Public Abstract

DE-FG02-05ER15661: FUNDAMENTAL STUDIES OF THE VIBRATIONAL, ELECTRONIC, AND PHOTOPHYSICAL PROPERTIES OF TETRAPYRROLIC ARCHITECTURES

Award Status: Inactive

Institution: Washington University, St Louis, MO
UEI: L6NFUM28LQM5
DUNS: 068552207

Most Recent Award Date: 05/02/2024
Number of Support Periods: 19
PM: Fecko, Christopher

Current Budget Period: 05/15/2023 - 08/31/2024
Current Project Period: 05/15/2022 - 08/31/2024
PI: Holten, Dewey

Supplement Budget Period: N/A

Public Abstract

Our integrated program of molecular design and synthesis coupled with spectroscopic and computational studies has probed from first principles how structural and electronic properties of tetrapyrrolic macrocycles (porphyrins, chlorins and bacteriochlorins) dictate spectral properties as well as excited-state energy flow in multicomponent architectures. The molecular constructs are designed to test ideas of fundamental importance, often requiring the development of new synthetic methodology to do so. The major issues that will be addressed are as follows: (i) Coherence phenomena have been observed in 2-D electronic spectroscopy studies of native photosynthetic antennas and reaction centers. The extent to which such coherences increase the rate of energy transfer above the level that can be achieved via incoherent processes, such as Förster energy transfer, remains unknown. We are addressing this issue by measuring the rates of energy transfer in sets of well-defined bacteriochlorin dyads with variation in the donor–acceptor excited-state energy gap. To date we have synthesized and characterized a family of nine dyads wherein the donor and acceptor bacteriochlorins are joined by a phenylethynyl linker and span a ~120-1100 cm^-1 range of energy gaps. The rate constant for energy transfer varies from (0.4 ps)^-1 to (1.1 ps)^-1. For each dyad the measured rate is greater than that predicted by Förster theory by about a factor of two, which is likely accounted for by the limitations of the theory for large molecules at the relatively short distances involved. Thus, the dyads so far show no significant increase in rate due to vibrational-electronic resonances. We propose to create two new sets of dyads (18 in total): one set with a shorter phenylene linker to afford a larger donor–acceptor electronic coupling and faster energy transfer for the same energy gap, and one set with a longer diphenylethyne linker to afford a weaker electronic coupling and slower energy transfer. Collectively, the three sets of dyads will ensure the robustness of the conclusions, and allow more extensive comparisons with theory and with energy transfer in various photosynthetic systems. (ii) Efficient solar-energy harvesting and conversion requires chromophores that absorb in the NIR to capture the strong solar emission in this spectral region. We have previously developed bacteriochlorins tuned with few-nm exactness across the 700–850 nm region, and four annulated bacteriochlorins absorb at >850 nm. Two bacteriochlorin-bis(imide)s absorb at 875 and 888 nm and exhibit 1.0 ns singlet excited-state lifetimes, whereas phenaleno-and benzo-bacteriochlorins absorb (913 and 1033 nm) have very short (0.15 and 0.007 ns) excited-state lifetimes. We plan to elaborate the bacteriochlorin-bis(imides) to give constructs that should have intense (e ~ 100,000 M^-1cm^-1) and deep (~1000 nm) NIR absorption with ~0.5 ns excited-state lifetimes suitable for use in solar-conversion strategies.

Our integrated program of molecular design and synthesis coupled with spectroscopic and computational studies has probed from first principles how structural and electronic properties of tetrapyrrolic macrocycles (porphyrins, chlorins and bacteriochlorins) dictate spectral properties as well as excited-state energy flow in multicomponent architectures. The molecular constructs are designed to test ideas of fundamental importance, often requiring the development of new synthetic methodology to do so. The major issues that will be addressed are as follows:

(i) Coherence phenomena have been observed in 2-D electronic spectroscopy studies of native photosynthetic antennas and reaction centers. The extent to which such coherences increase the rate of energy transfer above the level that can be achieved via incoherent processes, such as Förster energy transfer, remains unknown. We are addressing this issue by measuring the rates of energy transfer in sets of well-defined bacteriochlorin dyads with variation in the donor–acceptor excited-state energy gap. To date we have synthesized and characterized a family of nine dyads wherein the donor and acceptor bacteriochlorins are joined by a phenylethynyl linker and span a ~120-1100 cm^-1 range of energy gaps. The rate constant for energy transfer varies from (0.4 ps)^-1 to (1.1 ps)^-1. For each dyad the measured rate is greater than that predicted by Förster theory by about a factor of two, which is likely accounted for by the limitations of the theory for large molecules at the relatively short distances involved. Thus, the dyads so far show no significant increase in rate due to vibrational-electronic resonances. We propose to create two new sets of dyads (18 in total): one set with a shorter phenylene linker to afford a larger donor–acceptor electronic coupling and faster energy transfer for the same energy gap, and one set with a longer diphenylethyne linker to afford a weaker electronic coupling and slower energy transfer. Collectively, the three sets of dyads will ensure the robustness of the conclusions, and allow more extensive comparisons with theory and with energy transfer in various photosynthetic systems.

(ii) Efficient solar-energy harvesting and conversion requires chromophores that absorb in the NIR to capture the strong solar emission in this spectral region. We have previously developed bacteriochlorins tuned with few-nm exactness across the 700–850 nm region, and four annulated bacteriochlorins absorb at >850 nm. Two bacteriochlorin-bis(imide)s absorb at 875 and 888 nm and exhibit 1.0 ns singlet excited-state lifetimes, whereas phenaleno-and benzo-bacteriochlorins absorb (913 and 1033 nm) have very short (0.15 and 0.007 ns) excited-state lifetimes. We plan to elaborate the bacteriochlorin-bis(imides) to give constructs that should have intense (e ~ 100,000 M^-1cm^-1) and deep (~1000 nm) NIR absorption with ~0.5 ns excited-state lifetimes suitable for use in solar-conversion strategies.