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Temperature and solvent effects upon the 5D35D4-transition of Tb3+ in POCl3:SnCl4
Journal of Lummescence 11 (19i~)119—128 ~ North-Holland Publishing Company
TEMPERATURE AND SOLVENT EFFECTS UPON THE 5D 5D 3~IN POCI 3—~ 4-TRANSITION OF Th 3:SnCI4 J. CHRYSOCHOOS and P. TOKOUSBALIDES Department of Chemistry, The University of Toledo, Ohio 43606, USA Received 13 August 1975 3~in POCl~:SnCl Excitation of Th 4with5D ultraviolet 1ight~ivesrise to fluorescence emission arising from both the ~D3 and the 4-states of Th + whose relative contribution is a 341. function of both the composition of the at constant The fluorescence lifetime of the lightsolvent emittedand viathe thetemperature transitions 5D ITb 3 —~ 0, 1 6) increases 5D~is at lower brought IPOC13I/[SnCl4]-ratios about via interactionsand between temperatures. the excited TheTb3+ nonradiative and process POCI —~ 3+]. The activation energy barrier of the electronic excitation energy transfer 3 molecules at very from low Tb3~ [Th to the solvent is 3.4 kcal/mole at [POCI 3I/ISnC14] 12.5 and increases slightly at higher ratios, to a maximum of 4.15 kcal/mole at IPOCI3I/ [SnC14J = 55.5.
1. Introduction 3~in crystals, garnets and glasses at wavelengths shorter than of Tb several excited states higher than the 5D 487Excitation ±1 nm populates 7F 4-one viaarises the from 6 5D~-transitionsof Tb3+ [1]. The corresponding fluorescence emission various excited states of Tb3~and particularly from the 5D 5D 4 and 3-states [2,3].in 5D 3+ has not been observed Fluorescence emission arising from the 3-state of Tb organic and inorganic protic solvents under comparable excitation conditions. The predominant fluorescence emission of Tb3+ in such protic solvents originates from the 5D 4-state, regardless of5D the excitation transitions 5D 7F~rather than the 7Fj-oneswavelength, [4,5]. This namely is due toviathethe very efficient 4 3 deexicitation of the 5D 3~which can be attained via the 5D 5D 3-state of Tb 3~—* 4transition whose efficiency in solvents containing such groups as —OH, —OD, ~ C—H, ~ C—D etc. is of the order of 10~to l0~sec~based on the extremely 5D 3~[4,5]. Soluweak observed fluorescence emission arising from the 3-state of Tb tions of Tb3~in aprotic solvents such as the POd 3 :SnC14 system, where the highest energy vibrational mode is the stretching mode of the ~ P0 group (1300 cm~ 5D [6,7]) give rise to a very considerable fluorescence emission arising from the 33~[8]. state Tb Theoffluorescence lifetime of the emission arising from the 5D 3~ 3-state of Tb —~
-*
-+
119
120
J. Chrysochoos, P. Tokousbalides
/ Temperature and solvent effects
depends upon the concentration of Th3~[8], the composition of the solvent system and the temperature. The fluorescence self-quenching of Tb3~takes place via the process: Tb3~(5D
3~(7F 3~(5D 3~(7Fj); J < 6. 3)+ Tb 6) Tb 4)+ Tb Under special experimental conditions the transition 5D 5D 3~’-’- 4 can become very 5D ineffective to the point that the fluorescence emission3~] arising from the 3Further-state and temperature. may prevail. Such conditions consist of very low [Tb more, the [POC1 3]/[SnC14]-ratiocan affect the relative contribution 5D 5D 3~particularly when fluorescence combined with variations from the 4 and 3 -states of Th in the temperature. -~
2. Experimental Terbium Chloride, TbCl 3 6H20, of 99.9% to 99.99% purity was obtained in anhydrous form by a slow removal of the water at low temperatures, under vacuum and in the absence of traces of CO2. These precautions are necessary to avoid formation of either terbium oxychloride or carbonate. The anhydrous TbC13 was dissolved in mixtures of ultrapure POCP3 SnCl4Matwas lowobtained temperatures under refluxing 3~] of 6and X 10—2 in solutions with a conditions. A maximum [Tb [POC1 3]/[SnC14]-ratioof about 12. The amount of TbC13 dissolved decreased at lower [POd3] / [SnC14]ratios. Fluorescence emission spectra were obtained using the Aminco—Bowman spectrophotofluorimeter with a 1P21-Phototube and an X— Y recorder. An excitation slit of 0.5 mm was used which is equivalent to an excitation band width of about 4 nm. The fluorescence lifetimes were measured by discharging a homemade Xe-flash lamp at 5 to 6 kV (125 joules) with a flash duration of about 50 ps. The temperature was varied and was monitored by a thermistor with an accuracy of ±1°C.An excitation slit of 3 mm was employed in the measurement of the lifetimes giving rise to a band width of about 20 nm. The emitted light was filtered through an IR Thin Film Products filter (No. 4339A/50 A) with a maximum transmission at 434.6 nm and a band width of 4 nm. The emission spectra 3~,in-were correctedthat fordue the to sensitivity of the of phototube andatthe concentration of Tb In addicluding the expansion the solvent different temperatures. tion, the viscosity of the POC1 3 :SnC14 system was measured at all temperatures. The viscosity was found to decrease linearly as the temperature increased. .
3. Results and discussion 3~in POC1 Typical fluorescence emission spectra of Tb 3 :SnCl4 at constant [POC13] / [SnC14]-ratio are shown in fig. 1. Part (A) of this figure shows the fluorescence spectrum of the sample excited at 487 ±1 nm which corresponds to the transi-
J. Chrysochoos, P. Tokousbalides / Temperature and solvent effects
121
5D~’7F
40
(A)
30
(B)
(C)
5b0 eöo WAVELENGTH, nm Fig. 1. Fluorescence spectra (uncorrected) of 5 X i0~ Tb3~in POCI
4b0
300
12.7. (A)Xexc =487 ± 1 nm; T 0°C.(B) ~‘exc 368 nm; T = 50°C.Spectra given in arbitrary units. =
±
1 nm; T
3:SriCl4 [POC13]/[SnC14] 0°C.(C) Xexc = 368 ± 1
7F 5D 3~.The fluorescence spectrum shown represents the emission tion from 6 4 of arising the 5DTb 5D 7F 4 -state only, including some of the 4 -+ 1-transitions. Part (B) displays the fluorescence spectrum 368 1 nm, 5Dof the same5Dsample excited at 3~. The± fluoresgiving rise to population of both the 3 and the 4states of Tb cence emission in this case originates from both excited states. The corresponding emission bands have been indentified with the pertinent transitions [1]. The effect of the temperature upon the radiative properties of the 5D 3~is illustrated 3 -state of Tb in part (C) of fig. 1 The contribution of the 5D 3-state is definitely lower relative to —~
-
122
J. Chrysochoos, P. Tokousbalides
(A)
/
Temperature and solvent effects
5q~—~ 7F
80
5
5D3~~0~jjL (B)
300
4~0
ebO 3~in POC1 WAVELENGTH, nm Fig. 2. Fluorescence spectra (uncorrected) of 5 X i0~ M Th 3:SnCl4 Xexc = 368 nm. (A) [POCI3J/ESnC14] = 12.7. (B) IPOC13]/lSnCl4J = 55.5.
±
5D that of the 4-state at higher temperatures, although it is not obvious that the overall fluorescence emission is also lower at higher temperatures. Similar (uncorrected) fluorescence spectra are shown in fig. 2 at constant temperature and excitation wavelength, namely 25°Cand 368 ±1 nm, but at different [POC13]/[SnCl4]ratios. Results are presented for the 5D two extreme values ratio.fluorescence It is quite ap3~toof thethis overall parent that the contribution of the the 5D 3-state of Tb emission is greater than that of 4-state at lower [POd!3]/ [SnC14]-ratios. For values of the [POC13]/[SnCl4].ratio larger than 55.5, TbCl3, does not dissolve whereas for values smaller than 12.7 a white precipitate, possibly (POC13)2SnCl4, is formed.
1
J. Chrysochoos, P. Tokousbalides / Temperatureand solvent effects
~
3.0
123
7.0
‘2
/ ~
0
°—~--~ti~---o 0
115
T~°c 3~in POC1 Fig. 3. Variation of the corrected fluorescence intensities of 5 X l0~ M Th 3:SnC14 5D 7F 5D 7F at 415 ± 1 nm ( 3 —° 5) and at 544 ± 1 nm ( 4 -~ 5) with the temperature; ~‘exc= 368 ± 1 nm; o, • [SnCl4] = 0.192 M; A, A [SnCl4] = 0.346 M and o, • [SnC14J = 0.576 M.
3~] and [POd] / [SnC1 [Tb 4J-ratio the relative contributions the 5D At constant 5D 5D of 5D 3 and 4-states are temperature dependent. This indicates that the 3’—’ 4transition is also temperature dependent. Some typical results confirming this 5D assumption5Dare illustrated in fig.measured 3 where fluorescence emission arising intensity from the at 3 3~is in terms of the fluorescence and Tb 415 the ±1 nm4 -states and 544of± 1 mn, respectively. Results are shown for three different [POC1 5D 3] I [SnC14 ] -ratios.asInthe all temperature cases, the fluorescence emission arising from the it 3is 3+ decreases increases and for a given temperature state of Tb -
124
J. Chrysochoos, P. Tokousbalides / Temperature and solvent effects
higher at lower [P0d13] / [SnCl4I-ratios. On the other hand, the fluorescence emis5D 3+ increases as the temperature increases, atsion from the 4-state of Tb in a rather complicated manner. The increase tainsarising a maximum and then decreases can be accounted for by the 5D 5D 5D 3—÷ 4 -transition. The maximum implies a nearly complete depletion of7Ff. the The3-population some extent, of such 5D latter part ofand the participation, curves is moretodifficult to interpret processes as 4—~=* because of the large number of parameters involved. The position and magnitude of these fluorescence maxima are functions of such variables as the temperature, the [Tb3~], the [POC1 3]/[SnCl4]-ratioetc. These maxima can be predicted by the following mechanism: 3~(5D ~ Tb 4)+ heat (1) salvation sphere 3~(5D 3~(5D Th 3)~ Tb 4)+ heat (2) solvation sphere
3~(5D 3~(7F~) + hvf Tb 3)-÷ Tb 1 3~(5D 3~(7F 3~(5D 3~(7F~) Tb 3)+ Tb 6) Tb 4)+ Tb Tb3~(5D 3~(7F 4) Tb 3)+ hi4~ 3~(5D 3~(7Fj)+ heat Tb 4)salvation sphere Tb
(3)
(4)
-+
-~
(5) (6)
==~—~
Tb3~(5D
3~(7Fj)+ heat 4)
-=--~~=-~‘
salvation sphere
(7)
Tb
Processes (1) and (6) as well as (2) and (7) represent the deexcitation of the 5D 3 and 5D 3~,respectively, through the vibrational modes of solvent molecules 4 -states of in Tb located both the primary and the secondary solvation sphere of the ion [9,10]. The fluorescence quantum yield, 4fl and lifetimes, r, which correspond to the radiative processes (3) and (5) are affected by processes (1), (2), (6) and (7) although the radiative fluorescence lifetimes, r°and r’° may be unaffected. If one cannot distinguish between processes (1) and (2) or (6) and (7), the overall quenching effect of the solvent may be considered in conjunction with reactions (3) and (5). Reaction (4) has been studied independently [8]. Typical fluorescence lifetimes are,depicted in fig. 4 as a function of the temperature at constant [Tb3~] and at different [POC1 3]/ [Sndl4] -ratios. The fluorescence lifetimes are shorter at higher temperatures and at5D higher [POC13] / [SnCl4]-ratios. 5D 3~can be defined Thethe formation and the mechanism. decay of the The3 following and the equation 4-state of by aforementioned is Tb obtained in the case of the 5D 3~. 3 -state of Tb
J. Chrysocho’.c. P. Tokousbalides / Temperature and solvent effects
125
0.8
0.4
£
0 -IS
I
I
13
45
I
,
3~(5D
7F 3 -. 4) in POC13:SnCl4 1; x11 = 436 ± 1 nm; flash duration 50 ~ss;• [SnC14] = [SnCl4j = 0.346 M.
Fig. 4. Variation of the fluorescence lifetime of 5 X 10~ M Th with the temperature; ?~exc= 368 0.77 M; o [SnCl4j = 0.576 M and
± A
3+1
1’°~ + kr[POCl or
=
kf1
=
kf 1
+ ki’
3]~,
+
(8)
kq[Tb
3~i + k!~~ent [P0C1
+ kq
[Tb
3]
(9)
where kq stands for k4, [POCI3] ~ represents the concentration of the POdl3 molecules located in the secondary solvation sphere of the ion and [POC13]the initial concentration of the POdl3 molecules. The values of kq were determined 3~in previously POC1 [8] and k~in eq. (9) stands for the fluorescence rate constant of Tb 3 :SnCl4.
126
J. (7zrvsoc/,oos. P. Tokousbalides
/ Temperature and
solvent effects
The value of kf1 which corresponds to the 5D3-state is not known. However, it is 5D expected to be a little larger 3~in than the valuesolvents which corresponds to thethat 4-state. The various [111. Considering T~ is of latter is about 70 s~for Tb the order of 3500 s~ and kq [Tb3~] of the order of 400 s_i, eq. (9) can be rewritten as follows: kq [Tb3~] = kf 1+ or
(10)
k~0l~[POCl3]
3~1~k~0l~’[POCl 31 (11) 1 kq[Tb3~])versus l/Twe obtain the results illustrated in 1og10(r fig. By 5. plotting At very low [POdl 3] I [SnCl4I-ratios and low temperatures the plot is fairly linear although at higher ratios and temperatures deviations from linearity are apparent. Under the conditions at which eq. (11) is valid we have: 3+])~1og 1”)/23RT (12) log10(r~ kq[Tb 1oA~Olv[POdl3I (E~° based on the Arrhenius expression for ~ namely k~0l’~ = A!”’ exp((—E~”) 2/R7). —
kqETb
.
—
—
Eq. (12) employed in conjugation with fig. S gives rise to the following results:
~
3.70C ~—~--~ —~-.--~ 0 ~
0~A ~
~
~IOO3.00
~
.—=-.
O~A
~
3.40
3.80
~~ • K 3~i)with lIT; 5 X i0~ M Th3~in POCI Fig. 5. Variation oflog1Ø(T~ — kqETb 368 ± 1 nm; flash duration 50 ~s. • [SnCI4J = 0.77 M; o [SnCl4] 0.423 M and ISnCl4l = 0.346 M and 0.192 M.
=
0.576 M;
A
3:SnC14 Xexc = [SnC14] =
J. Chrysochoos, P. Tokousbalides
3.40 kcal/mole;A!”
/ Temperature and sOlvent effects 1.3 X l0~M1
51
127
for [SnC1 4]= 0.77 M
~E~)2 ~3.88 kcal/mole;A!”’ ~2.6>~ l0~M~s~for [SnC14]= 0.576 M 1 ~i for [SnC1 (Er)2 3.95 kcal/mole; A!” 3.4 X l0~M 4]= 0.423 M and (E~~”)2 ~4.l5 kcal/mole;A!”’ ~4.3 X lO~M~s~for [Sndl4]
=
0.346 M; 0.192 M.
The activation energy increases slightly at lower concentrations of SnC14. At the higher [Snd14] used the reaction rate constant for process (2) is given a k!” = 1.3 X l0~(—3400/RT)M s~. The nonlinearity of the curves in fig. 5 at higher [POC13] I [SnC14]-ratiosand temperatures indicates contribution from additional terms eq. (10). This canofbeSnCl~ visual3~,cin onsisting primarily ized in terms of a protective shield surrounding Tb [12—14],which may render the approach of POC1 3~rather 3 molecules toward Tb difficult. However, at higher [POC1 3]/[SnC14]-ratiosor lower [SnCl4] and particularly at3~, higher this protective is nottemperatures very rigid allowing POC1 shield, or the primary solvation sphere of Tb 3 molecules to penetrate giving rise to much
Table 1 5D 7Ff) and the efficiency of the nonradiaVariation tive transition of the 5Dfluorescence 5D~with lifetime temperature. of TbJ+ ( 3 —. 3 -= [POC1 3~?~exc= 368 ± 1 nm; A~-~ = 436 ± 1 nm; flash duration 50 p5. 3I/[SnCI4] = 12.7; 5 X 10~ M Tb T (°C)
r (ms)
—15 —10 — 5 0 5 10 15 20 25 30 35 40 45 50 55
0.73 0.65 0.58 0.52 0.46 0.41 0.37 0.23 0.29 0.26 0.24 0.22 0.20 0.19 0.18
kq 181 (M1 s~) —
2.lOx 2.25 X 2.62 X 2.90 X 3.11 X 3.55 X 3.81 X 4.06 X 4.50 x 4.86 x 5.25 x 5.68 x 6.12x 6.61 x
i04 i04 i04 i04 i0~ i04 l0’~ tO4 ~ i04 a) i04 a) i04 a) lO~~ i04 a)
a) Values obtained by extrapolation ofthe curve
k~0l~’ (M1 s’) 220 250 282 316 354 394 438 484 533 586 641 701 763 829 898
versus [Th3+] to higher temperatures [8].
128
J. Chrysochoos, P. Tokousbalides
/ Temperature and solvent effects
more efficient fluorescence quenching. The contribution of the additional k-term at higher temperatures can be estimated by introducing the values ofAr” and (E~”)2 obtained above into the following equation: 3~] k!1”[POC1 log10(r~ kq[Tb 3]) ~log10 k —
—
=
log10 A
—
(EA)/2.3RT (13)
Eq. (13) in conjunction with fig. 5 yields the following results: 1 for [SnC1 k=3.2X 1011 exp(—ll,3SORT)s 4]=0.346MandO.192M and k
=
5.0 X 10~exp(—14,700R7) s~for [SnC14]= 0.576 M.
The activation energy increases at higher [SnCl4] which is expected with the concept of a rigid solvation sphere at higher [SnCI4I. 3~,kq and k~°~” are sumAppropriate values of the fluorescence lifetime of Tb marized in table 1 at one [POCI 3]/[Sndl4]-ratio. At1.25°Cand for [POCL3]/[Sndl4] Consequently, the efficiency = 17.7 we obtain k!”[POC13] + k~ 5.5 X l0~ s of the 5D 5D 3~in Tb3~in POd 3—~ 4 -transition of Tb 3 :Sndl4 at room 3~]. This value is in temperature fair agreehas a minimum valueestimate of 6 X 10~ at ~ very s~ low obtained [Tb ment with an earlier of ~ ~ to for Tb3~in protic solvents [4,51in which case extremely weak fluorescence emission was observed arising from the 5D 3~due to the high efficiency of the 5D 5D 3 -state of Tb 3—* 4 -transition in the presence of the ~C—Hand —0—H groups of the solvent.
References ]l] G.H. Dieke and l-LM. Crosswhite, AppI. Opt. 2(1963) 675.
12]
L.G. Van Uitert, J. Luminescence 4 (1971) 1; see also included references. [3] B.D. Joshi, B.M. Patel, A.G. Page, J.R. Banglia and RN. Saxena, J. Luminescence 6 (1973) 125. [4] J. Chrysochoos and A. Evers, Chem. Phys. Letters 20 (1973) 174. 151 J. Chrysochoos, J. Luminescence 9 (1974) 79. ]6J M.E. Peack and T.W. Waddington, J. Chem. Soc. (1962) 3450. 17] K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds (Wiley, New York, 1963) p. 112. (8] P. Tokollsbalides and J. Chrysochoos, Chem. Phys. Letters 29 (1974) 226. [9] J. Clirysochoos, Chem. Phys. Letters 14 (1972) 270. [10] J. Chrysochoos and A. Evers, Spectry. Letters 6(1973)203. [11] J.L. Kropp and M.W. Windsor, J. Chem. Phys. 42 (1965) 1599. [12] P. Tokousbalides and 1. Chrysochoos, J. Phys. Chem. 76 (1972) 3397. [13] J. Chrysochoos and P. Tokousbalides, Spectry. Letters 6 (1972) 435. [14] F. Collier, H. Dubost, R. Kohimuller and G. Rault, C.R. Acad. Sci. Ser. C. 267 (1968) 1095.
TEMPERATURE AND SOLVENT EFFECTS UPON THE 5D 5D 3~IN POCI 3—~ 4-TRANSITION OF Th 3:SnCI4 J. CHRYSOCHOOS and P. TOKOUSBALIDES Department of Chemistry, The University of Toledo, Ohio 43606, USA Received 13 August 1975 3~in POCl~:SnCl Excitation of Th 4with5D ultraviolet 1ight~ivesrise to fluorescence emission arising from both the ~D3 and the 4-states of Th + whose relative contribution is a 341. function of both the composition of the at constant The fluorescence lifetime of the lightsolvent emittedand viathe thetemperature transitions 5D ITb 3 —~ 0, 1 6) increases 5D~is at lower brought IPOC13I/[SnCl4]-ratios about via interactionsand between temperatures. the excited TheTb3+ nonradiative and process POCI —~ 3+]. The activation energy barrier of the electronic excitation energy transfer 3 molecules at very from low Tb3~ [Th to the solvent is 3.4 kcal/mole at [POCI 3I/ISnC14] 12.5 and increases slightly at higher ratios, to a maximum of 4.15 kcal/mole at IPOCI3I/ [SnC14J = 55.5.
1. Introduction 3~in crystals, garnets and glasses at wavelengths shorter than of Tb several excited states higher than the 5D 487Excitation ±1 nm populates 7F 4-one viaarises the from 6 5D~-transitionsof Tb3+ [1]. The corresponding fluorescence emission various excited states of Tb3~and particularly from the 5D 5D 4 and 3-states [2,3].in 5D 3+ has not been observed Fluorescence emission arising from the 3-state of Tb organic and inorganic protic solvents under comparable excitation conditions. The predominant fluorescence emission of Tb3+ in such protic solvents originates from the 5D 4-state, regardless of5D the excitation transitions 5D 7F~rather than the 7Fj-oneswavelength, [4,5]. This namely is due toviathethe very efficient 4 3 deexicitation of the 5D 3~which can be attained via the 5D 5D 3-state of Tb 3~—* 4transition whose efficiency in solvents containing such groups as —OH, —OD, ~ C—H, ~ C—D etc. is of the order of 10~to l0~sec~based on the extremely 5D 3~[4,5]. Soluweak observed fluorescence emission arising from the 3-state of Tb tions of Tb3~in aprotic solvents such as the POd 3 :SnC14 system, where the highest energy vibrational mode is the stretching mode of the ~ P0 group (1300 cm~ 5D [6,7]) give rise to a very considerable fluorescence emission arising from the 33~[8]. state Tb Theoffluorescence lifetime of the emission arising from the 5D 3~ 3-state of Tb —~
-*
-+
119
120
J. Chrysochoos, P. Tokousbalides
/ Temperature and solvent effects
depends upon the concentration of Th3~[8], the composition of the solvent system and the temperature. The fluorescence self-quenching of Tb3~takes place via the process: Tb3~(5D
3~(7F 3~(5D 3~(7Fj); J < 6. 3)+ Tb 6) Tb 4)+ Tb Under special experimental conditions the transition 5D 5D 3~’-’- 4 can become very 5D ineffective to the point that the fluorescence emission3~] arising from the 3Further-state and temperature. may prevail. Such conditions consist of very low [Tb more, the [POC1 3]/[SnC14]-ratiocan affect the relative contribution 5D 5D 3~particularly when fluorescence combined with variations from the 4 and 3 -states of Th in the temperature. -~
2. Experimental Terbium Chloride, TbCl 3 6H20, of 99.9% to 99.99% purity was obtained in anhydrous form by a slow removal of the water at low temperatures, under vacuum and in the absence of traces of CO2. These precautions are necessary to avoid formation of either terbium oxychloride or carbonate. The anhydrous TbC13 was dissolved in mixtures of ultrapure POCP3 SnCl4Matwas lowobtained temperatures under refluxing 3~] of 6and X 10—2 in solutions with a conditions. A maximum [Tb [POC1 3]/[SnC14]-ratioof about 12. The amount of TbC13 dissolved decreased at lower [POd3] / [SnC14]ratios. Fluorescence emission spectra were obtained using the Aminco—Bowman spectrophotofluorimeter with a 1P21-Phototube and an X— Y recorder. An excitation slit of 0.5 mm was used which is equivalent to an excitation band width of about 4 nm. The fluorescence lifetimes were measured by discharging a homemade Xe-flash lamp at 5 to 6 kV (125 joules) with a flash duration of about 50 ps. The temperature was varied and was monitored by a thermistor with an accuracy of ±1°C.An excitation slit of 3 mm was employed in the measurement of the lifetimes giving rise to a band width of about 20 nm. The emitted light was filtered through an IR Thin Film Products filter (No. 4339A/50 A) with a maximum transmission at 434.6 nm and a band width of 4 nm. The emission spectra 3~,in-were correctedthat fordue the to sensitivity of the of phototube andatthe concentration of Tb In addicluding the expansion the solvent different temperatures. tion, the viscosity of the POC1 3 :SnC14 system was measured at all temperatures. The viscosity was found to decrease linearly as the temperature increased. .
3. Results and discussion 3~in POC1 Typical fluorescence emission spectra of Tb 3 :SnCl4 at constant [POC13] / [SnC14]-ratio are shown in fig. 1. Part (A) of this figure shows the fluorescence spectrum of the sample excited at 487 ±1 nm which corresponds to the transi-
J. Chrysochoos, P. Tokousbalides / Temperature and solvent effects
121
5D~’7F
40
(A)
30
(B)
(C)
5b0 eöo WAVELENGTH, nm Fig. 1. Fluorescence spectra (uncorrected) of 5 X i0~ Tb3~in POCI
4b0
300
12.7. (A)Xexc =487 ± 1 nm; T 0°C.(B) ~‘exc 368 nm; T = 50°C.Spectra given in arbitrary units. =
±
1 nm; T
3:SriCl4 [POC13]/[SnC14] 0°C.(C) Xexc = 368 ± 1
7F 5D 3~.The fluorescence spectrum shown represents the emission tion from 6 4 of arising the 5DTb 5D 7F 4 -state only, including some of the 4 -+ 1-transitions. Part (B) displays the fluorescence spectrum 368 1 nm, 5Dof the same5Dsample excited at 3~. The± fluoresgiving rise to population of both the 3 and the 4states of Tb cence emission in this case originates from both excited states. The corresponding emission bands have been indentified with the pertinent transitions [1]. The effect of the temperature upon the radiative properties of the 5D 3~is illustrated 3 -state of Tb in part (C) of fig. 1 The contribution of the 5D 3-state is definitely lower relative to —~
-
122
J. Chrysochoos, P. Tokousbalides
(A)
/
Temperature and solvent effects
5q~—~ 7F
80
5
5D3~~0~jjL (B)
300
4~0
ebO 3~in POC1 WAVELENGTH, nm Fig. 2. Fluorescence spectra (uncorrected) of 5 X i0~ M Th 3:SnCl4 Xexc = 368 nm. (A) [POCI3J/ESnC14] = 12.7. (B) IPOC13]/lSnCl4J = 55.5.
±
5D that of the 4-state at higher temperatures, although it is not obvious that the overall fluorescence emission is also lower at higher temperatures. Similar (uncorrected) fluorescence spectra are shown in fig. 2 at constant temperature and excitation wavelength, namely 25°Cand 368 ±1 nm, but at different [POC13]/[SnCl4]ratios. Results are presented for the 5D two extreme values ratio.fluorescence It is quite ap3~toof thethis overall parent that the contribution of the the 5D 3-state of Tb emission is greater than that of 4-state at lower [POd!3]/ [SnC14]-ratios. For values of the [POC13]/[SnCl4].ratio larger than 55.5, TbCl3, does not dissolve whereas for values smaller than 12.7 a white precipitate, possibly (POC13)2SnCl4, is formed.
1
J. Chrysochoos, P. Tokousbalides / Temperatureand solvent effects
~
3.0
123
7.0
‘2
/ ~
0
°—~--~ti~---o 0
115
T~°c 3~in POC1 Fig. 3. Variation of the corrected fluorescence intensities of 5 X l0~ M Th 3:SnC14 5D 7F 5D 7F at 415 ± 1 nm ( 3 —° 5) and at 544 ± 1 nm ( 4 -~ 5) with the temperature; ~‘exc= 368 ± 1 nm; o, • [SnCl4] = 0.192 M; A, A [SnCl4] = 0.346 M and o, • [SnC14J = 0.576 M.
3~] and [POd] / [SnC1 [Tb 4J-ratio the relative contributions the 5D At constant 5D 5D of 5D 3 and 4-states are temperature dependent. This indicates that the 3’—’ 4transition is also temperature dependent. Some typical results confirming this 5D assumption5Dare illustrated in fig.measured 3 where fluorescence emission arising intensity from the at 3 3~is in terms of the fluorescence and Tb 415 the ±1 nm4 -states and 544of± 1 mn, respectively. Results are shown for three different [POC1 5D 3] I [SnC14 ] -ratios.asInthe all temperature cases, the fluorescence emission arising from the it 3is 3+ decreases increases and for a given temperature state of Tb -
124
J. Chrysochoos, P. Tokousbalides / Temperature and solvent effects
higher at lower [P0d13] / [SnCl4I-ratios. On the other hand, the fluorescence emis5D 3+ increases as the temperature increases, atsion from the 4-state of Tb in a rather complicated manner. The increase tainsarising a maximum and then decreases can be accounted for by the 5D 5D 5D 3—÷ 4 -transition. The maximum implies a nearly complete depletion of7Ff. the The3-population some extent, of such 5D latter part ofand the participation, curves is moretodifficult to interpret processes as 4—~=* because of the large number of parameters involved. The position and magnitude of these fluorescence maxima are functions of such variables as the temperature, the [Tb3~], the [POC1 3]/[SnCl4]-ratioetc. These maxima can be predicted by the following mechanism: 3~(5D ~ Tb 4)+ heat (1) salvation sphere 3~(5D 3~(5D Th 3)~ Tb 4)+ heat (2) solvation sphere
3~(5D 3~(7F~) + hvf Tb 3)-÷ Tb 1 3~(5D 3~(7F 3~(5D 3~(7F~) Tb 3)+ Tb 6) Tb 4)+ Tb Tb3~(5D 3~(7F 4) Tb 3)+ hi4~ 3~(5D 3~(7Fj)+ heat Tb 4)salvation sphere Tb
(3)
(4)
-+
-~
(5) (6)
==~—~
Tb3~(5D
3~(7Fj)+ heat 4)
-=--~~=-~‘
salvation sphere
(7)
Tb
Processes (1) and (6) as well as (2) and (7) represent the deexcitation of the 5D 3 and 5D 3~,respectively, through the vibrational modes of solvent molecules 4 -states of in Tb located both the primary and the secondary solvation sphere of the ion [9,10]. The fluorescence quantum yield, 4fl and lifetimes, r, which correspond to the radiative processes (3) and (5) are affected by processes (1), (2), (6) and (7) although the radiative fluorescence lifetimes, r°and r’° may be unaffected. If one cannot distinguish between processes (1) and (2) or (6) and (7), the overall quenching effect of the solvent may be considered in conjunction with reactions (3) and (5). Reaction (4) has been studied independently [8]. Typical fluorescence lifetimes are,depicted in fig. 4 as a function of the temperature at constant [Tb3~] and at different [POC1 3]/ [Sndl4] -ratios. The fluorescence lifetimes are shorter at higher temperatures and at5D higher [POC13] / [SnCl4]-ratios. 5D 3~can be defined Thethe formation and the mechanism. decay of the The3 following and the equation 4-state of by aforementioned is Tb obtained in the case of the 5D 3~. 3 -state of Tb
J. Chrysocho’.c. P. Tokousbalides / Temperature and solvent effects
125
0.8
0.4
£
0 -IS
I
I
13
45
I
,
3~(5D
7F 3 -. 4) in POC13:SnCl4 1; x11 = 436 ± 1 nm; flash duration 50 ~ss;• [SnC14] = [SnCl4j = 0.346 M.
Fig. 4. Variation of the fluorescence lifetime of 5 X 10~ M Th with the temperature; ?~exc= 368 0.77 M; o [SnCl4j = 0.576 M and
± A
3+1
1’°~ + kr[POCl or
=
kf1
=
kf 1
+ ki’
3]~,
+
(8)
kq[Tb
3~i + k!~~ent [P0C1
+ kq
[Tb
3]
(9)
where kq stands for k4, [POCI3] ~ represents the concentration of the POdl3 molecules located in the secondary solvation sphere of the ion and [POC13]the initial concentration of the POdl3 molecules. The values of kq were determined 3~in previously POC1 [8] and k~in eq. (9) stands for the fluorescence rate constant of Tb 3 :SnCl4.
126
J. (7zrvsoc/,oos. P. Tokousbalides
/ Temperature and
solvent effects
The value of kf1 which corresponds to the 5D3-state is not known. However, it is 5D expected to be a little larger 3~in than the valuesolvents which corresponds to thethat 4-state. The various [111. Considering T~ is of latter is about 70 s~for Tb the order of 3500 s~ and kq [Tb3~] of the order of 400 s_i, eq. (9) can be rewritten as follows: kq [Tb3~] = kf 1+ or
(10)
k~0l~[POCl3]
3~1~k~0l~’[POCl 31 (11) 1 kq[Tb3~])versus l/Twe obtain the results illustrated in 1og10(r fig. By 5. plotting At very low [POdl 3] I [SnCl4I-ratios and low temperatures the plot is fairly linear although at higher ratios and temperatures deviations from linearity are apparent. Under the conditions at which eq. (11) is valid we have: 3+])~1og 1”)/23RT (12) log10(r~ kq[Tb 1oA~Olv[POdl3I (E~° based on the Arrhenius expression for ~ namely k~0l’~ = A!”’ exp((—E~”) 2/R7). —
kqETb
.
—
—
Eq. (12) employed in conjugation with fig. S gives rise to the following results:
~
3.70C ~—~--~ —~-.--~ 0 ~
0~A ~
~
~IOO3.00
~
.—=-.
O~A
~
3.40
3.80
~~ • K 3~i)with lIT; 5 X i0~ M Th3~in POCI Fig. 5. Variation oflog1Ø(T~ — kqETb 368 ± 1 nm; flash duration 50 ~s. • [SnCI4J = 0.77 M; o [SnCl4] 0.423 M and ISnCl4l = 0.346 M and 0.192 M.
=
0.576 M;
A
3:SnC14 Xexc = [SnC14] =
J. Chrysochoos, P. Tokousbalides
3.40 kcal/mole;A!”
/ Temperature and sOlvent effects 1.3 X l0~M1
51
127
for [SnC1 4]= 0.77 M
~E~)2 ~3.88 kcal/mole;A!”’ ~2.6>~ l0~M~s~for [SnC14]= 0.576 M 1 ~i for [SnC1 (Er)2 3.95 kcal/mole; A!” 3.4 X l0~M 4]= 0.423 M and (E~~”)2 ~4.l5 kcal/mole;A!”’ ~4.3 X lO~M~s~for [Sndl4]
=
0.346 M; 0.192 M.
The activation energy increases slightly at lower concentrations of SnC14. At the higher [Snd14] used the reaction rate constant for process (2) is given a k!” = 1.3 X l0~(—3400/RT)M s~. The nonlinearity of the curves in fig. 5 at higher [POC13] I [SnC14]-ratiosand temperatures indicates contribution from additional terms eq. (10). This canofbeSnCl~ visual3~,cin onsisting primarily ized in terms of a protective shield surrounding Tb [12—14],which may render the approach of POC1 3~rather 3 molecules toward Tb difficult. However, at higher [POC1 3]/[SnC14]-ratiosor lower [SnCl4] and particularly at3~, higher this protective is nottemperatures very rigid allowing POC1 shield, or the primary solvation sphere of Tb 3 molecules to penetrate giving rise to much
Table 1 5D 7Ff) and the efficiency of the nonradiaVariation tive transition of the 5Dfluorescence 5D~with lifetime temperature. of TbJ+ ( 3 —. 3 -= [POC1 3~?~exc= 368 ± 1 nm; A~-~ = 436 ± 1 nm; flash duration 50 p5. 3I/[SnCI4] = 12.7; 5 X 10~ M Tb T (°C)
r (ms)
—15 —10 — 5 0 5 10 15 20 25 30 35 40 45 50 55
0.73 0.65 0.58 0.52 0.46 0.41 0.37 0.23 0.29 0.26 0.24 0.22 0.20 0.19 0.18
kq 181 (M1 s~) —
2.lOx 2.25 X 2.62 X 2.90 X 3.11 X 3.55 X 3.81 X 4.06 X 4.50 x 4.86 x 5.25 x 5.68 x 6.12x 6.61 x
i04 i04 i04 i04 i0~ i04 l0’~ tO4 ~ i04 a) i04 a) i04 a) lO~~ i04 a)
a) Values obtained by extrapolation ofthe curve
k~0l~’ (M1 s’) 220 250 282 316 354 394 438 484 533 586 641 701 763 829 898
versus [Th3+] to higher temperatures [8].
128
J. Chrysochoos, P. Tokousbalides
/ Temperature and solvent effects
more efficient fluorescence quenching. The contribution of the additional k-term at higher temperatures can be estimated by introducing the values ofAr” and (E~”)2 obtained above into the following equation: 3~] k!1”[POC1 log10(r~ kq[Tb 3]) ~log10 k —
—
=
log10 A
—
(EA)/2.3RT (13)
Eq. (13) in conjunction with fig. 5 yields the following results: 1 for [SnC1 k=3.2X 1011 exp(—ll,3SORT)s 4]=0.346MandO.192M and k
=
5.0 X 10~exp(—14,700R7) s~for [SnC14]= 0.576 M.
The activation energy increases at higher [SnCl4] which is expected with the concept of a rigid solvation sphere at higher [SnCI4I. 3~,kq and k~°~” are sumAppropriate values of the fluorescence lifetime of Tb marized in table 1 at one [POCI 3]/[Sndl4]-ratio. At1.25°Cand for [POCL3]/[Sndl4] Consequently, the efficiency = 17.7 we obtain k!”[POC13] + k~ 5.5 X l0~ s of the 5D 5D 3~in Tb3~in POd 3—~ 4 -transition of Tb 3 :Sndl4 at room 3~]. This value is in temperature fair agreehas a minimum valueestimate of 6 X 10~ at ~ very s~ low obtained [Tb ment with an earlier of ~ ~ to for Tb3~in protic solvents [4,51in which case extremely weak fluorescence emission was observed arising from the 5D 3~due to the high efficiency of the 5D 5D 3 -state of Tb 3—* 4 -transition in the presence of the ~C—Hand —0—H groups of the solvent.
References ]l] G.H. Dieke and l-LM. Crosswhite, AppI. Opt. 2(1963) 675.
12]
L.G. Van Uitert, J. Luminescence 4 (1971) 1; see also included references. [3] B.D. Joshi, B.M. Patel, A.G. Page, J.R. Banglia and RN. Saxena, J. Luminescence 6 (1973) 125. [4] J. Chrysochoos and A. Evers, Chem. Phys. Letters 20 (1973) 174. 151 J. Chrysochoos, J. Luminescence 9 (1974) 79. ]6J M.E. Peack and T.W. Waddington, J. Chem. Soc. (1962) 3450. 17] K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds (Wiley, New York, 1963) p. 112. (8] P. Tokollsbalides and J. Chrysochoos, Chem. Phys. Letters 29 (1974) 226. [9] J. Clirysochoos, Chem. Phys. Letters 14 (1972) 270. [10] J. Chrysochoos and A. Evers, Spectry. Letters 6(1973)203. [11] J.L. Kropp and M.W. Windsor, J. Chem. Phys. 42 (1965) 1599. [12] P. Tokousbalides and 1. Chrysochoos, J. Phys. Chem. 76 (1972) 3397. [13] J. Chrysochoos and P. Tokousbalides, Spectry. Letters 6 (1972) 435. [14] F. Collier, H. Dubost, R. Kohimuller and G. Rault, C.R. Acad. Sci. Ser. C. 267 (1968) 1095.