Photochemical and Photophysical Processes in 2,2'-Bipyridine

4 Photochemical and Photophysical Processes in 2,2'-Bipyridine Complexes of Iridium(III) and

Downloaded by UNIV OF AUCKLAND on December 13, 2015 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0168.ch004

Ruthenium(II) R. J. WATTS, J. S. HARRINGTON, and J. VAN HOUTEN Department of Chemistry, University of California, Santa Barbara, CA 93106 Tris bipyridyl complexes of Ru(II) and Ir(III) have been found to be luminescent and photochemically active in fluid solutions—hence the term "luminactive." Luminescence lifetimes and quantum yields for emission and photochemistry provide a direct measure of the competition between chemical and physical energy transformations in these species. Photoactivity of Ru(bpy)3 in aqueous HCl and aqueous N a H C O is partially caused by photoanation. The primary photoproducts have been isolated and studied by absorption and emission spectroscopy. The results are consistent with their identification as species containing monodentate bpy. Photoactivity of Ir(bpy)3 in aqueous NaOH gives rise to [Ir(bpy) OH(bpy)] . A model based on a double potential minimum is presented to interpret the luminescence and photochemical properties of Ru(bpy)3 . 2+

3

3+

2

2+

2+

T h e production of electronic excited states in polyatomic molecules by absorption of visible and U V irradiation results in a variety of energy transfer processes. These include both radiative processes, which result in luminescence, and nonradiative processes. The latter can be con veniently divided into: (a) photophysical processes in which the chemi cal identity of the absorbing species is retained and the absorbed energy is eventually transferred as heat into the environment, and (b) photo chemical processes in which the chemical identity of the absorbing species is altered through either endothermic or exothermic reactions of the excited states resulting from light absorption. These chemically reactive excited states can be reached either by direct absorption of light or indirectly by light absorption followed by energy transfer within the reactive species or via sensitization by a second absorbing species. 0-8412-0398-9/78/33-168-057$05.00/0 © 1978 American Chemical Society In Inorganic and Organometallic Photochemistry; Wrighton, M.; Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

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W h i l e i t is convenient to divide nonradiative energy transfer into photophysical and photochemical processes, the classification of a given process is not always as clear as the division implies. Among the major complicating features encountered are: (a) how large a geometrical distortion constitutes a change i n chemical identity, and ( b ) over what time increment following light absorption is the classification to be made?

Metal-Ligand Distance Figure 1.

Energy transfer in double minima poten-

Consider the following example, diagrammed i n Figure 1, which serves to illustrate these features. T h e M L molecule, w h i c h is a central metal bound to three bidentate ligands and which has overall trigonal geometry i n the equilibrium ground state configuration, undergoes a F r a n c k Condon transition to an excited state having nearly the same equilibrium geometry along the metal-ligand bond axes. T h e potential w e l l for this excited state ( S ) interacts w i t h that for a second excited state ( S ' ) under distortion along the M - L axis. This second excited state has an equilibrium geometry i n w h i c h the M - L distance is substantially increased because of migration of metal electron density from the pseudo-octahedral 6

In Inorganic and Organometallic Photochemistry; Wrighton, M.; Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

4.

59

2,2-Bipyridine Complexes of Ir(IH) and Ru(II)

WATTS ET AL.

faces to a M - L bond axis. This distorted excited state can undergo a vertical transition to an equivalently distorted ground state via a radia tive emission i n which metal-electron density along the M - L axis migrates back to the pseudo-octahedral faces. In this example, the lowest-energy ground state configuration is considered to be the t r i gonally distorted M L species whereas the lowest-energy electronically excited state is the M L * species. 6


5

The experimental observations one might make on such a system would depend critically upon environmental factors such as temperature, solvent, viscosity, etc. F o r example, at low temperatures (kT < < Δ Ε / < ΔΕ ) one would expect to see simply emission (k ) or radiationless decay (k ) of S as it returns to G , and classification of the energy transfer as photophysical would be straightforward. A t higher temperatures ( Δ Ε / < kT < AE ) one might observe radiative (k/) or radiationless (k ) processes from S' to G ' as w e l l as from S to G , and the distorted G ' isomer might be observable i n a noncoordinating solvent medium. Again, assum ing that G ' could be observed and characterized as a chemical species distinct from G , classification of photochemical and photophysical events would be distinct, though complicated. Finally, consider the case where either Δ Ε / < AE < kT or ΔΕ ' ^ ΔΕ < kT; one might see emission from S' to G ' as the only radiative process and might be unable to observe G ' assuming ΔΕ and ΔΕ' > > kT. Here classification of the energy trans fer events is difficult since photochemistry may or may not have occurred as an intermediate step, depending on the extent of distortion present i n G ' , although no net photochemistry would be observed. Several points are apparent in this example. First, the dividing line between photochemistry and photophysics is indistinct since it depends on the extent of geometrical distortion between potential minima neces sary to define a "distinct chemical species" as opposed to a "highly dis torted excited state." Operationally, this distinction is complicated fur ther by activation energies which may or may not enable the experi mentalist to observe species corresponding to a potential minimum at a given temperature. Second and more important than the semantic diffi culties i n distinguishing photochemistry and photophysics is the point that isolated observations of a species under fixed environmental parame ters can lead to vastly different observations whose relationship may not be immediately apparent. Only through extensive studies of both photo chemistry and photophysics over a wide range of environmental parame ters can the experimentalist hope to arrive at a cohesive interpretation of the myriad of energy transfer events subsequent to electronic excitation of a polyatomic species. A variety of well-established techniques have been used i n studies of photochemical and photophysical energy transfer processes resulting r

α

n

f

a

n

a

α

α



60


from electronic excitation of metal complexes (1,2). Excited-state ab sorption techniques have proved to be extremely useful i n studies of both photochemistry and photophysics, but the requirement for extremely high intensity, fast-pulsed light sources limits the scope of this method. Although luminescence often has been used to characterize photophysical energy transfer, these studies generally have been performed under con ditions where photochemistry is absent (glasses or crystalline solids at 4.2°-77°K). B y variations i n temperature and/or solvent, one can hope to find that species which exhibit low-temperature luminescence begin to undergo photochemical activity while still displaying luminescence. Such species are referred to as "luminactive," and constitute a class of compounds around which the discussion w i l l be centered. W h i l e the present range of luminactive compounds at temperatures above 273° Κ is very limited, there are undoubtedly many more compounds that are luminactive between 77° and 273°K. Hence, studies of luminactive spe cies w i l l become more common as photophysical studies are extended upward from 77°K and as photochemical studies are extended downward from 273°K. The purpose of this discussion is to present an analysis of photo physical and photochemical energy-transfer events i n two such lumin active species. A t the present time, a comprehensive model of the energy transfer processes i n R u ( b p y ) can be presented. F a r less work has been completed on I r ( b p y ) , but its luminactive nature has been established. 3

3

2 +

3+

Experimntal Luminescence and Absorption Measurements. Corrected lumines cence spectra were determined with either a Perkin-Elmer Hitachi M P F 3A spectrophotofluorimeter (250-700nm) or with a red-sensitive appa ratus described previously (3) (450-1100nm). The dependence of the response of each instrument on wavelength was determined by calibra tion against a standard N B S quartz-iodine lamp. Absorption spectra were determined with a Cary 15 spectrometer. Photolysis of Ru(bpy) and Ir(bpy) * Solutions. Solutions of 10" M trisbipyridylruthenium(II) chloride i n 1 M H C l or 0.01 M N a H C O and 10" M trisbipyridyUridium(III) chloride i n 0.01M N a O H were pre pared and were stored i n the dark at 273° Κ when not i n use. F o r pho tolysis runs, samples of these solutions were placed i n 1-cm rectangular borosilicate glass cells and were heated to 363° Κ in a thermostatea cell holder with a Haake F K circulating water bath while dry nitrogen gas was bubbled through the solution to remove dissolved oxygen. For photolysis of the R u ( b p y ) solutions, the output of a 1000-W Hanovia H g - X e arc was passed through a 6-cm, water-cooled copper sulfate solution filter and was focused on the cell i n the thermostated holder. The copper sulfate filter was removed for photolysis of I r ( b p y ) solutions. Cells were capped tightly prior to photolysis to prevent diffusion 3

2+

3

3

3

a

3

3

2 +

3


3 +

4.

2,2-Bipyridine Complexes of Ir(III) and Ru(II)

WATTS ET AL.

61

of oxygen back into the nitrogen-saturated solutions. Under these condi tions, photolysis times of 15-20 m i n at 363°Κ were generally used to obtain maximum product yields. Dark solutions heated to 363° Κ for periods of several hours were found to be thermally stable. Following photolysis, the solutions were cooled quickly to ice tem perature to minimize thermal reactions of the photoproduct. Purification of the product obtained by photolysis of R u ( b p y ) i n 0.01M N a H C 0 was accomplished easily by passing the photolysis solution directly onto a 1 cm X 5 cm column of Sephadex C M cation exchange resin i n 0.01M N a H C 0 . The deep-red photoproduct was eluted immediately from the column with water, leaving the orange-red starting material behind. T o isolate the photoproduct from the solutions of R u ( b p y ) photolyzed i n 1 M H C l , it was necessary to remove the solvent with a small freezedrying apparatus. The red-purple residue was dissolved i n methanol and was purified by column chromatography on a 2.5 cm X 50 cm column of Sephadex L H - 2 0 , using 0.01M H C l i n methanol for elution. A n orange band of unreacted R u ( b p y ) eluted first from the column, followed by a band containing the red photoproduct. W h e n the chromatography was carried out i n room fight, a third yellow band merged into the top of the red band. This band also was observed i n increased proportion when photolysis times were extended beyond 20 min. F o r photolysis solutions irradiated for 15 m i n and chromatographed i n the dark, this third band was not observed. Solutions from the photolysis of I r ( b p y ) i n 0.01M N a O H were studied without further purification by absorption and emission spectroscopy. Solutions of purified R u ( b p y ) photoproducts were studied within 1 hr of isolation when kept at room temperature because of their thermal instability. Solutions frozen at 77° Κ for low-temperature emission studies were found to be stable for a period of several days. Preparation of Complexes. [ R u ( b i p y ) ] C l was purchased from G . F . Smith Chemical Co. and was used without further purification. [ I r ( b p y ) ] [ N 0 ] was prepared by the method described by F l y n n and Demas (4,5), and [ I r ( b p y ) H 0 ( b p y ) ] C l was prepared by the method described by Watts, Harrington, and V a n Houten (6). 3

2 +

3

3


3

3

2 +

2 +

3

3

2 +

3

3

3

3 +

2

3

2

2

3

Results Photolysis of R u ( b p y ) i n 0.01M N a H C 0 . Absorption and emis sion spectra obtained from solutions of the purified product of photolysis of R u ( b p y ) i n 0.01M N a H C 0 are shown i n Figure 2. The absorption spectrum of the purified photoproduct i n 0.01M N a H C 0 has intense bands at 510 and 355 nm. A structured emission with a maximum at 705 n m is observed i n the luminescence of the compound when excited at 436 n m i n 0.01M N a H C 0 i n water-methanol glasses at 77°K. W h e n the p H of a purified solution of the photoproduct was adjusted to a value of 1 w i t h H C 1 0 , the absorption spectrum was found to blue shift. As shown i n Figure 2, absorption bands at 480 and 345 n m are observed at p H = 1, and the low-temperature emission maximum shifts to 660 nm. The absorp3

3

2 +

2 +

3

3

3

3

4


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WATTS ET AL.

2,2'-Bipyridine Complexes of Ir(IlI) and Ru(II)

63

tion bands at 510 and 355 nm could be regenerated by treatment of the H C 1 0 solution w i t h N a O H to adjust the p H to 10. Photolysis of R u ( b p y ) i n 1 M H C l . The absorption and lowtemperature emission spectra of freshly purified solutions of the photoproduct are shown i n Figure 3. Intense absorption bands at 495 and 350 n m are observed i n the absorption spectrum and a weak, structured emission maximizing at 680 n m is observed i n the low-temperature emission. Both absorption and emission spectra of the freshly prepared photoproduct were found to be independent of p H over the range 1-8 when 1 M N a C l was used as a supporting electrolyte. I n the absence of supporting N a C l , the 495-nm absorption of the photoproduct was found to be red-shifted to about 510 nm by adjustment of the p H to 8 w i t h 0.01M 4


3

300 Figure 3.

500

2 +

700 900 Wavelength (nm)

1100

Absorption and emission spectra of the product of photolysis of Ru(bpy) inl.OM HCl 2+

s

N a H C 0 . Addition of HCIO4 to the p H = 8 solution to readjust the p H to 2 d i d not regenerate the original 495-nm absorption of the photoproduct, but led to a new absorption at about 480 nm. Addition of 1 M N a C l to either the p H = 8 or p H = 3 solutions led to shifts i n the respective absorption bands back to 495 nm. The 495-nm absorption band i n aqueous solutions was shifted to 508 n m i n methanol. Evaporation of the methanol followed by dissolution i n 0.01M H C l w i t h 1 M N a C l supporting electrolyte regenerated the 495-nm absorption band. However, when the photoproduct was dissolved i n acetonitrile two new bands appeared at 480 and 540 nm. Evaporation of the acetonitrile followed by partial dissolution of the residue i n water gave a solution w i t h an absorption band at 455 nm and an emission characteristic of R u ( b i p y ) . The water insoluble por3

3

2 +


In Inorganic and Organometallic Photochemistry; Wrighton, M.; Advances in Chemistry; American Chemical Society: Washington, DC, 1978. 2

3+

3

2+

2

1

3

3+

3

2

Γ

3+

3 3

Figure 4. Absorption and emission spectra of Ir(bpy) , the photolysis solution of Ir(bpy) * in O.I M NaOH, and [Ir(bpy) OH(bpy)] : (- · -) Ir(bpy) ; ( ) solution from photolysis of Ir(bpy) ; (- [Ir(bpy) OH(bpy)] +

τ


a

g

2

§

8

4.

2,2'-Bipyridine Complexes of Ir(III) and Ru(II)

WATTS ET A L .

tion of the residue was redissolved i n acetonitrile and was found to have an absorption band at 545 nm. Photolysis of I r ( b p y ) i n 0.01M N a O H . Absorption and emission spectra of 1 0 ' M solutions of I r ( b p y ) i n 0.01M N a O H before and after photolysis at 363° Κ are shown i n Figure 4. Absorption and emission spectra of [ I r ( b p y ) O H ( b p y ) ] are included i n the figure for compari son. The photolysis results i n the growth of an absorption band at 370 nm. A redistribution of intensity occurs i n the emission spectrum because of a relative growth of a band at 460 nm and a shift of the intense band at 500 n m i n the starting material to 495 nm i n the photolyzed solution. 3

3 +

3

3

2+

2


3 +

Discussion Previous studies of the luminescence lifetimes ( 7 , 8 ) , luminescence quantum yields ( 7 ) , and photochemical quantum yields (9) of R u ( b p y ) as a function of temperature over the range 278°-368°K i n aqueous acid solutions clearly indicate that measurable photochemistry begins to occur at temperatures where luminescence quantum yields and lifetimes are also measurable ( 7 ) . Hence, the term luminactive is applicable to this species. T h e interpretation of the combined photophysical and photo chemical results requires a cohesive model which quantitatively fits the quantum yield and lifetime data and qualitatively correlates with the observed photoreactivity. Such a model is presented i n Figure 5. I n this model, k represents the radiative decay rate of the charge-transfer manifold of emitting levels, and k represents their rate of radiationless decay to the ground state. Photoreactivity as w e l l as luminescence quenching is believed to be caused by a set of crystal field levels w h i c h He above the emitting charge-transfer levels with an energy gap, AE . These levels undergo radiationless decay back to the ground state w i t h rate constant k or undergo photochemistry with an additional activation energy, ΔΕ *, with rate constant k . F o r this model, the overall photo chemical quantum yield as a function of temperature, Φ ( Γ ) , is given b y 3

2+

lr

lq

LF

2q

p

α

k e" + k + k e * AELF/kT

2a

Κ

lq

2q

AEL

k e- *t + k e *t v

' k

/kT

2q

AE

/kT

p

E

/kT

(1)

The luminescence lifetime, T ( T ) , and luminescence quantum yield, Q(T), are given, respectively, b y m

rm(T) = [k

lr

Q(T)

+ k

lq

fc e-^-/ ]-

+

2e

fcr

1

=k r (T) lr

m


(2) (3)

65


66


Figure S.

Model for energy transfer in Ru(bpyh 2+

B y computer fitting of this model to experimental measurements of Φ ( Τ ) , T ( Γ ) , and Q(T) over the temperature range 278°-368°K, the set of rate constants and energy gaps shown i n Table I have been obtained ( 7 , 9 ) . W i t h these parameters the model provides a consistent, quantitative inter pretation of the photophysical and photochemical information available for R u ( b p y ) i n aqueous solutions. The photolysis results obtained i n 1 M H C l and i n 0.01M N a H C 0 clearly indicate that the nature of the photoproduct obtained is dependent upon the anion present i n solution. Furthermore, previous studies have shown that the photochemical quantum yield is dependent upon the chloride ion concentration i n H C l solutions, with higher quantum yields at higher CI" concentrations ( 9 ) . Hence, we conclude that at least a portion of the photochemical activity is attributed to photoanation. W h i l e most synthetic and kinetic studies of the binding of b p y to transition metal ions have assumed that species containing monodentate bpy are unstable with respect to either labilization or chelation of the bidentate ligand (10,11), we have recently isolated a stable complex M

3

2 +

3

Table I. Parameters Obtained from Measurements of Luminescence Lifetimes, Luminescence Quantum Yields, and Photochemical Quantum Yields of Ru(bpy) in Aqueous Solutions (7, 9) 3

Rate Constants (sec- )

Energy Gaps (kK)

1

fcir k k k, lq

2q

2+

- 6.9 X 10 - 1.2 Χ 10 - 1 χ 10 -1.0X10 4

6 13

AE - 3.6 =t 0.1 E =¥ - 2.0 ± 0.1 LF

a

1 3


4.


WATTS ET AL.

containing monodentate b p y ( 6 ) . This complex, which contains I r ( I I I ) bound to two bidentate bpys, one monodentate bpy, a n d one water molecule (which can be converted to hydroxide with a pKa of 3 ) , is thermally stable both i n acid and base solutions and as a pure crystalline solid. I n the present study, i t is evident from comparison of the absorp tion and emission spectra of the solution obtained by photolysis of Ir(bpy) with the absorption and emission spectra of [ I r ( b p y ) O H ( b p y ) ] (see Figure 4) that the latter species is formed i n the photolysis. 3 +

3

2


2+

B y analogy with the I r ( I I I ) results, one might expect formation of a complex of Ru(II)-containing monodentate b p y i n the photolysis of R u ( b p y ) . Consideration of the results we have obtained indicates they are indeed consistent with such a formulation. Photolysis i n 1 M H C l leads to formation of a species with absorption bands at 495 and 350 nm and an emission at 680 nm. The R u C l ( b p y ) complex is characterized by absorption bands at 553 and 380 nm i n acetonitrile (12,13) and an emission at 715 nm (14,15) whereas the R u ( b p y ) starting material has an absorption band at 455 nm and a low-temperature emission maximum at 590 nm. Hence, the photoproduct i n 1 M H C l has absorption and emis sion features at energies intermediate between those of R u ( b p y ) and RuCl (bpy) . 3

2 +

2

2

3

2 +

3

2

2 +

2

The long wavelength absorption and emission bands of R u ( b p y ) and R u C l ( b p y ) have been assigned as M L C T (12,13,14,15) whereas the shorter-wavelength (380nm) absorption band of the latter complex has been assigned as L M C T (12,13). These trends clearly indicate r e d shifts i n both the M L C T and L M C T bands of R u ( I I ) when π-donating chloride ligands are introduced into the coordination sphere. The inter mediate nature of these bands i n the photoproduct indicate that one rather than two chlorides is present i n the coordination sphere. Further more, the absence of a p H dependence i n the absorption of the photo product when 1 M N a C l is present rules out the presence of water or hydroxide i n the coordination sphere. Hence, our data are consistent with formulation of the photoproduct i n 1 M H C l as R u ( I I ) bound to two bidentate bipys, one monodentate b p y and one chloride, [ R u ( b p y ) C l 3

2

2 +

2

2

(bpy)] . B y analogous reasoning, the product of the photolysis of R u ( b p y ) i n 0.01M N a H C 0 is formulated as [ R u ( b p y ) O H ( b p y ) ] , which is converted to [ R u ( b p y ) H 0 ( b p y ) ] by treatment with acid. This is consistent with r e d shifts i n the absorption maxima to 510 and 355 nm when the photolysis is performed i n 0.01M N a H C 0 rather than 1 M H C l , because of the superior ττ-donating ability of O H " relative to CI". As expected, these absorption bands blue shift to 480 and 345 n m i n acidic solutions because of the weaker 7r-donating ability of H 0 relative to either CI" or O H " . 1+

3

3

1+

2

2

2

2 +

3

2


2 +

67

68


F r o m the dependence of the absorption spectra of the photoproducts on concentration of anions i n solution, it appears that the coordinated CI" or O H " is thermally labile i n these species. Thus, the shift i n the absorp tion of solutions containing [ R u ( b p y ) C l ( b p y ) ] from 495 n m i n 1 M N a C l to 510 nm i n 0.01M N a H C 0 with no N a C l added is attributed to conversion to [ R u ( b p y ) O H ( b p y ) ] which can then be converted to [ R u ( b p y ) H 0 ( b p y ) ] by acidification, as indicated above. W h i l e formation of R u ( b p y ) i n purified solutions of the photoproduct from 1M H C l strongly supports our formulation of a species containing monodentate bpy, the peculiar effect of acetonitrile is not fully under stood. Replacement of coordinated CI" by solvent is expected to occur i n coordinating solvents on the basis of the thermal lability postulated above. Thus, [ R u ( b p y ) a c n ( b p y ) ] is expected to be formed i n aceto nitrile. Apparently this species is not stable w i t h respect to chelation of the monodentate bpy to form R u ( b p y ) . Identification of the second species formed by adding acetonitrile to [ R u ( b p y ) C l ( b p y ) ] w i l l re quire further study to characterize fully. 1+

2

3

1 +

2

2

2 +

2


3

2 +

2+

2

3

2+

2

1+

W e now turn our attention back to the luminescence properties of R u ( b p y ) . The low-temperature and room-temperature luminescence has been widely studied, and both are reproduced i n Figure 6 for refer ence. As this figure indicates, the low-temperature luminescence is highly structured and is widely accepted to be attributed to a set of M L C T excited levels. W h i l e it is tempting to assume that the room-temperature emission is caused by the same M L C T states, altered only by thermal redistribution of the relative population, the red shift and total absence of vibrational structure i n the room-temperature luminescence suggest that the emission may have a substantially different origin than that at 77 °K. Furthermore, previous studies have shown that although the effect of deuteration of the coordinated bpy on the luminescence lifetime is small, a large effect is seen when the solvent is changed from D 0 to H 0 (3). Also, extrapolation of the model used to analyze the lowtemperature emission data (16) to room temperature leads to a predicted radiative rate constant of 12 Χ 10 sec" as compared w i t h the measured value of 6.9 Χ 10 sec" (7). These factors suggest strong interactions of the excited state of R u ( b p y ) with water i n fluid media, w h i c h we pre viously attributed to interactions of the M L C T configuration with chargetransfer-to solvent ( C T T S ) configurations (3, 7 ) . This by no means i m plies a complete electron transfer to solvent water to form aquated elec trons but rather some penetration of the excited-electron density into the first solvation sphere i n the fluid medium. It is interesting to note, how ever, that this quenching mechanism is i n a sense analogous to the elec tron transfer mechanisms responsible for the quenching of the R u ( b p y ) luminescence by added electron acceptors. In fact, recent measurements 3

2 +

2

2

4

4

1

1

3

2 +

3


2 +

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WATTS ET AL.

indicate that two-photon processes can give small yields of solvated elec trons under some circumstances (17), further stressing the potential analogy. T o continue this analogy, it also is known that energy transfer some times must occur to account for the quenching of excited R u ( b p y ) by species that are not easily reduced or oxidized. B y this analogy, it also is possible to explain the luminescence properties of R u ( b p y ) i n fluid 3

3


τ

550 Figure 6.

600

ι

2 +

2 +

r

1

650 700 Wavelenth (nm)

Emission spectra of Ru(bpy) : ( 2+

3

750 ) 77°K; (

800 ; 273°K

water by mechanisms more closely associated with energy transfer than w i t h electron tranfer. This alternate explanation is outlined i n Figure 7, which is a slight modification of a diagram used by Dellinger and Kasha i n their discussion of the use of a double-minima potential to interpret solvent effects on emission spectra (18). As applied to R u ( b p y ) , the potential w e l l labeled by C T would represent the charge-transfer levels and that labeled by L F , the ligand field levels. A t low temperature, 3


2 +

69


Ru—Ν Figure 7.

Distance

Double minima potential wetts for Ru(bpy)

2+

s

emission would occur from the charge-transfer levels because of both thermodynamic (AE ) and kinetic (ΔΕ *) barriers to population of the ligand field levels. As indicated by Dellinger and Kasha, ΔΕ * may i n clude contributions from both solvent viscosity (dotted line) and intra molecular potential terms (solid line). I n the case of R u ( b p y ) , the anticipated distortion along the R u - N bond i n the ligand field state is LF

α

α

3

Figure 8.

2+

Proposed intermediate in the photolysis of Ru(bpy)


2+

s

4.

WATTS ET AL.


likely to result i n twisting of the plane of one of the pyridine rings about the C - C bridging bond of bpy. A viscous solvent would be expected to contribute a substantital potential barrier to this type of motion. I n fluid media the solvent potential barrier would be greatly diminished, leading to a smaller value of ΔΕ *. As a result, an enhanced rate of population of the ligand field levels would be expected i n fluid media. Furthermore, as the temperature is increased, higher equilibrium concentrations of the ligand field levels would ensue. Since distortion along the R u - N bond occurs i n the ligand field levels, a solvent water molecule can occupy a position close to a coordination site when this state is formed. A n intermediate structure of this type is illustrated in Figure 8, which depicts an associative reaction of R u ( b p y ) with H 0 to form [ R u ( b p y ) H 0 ( b p y ) ] . Such an intermediate struc ture would be expected to show a substantial red shift i n emission relative to the charge-transfer levels as indicated i n Figure 7, and a large solvent deuterium effect is consistent with the close approach of water to a co ordination site. Furthermore, the radiative rate constant of the ligand field levels would be expected to be smaller than that of the chargetransfer Jevels because of the inherent forbidden nature of d-d transitions. In conclusion, the emission of R u ( b p y ) is consistent with a chargetransfer assignment i n low-temperature rigid glasses and increasing con tributions from a set of ligand field levels i n fluid media at elevated tem peratures. These levels also account for the photochemical activity ob served at elevated temperatures. Whether this or the previously proposed C T T S model best explains the luminescence results is perhaps a moot point since it is difficult to envision experiments that could clearly dis tinguish the two. However, it is quite likely that the double-minimum potential model depicted i n Figure 7 w i l l be of widespread use i n the interpretation of luminescence and photochemical data i n luminactive systems.


α

3

2

2

2

2 +

2 +

3

2 +

Acknowledgment Acknowledgment is made to the Committee on Research of the University of California, Santa Barbara, for support of this research. Literature Cited 1. Balzani, V., Carassiti, V., "Photochemistry of Coordination Compounds," Academic, London, 1970. 2. Adamson, A . W., Fleischauer, P. D., "Concepts of Inorganic Photochem istry Wiley, New York, 1975. 3. Van Houten, J., Watts, R. J., J. Am. Chem. Soc. (1975) 97, 3843. 4. Flynn, C. M . , Demas, J. N . , J. Am. Chem. Soc. (1974) 96, 1959. 5. Ibid. (1975) 97, 1988.


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INORGANIC A N D O R G A N O M E T A L L I C

PHOTOCHEMISTRY

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