The fluorescence and electronic structure of phenyl-substituted tetraazachlorin molecules

Abstract

The effect of the addition of phenyl groups to pyrrole rings of tetraazachlorins, a new class of tet rapyrroles, on the photophysical properties and electronic structure of the molecules has been investigated by a complex of experimental and theoretical methods. Characteristics of fluorescence at 293 and 77 K have been determined for phenyl substituted tetraazachlorins. The objects of this study include unsubstituted tet raazaporphine. The introduction of phenyl groups affords a marked increase in the fluorescence quantum yield. For tetraazaporphine and phenyl substituted tetraazachlorins, fluorescence buildup occurs as the tem perature is decreased from 293 to 77 K, but to a lesser extent than for tetraazachlorins having no phenyl groups, which were earlier studied by the authors. The fluorescence buildup mechanism is discussed. The sin glet oxygen generation quantum yield has been determined for the tetrapyrroles examined. This characteristic increases upon tetrapyrrole is phenylation. The electronic structure and absorption spectra of unsubstituted porphine and chlorin, tetraazaporphine, tetraazachlorin, octaphenyltetraazaporphine, and tetramethyl hexaphenyltetraazachlorin have been calculated by the INDO/Sm method (original modification of the INDO/S method) with molecular geometry optimization using DFT. The results of the quantum chemical calculation of the absorption spectra are in good agreement with experimental data for transitions to the low est excited electronic states Q x (S 1 ) and Q y (S 2 ).

Spectral and Luminescent Characteristics of a Hexaphenyltetraazachlorin Zinc Complex

“…According to the data for ZnTAC nb and H 2 TAC nb [5], this change caused the Q x (0-0) band to shift bathochromically by 600 cm -1 , the Q y (0-0) band to shift hypsochromically by 560 cm -1 , and the Q y -Q x gap to increase to ~4500 cm -1 whereas the Soret band analog was shifted insignifi cantly by 150 cm -1 and its splitting decreased (from 1600 to 1150 cm -1 ). The reported results for the ZnPh 6 TAC tm and H 2 Ph 6 TAC tm pair [1] were a bathochromic shift of the Q x (0-0) band by 550 cm -1 , a hypsochromic shift of the Q y (0-0) band by 450 cm -1 , and a Q y -Q x gap that increased from 3200 to 4200 cm -1 . Both maxima near the Soret band were shifted to shorter wavelength by 800 and 850 cm -1 , i.e., the splitting remained practically the same.…”

“…The rate constants or probabilities of the radiative transition S 1 -S 0 were calculated as k F ≈ 4·10 7 s -1 by using the φ F and τ F values. The k F value for H 2 Ph 6 TAC tm was about twice as large [1]. Such a difference could be explained by the molecular rearrangement of molecules in the S 1 state as discussed above.…”

“…It is important to note that the Stokes shift was very small, 2 nm or 40 cm -1 . It also decreased but not as much upon freezing other tetraazachlorins [1,[3][4][5][6] whereas it was large for ZnPh 6 TAC tm at 293 K.…”

“…The conjugated macrocycle of ZnPh 6 TAC tm had C 2v symmetry. Such a characteristic polarization spectrum and consideration of previous studies [1] provided a basis to confi rm rather precisely that the fi rst transition was polarized along the X axis in the molecular plane perpendicular to the C 2 axis; the second, along the C 2 axis. The P F value for the band maximum at 384 nm again reached a rather high negative value (-0.13).…”

“…The measurement procedure was described in detail [1,6]. 2-Methyltetrahydrofuran (MTHF, Aldrich), which readily forms a glass on freezing; a mixture of Et 2 O and toluene (ET, 2:1) that vitrifi es at 77 K; and pure toluene at 293 K were used as solvents because of the low-temperature measurements.…”