1976
J. Phys. Chem. 1982, 86,1976-1979
minimizes the volume V,,, inside the rotational envelope not occupied by the molecule itself. After Cohen and Turnbull,36 the probability that V , is not occupied by solvent molecules decreases exponentially with V,. Thus, free volume effects are expected to play a minor part in the rotation of 6, whereas they may be considerable for 5. In terms of eq 7 , the preexponential factor A will thus be dominant for 6, leading to increased "activation energies" as compared to 5, where the second term of eq 7 may be dominant. This is in complete agreement with the Arrhenius slopes of Figure 7 and the values derived in Table I which differ by more than one-standard deviation. The alternative treatment, plotting k / T vs. 1/T, yields linear relations with the same good correlation coefficient (0.96 to 0.99) with slopes reduced by a factor of 0.84 to 0.90. Thus, on the basis of the presented experimental data, it (36) M. H. Cohen and D. Turnbull, J. Chem. Phys., 31, 1164 (1959).
cannot be decided whether the equation for thermal or for accelerated diffusion (eq 3a or b) applies, but this does not affect the conclusions drawn above. From extrapolation to infinite-temperature, formal preexponential Arrhenius factors k" may be derived. If they are extracted from a temperature range where the second exponential term of eq 7 dominates, these should be systematically too low because at sufficiently high temperature the first exponential term will always gain importance. This may explain the nearly 2 orders of magnitude difference between compound 5 and the others investigated (Table I).
Acknowledgment. The author wishes to thank Professors E. Lippert and Z. R. Grabowski for stimulating discussions, and Dr. J. Zegelin for his assistance which was most helpful to the performance of the decay time measurements. Financial support from the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
Intersystem Crossing and Internal Conversion from the Lowest Excited Singlet State of Diazanaphthalenes+ David W. Boldridge, Gary W. Scott,' and Thomas A. Spiglanln Department of Chemistry, University of California, Riverside, Riverside, California 9252 I (Receive& November 18, 198 I )
Triplet quantum yields of five diazanaphthalenes in room temperature solution have been determined. These values indicate that, except in the case of quinoxaline, internal conversion competes as a nonradiative decay mechanism for the excited singlet state. Rates of intersystem crossing and internal conversion in these molecules are determined. Solvent effects on the phthalazine triplet quantum yield are interpreted in terms of perturbations on the states involved.
Introduction The excited-state kinetics of N-heteroaromatic molecules consisting of up to three fused rings have been the subject of many recent investigations.l-12 Recent studies of the excited states of azanaphthalenes have been concerned with intersystem crossing and internal conversion in these Diazanaphthalenes are generally expected to have high singlet to triplet conversion efficiencies due to the strong spin-orbit coupling of ln.rr* singlet and the lower 37r7r* states,6 as pointed out many years ago by Elsayed.13 In their lowest singlet state, these molecules may also undergo rapid internal conversion to the ground state. To date, however, no systematic investigation of the partitioning of nonradiative decay of the SIstate between the singlet and triplet manifolds has been reported for these molecules. In a few cases fluorescence quantum yields and triplet quantum yields'"l6 have been measured for certain diazanaphthalenes. In most cases, these triplet quantum yields have been determined at 77 K in a glass, using sensitized or direct triplet production with the resulting transient absorption used to measure the triplet quantum yield. Serious discrepancies among several of these determinations suggested to us the need for using more direct methods of quantum yield determination. 'This research was supported in part by the Research Corporation and the Committee on Research of the University of California, Riverside.
In this paper, we report the triplet quantum yields, a&, for five of the diazanaphthalenes (DAN): quinazoline (1,3-DAN), quinoxaline (1,4-DAN), 1,5-naphthyridine (1,5-DAN),1,8-naphthyridine (1,8-DAN),and phthalazine (2,3-DAN). All measurements were made using the method of Lamola and Hammond, with benzophenone (aisc = 1.00) as the actinometric standard.17Js By combining (1) Y. Hirata and I. Tanaka, Chem. Phys., 25, 381 (1977). (2) A. E. W. Knight and C. S. Parmenter, Chem. Phys., 15,85 (1976). (3) B. I. Greene, R. M. Hochstrasser, and R. B. Weissman, J . Chem. Phys., 70, 1247 (1979). (4) J. R. McDonald and L. E. Brus, J . Chem. Phys., 61, 3895 (1974).
(5) R. W. Anderson, Jr., D. E. Damschen, G. W. Scott, and L. D. Taliey, J . Chem. Phys., 71, 1134 (1979). (6) G. W. Scott, L. D. Taller, and R. W. Anderson, Jr., J. Chem. Phys.,
72, 5002 (1980). (7) S. Okajima and E. C. Lim, J . Chem. Phys., 69, 1929 (1978). (8)K. Yamamota. T. Takemura. and H. Baba. Bull. Chem. SOC.J D ~ . . 51,'729 (1978). (9) Y. Hirata and L. Tanaka, Chem. Phys. Lett., 41, 336 (1976). (10) V. Sundstrom, P. M. Rentzepis, and E. C. Lim, J . Chem. Phys., 66. 4287 (1977). (11) L. J. Noe, E. 0. Degenkolb, and P. M. Rentzepis, J. Chem. Phys., 68, 4435 (1978). (12) S. L. Shapiro and K. R. Winn, J . Chem. Phys., 73, 1469, 5958 (1980). (13) M. A. El Sayed, J . Chem. Phys., 38, 2834 (1963). (14) V. L. Alvarez and S. G. Hadley, J . Phys. Chem., 76,3937 (1972). (15) S. G. Hadley, J . Phys. Chem., 75, 2083 (1971). (16) T. Takemura, K. Yamamoto, I. Yamazaki, and H. Baba, Bull. Chem. SOC.Jpn., 45, 1639 (1972).
--.
\ - - .
0022-3654/82/2086-1970$01.25/00 1982 American Chemical Society
Intersystem Crossing in Diazanaphthalenes
the results of these measurements with the values of other available photophysical parameters for these diazanaphthalenes, we obtain a clear, quantitative picture of the partitioning of radiationless decay processes from the lowest excited singlet state via the ground-state singlet and the triplet manifolds.
The Journal of Physical Chemistry, Vol.
86,No. 11, 1982 1977
TABLE I: Relative mist of Benzophenone in Various Solvents ~~
[a &(solvent )/ solvent benzene THF CH,Cl,
kc(isooctane)] (benzophenone)
1.05 0.97 1.02
* 0.07 t
0.15 0.06
Experimental Section Materials. Quinazoline and quinoxaline (Aldrich Chemical Co.) were purified by recrystallization from hexane and vacuum sublimation. Phthalazine and benzophenone (Aldrich Chemical Co.) were purified by extensive zone refining followed by subsequent vacuum sublimation. 1,Ei-Naphthyridine (Columbia Organic Chemicals, 97% ) was purified by vacuum sublimation. 1,8-Naphthyridine was made via the procedure of Paudler and Kresslg and purified by recrystallization from hexane and vacuum sublimation. The solvents benzene and methylene chloride (Mallinckrodt SpectrAR grade) and isooctane (2,2,4-trimethylpentane, Aldrich Gold Label and Eastman Spectro ACS) were used without further purification. Tetrahydrofuran (THF; Mallinckrodt AR) was distilled from LAH to remove water and stabilizer. cis1,3-Pentadiene and trans-1,3-pentadiene (Chemical Samples Co., 99%) were used without further purification. Techniques. Using the method of Lamola and Hammond, the sensitized isomerization of 1,3-pentadiene was used to determine the intersystem crossing quantum yield for the diazanaphthalenes investigated. Stock solutions of cis-1,3-pentadiene(-0.2 M) in the solvents investigated were used to dissolve the diazanaphthalene samples and benzophenone to produce both sample and standard solutions, respectively, of -0.05 M in the solute; 3-mL aliquots of these solutions were transferred to separate square, 1-cm quartz cuvettes fitted with a Pyrex neck and vacuum joint. Both the samples and standards were then subjected to at least three freeze-pump-thaw cycles to remove dissolved oxygen, and they were subsequently sealed off under vacuum. At least three runs of sample and standard solutions were prepared by this procedure for each determination of cDiSc. Samples and standards were irradiated using the 366-nm lime from an Osram 200-W high-pressure mercury arc lamp. This mercury line was isolated by glass filters (Corning 7-54 and Schott GG 375). Irradiation times were set to provide between 1 and 10% conversion of cis- to trans-1,3-pentadiene. In addition, other samples were irradiated for a sufficiently long time to produce the photostationary state ratio of cis-/trans-l,3-pentadiene. After irradiation, the individual cells were broken open, and the cis/trans ratio in the solution was analyzed using a Varian 3700 gas chromatograph equipped with either thermal conductivity or flame ionization detection. The column was 25 f t X l/s in. P,P’-oxydipropionitrile (25% on Chromosorb P). The individual solutions were chromatographically analyzed at least twice for reproducibility. The absorbance of each solution at 366 nm was also determined on a Cary 17 spectrophotometer. Since several solvents were used, it was necessary to verlfy that @ , = 1.0 for benzophenone in each solvent. For this purpose, a s t o c k solution of cis-l,&pentadiene (-1.0 M) in isooctane was made. The benzophenone was dissolved in a 2.0-mL aliquot of this stock solution which was
Diazanaphthalenes. The triplet quantum yields displayed in Table I1 clearly show that there is a wide range of +isc
(17) G. S. Hammond, J. Saltiel, A. A. Lamola, N. J. Turro, J. S. Bradshaw, D. 0. Cowan, R. C. Counsell, V. Vogt, and C. Dalton, J. Am. Chem. SOC., 86,3197 (1964). (18) A. A. Lamola and G. S. Hammond, J. Chem. Phys., 43, 2129 (1965). (19) W.W.Paudler and T. J. Kress, J . Org. Chem., 32, 832 (1967).
(20) J. Saltiel in “Organic Photochemistry”, Vol. 3, 0. L. Chapman, Ed., Marcel Dekker, New York, 1973. (21) N. J. Turro, “Modern Molecular Photochemistry”, Benjamin Cummings, Menlo Park, CA, 1978. (22) A. M. Nishimura and J. S. Vincent, Chem. Phys. Lett., 11, 609 (1971).
f
then diluted to 10 mL with the appropriate solvent, giving a final solution -0.2 M in cis-1,3-pentadiene and -0.05 M in benzophenone. Relative quantum yields of intersystem crossing of benzophenone were determined in this way for the solvents benzene, THF, isooctane, and methylene chloride. The ratios, relative to the isooctane solvent determination, are given in Table I. Since these ratios were all 1.0, within experimental error, it was assumed that aiSc= 1.0 for benzophenone in all four ~olvents.~’J~
Results The room temperature intersystem crossing quantum yields for the diazanaphthalenes studied in various solvents are given in the third column of Table 11. Also given in Table 11, column 4, is the fraction of trans-1,3-pentadiene present after long-time irradiation using the different diazanaphthalenes as photosensitizers. The experimental error limits shown for each value in Table I1 are the larger of the error given by either (1)the standard deviation in the average for the different determinations for each sensitizer/solvent combination or (2) the largest error given by a simple propagation of errors on a single determination for that sensitizer/solvent.
Discussion Pentadiene Photostationary State Ratios for Diazanaphthalene Sensitizers. The triplet photosensitized cis-trans isomerization of 1,3-pentadienesis a well-studied ~ u b j e c t . ~At ’ ~the ~ ~photostationary ~ state (P.s.s.), the rate of conversion from cis to trans is equal to that of conversion from trans to cis. In general, the composition of the pentadiene mixture at the photostationary state is a function of the triplet energy of the sensitizer. A sensitizer whose triplet energy is above that of either cis- or trans1,3-pentadiene will preferentially transfer energy to the cis isomer, creating an inflated percentage of trans in the photostationary state. The T1state energy of quinoxaline is only slightly higher than that of naphthalene (fraction of trans at P.S.S. = 0.68)17 and slightly lower than that of Michler’s ketone (fraction of trans at P.S.S. = 0.551.l’ The measured fraction of trans- at P.S.S. (0.60) for the quinoxaline sensitizer fits well on published curves20,21 and lies in a region where the fraction of trans- is rapidly decreasing. Quinazoline, 1,5naphthyridine, and phthalazine have triplet energies high enough that PAS. (limiting) fractions of 0.55 would be predicted, and the measured values verify this prediction. The triplet energy of 1,8-naphthyridine (EoT> 23580 cm-1)22indicates that it should also display this P.S.S. ratio value, although it could not be measured for this molecule. Intersystem Crossing and Internal Conversion in the
1978
The Journal of Physical Chemistry, Vol. 86,No. 1 I, 1982
Boldridge et al.
TABLE 11: Triplet Q u a n t u m Yields and 1,3-Pentadiene Photostationary State Composition Determined for Various Diazanaphthalene Sensitizers [(trans-)/(cis
diazanaphthalene
solvent
4'iX
quinazoline quinoxaline phthalaz ine phthalazine 1,5-naphthyridine 1,8-naphthyridine
isooctane is0 oc t ane THF methylene chloride isooctane THF
0.70 t 0.05 0.99 t 0.06 0.43 f 0.02 0.29 r 0.06 0.55 f 0 . 0 4 0.04 f 0 . 0 2
0.557 0.604
. . .a
+ trans-)]p.s,s f
i
0.01 0.008
0.576 t 0.008 0 . 5 5 i 0.02
. . .b
a Not determined. Could not be determined d u e t o significant photochemical decomposition of the 1,8-naphthyridine during long time irradiation.
TABLE 111: Photophysical Parameters of t h e Diazanaphthalenes molecule quinazoline quinoxaline phthalazine 1,5-naphthyridine 1,8-naphthyridine
solventsiconditions isooctane/R. T. isooctane/R.T. THF/R.T. methylene ch1orideiR.T. EPAi77 K iso0ctaneiR.T. THF/R.T.
Disc
0.70 i 0.99 f 0.43 i 0.29 i 0.49' 0.55 f 0.04 i
0.05 0.06 0.02 0.05
0.04 0.02
TS,
7
PS
79 i 23 i 150 i 190 i __-100 f
____
15d lod 20e 30e _ 50f
kbc, ns" a
8.9 i 1 . 8 42.6 i 18.9 2.9 f 0.4 1 . 5 f 0.4 -___ 5.5 i 2.8
.___
kk,
ns-'
3.8 f 2.0 0.9 i 6.1 3.8 i 1.1 3.8 i 1 . 1 __._
4.5 t 5.7 __..
Assumes h i , = @ B ~ T S ; ~ . Assumes hi, = TS,;' - k b c . Reference 16. Reference 6. e Reference 24. Determined from transient absorption kinetics in 1,3-czs-pentadiene (D. W. Boldridge and G . W. Scott, unpublished results).
values among the diazanaphthalenes. The more commonly studied diazanaphthalenes-quinoxaline, quinazoline, and phthalazine-all have appreciable triplet quantum yields. Since these molecules are estimated to fluoresce with a quantum efficiency of internal conversion appears to compete effectively with intersystem crossing in quinazoline and phthalazine (see Table 111). In the case of both 1,5- and l,&naphthyridine, luminescence (presumably fluorescence but possibly from photoproducts) was observed during irradiation. The singlet lifetimes of these molecules have not yet been determined, although preliminary evidence indicates that Tal in 1,5-naphthyridine is approximately 100 ps.23 Some of the known photophysical parameters of these molecules are summarized in Table 111. It is generally accepted that the spin-orbit coupling mechanism between the Sl(ln7r*) and lower %7r* states provides the dominant contributions to intersystem crossing13 in many aromatic nitrogen heterocycles. Previously, we reported model calculations which produced limited correlations of the measured intersystem crossing rate constant (k,) with calculated values of the spin-orbit coupling matrix elements in these molecules.6 With these additional data now available, it appears that phthalazine has a somewhat smaller value of kk than would have been predicted from these correlations. Furthermore, the kisc for phthalazine, given in Table 111, is clearly solvent dependent. The discrepancy between the calculated correlation of the spin-orbit coupling (ln7r*-%r7r*) and the observed T ~ was , recently discussed by Anderson and Knox.Z1 They attributed these differences to the fact that the lowest triplet state of phthalazine is a mixed 3n7r*-37r7r* statez5and not a pure 37r7r* state as was assumed in the model spin-orbit calculations and correlation. The solvent dependence of the intersystem crossing rate is readily explained as due to the solvent effect on the electronic states. In general, x7r* states are red shifted by increasing solvent polarity, while nr* states are blue shifted. Since the singlet-triplet energy gap is much smaller (- lo00 cm-l) than the excited singlet-ground state energy gap (-24 000 cm-I), the relative increase in the (23) D. W. Boldridge and G. W. Scott, unpublished results. (24) R. W. Anderson and W. Knox, J . Lumin., 125, 647 (1981). (25) E. C. Lim and J. Stanislaus, J. Chem. Phys., 53, 2096 (1970).
energy gap between S1("r*) and T1(3.rr~*) should be proportionally much greater than between S1 and the ground state upon increasing solvent polarity. Since the rates of both internal conversion and intersystem crossing are related to the energy gap between the states involved, we would expect any rate changes to be demonstrated much more strongly in the intersystem crossing rate, as observed. The observed triplet quantum yields for phthalazine at room temperature are in reasonable agreement with the value', of 0.49 obtained in EPA at 77 K. While this latter value is higher than would be expected from our results, the difference may be attributable to thermally enhanced internal conversion as has been reported for quinoline and isoquinoline.26 In quinoxaline, large calculated values of the spin-orbit coupling matrix element lead to the prediction of efficient intersystem crossing, as observed. Our observed triplet quantum yield, ai,= 0.99, directly supports the conclusion by Li and Lim2' that internal conversion is not a major relaxation pathway in quinoxaline. Using the phosphorescence quantum yields of quinoxaline-h, and -dG (ap = 0.42 and 0.90, respectively) and the phosphorescence lifetimes, they postulated that aiSc Z 0.9, as we have observed. Other recent work has used nanosecond flash photolysis to establish a minimum bound of a , 2 0.67 for quinoxaline in water.28 This experiment depended on proton quenching of the triplet by 2-propanol, and the authors note that other triplet decay paths competing with proton quenching would necessitate an increase in the estimated triplet quantum yield. In 1,5-naphthyridine,the calculated spin-orbit matrix = lBJm and element between the lowest singlet state ("r* the lowest triplet (37r7r* = 3B,)30is identically zero due to the existence of a center of inversion in the molecule. This would then mean that the singlet-triplet spin-orbit coupling in this molecule would be expected to be significantly , less than in the other diazanaphthalenes. Hence T ~would (26) T. Lai and E. C. Lim, Chem. Phys. Lett., 62, 507 (1979). (27) R. Li and E. C. Lim, J. Chem. Phys., 57,605 (1972). (28) D. V. Bent, E. Hayon, and P. N. Moorthy, J . Am. Chem. SOC.,97, 5065 (1975). (29) A. D. Jordan, I. G. Ross, R. Hoffman, J. R. Swenson, and R. Gleiter, Chem. Phys. Lett., 10, 527 (1971). (30) G. Fischer, Chem. Phys. Lett., 21, 305 (1973).
J. Phys. Chem. 1982, 86, 1979-1985
be expected to be much longer and aiSc smaller than for the other diazanaphthalenes. This is clearly not observed experimentally. However, even small out-of-plane distortions of this molecule which destroy the inversion center can give rise to significant contributions to the 1na*-37r7r* spin-orbit coupling. An alternative explanation of the short 75, and nonnegligible aiSc concerns the identity of the S1 state in 1,5naphthyridine. Recent r e p ~ r t s ~provide l - ~ strong evidence that the lowest excited singlet has lB, symmetry, a conclusion also reached earlier by P e r r i n ~ . Alternatively, ~~ this state has been assigned as 1AU.35Spin-orbit matrix elements calculated from Huckel wavefunctions using the method of ref 6 indicated that coupling between this 'A,('na*) state and 3B,(3a.rr*)state would be even stronger than that observed in quinoxaline. Therefore, if 'A,('nn*) were the lowest singlet state of 1,5-naphthyridine, the molecule would be expected to undergo intersystem crossing with approximately unit efficiency. Since this is not observed, and based on the thoroughness of the spectroscopic we confirm that the lowest (31) P. J. Chappell, G. Fischer, J. R. Reimers, and I. G. Ross, J. Mol. Spectrosc., 87, 316 (1981). (32) G. Fischer and I. G. Ross, J. Mol. Spectrosc., 87, 331 (1981). (33) A. D. Jordan, G. Fischer, and I. G. Ross, J. Mol. Spectrosc., 87, 345 (1981). (34) N. M. Perrins, Ph.D. Thesis, Renssalaer Polytechnic Institute, 1971. (35) L. C. Robertson and J. A. Merritt, NTISAD Report 742217 (1971).
1979
singlet 'state is 'B, and that out-of-plane vibrations are responsible for the observed nonzero triplet formation in 1,5-naphthyridine. In l,&naphthyridine, calculations based on INDO wave functions6 indicate that the spin-orbit coupling matrix elements are small, but finite, in this molecule. This would lead one to predict that intersystem crossing will be observed, but that it will not be highly efficient. This result is indeed observed, although singlet photochemistry may be a complicating factor in this case. It is interesting to note that while the values of the ki, rate constants for the diazanaphthalenes studied show significant variation from molecule to molecule, as well as in the case of phthalazine in different solvents, the values of the ki, rate constants are, within experimental error, the same. Although admittedly based on a limited data set, this observation is not unreasonable since all of these molecules have similar S1 states ( h a * ) at approximately the same energy above the ground state.
Acknowledgment. We wish to thank Dr. A. Gupta of the Jet Propulsion Lab for loaning us the gas chromatography column. We thank Professors W. Okamura and R. C. Neuman of this department for the use of their gas chromatographs. We appreciate several helpful discussions with Dr. R. Anderson of Xerox Corp. We wish to thank Ms. L. DeLucci for typing the manuscript. One of us (T.A.S.)was supported during Summer 1979 by the National Science Foundation's Undergraduate Research Participation Program.
Electronic Structure of C-Li, Si-H, and Si-Li in the Lowest '2- and 211 States Arlstldes Mavrldls' and James F. Harrlson*2 Theoretical Chemistry Group, Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439, and Chemistry Department, Michigan State University, East Lanslng, Mlchlgan 48824 (Received December 4, 198 1)
We have studied the electronic structure of C-Li, Si-Li, and Si-H in the 42-and 211 states using ab initio SCF, MCSCF, and CI techniques. We find that, while Si-H is similar to C-H, having a 211 ground state with the 42-approximately 36 kcal mol-' higher, both C-Li and Si-Li have 48-ground states with the 211 being 33 and 14 kcal mo1-I higher, respectively. Potential energy curves and spectroscopic constants are presented for the three title molecules, and some speculation as to the possible consequences of the 48-ground state of Si-Li and C-Li is indulged in.
Introduction Recent work3 in this laboratory resulted in the prediction that the ground state of C-Li is of 48-symmetry with the companion 211 state 33 kcal mol-' higher. This is to be contrasted with the C-H situation4s5in which the is the ground state, some 17 kcal mol-l below the 41;-. In addition, contour maps3 of the GVB orbitals of C-Li, in both (1) On leave from the Chemistry Department, University of Athens, Athens, Greece. (2) Scientist in Residence, Argonne National Laboratory 1980/81. Address correspondence to this author at the Department of Chemistry, Michigan State University, East Lansing, MI 48824. (3) A. Mavridis and J. F. Harrison, J.Am. Chem. Soc., submitted for publication. (4) K.P. Huber and G. Herzberg, "Molecular Spectra and Molecular Structure", Vol. IV, Van Nostrand-Rheinhold, New York, 1979. (5) G. C. Lie and J. Hinze, J. Chem. Phys., 57,625 (1972); 59, 1872 (1973). 0022-3654/82/2086-1979$01.25/0
the 42-and are characteristic of a highly polar molecule with the 42-being more polar. A physical picture consistent with the above is that the 42-state results from the transfer of an electron from Li to the empty p orbital of carbon, forming C-which is then stabilized in the field of Li+. In the 2rI state the transfer is not as favorable, being to a carbon p orbital which already hosts one electron.6 The purpose of this report is to explore this idea by determining the 42-,211separation in Si-Li and Si-H. Since Si is less electronegative than C, we expect the charge transfer to be less in Si-Li than in C-Li but in the proper direction to differentially favor the 42-state. The results (6) The ionicity of the C-Li bond has been the subject of much discussion. See, for example: A. Streitwieser, J. E. Williams, S. Alexandratos, and J. M. McKelvey, J.Am. Chem. Soc., 98, 4778 (1976); G . D. Graham, D. S. Marynick, and W. N. Lipscomb, ibid., 102, 4572 (1980), and references contained therein.
0 1982 American
Chemical Society
