Synthesis and electronic properties of regioisomerically pure oxochlorins.
We describe a two-step conversion of C-alkylated zinc chlorins to zinc oxochlorins wherein the keto group is located in the reduced ring (17-position) of the macrocycle. The transformation proceeds by hydroxylation upon exposure to alumina followed by dehydrogenation with DDQ. The reactions are compatible with ethyne, iodo, ester, trimethylsilyl, and pentafluorophenyl groups. A route to a spirohexyl-substituted chlorin/oxochlorin has also been developed. Representative chlorins and oxochlorins were characterized by static and time-resolved absorption spectroscopy and fluorescence spectroscopy, resonance Raman spectroscopy, and electrochemistry. The fluorescence quantum yields of the zinc oxochlorins (Phi(f) = 0.030-0.047) or free base (Fb) oxochlorins (Phi(f) = 0.13-0.16) are comparable to those of zinc tetraphenylporphyrin (ZnTPP) or free base tetraphenylporphyrin (FbTPP), respectively. The excited-state lifetimes of the zinc oxochlorins (tau = 0.5-0.7 ns) are on average 4-fold lower than that of ZnTPP, and the lifetimes of the Fb oxochlorins (tau = 7.4-8.9 ns) are approximately 40% shorter than that of FbTPP. Time-resolved absorption spectroscopy of a zinc oxochlorin indicates the yield of intersystem crossing is >70%. Resonance Raman spectroscopy of copper oxochlorins show strong resonance enhancement of the keto group upon Soret excitation but not with Q(y)()-band excitation, which is attributed to the location of the keto group in the reduced ring (rather than in the isocyclic ring as occurs in chlorophylls). The one-electron oxidation potential of the zinc oxochlorins is shifted to more positive potentials by approximately 240 mV compared with that of the zinc chlorin. Collectively, the fluorescence yields, excited-state lifetimes, oxidation potentials, and various spectral characteristics of the chlorin and oxochlorin building blocks provide the foundation for studies of photochemical processes in larger architectures based on these chromophores.
Taniguchi
Masahiko
M
Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, USA.
Kim
Han-Je
HJ
Ra
Doyoung
D
Schwartz
Jennifer K
JK
Kirmaier
Christine
C
Hindin
Eve
E
Diers
James R
JR
Prathapan
Sreedharan
S
Bocian
David F
DF
Holten
Dewey
D
Lindsey
Jonathan S
JS
D007202:Indicators and Reagents; D008958:Models, Molecular; D008968:Molecular Conformation; D015394:Molecular Structure; D010084:Oxidation-Reduction; D011166:Porphyrins / Q000138:chemical synthesis* / Q000737:chemistry*; D013053:Spectrophotometry; D013237:Stereoisomerism
D007202:Indicators and Reagents; D011166:Porphyrins; C006969:chlorin