title: Ultrafast intramolecular charge transfer in tetrapyrazinoporphyrazines controls the quantum yields of fluorescence and singlet oxygen.
abstract: A series of octasubstituted zinc(II) tetrapyrazinoporphyrazines (TPyzPz), aza-analogues of phthalocyanines, differing in the number of peripheral N,N-diethylamino (n = 0-8) and tert-butylsulfanyl substituents (m = 8-n) has been synthesized. All possible congeners were characterized including adjacent and opposite isomers. Steady-state (UV-vis, fluorescence) and time-resolved (fluorescence, femtosecond transient absorption) spectroscopies, redox and photochemical (singlet oxygen formation) properties were investigated and compared. The peripheral tertiary amino substituents (donor) induce a new competitive relaxation pathway to fluorescence and intersystem crossing due to the mixing of the first excited state S(1) of the TPyzPz macrocycle with a nearby intramolecular charge transfer (ICT) state. The fluorescence quantum yield and fluorescence lifetime of 6Zn bearing one N,N-diethylamino substituent (n = 1, m = 7) decreased with increasing solvent polarity, while the same observables of 5Zn with no donor centre (n = 0, m = 8) were not affected. Protonation of the N,N-diethylamino substituent in 6Zn led to a strong increase of the fluorescence intensity. The cyclic voltammetry data, the steady-state and time-resolved emission and transient absorption studies revealed strong electronic coupling between the TPyzPz moiety and N,N-diethylamino substituents. ICT is an extremely rapid process occurring with a time constant of 10 ps and 7 ps in 6Zn (n = 1, m = 7) and 11Zn (n = 8, m = 0) in pyridine, respectively. The ICT efficiency decreased in non-polar solvents. The presence of two N,N-diethylamino substituents in 7Zn (n = 2, m = 6) considerably quenched the S(1) states in pyridine (polar, coordinating), toluene (non-polar, non-coordinating) and toluene-1% pyridine (v/v) (non-polar, coordinating). The photophysical properties of compounds with more donor substituents on the periphery (n > 2, m < 6) were similar to those of 7Zn.
authors.affiliation: Department of Pharmaceutical Chemistry and Drug Control, Faculty of Pharmacy in Hradec Kralove, Charles University in Prague, Heyrovskeho 1203, Hradec Kralove, 50005, Czech Republic.
authors.forename: Veronika
authors.initials: V
authors.lastname: Novakova
authors.forename: Petr
authors.initials: P
authors.lastname: Zimcik
authors.forename: Miroslav
authors.initials: M
authors.lastname: Miletin
authors.forename: Lenka
authors.initials: L
authors.lastname: Vachova
authors.forename: Kamil
authors.initials: K
authors.lastname: Kopecky
authors.forename: Kamil
authors.initials: K
authors.lastname: Lang
authors.forename: Pavel
authors.initials: P
authors.lastname: Chábera
authors.forename: Tomás
authors.initials: T
authors.lastname: Polívka
mesh_terms: D005453:Fluorescence; D007211:Indoles / Q000138:chemical synthesis / Q000737:chemistry*; D054833:Isoindoles; D010084:Oxidation-Reduction; D055668:Photochemical Processes; D011789:Quantum Theory; D026082:Singlet Oxygen / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet
chemical_list: D007211:Indoles; D054833:Isoindoles; D026082:Singlet Oxygen; C013647:phthalocyanine