4. Optical Emission Spectra of Phthalocyanines

From the Frank-Condon principle (see Sect. 1.​1.​3).

As the S₁ → S₀ transition is an allowed process, the emission promptly occurs.

The transition from S₁ (v = 0) to S₀ (v = 0) is referred to as a 0–0 transition, in common with optical absorption.

Not all metal complexes are fluorescent (Sect. 4.1.3.2).

Otherwise, the emission is from another substance, such as an impurity (Sect. 4.1.5).

The appearance of a single peak in the emission spectra of metal-free derivatives can occasionally give rise to a serious misinterpretation as mentioned later (Sect. 4.1.5).

Some high-spec spectrometers can directly determine quantum yield values using an integrating sphere (also known as an Ulbricht sphere).

See Fig. 1.​12 for the α and β positions.

Fluorescence data for the same naphthalocyanine derivative in pyridine are reported in the same reference [24]. However, these data are not cited in this work because of the dissimilarity of the excitation spectrum to the absorption spectrum in the same solvent and the appearance of the emission peak at a much shorter wavelength (749.8 nm) than absorption peak (783.6 nm); see Sect. 4.1.5.

Readers are reminded that Q band of porphyrins (an electronic transition to the lowest excited state) is weak not because the transition is forbidden but configuration interaction between the two kinds of one-electron transitions involving the nearly degenerate HOMOs and LUMO significantly reduced the magnitude of the electric dipole moment in the lowest excited state (Sect. 2.​2.​6). As optical emission is the most likely to occur from the lowest excited (S₁) state (Kasha’ rule; Sect. 4.1.1), it is quite reasonable that the most intense fluorescence from porphyrins is observed near the Q band. Note that this is not saying that no emission is observed near their Soret band. Actually, weak emission can be observed at the red flank of Soret band under appropriate conditions [43, 44] [i.e., emission from the S₂ state; cf. see S₂ emission from metal-free phthalocyanines (Fig. 4.7)]. Unlike the violet emission from phthalocyanines (Sect. 4.2.4), the corresponding excitation spectrum is similar to the Soret band.

This usually occurs when the metal ion is labile and hence liable to leave the cavity (e.g., the known Sb^III complex [8]) or when it is difficult to remove the unreacted free base from its metal complex through the synthetic procedure.

Actually, the emission peak appears at a slightly longer wavelength, and the magnitude of the difference may be considered as being reasonably close to the Stokes shift.

It was later found that emission could not be detected when the Sb^III complex was very carefully purified [8].

Fortunately, the Q-band position of the Sb^III complex is significantly red shifted relative to those of the other metal complexes [8].

Some authors have attempted to attribute the appearance of free-base-like excitation spectra to the demetallation of the photoexcited molecule followed by the protonation of the macrocyclic ligand in its cavity. Although this possibility cannot be completely excluded, it is unlikely because the two successive chemical steps must be completed within picoseconds before the initiation of the optical emission. If the assumed reactions do occur, continuous photoirradiation will increase the ratio of the corresponding free base to the bulk complex during irradiation.

Phosphorescence of the zinc derivative had been reported in their earlier work [32], but this was later reassigned as an artifact by the same group [29].

Emission from ¹O₂ around 1270 nm is mentioned in Sect. 4.2.3.

As far as phthalocyanines and related macrocyclic compounds are concerned, their application to such devices appears to be unpromising because the energy gap between S₁ and T₁ is too large to thermally excite triplet molecules.

The weak emission may have been due to poor solubility and strong aggregation of the macrocyclic compound in the solvent.

It is noteworthy that only a few systems have been reported with the ECL peak position at a longer wavelength than the nc system despite more than three decades having passed since this work was published [62‐64].