A chloroaluminum-phthalocyanine (AlCl-Pc) with tetra-α-butoxy chains (AlCl-Pc-OC4) has been synthesized and the photophysical parameters have been determined using steady-state and time-resolved absorption as well as emission spectroscopy. A luminescence from S2 excited state with long lifetime (5.71ns) is observed. A multi level model has been proposed to explain the photophysical processes after Soret-band excitation (λex=355nm). The optical limiting performance for 532nm-7ns laser pulses of AlCl-Pc-OC4 has been investigated in THF solution. The σex and ratio of σex/σ0 has been calculated. The good optical limiting performance is attributed to a reverse saturable absorption mechanism. It indicates that AlCl-Pc-OC4 could be promising candidates for optical limiting material.
©2005 Optical Society of America
Nonlinear optical materials continue attracting attention because of their potential application in optical communications, optical storage, optical computing, harmonic generation, optical switching, optical limiting, etc. . The basic properties that are required for organic nonlinear optical (NLO) materials include high NLO chromophore density so as to display large optical nonlinearity, low optical losses and ultrafast response time . Among all the NLO properties applications, optical limiting (OL) is one of the most promising for practical use . OL materials provide potential applications in strong laser modulation and the protection of human eyes and optical sensors . Several mechanisms could lead to optical limiting behavior, such as reverse saturable absorption (RSA), two-photon absorption (TPA), nonlinear refraction and optically induced scattering . Different mechanisms would dominate the OL performance under different conditions. RSA is characterized by an absorption coefficient that increases with light intensity, and results from an absorption cross-section in the excited state (σex) larger than that in the ground state (σ0) . Materials possessing a highly delocalized π–electron system often have a large optical limiting response .
Phthalocyanine complexes (Pcs) are a class of compounds with two-dimensional delocalized π–electron system and metal-N bonds . To be OL materials, the Pcs have some advantages for optical limiting, such as chemical stability and diversity of chemical variations. The architectural flexibility of Pcs facilitates the tuning of photophysical and optical properties over a very broad range by changing the peripheral substituents and the central metal ion of the macrocycle . To overcome the low solubility, peripheral substitutions were investigated to make Pcs soluble in a variety of solvents. Arthur and coworkers had demonstrated that metal-free phthalocyanines with peripheral substitution at the more sterically restricted α-positions reduces the aggregation tendency of Pcs more efficiently than at β-positions ; and increases the non-linear absorption coefficient . It has been found that coordinating the central metal ion with axial ligands, e.g., AlCl-Pc, TiO-Pc, etc., gives larger NLO absorption coefficients than Ni-Pc and Cu-Pc . The optical limiting behavior of AlCl-Pc and subsequently many other Pc compounds has been investigated . When the molecules have the central metal ion with axial ligand and appropriate peripheral substitution, we may obtain a material that should have advantages in NLO properties and solubility.
This paper concerns a soluble AlCl-Pc with tetra-α-butoxy chains (AlCl-Pc-OC4). We report on its synthesis, its properties after Soret-band excitation, and its optical limiting properties in THF solution.
2. Experiment section
1,8,15,22-tetra-butoxy-substituted chloroaluminum-phthalocyanine (AlCl-Pc-OC4) has been synthesized according to a known route  and characterized . The molecular structure is shown in Scheme 1. As found by Rückmann and coworkers, for the tetra-α-butoxy-substituted AlCl-Pc, the planar structure of the center metal-Pc is maintained, only the butyl chains extend out of the plane. The chains strongly suppress aggregation and greatly improve solubility .
2.2 Instruments and methods
1H NMR spectra were recorded on a DPX400 Bruker FT-NMR spectrometer with CDCl3 as solvent and tetramethylsilane (TMS) as internal standard. A mass spectrum was obtained on BiflexIII Maldi-TOF.
Steady-state electronic absorption spectra and emission spectra were recorded on Hitachi U3010 and F2500 spectrophotometers. Fluorescence quantum yield (ΦF) was determined by the comparative method using unsubstituted Zn-Pc in 1-chloro-naphthalene (ΦF=0.30) as the reference standard . Fluorescence decay processes were recorded with single photon counting technique on an Edinburgh FL900 fluorescence lifetime system.
Transient absorption at the nanosecond and picosecond time scale was observed in a saturated solution of AlCl-Pc-OC4 in argon. In the nanosecond flash photolysis system, the excitation light was the third harmonic of a Nd:YAG laser (Continuum Surelite II, 355nm, 7ns fwhm). A pulsed xenon arc lamp was used to provide the analyzing light. The signals were detected by an Edinburgh Lp900 and recorded on a Tektronix TDS 3012B oscilloscope and computer. The molar triplet-triplet extinction coefficient (ΔεT) was calculated by the method of total saturation , and the quantum yield of the triplet state was determined by the comparative technique . The triplet lifetime (τT) was calculated from a kinetic analysis of the transient absorption.
The picosecond measurements were performed using a pump-probe apparatus . In brief, the probe is a white light continuum generated by a Nd:YAG laser (1064nm) and the pump pulse is its third harmonic (355nm, 25ps fwhm). The signals were recorded by a double diode array multichannel analyzer (Princeton Instruments ST 120 controller and DDA 1024 detector).
The standard setup for OL measurements is shown in Fig. 1. AlCl-Pc-OC4 was dissolved in solvent (THF) at concentration of 1×10-4mol/l and 2×10-4mol/l, and placed in a 1.0cm path length quartz cell. The solution was bubbled with Argon for 30min to remove the dissolved O2. The second harmonic of a nanosecond Nd:YAG laser (Continuum, 532nm, 7ns fwhm) was used as the laser source. The laser beam was firstly filtered by an aperture to get a nearly homogeneous light spot and the input fluence of the laser beam was adjusted by an inverted telescope system (At) including a λ/2 waveplate and a Glan-Taylor prism. Before entering the sample, the laser beam was divided by a beam splitter (BS): the reflected beam was used as reference and the transmitted one was focused onto the sample by a 15cm focal length lens (L). The sample was placed about 2cm in front of the focus where the spot radius was 0.67mm. The incident and transmitted laser pulses were monitored by energy detectors D1 and D2.
3. Results and discussion
3.1 Ground state absorption and fluorescence emission
The electronic absorption spectrum and the uncorrected fluorescence emission spectrum of AlCl-Pc-OC4 in THF are shown in Fig. 2. The absorption and emission parameters are summarized in Table1. The ground state absorption is very typical for metal phthalocyanines (MPcs) with an intense S0-S1 transition (Q-band) in the red spectral region around 711nm with a shoulder at 640nm and a low broad Soret band at 320–360nm. The absorption coefficient at 532nm is very low in the measurement where the optical limiting property is detected. In the case of Soret-band excitation (λex=355nm), the emission consists of a strong and narrow S1 fluorescence around 725nm and a broader anomalous luminescence in the region 400–550nm. In deviation from Kasha’s rule, the observed anomalous luminescence is a radiative emission from a higher excited singlet state (S2) . In the uncorrected fluorescence spectrum, the relative intensities of bands at different wavelengths are not meaningful. The S1 emission has a fluorescence quantum yield of 0.35 and a lifetime of 7.04ns, while the S2 emission has a quantum yield of 1.25×10-3 and a lifetime of 5.71ns. Comparable S2 emission lifetimes have been reported, e.g. 5.4ns for ZnTSPc and 4.6ns for α–H2Pc(OBu)8 . Generally, a larger energy gap between the lowest and the higher excited state tends to reduce the deactivation and thus enhance the emission from the higher electronic state . The relatively large energy gap between S1 and S2 in AlCl-Pc-OC4 might be the reason for the unusual long S2 lifetime and also the reason for the unusual luminescence. From the intersection point of the normalized electronic absorption and the fluorescence emission peak, we estimate an S1 state energy of 1.73eV and an S2 state energy of 3.11eV.
3.2 Transient absorption studies
Several factors could contribute to the transient absorption spectra: (1) Positive ΔOD signals attribute to the absorption from the transient species, such as S1 and T1 state; (2) negative ΔOD signals are the results of bleaching of the ground state and/or stimulated emission at the wavelength of the luminescent transition in the sample. Meanwhile, the transient absorption spectra at different delay times will show different photophysical processes when the sample is excited by the laser with different pulse wide . To study the photophysical processes, we employed picosecond and nanosecond flash photolysis systems for excitation by the laser (355nm) with pulse width of both 25ps and 7ns, respectively.
Using a picosecond flash photolysis system, we recorded fluorescence emission from the S1 state, ground state photo-bleaching, and a broad transient absorption. Because the 3.49 eV energy of a 355nm photon is much higher than the S1 state energy, the molecules are initially excited only to the Sn state. On the timescale of a picosecond, most of the molecules relax to the first excited singlet state S1. They could be excited to upper singlet state (Sm) again by a probe light in picosecond scale, because the lifetime of the S1 state was determined to be in the timescale of nanosecond by the S1 fluorescence decay. So the transient absorption, mentioned above, was determined to be singlet-singlet absorption of S1 state to upper singlet states Sm. Figure 3 gives the transient absorption difference spectrum of argon-saturated solution of AlCl-Pc-OC4 in CH2Cl2, upon excitation by the 355nm laser. It shows very broad singlet-singlet absorption from 400nm to 800nm with two maxima at 460nm and 576nm. In the negative area, the combination of ground state photo-bleaching (711nm) and S1 fluorescence emission (725nm) were also observed. From the spectrum, the formation process of the transient S1 state could be seen directly. As shown in the inserts of Fig. 3, the ascending speed of the transient absorption (S1→Sm) at the 576nm is the same as the descending speed of the negative peak at 753nm. After being excited to Sn state, the molecules would undergo fast internal conversion to both S2 and S1 states. S2 state is a long-living state for the S2 emission lifetime had been measured to be 5.71ns, it is much slower than that of the formation of S1. Thus, most of the molecules on S1 state are originated directly from the Sn state. From the ascent gram (top insert of Fig.3) of transient S1 state (top insert of Fig.3), the risetime of transient S1 state could be estimated to be less than 10ps. The risetime of the following state is known to be the same magnitude with the lifetime of the original state. So the lifetime of the upper excited singlet Sn state is the same with the risetime of following S1 state and is less than 10ps. The lifetime of the higher excited singlet state Sn for several picoseconds is in the order of some other reported Pcs such as τSn=9ps of ZnPc(OEt)8 .
Recorded by the nanosecond-flash photolysis system, upon excitation by a 7ns pulse laser, the transient absorption spectrum is quite different with that obtained from the picosecond system. Figure 4 shows the transient spectrum of argon-saturated solution of AlCl-Pc-OC4 in THF upon excitation with the 7ns pulse laser. A combination of the ground state photo-bleaching and transient absorption was observed. In this timescale, neither S1 nor S2 emission could be detected because the fluorescence emission from S1 or S2 could only last several nanoseconds. The ΔOD maxima of the excited state absorption recorded by the picosecond and nanosecond flash photolysis system are significantly different. Usually, at this longer delay time, the excited Pc molecules were in long-lived triplet state . So the transient state absorption detected by nanosecond flash photolysis system is different from the results by picosecond one, contributes to triplet-triplet (T1→Tn) absorption. It is characterized by the broad and intense positive absorption of T1→Tn transition in the visible region with a peak at ca 565nm and the photo-bleaching of the Q-band and Soret-band in the negative area. The decay of the triplet-triplet absorption at 565nm was single-exponential with a lifetime of 93.7µs. The ΔεT was calculated to be 3.07×104M-1cm2 and the triplet state quantum yield of AlCl-Pc-OC4 in THF was determined to be ΦT=0.455.
3.3 Optical limiting property
From the above ground state and transient absorption properties, we could see that transient states (S1 and T1 states) of the AlCl-Pc-OC4 absorbs heavily within the optical window (approx 450–600nm), where the ground state hardly absorbs, which provides the possibility for AlCl-Pc-OC4 to act as the optical limiting material. The optical limiting behaviors of the AlCl-Pc-OC4 were investigated in argon saturated THF solutions in two different concentrations (2×10-4mol/l and 1×10-4mol/l). The linear transmittance (Tlin) in 532nm are Tlin=82% and Tlin=90.7% for the two solutions, respectively. The optical limiting response of AlCl-Pc-OC4 with concentration of 2x10-4mol/l in THF at 532nm for 7ns laser pulses is shown in Fig. 5, the nonlinear transmittance responses of AlCl-Pc-OC4 with two concentrations are shown in Fig. 6. It was found that the compound AlCl-Pc-OC4 exhibited good optical limiting performance. With a very low input fluence, the output fluence roughly obeyed Beer’s law, and then the transmittance decreased nonlinearly and remarkably with the increase of the input fluence. Various parameters used to evaluate the efficiency of OL materials are as follows: limiting threshold (the point where the transmittance is 50% of the initial transmittance), limiting transmittance (Tlim), and nonlinear attenuation factors (NAF). The value of nonlinear transmittance limitation (Tlim, which is defined as the limited saturated transmittance at high fluence) can be one of criteria to describe the optical limiting performance of a kind of material; a lower value of Tlim gives a better optical limiting behavior at high fluence. The NAF is the ratio of a linear transmittance Tlin to a nonlinear transmittance limitation Tlim, which effectively reflects the OL capability. The optical limiting behaviors of AlCl-Pc-OC4 in THF solution with two different concentrations are summarized in Table 2. From the data shown in Fig. 6 and Table 2, the solution of AlCl-Pc-OC4 with higher concentration has a lower limiting threshold and a lower transmittance at high fluence (Tlim).
From the results of the photophysical measurements, we find the unusual long-lifetime S2 emission accompanies with the usual S1 emission; and different transient states absorption processes at different delay times with Soret-band excitation. In order to describe the photophysical processes of AlCl-Pc-OC4 in the case of Soret-band excitation (λex=355nm), a multi levels model could be proposed, shown in Fig. 7, which includes the vibrational levels and potential minimum of the ground state S0, the first singlet excited state S1, upper singlet excited states S2, Sn and Sm, the excited triplet T1 and Tn states (Tn is an aggregate of all the higher excited triplet states including T2 state). The processes could be described as follows: the unperturbed AlCl-Pc-OC4 molecules reside in the lowest electronic state S0 at thermal equilibrium, (1) the molecules are promoted to the upper excited singlet state Sn (energy higher than S2 state) with excitation by 355nm light via a one-photon process; followed by fast internal conversion to the vibrational levels and the potential minimum of S2 (2) and S1 states (3). The fast internal conversion from upper singlet states Sn state to S1 state with rate of kic~1011s-1 had been directly detected by the picosecond-flash photolysis system, the lifetime of Sn state was estimated to be less than 10ps. Then molecules on S2 state undergo: (4) fluorescence emission from potential minimum state S2 state to S0 state (S2 emission), and (5) internal conversion to S0 state. There are three processes for the molecules on S1 state to go through: (6) fluorescence emission (S1 emission) and (7) back to ground state by nonradiative decay; (8) as well as intersystem crossing from S1 to T1 states to form the lowest excited triplet state T1, which is the beginning state for many expected processes such as nanosecond optical limiting. (9)With the excitation by the probe light in the picosecond system, the molecules on S1 state are stimulated to the upper singlet states Sm once more, the transient absorption spectrum of S1-Sm was recorded by the picosecond-flash photolysis system. (10)With excitation by the probe light in the nanosecond system, another transient absorption of T1 took the molecules to upper excited triplet state Tn, that absorption is a RSA process in nanosecond timescale, and may be the source of the property of optical limiting to nanosecond laser for AlCl-Pc-OC4. (11) The molecules on T1 state go back to S0 state by phosphoresce and intersystem crossing without emission. The model, which matches the experimental parameters well, summarizes the photophysical behaviors of AlCl-Pc-OC4 after Soret-band excitation by 355nm light clearly, shows the potential utilities of it as optical limiting materials based on RSA.
The tetra-α-butoxy substituents of AlCl-Pc not only extend out of the planar structure to strongly depress the tendency of molecules to aggregation and greatly improve the solubility , but also donate more electrons to the main phthalocyanine ring to enlarge the π-electron conjugation system and influence the photophysical properties. Compared with AlCl-Pc without α-substituents, the Q band of ground absorption is bathochromic shifted around 30nm from ca 680nm to ca 710nm, which might be explained by peripheral α-substituted butoxy groups on benzene rings in AlCl-Pc unit making the conjugation system much larger than unsubstituted Pc. The bathochromic shift makes the optical window of AlCl-Pc-OC4 broader than that of the unsubstituted one. In addition, the S1 fluorescence quantum yield of AlCl-Pc-OC4 is smaller than that of the AlCl-Pc (ΦF=0.58 in 1-chloronaphtalene) and the quantum yield of triplet state formation is bigger than that of the AlCl-Pc (ΦT=0.4 in 1-chloronaphthalene) . The tetra alpha substituents to the phthalocyanine rings decrease the S1 emission intensity due to the lower S1 state energy level, and then enhancing the internal conversion without radiation from S1 state to ground state. The lowering of the energy level of S1 state also narrows the energy gap between S1 and T1. Intersystem crossing from S1 to T1 state becomes easier in the case of the narrowed energy gap, which is the reason for the increase of the formation of the triplet state.
As for the nanosecond optical limiting behavior of 532nm, all the molecules of AlCl-Pc-OC4 are firstly excited to the higher vibrational level of the first excited singlet state; then fast internal conversion to the potential minimum of S1 state happens. At high saturation fluence, the ground state population is almost depleted and most of the molecules on excited state are distributed between the first excited singlet (S1) and lowest excited triplet (T1) states. Based on the photophysical parameters of AlCl-Pc-OC4 above, lifetimes of singlet states (7.04ns for S1 and 5.71ns for S2) are much shorter than that of triplet state (93.7µs for T1). At the long delay during the excitation by nanosecond laser (7ns fwhm), most excited molecules should be in long-living triplet state (T1), the average fractional populations of the excited triplet state would be much larger than that of the excited singlet state, so the average fractional populations of the excited singlet state can be ignored . The transient absorption is surely due to the triplet-triplet absorption (T1→Tn). Recently, McKerns and his coworkers reported that the Tn state of a porphyrin-like compound could probably be accessed by direct stimulation at 355nm from the triplet ground state, but the long lived triplet-state might not be accessible by direct stimulation at 532nm from the triplet ground state and must first go through state T2 . In this paper, the Tn is an aggregate of all the higher excited triplet states including T2 state in the proposed model. Considering the optical limiting characteristics originated from the excited triplet state absorption, the above multi-levels model can be simplified to be a four-level model, including two singlet states (S0 and S1) and two excited triplet states (T1 and Tn).
With the Q-band excitation by 532nm laser in this work, molecules on ground state (S0) were excited into the first excited singlet states (S1). The ground state absorption cross section (σ0) could be calculated from ground state absorption coefficient ε at 532nm (ε=4.34×10-4σ0N0, N0 is the Avogadro’s number). At low input fluence, the linear transmission (Tlin) can be expressed as following,
where N is the concentration of the sample, L is the effective path length.
Molecules on S1 state undergo further processes, such as radiative and nonradiative decay into the ground state, and intersystem crossing (ISC) from S1 to T1 state. Excited state absorption can occur from the long-living excited triplet state T1 up to a higher excited triplet state (Tn). The latter absorption process contributes to optical limiting behavior is the reverse saturable absorption. At a high input intensity, the ground state population can be ignored, transition of T1→Tn dominate the absorption, so the nonlinear transmittance limitation (Tlim) can be expressed as the following,
where σex is the excited absorption cross section from T1 to Tn. Then, the ratio of transmissions can be related to the ground state absorption cross section and the excited state (triplet state) absorption cross section by the following equation ,
On the basis of the above three equations and optical limiting parameters of AlCl-Pc-OC4 for 2×10-4mol/l solution, we can estimate the excited triplet state absorption cross section and evaluate the optical limiting behavior by the ratio of excited state absorption cross section to ground state absorption cross section. The results, together with the parameters of unsubstituted AlCl-Pc , are shown in Table 3.
Compared with the unsubstituted AlCl-Pc, it can be seen that the tetra-α-butoxy substituted AlCl-Pc-OC4 has a broader optical limiting window in the visible region between Q-band and Soret-band, a smaller ground state absorption cross section, a larger excited triplet state absorption cross section, and then a higher ratio of σex/σ0 value of 33. At the wavelength of 532nm, AlCl-Pc-OC4 would be a more suitable candidate for nanosecond optical limiting material with good solubility in organic solution.
In summary, the electronic absorption, S1 and S2 states fluorescence emission, and transient properties of the soluble tetra-α-butoxy substituted AlCl-Pc-OC4 have been investigated. An unusual emission from the higher excited singlet state (S2 emission) has been observed with long lifetime of 5.71ns. A multi levels model has been proposed to fit all the photophysical parameters and to discuss the total photophysical processes after Soret-band excitation. The optical limiting performance for 7ns laser pulses at 532nm of AlCl-Pc-OC4 had been investigated in THF solution and excellent OL performance (NAF=19.5) with a relatively good solubility has been observed. The reverse saturable absorption in the excited state (T1 to Tn) is responsible for the nanosecond optical limiting mechanism, a simplified four-level model has been employed to explain the nanosecond optical limiting behavior for 532nm. The tetra α-substitution on the molecules affected the photophysical and optical properties of AlCl-Pc significantly. A value of the ratio of σex/σ0 has been found to be 33 and it is much higher than that of unsubstituted AlCl-Pc. The results indicate that AlCl-Pc-OC4 is a suitable candidate for nanosecond optical limiting material at the wavelength of 532nm.
We would like to express our sincere thanks to the Major State Basic Research Development Program of China (Grant No. G2000078100) and National Natural Science Foundation of China (Grant No. 20333080, 20173066, 20473104, 50221001) for financial support. We also acknowledge help with the manuscript by Coracle Optics Co..
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