1. Warren Huey
CHEM 7350
11/29/17

2. Upconversion Photon upconversion
Absorption of multiple photons that lead to an emission of light at a shorter wavelength
Triplet-triplet energy transfer (TTET)
The transfer of energy from the triplet state of one molecule to the triplet state of another
Triplet-triplet annihilation (TTA)
The fluorescence of an molecule that was caused by the excitation of multiple triplet energy states
Photon upconversion: A type of anti-Stokes fluorescence the occurs when multiple photons of one wavelength are absorbed and then emitted at a shorter wavelength (higher energy)
TTET: The transfer of energy from the triplet state of a sensitizer, a molecule that absorbs light and transfers the energy to another molecule, to the triplet state of an emitter
TTA: An emitter that receives the energy from a sensitizer. It receives the energy from triplet states on the sensitizer in its triplet energy states which it then converts to a higher singlet energy state and emits the photon (fluorescence).
Issues: The quantum efficiencies of the upconversion relies on the ISC of the sensitizer, the TTET to the emitter, the TTA, and the emitter’s ability to fluoresce. The energy states also play a large role in efficiency of the upconversion.
Goals: The development of new sensitizers and emitters for upconversion and their possible application in devices.
/nano

3. Research
Some major areas of focus are the generalization of this phenomena and the improvement of the quantum yields. There are many sensitizers and acceptors being designed and tested. The absorb photons all along the visible spectrum and some into the near-IR (above 750 nm) while emitting in the visible and near-UV (below 380 nm). There efficiency can be seen in the change in the amount of eV emitted compared to absorbed. The more efficient upconversion couples have higher changes in eV.
At the time of this paper, the group had recently published on a platinum(II) tetraphenyltetrabenzoporphyrin (PtTPBP) sensitizer and perylene acceptor that was able to upconvert the wavelength by 0.86 eV.

4. Supermolecular-Chromophore-Sensitized Near-Infrared-to-Visible Photon Upconversion
Ruthenium(II) [15-(4′-ethynyl-(2,2′;6′,2′′-terpyridinyl))-bis[(5,5′,-10,20-di(2′,6′- bis(3,3- dimethylbutoxy)phenyl)porphinato)zinc(II)]ethyne][4′- pyrrolidin-1-yl-2,2′;6′,2′′-terpyridine] bis(hexafluorophosphate) (Pyr1RuPZn2)  Sensitizer/Donor
N,N-bis(ethylpropyl)perylene-3,4,9,10-tetracarboxylicdiimide (PDI)  Acceptor
The paper look at these two compounds for their upconversion scheme. The Pyr1RuPZn2 complex was able to absorb in the near-IR region of the spectrum, had accessible delocalized triplet energy states, and had a μs time-scale for its excited state.
The PDI compound is chemically, thermally, and photochemically stability, with large absorptive extinction and good singlet
fluorescence quantum yield.

5. Fluorescence Fluorescence Phosphorescence Triplet energy states
Near-IR
Phosphorescence
Quantum Yield
Triplet energy states
1.2 eV
Absorption (showing extinction coefficients) and normalized emission spectra of Pyr1RuPZn2 (sensitizer; λex = 462 nm) and PDI (acceptor/annihilator; λex = 475 nm) in MTHF.
The donor fluoresces in the near-IR region and is due to the porphyrin moieties. The acceptor has a quantum yield of 0.78 and a singlet lifetime of 4.35 ns which was determined from time-correlated single photon counting (TCSPC). The emission spectra was measured at 77 K. In either case, no phosphorescence was observed. For the PDI this indicated that the quantum yield for that process was extremely low. The acceptor was estimated to have triplet energy states around ~1.2 eV due to excited-state dynamics measurements. This value agrees with past literature that estimated a PDI derivative to have triplet energy states around 1.2 eV. The triplet excited state coefficient was calculated at 560 nm, [εT (PDI, 560 nm) = 6.58 × 104 M-1 cm-1].

6. Lifetimes and Decay Transient absorption spectrometry
Triplet excited state lifetimes and energies
Laser flash photolysis
5.9 x 108 M-1 S-1
Diffusion: 1.3 × 1010 M-1 s-1 at 20 °C
Transient absorption difference spectra of 7.0 μM Pyr1RuPZn2 measured at several delay, 1.0 mJ/pulse, λex = 700 nm. The inset shows the single exponential fit to the 1100 nm transient with residuals presented in green. (b) Transient absorption difference spectrum of 4.2 μM PDI measured at several delay times, 1.0 mJ/pulse, λex = 485 nm. The inset shows the single exponential fit to the 560 nm transient with residuals presented in green.
Donor: Nanosecond transient absorption difference spectrum was excited at 700 nm and lead to negative peaks at nm and nm corresponding to ground state bleaching to the MLCT states. It also showed positive excited triplet state absorptions observed between and nm. Analysis of the excited state peaks lead to the determination of a 17.2 µs triplet excited state lifetime.
Acceptor: Nanosecond transient absorption difference spectrum was excited at 485 nm and lead to nm excited triplet state absorption with maxima at 520 and 560 nm. The peak at 520 nm overlaps the ground state absorption. The excited state lifetime for the acceptor was determined to be 120 µs.
Nanosecond laser flash photolysis was used to determine the Stern-Volmer (Ksv) and bimolecular (kq) quenching constants. The Stern-Volmer constant was Ksv of M-1 and showed dynamic quenching. The biomolecular constant was 5.9 × 108 M-1 s-1 which is below the diffusion limit of 1.3 × 1010 M-1 s-1 at I20 °C in THF. MTHF was assumed to have the same diffusion limit. The bimolecular constant indicates that the TTET must have a low driving force and thus the triplet energy state of the donor must be near the acceptor states of 1.2 eV. The TTET of the donor was also investigated with perylene and tetracene. No upconversion occurred with the perylene, which triplet energy states are at 1.53 eV, but upconversion did occur with tetracene, which triplet energy states are at 1.27 eV. This indicated that the triplet energy states of the donor must be between 1.27 and 1.53 eV. The excitation wavelength for these experiments was 780 nm.

7. Lifetimes and Decay Fluorescence Stability Oxygen
Figure 3. (a) Photoluminescence intensity profile of a freeze-pump-thaw degassed MTHF solution of Pyr1RuPZn2 (3.3 μM) and PDI (420 μM) measured as a function of 780 nm incident laser power density.
Figure 4. (a) Continuous kinetic scan of the upconverted PDI fluorescence upon selective excitation of Pyr1RuPZn2 at 780 nm at a laser power of 22 mW in a mixture of Pyr1RuPZn2/PDI, λobs ) 541 nm. (b) Relative percent upconversion quantum yields of Pyr1RuPZn2/PDI mixtures measured as a function of PDI concentration upon selective excitation of Pyr1RuPZn2 at 780 nm in deaerated MTHF.
They also continuously excited the sensitizer at 780 nm and determined it was stable over 5 h excitation. They also determined that oxygen quenched the donor with a decay rate of 1.0 × 109 M-1 s-1 which indicate that there was a weak thermodynamic driving force. The quantum yield of the oxygen quenching was 34% and showed that it would inhibit the TTET.

8. Quantum Yield PDI quantum yield Sensitized PDI quantum yield
Φ = Φ = ±
λexc = 500 nm λexc = 680 nm
Relative to rhodamine B Relative to [Os(phen)3](PF6)2
Causes
TTET and TTA
The quantum yield of PDI in MTHF was measured relative to rhodamine B in ethanol (Φstd = 0.49 at λexc = 500 nm)
The sensitized upconverted fluorescence quantum yield measurements of PDI in MTHF were measured relative to [Os(phen)3](PF6)2 in acetonitrile using 680 nm excitation
integrated intensity of the upconverted fluorescence for PDI was analyzed over the nm, [Os(phen)3](PF6)2 was analyzed over the nm
The emission spectra did not overlap well but results were reproducible. The quantum yield of the upconversion scheme depends on the whole process.
The causes of the low quantum yield were determined to not be due to the with donor excitation/ISC, quantum yields near 100%, or acceptor fluorescence, 78% yields. The causes must be due to the TTET or TTA. The TTET (quenching) step was determined to occur only 69% of the time. This was believed to potentially be due to the low driving force and/or bad overlap/ weaker coupling between the triplet states of the donor and acceptor.
The TTA rate constant was determined by using extinction coefficient of PDI, [εT(560 nm) = 6.58 × 104 M-1 cm-1], compared to anthrecene, εT at 420 nm = M-1 cm-1, kTT = (1.02 ( 0.2) × 109 M-1 s-1, which was times that of diffusion. This indicates that there are low yields of singlet PDI excited state which may be due to solvent interactions forcing the acceptor away from the sensitizers.

9. Conclusion Solutions Future work Criticisms
Sensitizer with long lifetime
Accessible triplet energy states in both the donor and acceptor
Soluble and stable over long periods of time
Device applications
Future work
Improving TTET and TTA
Criticisms
Left out data explaining much about the PDI properties
Mentioned solubility but did not investigate

10. Questions