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J. Phys. Chem. 1903, 87, 4872-4877
nately the mathematical description of this situation has not been solved and we are badly underdetermined in assessing the validity of this model. No conclusion can be drawn except that a distribution of site energies may be a significant factor. In general, when applying the competition correction method, the rate parameters for the et- capture reactions 1 and 3 must be measured separately. Therefore, the experimental range over which these are known is limited, while in an intermolecular E T experiment the e; capture rates at very early times may be important since a large fraction of the e; may be captured then. Thus, it may be difficult to obtain the necessary e; rate parameters, particularly if the decay curves are shaped by solvation effects. The present results are somewhat insensitive to the e; parameters as long as the rates are about equal for the two solutes. In some cases it may be necessary to use compound rate functions which have different rate parameters in two separate regions of time; this is really only mathematically tractable in the case of the rate of e; reacting with the acceptor (reaction 5). Such a procedure yields reasonable results5 in the case of flattened decay curves which it is assumed that at early times the rate approaches
but does not exceed the maximum e; rates that have been observed in MTHF.4 In summary, good agreement has been obtained by applying the competition correction method in two cases: Bip- + MNQ in the present study and the cinnamaldehyde + pyromellitic dianhydride reaction studied previously.' For these reasonably fast reactions, the method may be used to accurately measure the rate of intermolecular ET as a function of distance, even at equal concentrations of donor and acceptor solutes. The method is less satisfactory for very weakly exothermic reactions (e.g., Bip- + Phen, AGO = -0.13 eV) which are slower by several orders of magnitude (see Table I). For such reactions reliable measurements of intermolecular E T rates can be obtained only when the concentration of the donor solute is much greater than the concentration of the acceptor solute. Acknowledgment. Work performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Science, US-DOE under Contract W-31-109-ENG-38. Registry No. Biphenyl anion radical, 34509-93-8;2-methyl1,4-naphthoquinone,58-27-5;phenanthrene, 85-01-8; biphenyl cation radical, 34507-30-7; biphenyl, 92-52-4.
Room-Temperature Phosphorescence and Delayed Charge-Transfer Fluorescence of Coronene Adsorbed on y-Alumina W. Honnen, G. Krablchler, S. Uhl, and D. Oelkrug' Instbut fur Physikalische und Theoretische Chemle der Universltat, 0-7400 Tubingen, Auf der Morgenstelle 8, West Oermany (Received: October 20, 1982; I n Final Form: January 25, 1983)
The luminescence of coronene adsorbed from high vacuum on thermally activated y-alumina has been studied in the temperature range T = 100-500 K. At low temperatures the spectrum consists of an unstructured fluorescence (vmm = 20500 cm-', quantum yield ~ F O= 0.07, decay time TFO = 2.5 ns) and a weakly structured phosphorescence (vM = 19OOO cm-', 4: = 0.12,~: = 2.2 s). The fluorescence originates in the charge-transfer (CT) singlet (ScT)of an EDA complex between coronene (donor) and Lewis centers of the surface (acceptor). The phosphorescence arises from a locally excited triplet (Tm)of coronene. The phosphorescence yield is almost constant up to room temperature. The total luminescence yield even increases strongly with temperature. At 440 K the total yield exceeds 4Fo+ 4: by a factor of 4. This increase is caused by thermally activated intersystem crossing (isc),TLE SCT,which leads to pronounced E-type delayed fluorescence. Radiative and nonradiative rate constants have been determined. The value of the SCT T L E intersystem crossing rate (k = 3.7 X lo8 s-l) exceeds the rate of the reverse process T L E SCTby more than 3 orders of magnitude.
-
Introduction Aromatic hydrocarbons are usually strongly luminescent after adsorption on activated y-al~mina.'-~ The luminescence properties depend on the activation temperature (TA)of the surface. At T A< 200 "C the luminescence spectra as well as the decay kinetics correspond to those in solution. At TA 1 300 "C new absorption and emission bands with entirely new decay characteristics are observed. These bands are assigned to charge-transfer (CT) transitions between the aromatics (donor) and active surface sites ( a ~ c e p t o r ) .The ~ nature of the acceptor sites is not (1) D. Oelkrug, M. Radjaipour, and H. Erbse, 2. Phys. Chem. (Frankfurt - a - m Main). 88. 23 -- (1974). (2) D. Oelkrug, H. Erbse, and' M. Plauschinat, 2. Phys. Chem. (Frankfurt am Main), 96,296 (1975). ( 3 ) D . Oelkrug and M. Radjaipour, 2.Phys. Chem. (Frankfurt a m Main), 123, 169 (1980). \-
- - - I -
-,. --. .--
0022-365418312087-4872$01.50/0
-
-
yet completely understood. Most probably they consist of coordinatively unsaturated Al atoms which are produced by dehydroxylation of the surface. At moderate activation (TA = 200 "C)particularly single OH-free A1 atoms which are surrounded by OH-covered regions are formed. Stronger activation (TA = 300 "C) causes an increase in the number of acceptor sites which are formed by several adjacent Al atoms.4 As CT complexes are mainly formed a t TA > 300 "C,the larger acceptor sites seem to be necessary for CT complexation. Primarily this is because of steric reasons, i.e., the size of the acceptor size must be comDarable to the size of the aromatic molecule. For this reason a small aromatic will more probably find a suitable acceptor center than a large one. (4) H. Knoezinger and P. Ratnasamy, Catal. Reu.-Sci. Eng., 17, 31 (1978).
0 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 24, 1983 4073
Coronene Adsorbed on y-Alumina 1
I
Lb \ \
16
20
18
22
.
v
Flgure 1. Luminescence spectra of coronene/alumina excited at 365 nm. (a) Phosphorescence (PIa d prompt delayed fluorescence (fd at different temperatures: (. - .) 150, (.- .) 270, (. 293, (- - -) 339, and (-) 446 K (b) Phosphorescence (P)and delayed fluorescence (fJ ,. recorded with a phosphoroscope at different temperatures: (-) T = 170 K; (- --) T = 263 K; coronene in ethanol, T = 77 K.
- --
+
.
e)
(.-e)
In this paper we report on the luminescence properties of adsorbed coronene, a large and highly symmetric, aromatic molecule. The donor properties of coronene are expected to be rather simple. The molecule consists of three different kinds of carbon atoms (a-c, Figure 1). The r-electron density of the two degenerate HOMO'S is especially high a t the outermost atoms of type a (0.226 in the Huckel approximation). The electron density at atoms b and c is considerably lower (0.151 and 0.064). For this, atoms of type a are favored as donor centers not only for sterical but also for chemical reasons. The luminescence properties of coronene have been studied in s o l ~ t i o nin , ~mixed ~ ~ crystal^,^^ in thin films,l0 in hydrocarbon matrix," and in p l a ~ t i c s . ~Fluorescence ~J~ and phosphorescence spectra are extensively structured in all cases. In plastics, phosphorescence exists even a t room temperature. The small energy difference between the f i t excited singlet and triplet (AI3 = 3750 cm-') causes the rare phenomenon of E-type delayed fluorescence. Since the first excited singlet of aromatics is generally lowered in energy by adsorption on alumina, the effect of delayed fluorescence should become more pronounced in the adsorbed state.
Experimental Section Coronene (Fa. EGA) was adsorbed without further purification on y-Al,03 (Fa. Merck, active, neutral specific surface = 98 m2/g). The adsorbate concentration was (5)A. R. Horrocke and F. Wilkinson, Proc. R. SOC.London, Ser. A, 306, 257 (1968). (6)A. Kearvell and F. Wilkinson, Chem. Phys.Lett., 11,472(1971). (7)M.Zander, Ber. Bunsenges. Phys.Chem., 68, 301 (1964). (8)M.Zander, Z.Naturforsch. A, 27, 172 (1972). (9)H.Dreeskamp and M. Zander, Z.Naturforsch. A, 28,45 (1973). (10)T.Kajiwara, K. Ohno, S. Iwashima, and H. Inokuchi,Bull. Chem. SOC.Jpn., 42, 2734 (1969). (11)M.Zander, Naturwissenschaften, 47,443 (1960). (12)J. L.Kropp and W. R. Dawson, J . Phys.Chem., 71,4499(1967). (13)W. R. Dawson and J. L. Kropp, J. Phys. Chem., 73, 69 (1969).
16
18
20
22
24
I I,
io3 cm' Flgure 2. Phosphorescence (P), phosphorescence excitation (-), and fluorescence excitation (- - -) spectra of coronene/alumina at T = 20 K. Fluorescence excitation spectrum of coronene in ethanol (. * -) at T = 77 K. The experimental activation energy A€ of Figure 4 with respect to eq 10 and 11 is marked for illustration.
about 0.1% of a monolayer. The high-vacuum adsorption technique has already been described.'~~Because of the low vapor pressure of coronene the adsorption was carried out a t 120 "C and was completed after 20 days. Luminescence spectra were obtained with a Zeiss fluorimeter (450-W xenon or 75-W mercury lamp, MM12 double monochromators) equipped with a RCA photomultiplier tube 7265 (S20-Kathode),a Keithley picoamperemeter 417, and a phosphoroscope. Fluorescence decay curves were measured by using the time-correlated, single-photon counting method (PRA nanosecond lamp 510, Ortec electronics). The curves were fitted double-exponentially by using the method of least-squares iterative reconvolution.14 Phosphorescence decay curves were recorded with a multichannel analyzer after excitation with a monochromatic pulse of a xenon lamp. Pulses of 2-10-ms duration were generated with a photo shutter. Luminescence yields were determined by using the adsorption system anthracene/alumina ( T A= 600 "C) as a reference. The fluorescence yield of the latter system (& = 0.7 f 0.1 a t room temperature) was determined according to ref 15.
Results and Discussion Spectra. According to the size of coronene new absorption and emission bands are found if the surface is dehydroxylated to an amount of about 75% corresponding to an activation temperature T A = 600 "C. The spectra (Figure 1)consist of an unstructured fluorescence (urn= = 20500 cm-l) and a weakly structured phosphorescence (uN = 19000 cm-I). The excitation spectra (Figure 2) of both emission processes are identical (urn= = 22800, 24200, 25800, 30500 cm-'). The emitting species is assigned to an electron donoracceptor complex between the aromatic molecule (donor) and the alumina surface (acceptor) for the following reasons: (1)As in the case of smaller arom a t i c ~the ~ energy of the first allowed electronic transition (14)D. V. O.'Connor, W. R. Ware, and J. C. Andrb, J.Phys.Chem., 83, 10 (1979). (15)D. Oelkrug and A. Wolpl, Ber. Bunsenges. Phys. Chem., 76, 1088 (1972).
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The Journal of Physical Chemistry, Vol. 87, No. 24, 1983
Honnen et al. ~ F O= k17F
- - - - - - -naphthalene - - - - - - - - --phenanthrene - - -
(8)
and 4ST is the quantum efficiency of the triplet formation 4ST
=
(9)
k37F
After the excitation is turned off (labs = 0), the luminescence decays with a fast and a slow (delayed) time constant. As long as 7F > 7p-l > 4STkG
the approximations (Y1
i=
TP-l
-4 s ~ = k ~7d-l
7F-l
+ 4STkg
%
7F-I
can be used with no loss of accuracy, and we get IF(t)
+
= 4F04sTk6e-rd-1t k 1c r F - l t
Ip(t) = 4STk4(e-rd-1t- e-TF"t) The most temperature-dependent rate constants are k5 and ks. Usually both are expressed by an Arrhenius equation: l7
k6 = k6- exp(-AE/RT) k5 =
k50
+ k5" exp(-Ac/RT)
(11) (12)
where the constants AJ3 and k6- are associated with the energy difference between SCTand T1 and with the rate constant for the intersystem crossing (isc) back to the singlet. The interpretation of the constants in eq 12 is controversial. Internalla as well as envir~nmental'~ vibrations are possibly responsible for the value of A€ which varies in aromatics between 700 and 3500 cm-'. The preexponential factor k5- varies between lo1 and 1O1O s-l.18 Its lower limit (101-102 s-l) is typical for the adsorbed state.20 No satisfactory explanation has been given for this large range of k,". Equation 10 is especially useful to determine (17) J. B. Birks, 'Photophysics of Aromatic Molecules", Wiley-Interscience, London, 1970. (18) S.G.Hadley, H. E. Rast, Jr., and R. A. Keller, J. Chem. Phys., 39, 705 (1963). (19) E. T. Harrigan and N. Hirota, J. Chen. Phys., 49, 2301 (1968). (20) M. Plauschinat, PhD Thesis, Universitat Tubingen, West Germany, 1979.
Coronene Adsorbed on y-Alumina
-5
I
3
The Journal of Physical Chemistry, Voi. 87, No. 24, 7983 4875
\ 4
k
I
I
260 34 0 420 T(K) Figure 5. Absolute fluorescence phosphorescence yield (4 Ftot 4 ,) of coronene/alumina vs. temperature: (0, 0 )experimental data of two different series of measurements; (a, -) calculated from eq 3-9 and 11 by using the rate constants of Table I and A€ = 2010 cm-'; (b, e - . ) like a, but A€ = 2500 cm-'; (c, - - -) like a, but k," = 5 x i o 5 S-l.
+
x lD3[K-'l
Flgure 4. Logarithmic ratio of delayed fluorescence to phosphorescence for coronene/alumina vs. reclprocal temperature.
the temperature dependence of k6 as k5 does not appear in it and 4F0 (and TF respectively) is almost constant in the temperature range in question. Using eq 11, a linear re) and T'is expected which is lation between In (hde1/4 experimentally very well fulfilled (Figure 4). A value of hE = 2010 cm-' is calculated from the slope. In Figure 2 this value is correlated with the excitation and the phosphorescence spectrum. Unfortunately the 0-0 transitions of both bands are not well-known. The energy difference between ET1 = 19000 cm-' (the first phosphorescence maximum) and E& = 21500 cm-' (mean value between the CT absorption and fluorescence maximum) is somewhat higher than AE. It is possible that E,, is not identical with E , as the mirror symmetry between absorption and fluorescence is not verified. A more important reason for the discrepancies between kinetic and spectroscopic AE value may involve inhomogeneous adsorption (see following chapter). It is noteworthy that the total luminescence yield C # I F ~ ~ + 4pof adsorbed coronene increases strongly in the temperature range of T = 250-400 K. This unusual behavior can be explained if the following conditions are fulfilled: (a)High Triplet Population Yield 4sT,Le., a Small &O. In chemisorbed aromatics 4ST depends strongly on the position of SCT relative to the triplet terms. 4ST ranges from about 0.05 (anthracene/alumina) up to 0.95 (phenanthrene/alumina).21 A similar range is found in the dissolved aromatics, e.g., 4ST 2: 0.05 in 9,lO-diphenylanthracene, ~ 0 . 6in coronene, ~ 0 . 9 5in triphenylene. (b)Moderate Low-Temperature Phosphorescence Yield ( k 2 > k4). If one adds a and b, the low-temperature limit of the total luminescence yield (eq 3 + 4) will be moderate, too limk4+k,>>k8(d'Fbt + 4 p ) = 4Fo + 4STk4/(k4 + k 5 )
= 4Fo + 4:
(13)
--
( c ) High Probability of Thermal Repopulation of SCT via Tl SCT. This process becomes important only if k6 is increasing much more with temperature than k,. Therefore, a system with a small S C T , energy difference (=AEof eq 11) and high A€ or low k5- (eq 12) must be realized. The latter condition is fulfilled extremely well in aromatics chemisorbed on alumina (kEm = 10-100 s-'1, (21) D. Oelkrug, M. Plauschinat, and R. W. Kessler, J.Lurnin., 18/19, 434 (1979).
+
TABLE I: Luminescence Decay Times, Quantum Yields ( T = 150 K), and Calculated Rate Constants of Coronene Adsorbed on r-Alumina' coronenel coronenel coronene/Al,O, plastics ethanol - TF',
ns
Tp',
s
@F
k , , s-' k , , s-' k , , s" k , , s-' k,O, s-'
2.5b 2.2b 0.07 0.12 0.9c ( 2 . 7 f 0 . 4 ) X 10' ( 3 r 3 ) x lo6 ( 3 . 7 f 0 . 5 ) X 10' ( 5 . 7 r 0 . 8 ) X 10" ( 4 . 0 f 0 . 5 ) X lo-'
k,", s-' A E , cm-'
(4.5 r 0.5) x 2010 i 30
9a0 :@ ;
,
a
l o 4e
3 20 9.8 0.27 0.11
286 0.23
8.4 X l o 5 1.9 X lo6 1 . 6 x 105 7.0 x 105 2.1 X 10, 4.7 X 10, 1.7 X 8.6 X 10.' (0.141 at 296 K ) 5.9 x l o 5 3930 4400d
For comparison the results of coronene in plastic^^^^'^
(T= 7 7 K) and in ethano15s6(T= 296 K) are given. For definitions see text and Figure 3. Decay is nonexponential. The term of highest amplitude of a bi- or triexponential fit is given. High-temperature limit extrapolated from Figure 5. Spectroscopic energy difference S, - T , . e If the low value of T~~ is used, k," increases to about 1 X l o 5 s-'.
so that the phosphorescence quenching constant k5 increases only by a factor of 2-5 if the temperature is raised to 200 "C. ( d ) Negligible Nonradiatiue Transition Scr So ( k ,
