center.

To explore the photophysics of platinum acetylide chromophores with strong two-photon absorption cross-sections, we have investigated the synthesis and spectroscopic characterization of a series of platinum acetylide complexes that feature highly pi-conjugated ligands substituted with pi-donor or -acceptor moieties. The molecules (numbered 1-4) considered in the present work are analogs of bis(phenylethynyl)bis(tributylphosphine)platinum(II) complexes. Molecule 1 carries two alkynyl-benzothiazolylfluorene ligands, and molecule 2 has two alkynyl-diphenylaminofluorene ligands bound to the central platinum atom. Compounds 3 and 4 possess two dihexylaminophenyl substituents at their ends and differ by the number of platinum atoms in the oligomer "core" (one vs two in 3 and 4, respectively). The ligands have strong effective two-photon absorption cross-sections, while the heavy metal platinum centers give rise to efficient intersystem crossing to long-lived triplet states. Ultrafast transient absorption and emission spectra demonstrate that one-photon excitation of the chromophores produces an S1 state delocalized across the two conjugated ligands, with weak (excitonic) coupling through the platinum centers. Intersystem crossing occurs rapidly (Kisc approximately 1011 s-1) to produce the T1 state, which is possibly localized on a single conjugated fluorenyl ligand. The triplet state is strongly absorbing (epsilonTT > 5 x 104 M-1 cm-1), and it is very long-lived (tau > 100 micro s). Femtosecond pulses were used to characterize the two-photon absorption properties of the complexes, and all of the chromophores are relatively efficient two-photon absorbers in the visible and near-infrared region of the spectrum (600-800 nm). The complexes exhibit maximum two-photon absorption at a shorter wavelength than 2lambda for the one-photon band, consistent with the dominant two-photon transition arising from a two-photon-allowed gerade-gerade transition. Nanosecond transient absorption experiments carried out on several of the complexes with excitation at 803 nm confirm that the long-lived triplet state can be produced efficiently via a sequence involving two-photon excitation to produce S1, followed by intersystem crossing to produce T1.

We present results of a joint computational and experimental study for a series of annulated metalloporphyrins in order to establish structure-property relationships. Specifically, we have examined the effects of substitution by meso-tetraphenylation, tetrabenzo and tetranaphtho annulation, and effects of changing the central metal from zinc (Zn) to palladium (Pd). Utilizing absorption and emission spectroscopy and laser flash photolysis techniques, the photophysical properties of these porphyrins have been determined. Upon the addition of benzo or naphtho groups, we observed an overall red shift in the ground state absorption spectra of both the B-bands and the Q-bands with increased conjugation and an increase in the Q-band to B-band intensity ratios. Time-dependent density functional theory calculations were performed on both series of porphyrins to identify the effects of phenyl, benzo, and naphtho substituents on the spectra. The benzo and naphtho adducts provide a larger contribution (typically 40-90%) to the observed red shifts due to increased π-conjugation, while there is a smaller contribution (typically 0-25%) from distortion of the porphyrin. Similarly, a red shift for the T 1 -T n absorption spectrum and an overall general broadening in the spectrum were found with increased conjugation. An increase in the triplet molar extinction coefficient through the near-infrared region with annulation was also found. Varying the metal has an effect on the overall absorption spectra; i.e., the ground state spectra of the Zn porphyrins are red-shifted relative to the Pd porphyrins. For the triplet excited state spectra there were small effects in the spectra by changing the metal with a significant contribution to the kinetic properties by the heavy atom effect of the Pd.

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Femtosecond time-resolved fluorescence and transient absorption measurements have been carried out on a series of platinum acetylide complexes to unravel the dynamics of intersystem crossing and the formation of triplet states in real time as a function of chain length. Ultrafast inter system crossing with a time constant less than 100 fs has been observed for the case of short chain length platinum acetylide complex and this time constant increases with increasing the chain length. Apart from the singlet to triplet intersystem crossing, additional triplet state relaxation has also been observed which happens in picosecond time scale.

Initiation of the first wave of spermatogenesis in the neonatal mouse testis is characterized by the differentiation of a transient population of germ cells called gonocytes found in the center of the seminiferous tubule. The fate of gonocytes depends upon these cells resuming mitosis and developing the capacity to migrate from the center of the seminiferous tubule to the basement membrane. This process begins approximately Day 3 postpartum in the mouse, and by Day 6 postpartum differentiated type A spermatogonia first appear. It is essential for continual spermatogenesis in adults that some gonocytes differentiate into spermatogonial stem cells, which give rise to all differentiating germ cells in the testis, during this neonatal period. The presence of spermatogonial stem cells in a population of cells can be assessed with the use of the spermatogonial stem cell transplantation technique. Using this assay, we found that germ cells from the testis of Day 0-3 mouse pups can colonize recipient testes but do not proliferate and establish donor-derived spermatogenesis. However, germ cells from testes of Day 4-5 postpartum mice colonize recipient testes and generate large areas of donor-derived spermatogenesis. Likewise, germ cells from Day 10, 12, and 28 postpartum animals and adult animals colonize and establish donor-derived spermatogenesis, but a dramatic reduction in the number of colonies and the extent of colonization occurs from germ cell donors Days 12-28 postpartum that continues in adult donors. These results suggest spermatogonial stem cells are not present or not capable of initiating donor-derived spermatogenesis until Days 3-4 postpartum. The analysis of germ cell development during this time frame of development and spermatogonial stem cell transplantation provides a unique system to investigate the establishment of the stem cell niche within the mouse testis.

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Sterically hindered fullerenyl chromophore dyad and triads, C 60 (>DPAF-C 9 ) x (x ) 1 and 2, respectively), in an acceptor-donor (A-D) molecular linkage of C 60 -(keto-fluorene) x were synthesized and fully characterized. Attachment of two 3,5,5-trimethylhexyl groups on C 9 of the fluorene ring moiety greatly improves their solubility and makes direct intermolecular aromatic stacking contacts more difficult. They are the first series of fullerene derivatives showing high three-photon absorptivity (3PA). Accordingly, C 60 (>DPAF-C 9 ) 2 exhibits 2PA and 3PA cross sections in the values of 0.824 × 10 -48 cm 4 s (or 82.4 GM) and 6.30 × 10 -25 cm 6 /GW 2 , respectively, in femtosecond region among the highest ones reported for many diphenylaminofluorene-derived AFX chromophores. Utilization of a keto linker located immediately between C 60 cage and fluorene chromophore moieties facilitates molecular polarization of the DPAF ring toward the C 60 cage. That may serve as the fundamental cause for correlation of enhanced A-D electron interactions to, ultimately, observed multiphoton absorption cross sections. By using nanosecond laser flash photolysis results taken at 355 nm as the reference, transient absorption data obtained from femtosecond pump-probe experiments at 800 nm unambiguously verified the occurrence of two-photon excitation processes of C 60 (>DPAF-C 9 ) in air-saturated benzene and subsequent efficient energy transfer from the two-photon pumped DPAF-C 9 moiety to the C 60 cage moiety.