Spectroscopic effects resulting from interacting singlet and triplet excited states: vibronic structure involving the O-H stretching mode in d-d absorption bands of Ni(H2O)6(2+).

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Spectroscopic effects resulting from interacting singlet and triplet excited states: vibronic structure involving the O-H stretching mode in d-d absorption bands of Ni(H2O)62+ †

a

Departement für Chemie und Biochimie, Universität Bern, CH-3012 Bern, Switzerland

b

Département de chimie, Université de Montréal, Montréal QC H3C 3J7, Canada

*

author for correspondence, [email protected], tel. +1-514-343-7332

x

Dedicated to the memory of our friend and colleague PLWTP

Abstract The ligand-field absorption spectrum of the Ni(H2O)62+ cation has been thoroughly measured and analyzed over the past sixty years, often on crystals with low symmetry at the metal site, and its absorption band maxima have been used as a benchmark for increasingly sophisticated electronic structure calculations over the last decades. We present variable-temperature absorption spectra measured on crystals with cubic Th symmetry at the site of the nickel(II) cation. This high site symmetry is confirmed for CsNi(H2O)6PO4 by X-ray and neutron diffraction and allows for a direct comparison with ligand-field calculations in Th symmetry, at the basis of an analysis of the vibronic structure in the energy range of the lowest-energy spin-forbidden transition, the “red” or middle band of the spectrum. This spectroscopic region displays effects of strong interactions between singlet and triplet excited states, influencing intensities and vibronic structure. A particular feature that has not been analyzed in detail is a clearly discernible vibronic progression involving the O-H stretching mode on the high-energy side of the absorption band. A quantitative model is presented and applied in order to rationalize this unusual effect, arising from coupling between normal coordinates, a consequence of coupled excited sates, to the best of our knowledge the first analysis of this distinct spectroscopic feature arising from interacting excited states.

† Electronic supplementary information (ESI) available: Structure of CsNi(H2O)6(PO4): anisotropic displacement parameters and hydrogen bonds, ligand-field parameters for CsNi(H2O)6(PO4) and) Ni(H2O)6(BrO3)2.

1

Dalton Transactions Accepted Manuscript

Published on 03 October 2014. Downloaded by Northeastern University on 05/10/2014 03:26:40.

Christopher Dobe,a Emmanuel González,b Philip L. W. Tregenna-Piggott,a,x Christian Reberb*

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DOI: 10.1039/C4DT01979B

1. Introduction Absorption spectroscopy in the visible and near-infrared wavelength ranges is one of the most established and important techniques to study excited states of transition metal compounds.

1,2


intensities provide information on assigning transitions and determining excited-state energies.1,2 Band shapes, in particular resolved vibronic progressions, can be used to quantitatively determine excited-state distortions.3 Spectra arising from transitions to excited states that are close in energy can be challenging to analyze, illustrated by the “red” or middle band of the Ni(H2O)62+ complex with its long and controversial history.4-11 In this compound, energetically close singlet and triplet excited states interact, leading to unusual intensity distributions, most notably a double maximum in the solution spectrum, and characteristic vibronic structure in low-temperature spectra of single crystals. Quantitative models have been developed and applied in order to reproduce the intensities and vibronic spacings and the interference effects leading to these spectroscopic features have been reported.8 The absorption spectrum of Ni(H2O)62+ in solution has been used over the past decades as a test case for a variety of increasingly sophisticated electronic structure calculations.12-18 One unusual aspect remains, and it is the focus of this report. A low-intensity vibronic system occurs in low-temperature spectra and has been assigned as a short progression in the O-H stretching mode of the aquo ligands.6,7,9 This is unusual, as metal-centered d-d transitions are not expected to influence ligand bonds, even less so as one of the transitions is intraconfigurational, its spin-flip nature not leading to any structure changes for the excited state. It is this effect that we explore for Ni(H2O)62+ in two high-symmetry crystal environments. The spectra of the deuterated analog Ni(D2O)62+ show this set of bands at lower offset, corresponding to the lower D-O frequency and supporting the assignment.9 Cesium nickel(II) hexaqua phosphate forms part of a high symmetry (F-43m) isostructural series CsM2+(H2O)6A3-, where A3- = PO43-, AsO43- and M2+=Fe2+, Co2+, Ni2+19,20. Mg2+ also forms an isostructural compound in the AsO43- series; its structure was recently redetermined. There exists a large body of work investigating hexa-aqua Ni2+ systems in terms of both crystallographic

21-23

and electronic structure.4-8 However, the systems studied have tended to be

low symmetry, complicating the interpretation of the data. Due to the unusually high symmetry

2


Spectra of solutions and solids have been extensively studied in the past. Band maxima and

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of the Ni(H2O)62+ cation in CsNi(H2O)6PO4, the metal – water interaction in a novel high symmetry configuration may be studied without the complications of a low symmetry lattice. In electronic aspects, it is advantageous to perform ligand-field calculations on very high symmetry


calculated energy differences and transition energies.

2. Experimental Synthesis Powder samples of CsNi(H2O)6PO4 were prepared as follows: Ni(NO3)2.7H2O (0.5g) and CsNO3 (5g) were dissolved in 80ml of distilled water containing 1ml H3PO4 (1.8M) and a few drops of phenolphthalein. Dilute aqueous ammonia solution was added slowly, upon which a light green precipitate formed, until a pink color change was observed. The precipitate was filtered off, washed with water and ethanol, and then allowed to dry. The structure was confirmed by PXRD. Single crystals were grown over several weeks by evaporation of an aqueous solution. Two solutions were prepared, one containing NiNO3.7H2O (4.36g) and CsNO3 (6g). The second, Na2HPO4 (5.37g), NaCH3COO (2.72g) and 10ml CH3COOH (4M). The two solutions were combined, filtered and placed in beakers to evaporate slowly. Several crops of perfectly clear bright green crystals, with well developed faces orthogonal to the three-fold axis, were harvested. It was found that the solutions must be prepared sufficiently dilute to prevent the immediate precipitation of the compound as an extremely fine powder. Ni(H2O)6(BrO3)2 was prepared by slow evaporation of an aqueous solution. Ba(BrO3)2 (7.47g) was dissolved in 500mL distilled water, and NiSO4.6H2O (5g) in 80mL distilled water. The two solutions were slowly combined and the resulting BaSO4 precipitate filtered off. and crystallized following the published procedure. The crystal structure has been previously published.24 Single crystal x-ray diffraction was carried out by the Laboratory for Chemical and Mineralogical Crystallography, Universität Bern. Optical absorption measurements were carried out on a single crystal ground and polished to 0.5 mm thickness. The crystal was mounted with copper grease on the cold finger of a displex

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systems, as it allows a reasonable parameter to observation ratio and a direct comparison of

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cryostat and unpolarized spectra recorded at 294 K and 10 K. The spectral bandwidth was 0.5 nm.

3.1 Crystal structure of CsNi(H2O)6PO4 . Previous investigations involving this system are restricted to powder x-ray diffraction (PXRD) measurements.19,20 Structural data collected from single crystal x-ray diffraction methods show the high symmetry of the complex cation. Optical absorption spectroscopy probes the electronic structure. This compound is also of interest as it is isostructural with CsFe(H2O)6PO4 for which single crystals are not available for a full structural analysis. Single crystal X-ray diffraction data collection details for CsNi(H2O)6PO4 are given in Table 1 and atomic coordinates in Table 2. Table 3 presents selected bond lengths and angles of interest. Table ESI1 the anisotropic displacement parameters for the heavy atoms, and Table ESI2 the hydrogen bonds of interest. Fig. 1 plots the structure graphically.

3.2 Absorption spectroscopy of CsNi(H2O)6PO4 and Ni(H2O)6(BrO3)2 The ligand field absorption spectrum of Ni(H2O)62+ measured in solution is given in many textbooks on coordination chemistry.11 Spectra measured at low temperature on single crystals reveal much additional information, as illustrated in Fig. 2 for CsNi(H2O)6PO4 and Ni(H2O)6(BrO3)2. The low-temperature optical absorption measurements in Fig. 2 show five transitions, which are easily assignable.4 One weak spin forbidden transition, 1T2g←3A2g, is seen at 22190 cm-1 while a second, 1Eg←3A2g, falls on the wing of the 3T1g←3A2g transition. Table 4 compares absorption maxima at room temperature and at low temperature. Small shifts are observed, most likely due to loss of vibronic intensity at low temperature. The vibronic structure observed in the 1T2g and 1

Eg bands is pronounced at low temperature. The 1Eg←3A2g vibronic progression interval is, on

average, approximately 470 cm-1, while the 3T1g←3A2g band demonstrates a progression of approximately 190 cm-1. The former progression appears anharmonic showing non-constant spacing in the progression.

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3. Results

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The overall band assignments are straightforward as given in the figure. Fig. 3 shows the middle or “red” band of the spectrum in detail. Rich vibronic structure is observed. The prominent progression on the high-energy side of the band system is doubled for Ni(H2O)6(BrO3)2, most


upon which the progressions are built, as the degeneracy of the 1Eg state is not expected to be lifted in this high symmetry. A surprising observation is the weak, but clearly measurable vibronic progression involving the O-H stretching frequency between 18000 and 20000 cm-1, a replica of the prominent progression starting at 16000 cm-1. This replica or member of a short progression is observed in spectra of crystalline Ni(H2O)62+ in many different crystal structures and its energy interval indicates that it involves the O-H stretching mode. Its intensity is on the order of 10% of the origin of the vibronic progression, and subsequent members of the progression can not be observed as they are masked by the higher-energy d-d transition.

4. Discussion 4.1 Calculated spectra based on one-dimensional potential energy curves The solution spectra and crystal spectra without the O-H progression of interest here can be quantitatively analyzed with the model illustrated by the potential energy curves in Fig. 4 along the totally symmetric Ni-O stretching mode. This model has been described in detail8 and is directly applicable to the title compounds due to the high site symmetry. Parameter values defining these potential curves are given in Table 5. Absorption transitions from the 3A2g ground state to the 3T1g(3P) and 1Eg excited states form the band system analyzed here. Transitions occur to the A2g, T1g, T2g and Eg spin-orbit levels arising from the 3T1g excited state and to the Eg state arising from the 1Eg excited state. The energies of all electronic states are calculated with the ligand-field parameters given in Table ESI 3 and correspond to vertical transitions, i.e. absorption band maxima. The parameter values are chosen to reproduce all absorption band maxima of the spectra shown in Fig. 2. The energies given in Table 5 for the electronic states correspond to electronic origins, retaining the energy differences between states obtained from the ligand field calculations, but also including the lowering of the energies due to the offset of the potential energy minimum along the Ni-O normal coordinate by ∆Q as illustrated for the 3T1g state in Fig. 4.

5


likely due to frequency differences between ungerade parity modes forming vibronic origins

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The agreement between experimental and calculated spectra, shown as the top two traces in Fig. 5, is good. This is the first comparison of an experimental spectrum for a cubic symmetry chromophore with a model using the same point group symmetry, in contrast to previously


arising form the singlet and triplet excited states are coupled and their energies and wavefunctions affected by spin-orbit coupling. An interference dip governs the intensity distribution, as shown in Fig. 5 for the spectrum of the transitions to the coupled Eg levels, with a notable absence of intensity between 14500 cm-1 and 15500 cm-1. It is important to note that no intensity at all is calculated at wavenumbers higher than 18500 cm-1 where the O-H replica is observed. Spectra are calculated using the time-dependent theory of spectroscopy and numerical methods described before. 8 The higher-energy part of the band systems arises from the transition to the upper adiabatic surface in Fig. 4, corresponding to mostly singlet character in the region of the ground-state equilibrium structure, defined by the origin of the horizontal axis in Fig. 4. Qualitatively, an intraconfigurational transition to an excited state with a minimum not far from the ground state along the horizontal axis of Fig. 4 is expected, and such transitions do not typically show vibronic structure. It is therefore very unusual to see the O-H progression built on this part of the band. To the best of our knowledge, no quantitative models for this progression have been published. The following section presents such a model in order to understand this unusual vibronic feature.

4.2 Calculated spectra based on two-dimensional potential energy surfaces The absorption spectra in Fig. 3 clearly show short progressions involving the O-H mode. DFTcalculated molecular orbitals for Ni(H2O)62+ can be used to gain qualitative insight on the contribution of the O-H electronic density in the mainly metal-centered t2g and eg orbitals. Graphical representations of these orbitals obtained with the B3LYP functional and LANL2DZ basis functions are given in Fig. 6. The eg orbital shows the expected metal-ligand σ* interaction, but also an antibonding pattern along the two O-H bonds of each aquo ligand. In contrast, the metal-ligand π* t2g orbitals are strictly nonbonding in the O-H region. This qualitative difference in O-H bonding implies that a transition from the ground state to an excited state with a higher number of electrons in the eg levels should lead to a short progression in the O-H mode. Such progressions are usually not observable as the change of eg electron

6


reported comparisons with lower-symmetry Ni(H2O)62+ cations. The two Eg spin-orbit levels

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population leads to a broad absorption band, masking any small effects arising from possible O-H progressions. The O-H progression is expected for the 3T1g state, as its eg population is higher than two electrons, the configuration of the ground state. A small offset along the O-H stretching


from the same electron configuration as the ground state, and no offsets along normal coordinates or progressions are expected. This expectation is too simple in the case of the middle band of Ni(H2O)62+: a significant progression occurs on the high-energy side of the band as shown in Fig. 5. This progression is a consequence of spin-orbit coupling between the Eg levels from the singlet and triplet states, and it can be viewed as a “transfer” of excited-state bond length change from the singlet to the triplet state. Does such a “transfer” also occur for the O-H coordinate? In order to explore this question, the model in Fig. 4 has to be extended to a second coordinate, leading to the adiabatic potential energy surfaces in Fig. 7, with the parameters used to calculate these surfaces given in Table 5. The lower surface shows a small offset along the O-H coordinate. Spin-orbit coupling of the Eg levels causes a deformed upper adiabatic surface, with a small offset of the minimum along the O-H coordinate and approximately elliptical overall shape with main axes no longer aligned with the two normal coordinates. The calculated spectra from this model are compared to the experimental spectra in Fig. 8. Fig. 8a shows the total calculated spectrum, corresponding to the sum of all spectra calculated for the spin-orbit levels for CsNi(H2O)6PO4. The agreement is good, in particular in the region of the OH progression between 18000 cm-1 and 20000 cm-1, indicating that the two-dimensional model presented here correctly reproduces all aspects of coupled excited states that lead to this surprising progression. This comparison illustrates the origin of the O-H progression: it arises through spin-orbit coupling between the two Eg states, leading to effects involving both metalligand and ligand centered normal coordinates. Fig. 8b shows the comparison between the spectrum calculated for just the Eg states for Ni(H2O)6(BrO3)2, again showing that the highfrequency progression is a consequence of spin-orbit coupled excited states. The spectra presented illustrate the rich variety of phenomena arising from excited states in inorganic coordination chemistry.

Acknowledgments

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normal coordinate can therefore be expected for the 3T1g state. In contrast, the 1Eg state arises

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We thank the Natural Sciences and Engineering Research Council (Canada) and the Swiss National Science Foundation (Switzerland) for research grants.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

J. Ferguson, Progr. Inorg. Chem., 1970, 12, 159. P. Day, Angew. Chem. Int. Ed., 1980, 19, 290. J. I. Zink, Coord. Chem. Rev., 2001, 211, 69. C. K. Jørgensen, Acta Chem. Scand., 1955, 9, 1362. A. D. Liehr and C. J. Ballhausen, Ann. Phys., 1959, 6, 134. M. H. L. Pryce, G. Agnetta, T. Garofano, M. B. Palma-Vittorelli and M. U. Palma, Phil. Mag., 1964, 10, 77. E. I. Solomon and C. J. Ballhausen, Mol. Phys., 1975, 29, 279. G. Bussière and C. Reber, J. Am. Chem. Soc., 1998, 120, 6306. G. Bussière, R. Beaulac, B. Cardinal-David and C. Reber, Coord. Chem. Rev., 2001, 219221, 509. G. Bussière, C. Reber, D. Neuhauser, D. A. Walter and J. I. Zink, J. Phys. Chem. A, 2003, 107, 1258. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, Fifth Edition, 1988, p. 745. J. Landry-Hum, G. Bussière, C. Daniel and C. Reber, Inorg. Chem., 2001, 40, 2595. C. Anthon and C. E. Schaffer, Coordination Chemistry Reviews, 2002, 226, 17. S. Iuchi, A. Morita and S. Kato, Journal of Chemical Physics, 2004, 121, 8446. F. Neese, T. Petrenko, D. Ganyushin and G. Olbrich, Coordination Chemistry Reviews, 2007, 251, 288. C. M. Aguilar, W. B. De Almeida and W. R. Rocha, Chemical Physics, 2008, 353, 66. D. Casanova and M. Head-Gordon, Physical Chemistry Chemical Physics, 2009, 11, 9779. S. Iuchi and S. Sakaki, Chemical Physics Letters, 2010, 485, 114. A. Ferrari, L. Cavalca and M. Nardelli, Gazz. Chim. Ital., 1955, 85, 169. A. Ferrari, M. Tani and R. Bonati, Gazz. Chim. Ital., 1956, 86, 1026. G. J. McIntyre, H. Ptasiewicz-Bak and I. Olovsson, Acta Cryst. , 1990, B46, 27. H. Ptasiewicz-Bak, I. Olovsson and G. J. McIntyre, Acta Cryst. , 1993, B49, 192. H. Ptasiewicz-Bak, I. Olovsson and G. J. McIntyre, Acta Cryst., 1997, B53, 325. A. C. Blackburn, J. C. Gallucci and R. E. Gerkin, Acta Cryst. , 1991, C47, 1786.

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References

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Tables

Identification code Empirical formula Formula weight Temperature Wavelength Crystal system Space group Unit cell dimensions

Volume Z Density (calculated) Absorption coefficient F(000) Crystal size Theta range for data collection Index ranges Reflections collected Independent reflections Completeness to theta = 27.68° Absorption correction Max and min transmission Refinement method Data / restraints / parameters Goodness of fit on F2 Final R indices (I>2θ(I)) R indices (all data) Absolute structure parameter Extinction coefficient Largest diff. Peak and hole

CsNi(H2O)6PO4 H12 Cs Ni O10 P 394.69 g/mol 293(2) K 0.71073 Å Cubic F –4 3 m a = 10.0024(5) α = 90° β = 90° b = 10.0024(5) c = 10.0024(5) γ = 90° 1000.72(9) Å3 4 2.620 g/cm3 5.713 mm-1 760 0.22x0.18x0.14 mm3 3.53 to 27.68° -12≤h≤6, -12≤k≤13, -12≤l≤13 1432 148 (R(int) = 0.0265) 97.7 % Semi-empirical from int. of equiv. Reflec. 0.288 and 0.220 Full-matrix least-squares on F2 148 / 0 / 15 1.049 R1 = 0.0159, wR2 = 0.0386 R1 = 0.0170, wR2 = 0.0388 -0.03(5) 0.0035(5) 0.360 and –0.360 e. Å-3

Table 2 Atomic coordinates (x104) and equivalent isotropic displacement parameters (Å2x103) for CsNi(H2O)6PO4. U(eq) is defined as one third of the trace of the orthogonalised Uij tensor. Ni Cs P O(1) O(2) H

x 0 7500 2500 2045(3) 3392(2) 2460(60)

y 0 7500 2500 0 3392(2) 420(30)

z 0 7500 2500 0 3392(2) 420(30)

Table 3 Selected bond lengths (Å) and angles (°) for CsNi(H2O)6PO4. Ni-O(1) P-O(2) O(1)-H(1) O(2)-P-O(2) Ni-O(1)-H

2.045(3) 1.545(4) 0.72(5) 109.471(1) 125(4)

9

U(eq) 28(1) 55(1) 24(1) 62(1) 32(1) 66(14)



Table 1 Crystal data and structure refinement for CsNi(H2O)6PO4.

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Assignment 3

T2g←3A2g T1g←3A2g 1 Eg←3A2g 1 T2g←3A2g 3 T1g←3A2g Published on 03 October 2014. Downloaded by Northeastern University on 05/10/2014 03:26:40.

3

Exp. 296K cm-1 8470 (10) 13650 (200) 15330 (200) 21980 (50) 24800 (1)

10

Exp. 10K cm-1 8700 (10) 13750 (200) 15250 (200) 22190 (50) 25144 (2)


Table 4 Observed transition energies for d-d bands of CsNi(H2O)6PO4.

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Table 5 Spectroscopic parameters used to calculate absorption spectra of Ni(BrO3)2.6H2O and

Parameter

[Ni(H2O)6](BrO3)2

Cs[Ni(H2O)6](PO4)

ω0 (ground state) (Ni-O/O-H modes) (cm-1)

397/3000

397/3000

ω0 (1Γ) (cm-1)

397/3000

397/3000

ω0 (3Γ) (cm-1)

355/3000

350/3000

∆Q (1Γ) (Å)

0.0/0.0

0.0/0.0

∆Q (3Γ) (Å)

0.26/0.01

0.22/0.01

λ (cm-1)

-300

-300

γ (cm-1)

6λ

6λ

Γ (cm-1)

60

60

E00 Eg(1Eg) (cm-1)

14622 (λ=0)

14608 (λ=0)

E00 Eg(3T1g) (cm-1)

12718 (λ=0)

12842 (λ=0)

E00 A1g(3T1g) (cm-1)

12007

12125

E00 T1g(3T1g) (cm-1)

12479

12601

E00 T2g(3T1g) (cm-1)

13225

13355

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Cs[Ni(H2O)6](PO4) in Fig. 7.

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Figure captions

Fig. 1 Single Crystal x-ray diffraction structure of CsNi(H2O)6PO4 viewed down the four-fold


thermal ellipsoid. The hydrogen ellipsoids are isotropic, all others are anisotropic.

Fig. 2 Unpolarized single crystal optical absorption spectra of CsNi(H2O)6PO4 (top trace) at 10 K and Ni(H2O)6(BrO3)2 (bottom trace) at 15 K. The spectral bandwidth was 0.5 nm. Fig. 3 Experimental absorption spectra of CsNi(H2O)6PO4 (solid trace) at 10 K and Ni(H2O)6(BrO3)2 (dotted trace) at 15 K in the region of the lowest-energy spin-forbidden transition. Fig. 4 Adiabatic (dotted lines) and diabatic (solid lines) potential energy curves used to calculate the absorption spectrum the title complex in the region of the lowest-energy singlet excited state. Numerical values for all parameters are given in Table 5. This one-dimensional model includes only the totally symmetric Ni-OH2 stretching mode. Fig. 5 Comparison of experimental (dotted trace) and calculated absorption spectra (solid traces) in the region of the lowest-energy singlet excited state for Ni(H2O)6(BrO3)2. Comparison of the contribution of each spin-orbit level (dotted traces, offset along the ordinate for clarity). The parameter values used for the calculation are given in Table 5. Only the totally symmetric Ni-O stretching mode is used for these calculations. Note the absence of any calculated absorbance in the region of the O-H stretching vibronic feature between 18500 cm-1 and 20000 cm-1. Fig. 6 Calculated molecular orbital amplitudes for t2g (bottom) and eg (top) orbitals. Fig. 7 Two-dimensional adiabatic potential energy surfaces along the normal coordinates for the QNi-O and QO-H stretching modes for Ni(H2O)6(BrO3)2. (a) Higher-energy adiabatic surface, (b) lower-energy adiabatic surface. Parameters defining the potential energy surfaces are given in Table 5.

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axis. Thermal ellipsoids are drawn at 50% probability. Note the elongation of the water oxygen

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Fig. 8 (a) Experimental and calculated absorption spectra based on surfaces along both the QNi-O and the QO-H coordinates for CsNi(H O)6(PO4). (b) Experimental and calculated absorption 2

spectra for the coupled two-dimensional Eg potential energy surfaces for Ni(H2O)6(BrO3)2 along

Text for table of contents entry

Spin-orbit coupling between metal-centered electronic states can lead to absorption spectra with vibronic structure involving ligand-centered vibrational modes, as shown and theoretically analyzed for a band system of Ni(H2O)62+ with its O-H progression.

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the corresponding QNi-O and the QO-H normal coordinates.

Absorbance

Ni-OH2 stretching

exp. calc.

14

16

cm

-1

18




O-H stretching

20x10

3

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QO-H

QO-H



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QNi-OH2

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exp.

calc. 12000

14000

16000

18000

20000

22000

-1

Wavenumber (cm )

Absorbance

(b)

14000

16000 18000 -1 Wavenumber (cm )

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An Unusually Small Singlet-Triplet Gap in a Quinoidal 1,6-Methano[10]annulene Resulting from Baird's 4n π-Electron Triplet Stabilization.

Singlet-triplet annihilation in single LHCII complexes.

Reactions of excited triplet states of metal substituted myoglobin with dioxygen and quinone.

Quenching of singlet excitations by triplet excitation.

Triplet excited states of polycyclic aromatic compounds as probes of their microenvironment in serum albumin complexes.