Transient absorption spectra of tetraazaporphine and its reduced derivatives upon picosecond excitation

Abstract

Transient absorption spectra of tetraazaporphine and substituted derivatives of tetraazachlorin and tetraazabacteriochlorin were obtained upon picosecond excitation with various probe pulse delay times in order to resolve the open question about the reasons for fluorescence quenching in tetraazaporphine hydrogenated derivatives. The quantum yield of triplet state T 1 formation has been estimated. It has been shown that in all investigated cases radiationless de-excitation of the fluorescent level S 1 occurs by two channels: S 1 ‫ב‬ T 1 and S 1 ‫ב‬ S 0 , the latter being predominant. As the S 1 level becomes lower, the transition rate for this channel increases. For tetraazaporphine and its derivatives, a quasi-photochemical mechanism is proposed that accounts for the anomalous efficiency of the S 1 ‫ב‬ S 0 channel in the dissipation of the electronic excitation energy.Introduction. Bridging tetraaza-substitution in the tetrapyrrole macrocycle and hydrogenation of the pyrrole rings is one of the factors capable of causing fundamental changes in the electronic spectra of porphine derivatives. Combination of these factors deserves consideration as a method for creating compounds with controlled properties. Spectral and luminescence characteristics of hydrogenated tetraazaporphine derivatives (also called porphyrazine) have been well studied [1][2][3][4][5][6][7][8].Studies showed that hydrogenation of one or two opposed pyrrole rings in tetraazaporphine (H 2 TAP) causes significant fluorescence quenching [1,3,5,8]. This quenching may be due to an increase of the probabilities for the two radiationless intramolecular processes of intersystem crossing S 1 ‫ב‬ T 1 , k ST , and internal conversion S 1 ‫ב‬ S 0 , i.e., energy exchange of the S 1 electronic state onto vibrations of the molecule in the ground state S 0 , the probability of which increases with lowering of the S 1 level. Preference has been given to the first de-excitation channel [1, 3, 5] on the basis of indirect literature data, in particular, a comparison with fluorescence of bacteriochlorin [9] and 2,3-naphthalocyanine [10] derivatives. In these instances, lowering of the S 1 level was also accompanied by fluorescence quenching although the quenching was weaker. Both pathways were mentioned in another study [8].The kinetics of absorption spectra upon picosecond excitation of H 2 TAP, dibenzobarrelene-substituted tetraazachlorin (H 2 TAC t ), and dibenzobarrelene-substituted tetraazabacteriochlorin (H 2 TABC

Spectral-luminescence properties of a norbornene-substituted tetraazachlorin and its metal complexes

“…Molecules of tetraazachlorins in the excited state in solution underwent rearrangement (probably a change of geometric structure). This was confirmed indirectly by the existence of a quasiphotochemical mechanism for deactivation of the S 1 state that was proposed before [15]. A norbornene moiety annelated to a reduced pyrrole ring decreased the probability of radiationless deactivation at 77 K. The fluorescence polarization spectrum of H 2 TAC nb exhibited in the region of the Soret band at least four electronic transitions.…”

“…The effect was much stronger for the free base. Therefore, it could be proposed that the quasiphotochemical deactivation mechanism of the S 1 state that was proposed earlier by our group [15] for H 2 TAP, H 2 TAC t , and H 2 TABC tt played a significant role for H 2 TAC nb . This mechanism included in some way a deviation of the central imino hydrogens from the macrocycle plane.…”

“…First of all, it will be necessary in further studies to eliminate or account for the decrease of ϕ ST and to shorten the lifetime of the T 1 level (for the studied compounds [29,30] it was 200-600 nsec). It is noteworthy that ϕ ST was estimated as ~0.4 for H 2 TAC t [15], which clearly did not correspond to ϕ Δ = 0.07.…”

The fluorescence and electronic structure of phenyl-substituted tetraazachlorin molecules

“…For the fraction of molecules undergoing the nonradiative transition S 1 ∼ >S 0 , ϕ SS = k SS /(k F + k ST + k SS ), which is approximately 0.3 and is smaller than for H 2 TAC t [21]. Note that, in a recent work [22] mainly devoted to the photochemical properties of tetraaza chlorins in benzene and in micelles of nonionic deter gents in water, it was found that ϕ Δ = 0.70; according to our data for the same object in toluene, ϕ Δ = 0.58.…”

“…For H 2 Ph 8 TAC mp and H 2 Ph 6 TAC tm , the fluorescence buildup in MTHF is smaller by a factor of 4-5 and that in toluene is 2.6-3 times smaller than in MTHF. For H 2 TAP and H 2 TAC t , it has been demonstrated by picosecond adsorption spectroscopy [21] that the effi ciency of nonradiative deactivation in the S 1 ∼ >S 0 channel is fairly good, which is unnatural for porphy rins [23], and a quasi photochemical mechanism including a displacement of central imino hydrogen atoms was suggested for the process. This mechanism is the subject of a separate subsection below.…”