Visible-Light-Driven Triplet Sensitization of Polycyclic
Aromatic Hydrocarbons Using Thionated Perinones
Posted on 2020-06-17 - 17:44
Metal-free
chromophores that efficiently generate triplet excited
states represent promising alternatives with respect to transition
metal-containing photosensitizers, such as those featuring metal-to-ligand
charge transfer excited states. However, such molecular constructs
have remained underexplored due to the unclear relationship(s) between
molecular structure and efficient/rapid intersystem crossing. In this
regard, we present a series of three thionated perinone chromophores
serving as a newly conceived class of heavy metal-free triplet photosensitizers.
We demonstrate that thionation of the lone CO substituent
in each highly fluorescent perinone imparts red-shifted absorbance
bands that maintain intense extinction coefficients across the visible
spectrum, as well as unusually efficient triplet excited state formation
as inferred from the measured singlet O2 quantum yields
at 1270 nm (ΦΔ = 0.78–1.0). Electronic
structure calculations revealed the emergence of a low energy S1 (n → π*) excited state in the proximity of a
slightly higher energy S2 (π → π*) excited
state. The distinct character in each of the two lowest-lying singlet
state manifolds resulted in the energetic inversion of the corresponding
triplet excited states due to differences in electron exchange interactions.
Rapid S1 → T1 intersystem crossing was
thereby facilitated in this manner through spin–orbit coupling
as predicted by the El Sayed rules. The lifetimes of the resultant
triplet excited states persisted into the microsecond time regime,
as measured by transient absorbance spectroscopy, enabling effective
bimolecular triplet sensitization of some common polycyclic aromatic
hydrocarbons. The synthetically facile interchange of a single O atom
to an S atom in the investigated perinones resulted in marked changes
to their photophysical properties, namely, conversion of dominant
singlet state fluorescence in the former to long-lived triplet excited
states in the latter. The combined results suggest a general strategy
for accessing long-lived triplet excited states in organic chromophores
featuring a lone CO moiety residing within its structure,
valuable for the design of metal-free triplet photosensitizers.
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Palmer, Jonathan
R.; Wells, Kaylee A.; Yarnell, James E.; Favale, Joseph M.; Castellano, Felix N. (2020). Visible-Light-Driven Triplet Sensitization of Polycyclic
Aromatic Hydrocarbons Using Thionated Perinones. ACS Publications. Collection. https://doi.org/10.1021/acs.jpclett.0c01634
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AUTHORS (5)
JP
Jonathan
R. Palmer
KW
Kaylee A. Wells
JY
James E. Yarnell
JF
Joseph M. Favale
FC
Felix N. Castellano
KEYWORDS
singlet O 2 quantum yieldsO atomPolycyclic Aromatic Hydrocarbonstriplet sensitizationabsorbance spectroscopyenergy S 1extinction coefficientselectron exchange interactionsstate formationphotophysical propertiessinglet state fluorescencemetal-free triplet photosensitizersEl Sayed rulestransition metal-containing photosensitizersS atomlowest-lying singlet state manifoldsred-shifted absorbance bandsThionated Perinones Metal-free chromophoresmetal-to-ligand charge transferthionated perinone chromophoresVisible-Light-Driven Triplet Sensitizationmicrosecond time regime