Open Access
Add to BibSonomy
Abstract
We use the two-photon excited fluorescence method to determine the two-photon absorption (2PA) cross sections of three series of (fluorenyl benzothiazole) gold(I) complexes in the visible wavelength range from 570 to 700 nm. We compare the effect of ancillary ligand substitutions on the 2PA magnitudes and find that the ancillary ligand does not drastically affect either the magnitude or the shape of 2PA. Even so, moderate 2PA cross sections were measured that ranged from 10 to 1000 s of GM (Göppert–Mayer, $= {10^{- 50}}\;{\rm{cm}}^4{\rm{s}}/{\rm{photon}}$), making these types of complexes nonlinear optical materials for two-photon absorbing applications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. INTRODUCTION
The nonlinear optical (NLO) properties of numerous organometallic complexes have been explored over the past three decades [1–4] possessing characteristics that allow them to be used in a variety of applications such as all-optical switching [5], optical data storage [6], microfabrication [7], and as two-photon absorbers [8]. These studies have included a diverse set of metal complexes including cyclometalated Ir(III) complexes [9–16], Ru(II) bipyridine, phenanthroline, and pyridinium complexes [17–21], Ru(II) acetylides [22–27], cyclometalated Pt(II) complexes [28–31], Pt(II) terpyridine complexes [32,33], Pt(II) bipyridine acetylide complexes [34–38], and platinum acetylides [39–48]. The NLO response in these metal complexes largely falls into one of two groups: complexes that display instantaneous two-photon absorption (2PA) coupled with excited state absorption (an effective three-photon process) or reverse saturable absorption (RSA) that is largely determined by the incoming pulse wavelength. These processes result from metal-to-ligand charge transfer (MLCT) or ligand-to-metal charge transfer (LMCT) excited states or complexes where metal coordination to a chromophoric ligand produces an organic–inorganic hybrid with altered 2PA or RSA relative to the uncoordinated ligand.
The advantages of organometallic complexes can be demonstrated by comparing the linear and 2PA properties of an organic donor–acceptor complex, AF240 [49,50], and an organometallic Pt(II) acetylide complex, E1BTF [48], that share a benzothiazole-2,7-fluorenyl (BTF) unit, as shown in Fig. 1. AF240 has a diphenyl amine donor unit attached to the fluorene core while E1BTF has a Pt(II) ethynyl unit attached to the fluorene core. The intrinsic 2PA cross section (in units of Göppert–Mayer, $1{\rm{GM}} = {10^{- 50}}\;{\rm{cm}}^4{\rm{s}}/{\rm{photon}}$) at 800 nm have been reported for both AF240 (43 GM) and E1-BTF (472 GM) and show an enhancement in the metalated complex [48,50]. Under nanosecond excitation, AF240 has been reported to possess both 2PA coupled with an excited state absorption dominated by both radiative decay from the singlet excited state as well as the formation of an intramolecular charge-transfer (ICT) state that varies with solvent polarity [51]. The effective nonlinearity was observed to be enhanced in E1BTF where the excited state dynamics are dominated by rapid intersystem crossing to form a strongly absorbing triplet excited state in which the maximum of the triplet–triplet absorption spectrum was shown to overlap with the maximum of the 2PA spectrum [48]. The cascaded NLO response stemming from 2PA followed by one photon absorption (1PA) from the triplet excited state results in further enhancement of the nonlinearity in E1BTF for ns pulse excitation [52]. Even though these complexes have adequate 2PA characteristics, the main drawback for both is that the lowest energy ground state absorption transition is centered at ${\sim}{{405}}\;{\rm{nm}}$ having a broad linewidth (${\rm{FWHM}} \approx 100\;{\rm{nm}}$). This results in significant linear absorption in the blue region of the visible spectrum. Therefore, the design and characterization of new organic–inorganic hybrid materials is desirable that maintain the compelling intersystem crossing dominated excited state dynamics of E1BTF while enhancing linear transmission in the visible wavelength range.
Fig. 1. Molecular structures of (top) organic donor–acceptor complex, AF240 [49], and (bottom) organometallic complex, E1BTF [48].
Fig. 2. Chemical structures of the Au(I) complexes. Note that one of the complexes from [57] (Au-DiBTF3) is not shown. This compound suffers from minimal solubility in toluene and, thus, its 2PA could not be determined.
Au(I) possesses a unique set of characteristics than make it an attractive choice as an alternative metal center to Pt(II) in BTF complexes. Au(I) has a filled, ${{\rm{d}}^{10}}$ electron configuration, preventing deactivation by ligand field states on the metal. The gold-carbon bond is essentially nonpolar because gold has an electronegativity value close to carbon. The nonpolar nature of this bond could improve transmission in the visible. Gold also has a large spin-orbit coupling matrix element (${{5104}}\;{\rm{c}}{{\rm{m}}^{- 1}}$ [53]), increasing the likelihood of intersystem crossing in gold containing organometallic chromophores [54].
With these attributes in mind, three series of Au(I)-BTF complexes were recently synthesized to ascertain the effects of ancillary ligand substitutions, bridging moiety, and the number of gold centers on their photophysical characteristics following 1PA in degassed solutions [55–57]. The chemical structures of the Au(I)-BTF complexes are shown in Fig. 2. Phosphine-containing complexes with a BTF moiety bound through a gold–carbon $\sigma$ bond (Au-BTF0 and Au-BTF1) were found to exhibit fluorescence lifetimes that were a factor of 2 shorter and fluorescence quantum yield (QY) values that were a factor of 2 smaller than the Au(I)-BTF complex containing an N-heterocyclic carbene (Au-BTF2) ancillary ligand. Simultaneously, intersystem crossing was found to be enhanced in the phosphine containing derivatives, Au-BTF0 and Au-BTF1, relative to the N-heterocyclic carbene complexes, Au-BTF2 [55]. In a related work, inclusion of an alkynyl spacer between the Au(I) metal center and the BTF ligand (Au-ABTF0-2) leads to enhanced fluorescence QYs and diminished intersystem crossing QYs compared to the Au-BTF complexes. Interestingly, the fluorescence QY and intersystem crossing QY are the largest for the N-heterocyclic carbene containing complexes, Au-ABTF2, compared to the phosphine containing derivatives, Au-ABTF0 and Au-ABTF1 [56]. Furthermore, asymmetric dinuclear Au(I) complexes with variation in ancillary ligand and gold–chromophore linkage were synthesized, and it was found that the excited state properties were affected when the capping ligand was changed from phosphines (Au-DiBTF0 and Au-DiBTF1) to N-heterocyclic carbenes (Au-DiBTF2 and Au-DiBTF3) [57]. Changing the gold–chromophore linkage from an alkynyl (Au-DiBTF2) to a triazolyl (Au-DiBTF3) also leads to changes in the excited-state dynamics. All 10 Au(I) complexes have enhanced visible transparency and the Au-BTF and Au-ABTF series have similar triplet excited state absorption compared to E1BTF.
These desirable 1PA properties along with the diversity and tunability of the photophysical properties among these Au(I) derivatives necessitates the collection of intrinsic 2PA cross sections for all of these chromophores. To this end, we measured the intrinsic 2PA spectra of three series of Au(I) complexes, Au-BTF(0-2), Au-DiBTF(0-2), and Au-ABTF(0-2), by using two-photon excited fluorescence (2PEF) spectroscopy [58]. The 2PA spectra are provided in the wavelength range from 570 to 700 nm.
2. EXPERIMENTAL SETUP
For the 2PEF measurements, the laser pulses are derived from an amplified Ti:sapphire system (Solstice, Spectra-Physics) producing 3.5 mJ, 100 fs (FWHM) pulses at a 1 kHz repetition rate at a wavelength ($\lambda$) of 800 nm. To generate the visible wavelengths of interest, approximately 1.8 mJ from the laser output is used to pump an optical parametric amplifier (OPA) (Light Conversion Ltd., TOPAS-PRIME). The wavelengths of interest are generated via frequency mixing from the output of the OPA. The pulses from the OPA were then spatially filtered using an all-reflective focusing geometry. After spatial filtering, the laser pulses are directed into the spectrofluorometer (Edinburgh Instruments Ltd., FS5) via a modified sample compartment chamber (SC-05), as shown in Fig. 3. The cassette has an opening drilled in the front to allow laser light entry as well as a custom-fitted optical breadboard (${\rm{1/}}{{{4}}^{{\prime \prime}}}{\rm{- 20}}$ taps on ${{{1}}^{{\prime \prime}}}$ centers) in which mirrors direct the laser light into the sample cuvette. The last mirror (top right in Fig. 3) is an off-axis parabolic mirror to focus the laser light into the sample. The 2PEF emission is collected with a photo-multiplier tube (PMT) having detectivity ranging from 200 to 1000 nm. To control the fluorescence intensity reaching the PMT, the emission bandwidth is adjusted and attenuation is placed in the laser light beam path prior to entering the SC-05. The attenuation consists of neutral density filters and a half-wave plate/polarizer combination.
Fig. 3. Schematic of 2PEF experiment. OPA, optical parametric amplifier; M, mirror; CM, concave mirror; SF, spatial filter; OAP, off-axis parabola; and PMT, photomultiplier tube.
3. RESULTS
A. Methodology
To perform 2PEF measurements, a high-intensity pump beam excites a sample via 2PA and then the excited state radiatively relaxes back down to the ground state [58]. Thus, for 2PEF to be an efficient means to extract the 2PA coefficient, the sample should be highly fluorescent. For all samples, the fluorescence QY values are $\gg {{1}}\%$ and have been experimentally determined previously in a degassed solution using an integrating sphere [55–57]. The magnitude of the 2PEF signal for each wavelength measured allows for the construction of the 2PA spectrum of a given material. 2PEF measurements are performed in matched 1 cm path length cuvettes in aerated solutions dissolved in spectroscopic grade toluene.
The 2PA spectra of the dyes are determined by the comparison method, where the 2PA of a well-studied reference molecule is used to determine the 2PA spectrum of the unknown by
Table 1. Fluorescence QY Values for Au(I)-BTF Complexes Dissolved in Aerated Toluenea
The fluorescence QYs of the chromophores were verified in aerated solutions using the comparison method. A permanently heat fusion sealed 1.28 µM solution of quinine sulfate dissolved in 0.105 M perchloric acid (SRM 936a) was purchased from Starna Cells (RM-QS00) as the reference having $\phi = 0.60$ [62]. For the Au-BTF series, the same excitation wavelength as SRM 936a was used. The Au-ABTF and Au-DiBTF series were excited at different excitation wavelengths from SRM 936a in which an excitation power correction factor of 0.952 and 0.849, respectively, was used for QY calculation. The aerated QY values obtained from the comparison method are provided in Table 1 along with those previously found using an integrating sphere.
Bis-MSB (1,4-Bis(2-methylstryl)benzene; CAS: 13280-61-0) dissolved in spectroscopic grade cyclohexane and Rhodamine B (CAS: 81-88-9) dissolved in spectroscopic grade methanol were used as the reference molecules for the 2PEF measurements as depicted in Fig. 4. The fluorescence QY values of $\phi = 0.70$ [63] and $\phi = 0.86$ [64] have been reported for Rhodamine B in methanol and Bis-MSB in cyclohexane, respectively. The 2PA spectra of Bis-MSB in cyclohexane has been previously reported in [65] (a typographical error was corrected in [66]) and in [67,68] for Rhodamine B in methanol. The chemical structures of Bis-MSB and Rhodamine B as well as their normalized absorption, fluorescence, and 2PA spectra are shown in Fig. 4.
Fig. 4. Chemical structures of the 2PA reference molecules: (a) Bis-MSB and (b) Rhodamine B. (c) Normalized absorption (solid black line) and fluorescence (solid red line) spectra of Bis-MSB dissolved in cyclohexane. The right vertical axis shows the 2PA spectrum of Bis-MSB in cyclohexane from 560 to 700 nm (solid blue circles) [65,66]. (d) Normalized absorption (solid black line) and fluorescence (solid red line) spectra of Rhodamine B dissolved in methanol. The right vertical axis shows the 2PA spectrum of Rhodamine B in methanol from 650 to 700 nm (solid blue circles) [67,68].
Since the 2PEF intensity is directly related to the incident irradiance and sample concentration, the 2PEF experimental sample concentrations were orders of magnitude larger than those used for QY measurements to maximize the detected signal. Consequently, the resultant 2PEF spectrum is heavily distorted due to inner filter effects [69]. Therefore, to determine ${\langle F_2\rangle}$, the normalized one-photon fluorescence (1PF) curve (of a dilute solution) is scaled to the tail of the 2PEF spectrum where absorption is minimal. Integrating over this scaled 1PF curve yields ${\langle F_2\rangle}$ in Eq. (1). This is performed for both the reference and unknown sample at each wavelength of measure. Of note, for each wavelength the same laser excitation energy was used for both the reference and unknown sample. Thus, there is no irradiance excitation correction factor used in Eq. (1) since both reference and unknown sample are measured under the same exact conditions for each wavelength.
B. Comparison of 2PA Spectra
All Au(I) complexes exhibit dual emission from fluorescence and phosphorescence in the 500–700 nm wavelength range in degassed solutions. The Au-BTF series [arylgold(I) complexes] display the most pronounced features, with phosphorescence intensity decreasing from Au-BTF0 to Au-BTF2; that is, as the ancillary ligand changes from ${\rm{PP}}{{\rm{h}}_3}$ to ${\rm{PC}}{{\rm{y}}_3}$ to the N-heterocyclic carbene IPr. The ratio of phosphorescence peak intensity compared to the fluorescence peak intensity (${I_{{\rm{phos}}}}/{I_{{\rm{fluor}}}}$) reaches a maximum of nearly 0.7 for Au-BTF0, 0.5 for Au-BTF1, and 0.3 for Au-BTF2. The Au-ABTF series show weaker phosphorescence in the 550–700 nm wavelength range in degassed solutions with a maximum ${I_{{\rm{phos}}}}/{I_{{\rm{fluor}}}}$ ratio of 0.28 for Au-ABTF2, 0.08 for Au-ABTF0, and 0.06 for Au-ABTF1. The Au-DiBTF series exhibit much weaker phosphorescence than the Au-BTF and Au-ABTF series in the 550–700 nm wavelength range in degassed solutions. The ${I_{{\rm{phos}}}}/{I_{{\rm{fluor}}}}$ ratio only reaches a maximum of 0.025 for Au-DiBTF2 and 0.02 for both Au-DiBTF0 and Au-DiBTF1. The phosphorescence in the 500–700 nm range disappears for all Au(I) complexes in an aerated solution, as shown in Figs. 5(a)–5(i). Thus, these complexes are singly emissive in the presence of oxygen and the 2PEF signal is attributed explicitly to the ${S_1} \to {S_0}$ transition.
Fig. 5. Normalized absorption (solid black lines) and fluorescence (solid red lines) spectra along with the 2PA cross sections in units of GM (solid blue circles) in aerated toluene of (a) Au-BTF0, (b) Au-BTF1, (c) Au-BTF2, (d) Au-ABTF0, (e) Au-ABTF1, (f) Au-ABTF2, (g) Au-DiBTF0, (h) Au-DiBTF1, and (i) Au-DiBTF2. The error bars associated with each measured 2PA value are $\pm 25\%$.
The bandwidth and wavelength maxima of the 2PA spectra of the nine fluorenyl benzothiazole complexes reported here are all relatively similar. The spectra appear as a single band centered at 600 nm with a FWHM of approximately 50 nm. There are variations in the magnitudes of the 2PA cross sections among the chromophores, with values ranging from 200 to 3000 GM at the maximum of the 2PA spectra. A summary of 1PA and peak 2PA values for the Au(I) complexes is listed in Table 2.
Table 2. Comparison of 1PA and 2PA Properties of the Au(I) Complexes in Aerated Toluenea
For all Au-BTF complexes, see Figs. 5(a)–5(c), the 2PA cross section increases from 570 nm to a maximum at ${\sim}{{590}}\;{\rm{nm}}$. Au-BTF0 shows the largest 2PA cross section at the peak of ${\sim}{{3000}}\;{\rm{GM}}$ while Au-BTF1 and Au-BTF2 reach modest peaks of 210 GM and 230 GM, respectively. The 2PA rapidly decreases for Au-BTF0 to a minimum of ${\sim}{{68}}\;{\rm{GM}}$ at 680 nm and then slowly rises to 210 GM at 700 nm. For Au-BTF1 and Au-BTF2, the minimum 2PA values of 8 GM and 5 GM occur at 650 nm, respectively. There appears to be no drastic blue or red shift of the 2PA spectrum as it pertains to ancillary ligand modification for the Au-BTF series complexes.
Note that the peak 2PA value of Au-BTF0 is more than order of magnitude larger than Au-BTF1 and Au-BTF2. This interesting result is proven reproducible by conducting multiple scans and noting no discernible difference. Furthermore, as mentioned previously, the 2PEF signal from Au-BTF0 and Au-BTF1 (having similarly low fluorescence QY values) are collected in the same manner with vastly different peak 2PA cross sections. Thus, we attribute this difference to the molecule itself and not to measurement inaccuracy of this technique for low QY dyes, as was reported by Dubinina et al. [70].
Alkynyl gold(I) complexes Au-ABTF1 and Au-ABTF2 exhibit a slight increase in the 2PA cross section from 570 nm to a peak around 400 GM at ${\sim}{{590}}\;{\rm{nm}}$, whereas the 2PA values are relatively flat for Au-ABTF0 from 570 to 620 nm, as shown in Figs. 5(d)–5(f). For all Au-ABTF complexes, the 2PA cross section reaches a minimum at 650 nm before another peak at 690 nm, which corresponds to the vibronic band evident in the linear absorption at 345 nm. Similar to the Au-BTF series, there appears to be no apparent blue or red shift of the 2PA spectrum as it pertains to the ancillary ligand for the Au-ABTF series complexes.
All di-gold Au-DiBTF complexes show an increase in 2PA from 570 nm to a peak at 600 nm, as shown in Figs. 5(g)–5(i). Furthermore, the magnitudes of the 2PA peaks are very similar: ${\sim}{{1400}}\;{\rm{GM}}$. The 2PA cross section for all Au-DiBTF complexes reaches a minimum at 650 nm of ${\sim}{{100}}\;{\rm{GM}}$ before continuously increasing to 700 nm. As with the Au-BTF and Au-ABTF series, there appears to be no apparent blue or red shift of the 2PA spectrum as it pertains to the ancillary ligand for the Au-DiBTF series complexes.
Two-photon absorption measurements were reported on a series of Au(I) and Pt(II) acetylide complexes from 540 to 810 nm [71]. As it pertains to the authors’ Au(I) acetylide complex, they observed a similar increase of the 2PA cross section from 540 nm to a peak of ${\sim}{{560}}\;{\rm{nm}}$ with a 2PA cross section of ${\sim}{{800}}\;{\rm{GM}}$. While the magnitude of the 2PA cross sections across the visible spectrum of Au-BTF1 and Au-BTF2 are similar to those reported, the peak cross section is ${\sim}{{4}} \times \;{\rm{larger}}$ in Au-BTF0 than the Au(I) acetylide complex. Comparing the Au(I) acetylide complex to the Au-ABTF series, the Au-ABTF series yields slightly smaller 2PA cross section values throughout the visible regime. The minimum at 650 nm for the Au-ABTF series is on the order of 10s of GM whereas for the Au(I) acetylide complex the 2PA cross section is ${\sim}{{180}}\;{\rm{GM}}$. Pertaining to the Au-BTF series complexes, the 2PA cross sections of the Au-ABTF series are similar in magnitude. There is a much less pronounced peak in the 2PA spectra (with the exception of Au-ABTF0) at 590 nm. For the Au-DiBTF series, the 2PA cross section is nearly a factor of 2 larger than the Au(I) acetylide complex with the magnitude of the 2PA cross sections across the visible regime being similar. Compared to the Au-BTF and Au-ABTF series, the 2PA magnitudes for Au-DiBTF are typically larger spanning the measured wavelength range. The minimum position of the 2PA cross section at 650 nm coincides with the minimum observed for the Au-BTF series (with the exception of Au-BTF1) as well as the Au-ABTF series.
The 2PA cross-section values of the Au-BTF, Au-ABTF, and Au-DiBTF complexes reported here as well as the previously studied Au(I) acetylide complexes are all moderate ($\le {{3000}}\;{\rm{GM}}$), as stated above. The magnitude of 2PA cross sections are generally enhanced in molecules with strong charge transfer (CT) character [8,72]. The lowest energy electronic transitions in the Au(I)-BTF complexes are all $\pi - {\pi ^*}$ in character [55–57] (i.e., the benzothiazole unit is a moderate electron acceptor, and the chromophores lack strong electron donating moieties). The Au(I) acetylides are constructed using thiophenes and carbozole, both electron donating moieties that also result in electronic transitions that are $\pi - {\pi ^*}$ in character [71]. As such, the moderate 2PA cross-section values in the Au(1) BTF complexes reported here and the Au(I) acetylide complexes are most likely due to the absence of a strong CT character in the electronic transitions of these molecules.
4. CONCLUSION
We measured the two-photon absorption (2PA) cross sections of three series of Au(I) fluorenyl benzothiazole complexes with different ancillary ligands in the visible wavelength range. The intrinsic 2PA cross sections ranged from 10 to 1000s of GM with the phosphine-containing fluorenyl benzothiazole compound (Au-BTF0) possessing the largest 2PA cross section of 3000 GM at 590 nm. Furthermore, changing the ancillary ligand within each Au(I) series does not drastically affect the shape or magnitude of the 2PA spectrum. These moderate 2PA cross sections suggest these Au(I) complexes are promising for nonlinear optical applications. In the future, we plan to measure the effective nonlinearity using nanosecond pulse excitation for direct comparison to E1-BTF and AF240.
Funding
Air Force Research Laboratory (FA8650-16-D-5402-0001); Air Force Office of Scientific Research (AFOSR) (9550-19-1-18RX056, FA9550-18-1-0247).
Acknowledgment
Case Western Reserve University recognizes the U.S Air Force Research Laboratory (AFRL) (Contract FA9550-18-1-0247 to author Thomas G. Gray). All AFRL-affiliated authors recognize the Air Force Office of Scientific Research.
Disclosures
Case Western Reserve University has filed for provisional patent protection for all Au(I) complexes studied in this work (Au-BTF0-2, Au-ABTF0-2, and Au-DiBTF0-3).
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
REFERENCES
1. L. W. Tutt and T. F. Boggess, “A review of optical limiting mechanisms and devices using organics, fullerenes, semiconductors and other materials,” Prog. Quantum Electron. 17, 299–338 (1993). [CrossRef]
2. D. Dini, M. J. F. Calvete, and M. Hanack, “Nonlinear optical materials for the smart filtering of optical radiation,” Chem. Rev. 116, 13043–13233 (2016). [CrossRef]
3. C. E. Powell and M. G. Humphrey, “Nonlinear optical properties of transition metal acetylides and their derivatives,” Coord. Chem. Rev. 248, 725–756 (2004). [CrossRef]
4. J. P. Morrall, G. T. Dalton, M. G. Humphrey, and M. Samoc, “Organotransition metal complexes for nonlinear optics,” in Advances in Organometallic Chemistry, R. West, A. F. Hill, and M. J. Fink, eds. (Academic, 2007), pp. 61–136.
5. J. M. Hales, S. Barlow, H. Kim, S. Mukhopadhyay, J.-L. Bredas, J. W. Perry, and S. R. Marder, “Design of organic chromophores for all-optical signal processing applications,” Chem. Mater. 26, 549–560 (2014). [CrossRef]
6. S. Kawata and Y. Kawata, “Three-dimensional optical data storage using photochromic materials,” Chem. Rev. 100, 1777–1788 (2000). [CrossRef]
7. B. H. Cumpston, S. P. Ananthavel, S. Barlow, D. L. Dyer, J. E. Ehrlich, L. L. Erskine, A. A. Heikal, S. M. Kuebler, I. Y. S. Lee, D. McCord-Maughon, J. Q. Qin, H. Rockel, M. Rumi, X. L. Wu, S. R. Marder, and J. W. Perry, “Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication,” Nature 398, 51–54 (1999). [CrossRef]
8. M. Albota, D. Beljonne, J. L. Bredas, J. E. Ehrlich, J. Y. Fu, A. A. Heikal, S. E. Hess, T. Kogej, M. D. Levin, S. R. Marder, D. McCord-Maughon, J. W. Perry, H. Rockel, M. Rumi, C. Subramaniam, W. W. Webb, X. L. Wu, and C. Xu, “Design of organic molecules with large two-photon absorption cross sections,” Science 281, 1653–1656 (1998). [CrossRef]
9. B. Liu, L. Lystrom, S. Kilina, and W. Sun, “Effects of varying the benzannulation site and π conjugation of the cyclometalating ligand on the photophysics and reverse saturable absorption of monocationic iridium(III) complexes,” Inorg. Chem. 58, 476–488 (2019). [CrossRef]
10. B. Liu, L. Lystrom, S. L. Brown, E. K. Hobbie, S. Kilina, and W. Sun, “Impact of benzannulation site at the diimine (N^N) ligand on the excited-state properties and reverse saturable absorption of biscyclometalated iridium(III) complexes,” Inorg. Chem. 58, 5483–5493 (2019). [CrossRef]
11. X. Zhu, P. Cui, S. Kilina, and W. Sun, “Multifunctional cationic iridium(III) complexes bearing 2-aryloxazolo[4,5-f] [1,10]phenanthroline (N^N) ligand: synthesis, crystal structure, photophysics, mechanochromic/vapochromic effects, and reverse saturable absorption,” Inorg. Chem. 56, 13715–13731 (2017). [CrossRef]
12. X. Zhu, L. Lystrom, S. Kilina, and W. Sun, “Tuning the photophysics and reverse saturable absorption of heteroleptic cationic iridium(III) complexes via substituents on the 6,6′-bis(fluoren-2-yl)-2,2′-biquinoline ligand,” Inorg. Chem. 55, 11908–11919 (2016). [CrossRef]
13. C. Wang, L. Lystrom, H. Yin, M. Hetu, S. Kilina, S. A. McFarland, and W. Sun, “Increasing the triplet lifetime and extending the ground-state absorption of biscyclometalated Ir(iii) complexes for reverse saturable absorption and photodynamic therapy applications,” Dalton Trans. 45, 16366–16378 (2016). [CrossRef]
14. T. M. Pritchett, M. J. Ferry, W. M. Shensky III, A. G. Mott, D. J. Stewart, S. L. Long, J. E. Haley, Z. Li, and W. Sun, “Strong triplet excited-state absorption in a phenanthrolinyl iridium(III) complex with benzothiazolylfluorenyl-substituted ligands,” Opt. Lett. 40, 186–189 (2015). [CrossRef]
15. Y. Li, N. Dandu, R. Liu, L. Hu, S. Kilina, and W. Sun, “Nonlinear absorbing cationic iridium(III) complexes bearing benzothiazolylfluorene motif on the bipyridine (N∧N) ligand: synthesis, photophysics and reverse saturable absorption,” ACS Appl. Mater. Interfaces 5, 6556–6570 (2013). [CrossRef]
16. B. Liu, M. A. Jabed, S. Kilina, and W. Sun, “Synthesis, photophysics, and reverse saturable absorption of trans-bis-cyclometalated iridium(III) complexes (C^N^C)Ir(R-tpy) + (tpy = 2,2′:6′,2″-terpyridine) with broadband excited-state absorption,” Inorg. Chem. 59, 8532–8542 (2020). [CrossRef]
17. L. Wang, H. Yin, M. A. Jabed, M. Hetu, C. Wang, S. Monro, X. Zhu, S. Kilina, S. A. McFarland, and W. Sun, “π-expansive heteroleptic ruthenium(II) complexes as reverse saturable absorbers and photosensitizers for photodynamic therapy,” Inorg. Chem. 56, 3245–3259 (2017). [CrossRef]
18. B. J. Coe, “Switchable nonlinear optical metallochromophores with pyridinium electron acceptor groups,” Acc. Chem. Res. 39, 383–393 (2006). [CrossRef]
19. W. Sun, T. H. Patton, L. K. Stultz, and J. Pablo Claude, “Resonant third-order nonlinearities of tetrakis(2,2′-dipyridyl)diruthenium complexes,” Opt. Commun. 218, 189–194 (2003). [CrossRef]
20. M. Four, D. Riehl, O. Mongin, M. Blanchard-Desce, L. M. Lawson-Daku, J. Moreau, J. Chauvin, J. A. Delaire, and G. Lemercier, “A novel ruthenium(ii) complex for two-photon absorption-based optical power limiting in the near-IR range,” Phys. Chem. Chem. Phys. 13, 17304–17312 (2011). [CrossRef]
21. C. Feuvrie, O. Maury, H. Le Bozec, I. Ledoux, J. P. Morrall, G. T. Dalton, M. Samoc, and M. G. Humphrey, “Nonlinear optical and two-photon absorption properties of octupolar tris(bipyridyl)metal complexes,” J. Phys. Chem. A 111, 8980–8985 (2007). [CrossRef]
22. S. K. Hurst, M. G. Humphrey, T. Isoshima, K. Wostyn, I. Asselberghs, K. Clays, A. Persoons, M. Samoc, and B. Luther-Davies, “Organometallic complexes for nonlinear optics. 28.1 dimensional evolution of quadratic and cubic optical nonlinearities in stilbenylethynylruthenium complexes,” Organometallics 21, 2024–2026 (2002). [CrossRef]
23. A. M. McDonagh, M. G. Humphrey, M. Samoc, B. Luther-Davies, S. Houbrechts, T. Wada, H. Sasabe, and A. Persoons, “Organometallic complexes for nonlinear optics. 16.1 Second and third order optical nonlinearities of octopolar alkynylruthenium complexes,” J. Am. Chem. Soc. 121, 1405–1406 (1999). [CrossRef]
24. A. M. McDonagh, M. G. Humphrey, M. Samoc, and B. Luther-Davies, “Organometallic complexes for nonlinear optics. 17.1 Synthesis, third-order optical nonlinearities, and two-photon absorption cross section of an alkynylruthenium dendrimer,” Organometallics 18, 5195–5197 (1999). [CrossRef]
25. S. K. Hurst, M. P. Cifuentes, J. P. L. Morrall, N. T. Lucas, I. R. Whittall, M. G. Humphrey, I. Asselberghs, A. Persoons, M. Samoc, B. Luther-Davies, and A. C. Willis, “Organometallic complexes for nonlinear optics. 22.1 Quadratic and cubic hyperpolarizabilities of trans-bis(bidentate phosphine)ruthenium σ-arylvinylidene and σ-arylalkynyl complexes,” Organometallics 20, 4664–4675 (2001). [CrossRef]
26. S. K. Hurst, M. P. Cifuentes, A. M. McDonagh, M. G. Humphrey, M. Samoc, B. Luther-Davies, I. Asselberghs, and A. Persoons, “Organometallic complexes for nonlinear optics: Part 25. Quadratic and cubic hyperpolarizabilities of some dipolar and quadrupolar gold and ruthenium complexes,” J. Organomet. Chem. 642, 259–267 (2002). [CrossRef]
27. J. P. L. Morrall, M. P. Cifuentes, M. G. Humphrey, R. Kellens, E. Robijns, I. Asselberghs, K. Clays, A. Persoons, M. Samoc, and A. C. Willis, “Organometallic complexes for nonlinear optics. Part 36. Quadratic and cubic optical nonlinearities of 4-fluorophenylethynyl- and 4-nitro-(E)-stilbenylethynylruthenium complexes,” Inorg. Chim. Acta 359, 998–1005 (2006). [CrossRef]
28. Z. Li and W. Sun, “Synthesis, photophysics, and reverse saturable absorption of platinum complexes bearing extended π-conjugated C^N^N ligands,” Dalton Trans. 42, 14021–14029 (2013). [CrossRef]
29. Z. Li, E. Badaeva, A. Ugrinov, S. Kilina, and W. Sun, “Platinum chloride complexes containing 6-[9,9-di(2-ethylhexyl)-7-r-9h-fluoren-2-yl]-2,2′-bipyridine ligand (R = NO2, CHO, benzothiazol-2-yl, n-Bu, carbazol-9-yl, NPh2): tunable photophysics and reverse saturable absorption,” Inorg. Chem. 52, 7578–7592 (2013). [CrossRef]
30. B. Zhang, Y. Li, R. Liu, T. M. Pritchett, A. Azenkeng, A. Ugrinov, J. E. Haley, Z. Li, M. R. Hoffmann, and W. Sun, “Synthesis, structural characterization, photophysics, and broadband nonlinear absorption of a platinum(II) complex with the 6-(7-Benzothiazol-2′-yl-9,9-diethyl-9 H-fluoren-2-yl)-2,2′-bipyridinyl ligand,” Chem. Eur. J. 18, 4593–4606 (2012). [CrossRef]
31. P. Shao, Y. Li, J. Yi, T. M. Pritchett, and W. Sun, “Cyclometalated platinum(II) 6-phenyl-4-(9,9-dihexylfluoren-2-yl)-2,2′-bipyridine complexes: synthesis, photophysics, and nonlinear absorption,” Inorg. Chem. 49, 4507–4517 (2010). [CrossRef]
32. B. Zhang, Y. Li, R. Liu, T. M. Pritchett, J. E. Haley, and W. Sun, “Extending the bandwidth of reverse saturable absorption in platinum complexes using two-photon-initiated excited-state absorption,” ACS Appl. Mater. Interfaces 5, 565–572 (2013). [CrossRef]
33. W. Sun, Z.-X. Wu, Q.-Z. Yang, L.-Z. Wu, and C.-H. Tung, “Reverse saturable absorption of platinum ter/bipyridyl polyphenylacetylide complexes,” Appl. Phys. Lett. 82, 850–852 (2003). [CrossRef]
34. T. Lu, C. Wang, L. Lystrom, C. Pei, S. Kilina, and W. Sun, “Effects of extending the π-conjugation of the acetylide ligand on the photophysics and reverse saturable absorption of Pt(ii) bipyridine bisacetylide complexes,” Phys. Chem. Chem. Phys. 18, 28674–28687 (2016). [CrossRef]
35. X.-G. Liu and W. Sun, “Synthesis, Photophysics, and reverse saturable absorption of bipyridyl platinum(II) bis(acetylide) complexes bearing aromatic electron-withdrawing substituents on the acetylide ligands,” J. Phys. Chem. A 118, 10318–10325 (2014). [CrossRef]
36. R. Liu, Y. Li, J. Chang, E. R. Waclawik, and W. Sun, “Pt(II) bipyridyl complexes bearing substituted fluorenyl motif on the bipyridyl and acetylide ligands: synthesis, photophysics, and reverse saturable absorption,” Inorg. Chem. 53, 9516–9530 (2014). [CrossRef]
37. Z. Li, E. Badaeva, D. Zhou, J. Bjorgaard, K. D. Glusac, S. Killina, and W. Sun, “Tuning photophysics and nonlinear absorption of bipyridyl platinum(II) bisstilbenylacetylide complexes by auxiliary substituents,” J. Phys. Chem. A 116, 4878–4889 (2012). [CrossRef]
38. T. M. Pritchett, W. Sun, B. Zhang, M. J. Ferry, Y. Li, J. E. Haley, D. M. Mackie, I. I. I. W. Shensky, and A. G. Mott, “Excited-state absorption of a bipyridyl platinum(II) complex with alkynyl-benzothiazolylfluorene units,” Opt. Lett. 35, 1305–1307 (2010). [CrossRef]
39. R. W. Winkel, G. G. Dubinina, K. A. Abboud, and K. S. Schanze, “Photophysical properties of trans-platinum acetylide complexes featuring N-heterocyclic carbene ligands,” Dalton Trans. 43, 17712–17720 (2014). [CrossRef]
40. R. Vestberg, R. Westlund, A. Eriksson, C. Lopes, M. Carlsson, B. Eliasson, E. Glimsdal, M. Lindgren, and E. Malmström, “Dendron decorated platinum(II) acetylides for optical power limiting,” Macromolecules 39, 2238–2246 (2006). [CrossRef]
41. R. Zieba, C. Desroches, F. Chaput, M. Carlsson, B. Eliasson, C. Lopes, M. Lindgren, and S. Parola, “Preparation of functional hybrid glass material from platinum (II) complexes for broadband nonlinear absorption of light,” Adv. Funct. Mater. 19, 235–241 (2009). [CrossRef]
42. R. S. Price, G. Dubinina, G. Wicks, M. Drobizhev, A. Rebane, and K. S. Schanze, “Polymer monoliths containing two-photon absorbing phenylenevinylene platinum(II) acetylide chromophores for optical power limiting,” ACS Appl. Mater. Interfaces 7, 10795–10805 (2015). [CrossRef]
43. T. J. McKay, J. Staromlynska, J. R. Davy, and J. A. Bolger, “Cross sections for excited-state absorption in a Pt:ethynyl complex,” J. Opt. Soc. Am. B 18, 358–362 (2001). [CrossRef]
44. T. J. McKay, J. A. Bolger, J. Staromlynska, and J. R. Davy, “Linear and nonlinear optical properties of platinum-ethynyl,” J. Chem. Phys. 108, 5537–5541 (1998). [CrossRef]
45. J. Staromlynska, T. J. McKay, and P. Wilson, “Broadband optical limiting based on excited state absorption in Pt:ethynyl,” J. Appl. Phys. 88, 1726–1732 (2000). [CrossRef]
46. J. E. Rogers, T. M. Cooper, P. A. Fleitz, D. J. Glass, and D. G. McLean, “Photophysical characterization of a series of platinum(II)-containing phenyl−ethynyl oligomers,” J. Phys. Chem. A 106, 10108–10115 (2002). [CrossRef]
47. T. M. Cooper, D. M. Krein, A. R. Burke, D. G. McLean, J. E. Rogers, J. E. Slagle, and P. A. Fleitz, “Spectroscopic characterization of a series of platinum acetylide complexes having a localized triplet exciton,” J. Phys. Chem. A 110, 4369–4375 (2006). [CrossRef]
48. J. E. Rogers, J. E. Slagle, D. M. Krein, A. R. Burke, B. C. Hall, A. Fratini, D. G. McLean, P. A. Fleitz, T. M. Cooper, M. Drobizhev, N. S. Makarov, A. Rebane, K.-Y. Kim, R. Farley, and K. S. Schanze, “Platinum acetylide two-photon chromophores,” Inorg. Chem. 46, 6483–6494 (2007). [CrossRef]
49. R. Kannan, G. S. He, L. Yuan, F. Xu, P. N. Prasad, A. G. Dombroskie, B. A. Reinhardt, J. W. Baur, R. A. Vaia, and L.-S. Tan, “Diphenylaminofluorene-based two-photon-absorbing chromophores with various π-electron acceptors,” Chem. Mater. 13, 1896–1904 (2001). [CrossRef]
50. J. E. Rogers, J. E. Slagle, D. G. McLean, R. L. Sutherland, M. C. Brant, J. Heinrichs, R. Jakubiak, R. Kannan, L. S. Tan, and P. A. Fleitz, “Insight into the nonlinear absorbance of two related series of two-photon absorbing chromophores,” J. Phys. Chem. A 111, 1899–1906 (2007). [CrossRef]
51. D. J. Stewart, R. Kannan, T. A. Grusenmeyer, J. M. Artz, S. L. Long, Z. Yu, T. M. Cooper, J. E. Haley, and L.-S. Tan, “Effects of intramolecular hydrogen bonding and sterically forced non-coplanarity on organic donor/acceptor two-photon-absorbing molecules,” Phys. Chem. Chem. Phys. 20, 19398–19407 (2018). [CrossRef]
52. R. L. Sutherland, M. C. Brant, J. Heinrichs, J. E. Rogers, J. E. Slagle, D. G. McLean, and P. A. Fleitz, “Excited-state characterization and effective three-photon absorption model of two-photon-induced excited-state absorption in organic push-pull charge-transfer chromophores,” J. Opt. Soc. Am. B 22, 1939–1948 (2005). [CrossRef]
53. M. Montalti, A. Credi, L. Prodi, and M. T. Gandolfi, Handbook of Photochemistry (CRC, 2006).
54. T. J. Penfold, E. Gindensperger, C. Daniel, and C. M. Marian, “Spin-vibronic mechanism for intersystem crossing,” Chem. Rev. 118, 6975–7025 (2018). [CrossRef]
55. J. J. Mihaly, D. J. Stewart, T. A. Grusenmeyer, A. T. Phillips, J. E. Haley, M. Zeller, and T. G. Gray, “Photophysical properties of organogold(i) complexes bearing a benzothiazole-2,7-fluorenyl moiety: selection of ancillary ligand influences white light emission,” Dalton Trans. 48, 15917–15927 (2019). [CrossRef]
56. J. J. Mihaly, A. T. Phillips, D. J. Stewart, Z. M. Marsh, C. L. McCleese, J. E. Haley, M. Zeller, T. A. Grusenmeyer, and T. G. Gray, “Synthesis and photophysics of gold(i) alkynyls bearing a benzothiazole-2,7-fluorenyl moiety: a comparative study analyzing influence of ancillary ligand, bridging moiety, and number of metal centers on photophysical properties,” Phys. Chem. Chem. Phys. 22, 11915–11927 (2020). [CrossRef]
57. J. J. Mihaly, A. T. Phillips, J. T. Malloy, Z. M. Marsh, M. Zeller, J. E. Haley, K. de La Harpe, T. A. Grusenmeyer, and T. G. Gray, “Synthesis and photophysical properties of laterally asymmetric digold(I) alkynyls and triazolyl: ancillary ligand and organic functionality dictate excited-state dynamics,” Organometallics 39, 489–494 (2020). [CrossRef]
58. C. Xu and W. W. Webb, “Measurement of two-photon excitation cross sections of molecular fluorophores with data from 690 to 1050 nm,” J. Opt. Soc. Am. B 13, 481–491 (1996). [CrossRef]
59. J. Rheims, J. Köser, and T. Wriedt, “Refractive-index measurements in the near-IR using an Abbe refractometer,” Meas. Sci. Technol. 8, 601–605 (1997). [CrossRef]
60. K. Moutzouris, M. Papamichael, S. C. Betsis, I. Stavrakas, G. Hloupis, and D. Triantis, “Refractive, dispersive and thermo-optic properties of twelve organic solvents in the visible and near-infrared,” Appl. Phys. B 116, 617–622 (2014). [CrossRef]
61. M. N. Polyanskiy, “Refractive index database,” https://refractiveindex.info/.
62. R. A. Velapoldi and K. D. Mielenz, A Fluorescence Standard Reference Material, Quinine Sulfate Dihydrate (U.S. Dept. of Commerce, National Bureau of Standards: For sale by the Supt. of Docs., U.S. Govt. Print. Off., 1980).
63. R. A. Velapoldi and H. H. Tønnesen, “Corrected emission spectra and quantum yields for a series of fluorescent compounds in the visible spectral region,” J. Fluoresc. 14, 465–472 (2004). [CrossRef]
64. C. Buck, B. Gramlich, and S. Wagner, “Light propagation and fluorescence quantum yields in liquid scintillators,” J. Instrum. 10, P09007 (2015). [CrossRef]
65. S. M. Kennedy and F. E. Lytle, “p-Bis(o-methylstyryl)benzene as a power-squared sensor for two-photon absorption measurements between 537 and 694 nm,” Anal. Chem. 58, 2643–2647 (1986). [CrossRef]
66. K. S. Overway and F. E. Lytle, “Fluorescent enzymatic probes suitable for two-photon excitation,” Appl. Spectrosc. 52, 298–302 (1998). [CrossRef]
67. N. S. Makarov, M. Drobizhev, and A. Rebane, “Two-photon absorption standards in the 550–1600 nm excitation wavelength range,” Opt. Express 16, 4029–4047 (2008). [CrossRef]
68. National Institute of Chemical Physics and Biophysics, Tallinn, Estonia, “Two-photon absorption spectra,” https://kbfi.ee/chemical-physics/research/nonlinear-spectroscopy/multiphoton-spectra/?lang=en.
69. C. A. Parker and W. J. Barnes, “Some experiments with spectrofluorimeters and filter fluorimeters,” Analyst 82, 606–618 (1957). [CrossRef]
70. G. G. Dubinina, R. S. Price, K. A. Abboud, G. Wicks, P. Wnuk, Y. Stepanenko, M. Drobizhev, A. Rebane, and K. S. Schanze, “Phenylene vinylene platinum(II) acetylides with prodigious two-photon absorption,” J. Am. Chem. Soc. 134, 19346–19349 (2012). [CrossRef]
71. S. Goswami, G. Wicks, A. Rebane, and K. S. Schanze, “Photophysics and non-linear absorption of Au(i) and Pt(ii) acetylide complexes of a thienyl-carbazole chromophore,” Dalton Trans. 43, 17721–17728 (2014). [CrossRef]
72. P. Norman, P. Cronstrand, and J. Ericsson, “Theoretical study of linear and nonlinear absorption in platinum-organic compounds,” Chem. Phys. 285, 207–220 (2002). [CrossRef]
