J. Phys. Chem. 1993,97, 7178-7184
7178
Influence of Solvent Polarity on the Radiationless Decay of the Intramolecular Exciplexes of w-Phenyl-a-N,N-dimethylaminoalkanes M. Van der Auweraer,' L. Viaene, Ph. Van Haver, and F. C. De Schryver' Chemistry Department, K.U.Leuven, Celestijnenlaan 200 F, 3001 Leuven, Belgium Received: November 24, 1992; In Final Form: March 30, 1993
Using optoacoustic spectroscopy, the radiationless decay processes of intramolecular exciplexes of w-phenyla-N,N-dimethylaminoalkanesare investigated. Upon increase of the solvent polarity, the relative efficiency of the intersystem crossing process from the singlet exciplex to the locally excited triplet decreases compared to the efficiency of the internal conversion process. These results can be rationalized in the framework of the current electron-transfer theory and compared to the results obtained for other w-aryl-a-N,N-dimethylaminoalkanes.
compound reacts with an aqueousdimethylamine solution to form 2-(4-cyanophenyl)-N,N-dimethylaminoethane(PCNZNM). DeThe influence of the solvent polarity on the radiationless decay tails related to this and subsequent synthesis15purification, and of intramolecular exciplexes has already been investigated,using identification are reported in the supplementary material. optoacousticspectroscopy and transient absorption spectroscopy, P2NM and P3NM. The synthesis of P2NM and P3NMlS for some intramolecularexciplexes between pyreneor naphthalene from 2-phenylethyl bromideand 3-phenylpropylbromide is similar and electron donors.' to the last step in the synthesis of PCN2NM. Upon investigation of the temperature dependence of the rate PBr2NM. The synthesis PBr2NM from 2-(4-bromophenyl)constant for radiationless decay of the exciplexes of w-phenylethyl alcohol is analogous to the two last steps in the synthesis a-N,N-dimethylaminoalkanes, determined using stationary and of PCN2NM. time-resolved fluorescence spectroscopy, relatively small values Solvents. The solvents used (acetonitrile, Merck; tetrahydrowere obtained for the preexponential factor ( lo7-lo9 s-l) and the furan, Rathbum; isooctane, Fluka Chemical) wereof spectroscopic activation energy (0.5-10 kJ/mol) of this processaZd This grade. Isooctane and acetonitrile were used as received. Diethyl suggested that the radiationless decay occurred mainly by ether and tetrhaydrofuran were refluxed for 2 h over sodium intersystem crossing. The absorption coefficient of the triplet of metal and distilled under nitrogen before use. toluene is very small at wavelengths above 300 nm.7 Therefore, Experimental Methods. The absorption spectra were deterit is not possible to study the radiationless decay processes of the intramolecular exciplexes of the w-phenyl-a-N,N-dimethylami- mined with a Perkin-Elmer Lambda 5 UV/Vis spectrophotometer. Fluorescence spectra were determined with a Spex Fluorolog. noalkanes using transient absorption spectroscopy. It remains The fluorescence quantum yields were determined using quinine possible, however, to investigate the radiationless decay of the sulfatein 1 NHzSOdasareference (@=0.55).l6 Theabsorbance exciplexes of the o-phenyl-a-N,N-dimethylaminoalkanes listed of sample solutions and of the solution of the reference equals 0.1 below by using laser-induced optoacoustic spectroscopy (LIfor the determination of fluorescence spectra and quantum yields. OAS) .*-I1 The determined fluorescence quantum yields are corrected for the difference between the refractive index of each solvent used in sample preparation and that of the reference solution by using eq 1. Fluorescence spectra and decay measurements were n = 2, X = H: Pphenyl-N,Ndimelhylaminoethane(PZNM) performed on samples degassed by several freezepumpthaw n = 2, X = CN: 2-(4-~yan~henyI)-N,Ndimeihylaminoeihane (PCNZNM) n = 2. X = Br: 2-(4-bromophenyl)-N,Ndimethylaminoethane(PBRNM) cycles.
Introduction
n = 3, X = H: 3-phenyl-N,Ndimethylaminopropane(P3NM)
Due to the irreversible character of exciplex formation in solvents with a polarity ranging from that of alkanes to that of acetonitrile, it was possible to obtain the individual rate constants for the different radiative and radiationless decay processes of the exciplex in those solvents in a straightforward way. This allows one to discuss the influence of the solvent polarity on the individual rate constants rather than on the ratio of the rate constants for internal conversion to the neutral ground state and intersystem crossing to the locally excited triplet state.
Experimental Section Synthesis. PCN2NM. For the preparation of PCNZNM, the seven-step synthesis'z starting from 2-phenylethyl bromide was replaced by a three-step synthesis in which 2-(4-bromophenyl)ethyl alcohol is subsequently transformed into 2-(4-cyanophenyl)ethyl alcoholl3and 2-(4-~yanophenyl)ethylbromide.14 The latter To whom all correspondence should be addressed.
0022-3654/93/2097-7 178%04.00/0
Ssample, Asample,and nhm correspond to the area under the fluorescence spectrum of t\e sample, the absorbanceof the sample at the excitationwavelength, and the refractiveindex of the solvent of the sample, respectively. S,r, Arcf, nfcr,and @f,rof correspond to the area under the fluorescencespectrum of the reference, the optical density of the reference at the excitation wavelength, the refraction index of the solvent of the reference, and the fluorescence quantum yield of the reference, respectively. The experimental setup for the laser-induced optoacoustic spectroscopy (LIOAS) is described in detail elsewhere.lJ1 The absorbanceof the sample solutions used in the LIOAS experiments was always below 0.1, and the samples were deoxygenated by bubbling argon through the solution for 15 min. Time-resolved fluorescence measurements were performed using an Edinburgh flash lamp as an excitation source. Two H-20 monochromators of Jobin-Yvon were used for excitation 0 1993 American Chemical Society
Solvent Polarity Influence on Radiationless Decay
The Journal of Physical Chemistry, Vol. 97, No. 28, 1993 7179
and emission. The fluorescence of the sample was detected by a XP-20-20 photomultiplier operated at 2400 V. The remaining part of the detection system is identical to that described elsewhere.’’ To maximize the emission intensity, the absorbance of the samples amounted to 0.3. The individual fluorescence decays were analyzed with single curve analysis as a oneexponential decay using the Marquardt modification of a nonlinear least-squares algorithm.’* The goodness of fit was evaluated by visual inspection of the residuals and their autocorrelation function and by the calculation of the statistical parameters Zxz,DurbinWatson parameter (DW), and “Ordinary Runs Test” (OR).19
TABLE I: Quantum Yields of the Different Radiative and Radiationless Decay Processes of the Singlet Manifold of PZNM, P3NM, and PCNZNM’
Experimental Results
a The relative error on Ofc amounts to h5-10% and that on @1scC + @1cC to f l - 2 % in isooctane and less than f l % in acetonitrile.
Emission Spectra. The emission spectra of P2NM, P3NM, and PCN2NM in isooctane, tetrahydrofuran, and acetonitrile consist of a broad structureless band attributed to an exciple^.^-^^^^ For P2NM and PCNZNM, the maximum of this band shifts from 305 and 380 nm in isooctane to 368 and 481 nm in tetrahydrofuran to 396 and 533 nm in acetronitrile, respectively. For P3NM, the emission maximum is situated at 323, 372, and 386 nm in isooctane, tetrahydrofuran, and acetonitrile, respectively. The emission spectra obtained in isooctane are similar to those obtained in isopentane.2+20 For PCN2NM in isooctaneand acetonitrile, an additional weak band due to the emission of the locally excited state2I is observed a t 290 nm.5~6 The emission is, however, characterized by a quantum yield of 0.006 and 0.001 in isooctane and acetonitrile, respectively. Therefore, one cannot exclude completely that this emission is also present in the emission spectra of P2NM or P3NM where it would, especially in isooctane, be hidden by overlap of the weak emission of the locally excited state and the intense exciplex emission. However, as the quantum yield of exciplex formation, determined from the quenching of the emission of the locally excited state, still amounts to 0.96 and 0.99 in isooctane and acetonitrile, respectively, this affects only a small fraction of the molecules and does not introduce a discrepancy of the experimental results exceeding the experimenJa1 error which amounts to f 5 % for CY and to f5-10% for the fluorescence quantum yields. To the extent that the quenching of the locally excited state is due to exciplex formation, the quantum yield for exciplex formation, @for, is given by
where Ofmand @m: correspond to the fluorescence quantum yield of the locally excited state in a model compound that is not able to form an intramolecular exciplex and to that in the bichromophore. In this framework, the quantum yield for exciplex fluorescence, Of,,is given by (3) The quantum yield for the radiationless decay of the exciplex from the exciplex to the ground state, @lce, or the locally excited triplet state, @ I S C ~ ,is given by
In eq 4, klcc and k l s c c correspond to the rate constant for internal conversion from the exciplex to the ground state and to the rate constant for intersystem crossing from the exciplex to the locally excited triplet state, respectively. The quantum yields of the different decay processes of PZNM, P3NM, and PCN2NM in isooctane, tetrahydrofuran, and acetonitrile, calculated on the basis of eqs 2-4, are reported in Table I. At room temperature, the decay of the exciplex fluorescence, determined using single-photon timing, could always be analyzed as a one-exponential decay. In acetonitrile, the intensity of the exciplex fluorescence of PCN2NM was too weak
P2NM (isooctane) P3NM (isooctane) PCNZNM (isooctane) P2NM (tetrahydrofuran) P3NM (tetrahydrofuran) PCNZNM (tetrahydrofuran) P2NM (acetonitrile) P3NM (acetonitrile) PCN2NM (acetonitrile)
@for
@re
@IS&+ @IC,
1.00 1.00 0.96 1.00 1.00 1 .OO 0.96 1.00 0.99
0.19 0.24 0.18 0.25 0.17 0.13 0.04 0.007 0.02
0.8 1 0.76 0.78 0.75 0.83 0.87 0.96 0.993 0.98
TABLE 11: Decay Times of the Exciplex for PZNM, P3NM, and PCNZNM in ns’ isopentane tetrahydrofuran acetonitrile
P2NM
P3NM
PCNZNM
13.4 9.1 4.0
14.7 17.4
6.51 20.2 b
b
a The absolute error is less than *0.3 ns. b The exciplex emission is too weak to determine the decay times.
TABLE 111: Values of the Rate Constants of the Exciplex Fluorescence (kfe)and of the Sum of Rate Constants of the Radiationless Exciplex Decay k1ce k w e for PZNM, P3NM, and PCNZNM in Solvents of Different Polarity
+
P2NM (isooctane) P3NM (isooctane) PCNZNM (isooctane) lN2NM (diethyl ether)’ 1Py3NM (diethyl ether)b P2NM (tetrahydrofuran) P3NM (tetrahydrofuran) PCNZNM (tetrahydrofuran) P2NM (acetonitrile) P3NM (acetonitrile) PCNZNM (acetonitrile)
1.4 x 1.7 x 2.7 x 1.6 x 7.0 X 2.6 x 9.8 X 6.4 X 1.0 x
107 107 107 107 lo6 107 lo6 lo6 107
6.0 x 107 5.1 x 107 1.26 X lo8 6.2 x 107 2.2 x 107 8.4 x 107 4.8 x 107 4.3 x 107 2.4 X lo*
c
c
c
c
2-(2-Naphthyl)-N,N-dimethylaminoethane.35 b 3 4 1 -Pyrenyl)-N,Ndimethylaminopr~pane.~~ Theexciplexemission is too weak todetcrmine the decay times.
to allow the determination of the fluorescence decay. These data suggest that the rising of the exciplex emission is always faster than the timeresolutionofthesetup (300ps). Under experimental conditions where the rate constant for the dissociation of the exciplex to the locally excited singlet state is much smaller than the sum of the rate constants of the different decay processes of the exciplex to the ground state or the locally excited triplet state, the observed decay time (Tables I1 and 111) corresponds to (kfe + ~ I S C+~k l c e ) - l . If this would not be the case, the observed decay time corresponds to a weighted average of (kf, + ~ I S C+ ~ k ~ c , ) - l and (kfm + kISCm + kICm)-l, where kfm, kISCm, and k I c m correspond to the rate constants for fluorescence, intersystem crossing, and internal conversion of the locally excited ~ t a t e . ~ * , ~ ~ For the model compounds toluene and p-cyanotoluene, (kf, + kISCm + k1cm)-l amounts in isopentane to 31 and 26.2 ns, re~pectively.~ For toluene, (kfm klscm+ k ~ c , ) - l amounts to 28 and 23 ns in tetrahydrofuran and acetonitrile, re~pectively.~ The emission spectra of PBr2NM in isopentane and tetrahydrofuran consist of a very weak emission with a maximum at 285 nm, due to the locally excited state, and a weak structureless band, with a maximum at 358 and 472 nm in isooctane and tetrahydrofuran, respectively. The solvent dependence of the emission maximum of this band suggests that this emission is due to an exciplex. In acetonitrile, no exciplex fluorescence is observed. In all solvents, the fluorescence intensity of the exciplex was too low to determine for this compound the exciplex decay time.
+
Van der Auweraer et al.
7180 The Journal of Physical Chemistry, Vol. 97, No. 28, 1993
TABLE I V Quantum Yields of the Different Radiative and Radiationless Processes That Occur After Excitation of PBr2NM isooctane tetrahydrofuran acetonitrile ~~
~
@re @re @fm @ISCm @lCm @for @ISCC
+ @IC0
a
@R:
0.016 0.012 0.004 0.370 0.000 0.637 0.624
0.002 0.000 0.002 0.180 0.000 0.818 0.818
0.023 0.020 0.003 0.271 0.000 0.726 0.701
total fluorescence quantum yield.
E(S*lm-S1,) equals the energydifferencebetween the FranckCondon excited state S*lmand the relaxed excited state S 1 m of the acceptor. The latter value corresponds to the 0-0 transition of the absorption or fluorescence spectrum of the acceptor. This energy difference is dissipated ( 1 0 4 s) within the time window of the experimental setup. E ( S * b - S o ) equals the energy difference between the FranckCondon ground state, reached after fluorescence from SI,, and the relaxed ground state S b of the acceptor. This quantity corresponds to the difference between the first moment of the fluorescence spectrum of the acceptor and the 0-0 transition of the fluorescence spectrum of the acceptor, which both are experimentally accessible. This amount of heat is also dissipated within the time window of the setup. E(S1, - T I ) equals the energy difference between the relaxed excited stateS1,and thelocally excited triplet state oftheacceptor. This energy difference will be completely dissipated within the time resolution of the setup. - A P ErePequals the difference between the enthalpy of exciplex formation and the repulsive potential in the ground state corresponding to the exciplex configuration (S*h- SO). This amount is calculated from the difference of the 0 transition of the absorption or fluorescence spectrum of the acceptor (or the fluorescence maximum of the locally excited state augmented with S * b - S h ) and the fluorescence maximum of the exciplex l(D+A-). This energy difference depends on the polarity of the solvent and will be completely dissipated within the time resolution of the setup. hue,, is the energy of a photon a t the excitation wavelength (266 nm). On the basis of Figure 1, eq 6, and the experimentally obtained value of a,it is possible to calculate the value of 6, the ratio of the rate constant of internal conversion of the exciplex to the sum of the rate constants of all the radiationless decay pathways of the exciplex. Plotting the amplitude of the optoacoustic signal versus the incident laser energy, Eab, one can determine the value of a by using eq 8. The reference 2-hydroxybenzophenone is characterized by a value of a equal to 1." Equation 8 is valid to the extent that the sample and the reference are characterized by an identical absorbance, which should be smaller than 0.1, at the excitation wavelength.
+
i
Erep
403
I
Figure 1. Kinetic and thermodynamic scheme of the photophysical processes of the bichromophoric compounds: klcmand k1scmindicate the rateconstant of respectively internal conversion and intersystemcrossing from the relaxed locally excited state SI,. krm is the rate constant of fluorescence from the relaxed locally excited state SI,, and k ~ a k, m , and &re indicate the rate constant of respectively internal conversion, intersystem crossing, and fluorescence from the exciplex. A P is the stabilization energy of the exciplex, and E,, is the energy content of the Franck-Condon ground state reached after the exciplex decay.
To calculate the quantum yields of the different radiative and radiationless processes, p-ethylbromobenzene has been used as a model compound. From the quantum yield of fluorescence of p-ethylbromobenzene, which amounts to 0.01 1, and the quantum yield of fluorescence of the local excited state of PBr2NM, which amounts to 0.004 and 0.003 in isooctane and tetrahydrofuran, respectively, the quantum yield of exciplex formation can be calculated. The quantum yields of the different processes are given in Table IV. Laser-lnducedO p t O a ~ t i Cspectnwcopy (LIOAS) of (+Phenyla-N,N-dimethylaminoalkanes.In the LIOAS experiments, the fraction a of the absorbed laser energy, Eab, that is converted into heat within the time constant of the experimental setup, Elh, is determined. The interpretation of the laser-induced optoacoustic experiments is based on the mechanistic and thermodynamic scheme presented in Figure 1. a = EtdEa,
with
(5)
Eth,sampl$abs,ref Eth,rcfEabs,samplc
-- ~ a a m p l e / ~ r e f
The time-resolved fluorescence experiments have indicated that the exciplex formation and decay times are much shorter than the response time of the experimental setup for the LIOAS. One can assume, therefore, that the absorbed energy that is not detected as prompt heat, (1 - a)hvex,,is used for fluorescence and/or stored in the locally excited triplet state. In Table V, the values of energy levels necessary to calculate j3 from the experimentally determined values of a are reported. Figure 2 shows the dependences of the amplitude of the optoacoustic signal on EO,the incident laser energy for PZNM in tetrahydrofuran and acetonitrile. The linearity between the amplitude of the optoacoustic signal and the energy of the incident laser pulse suggests that for the current energy range no biphotonic processes occur. Similar results were obtained for PZNM in isooctane and for P3NM, PCNZNM, and PBr2NM in solvents of different polarity. The experimentally determined values of a and the calculated values of j3 are given in Tables VI and VII, respectively. On the basis of the values of @ and the sum of the rate constants of the radiationless decay processes of the exciplex, klcc + ~ I S C ~ , the individual rate constants klcc and klsce can be calculated (Table VIII).
Solvent Polarity Influence on Radiationless Decay
The Journal of Physical Chemistry, Vol. 97, No. 28, 1993 7181
TABLE V: Energy Levels (in em-') of the Different Excited States Involved in the Photophysical Processes of the Intramolecular Exciplexes of the Investigated
w-Phenyl-a-N,N-dimethylaminoalkanes P2NM P3NM PCN2NM PBr2NM 35970 37170 E(SIm) - @om) 37310 37310 37590 37590 E(S*lIn)- E(Som) 37590 37590 1490 2270 E(S*om) 1940 1940 fl (isooctane) 32780 30300 26310 27940 fl (tetrahydrofuran) 27170 26880 20790 21190 fl (acetonitrile) 2520 25910 18760 b -AHo + Erep (isooctane) 4530 7010 9660 8230 -AHo + E,, (tetrahydro- 10140 10430 15180 14980 furan) -OH0 + E,, (acetonitrile) 12060 11400 17210 b E(T*i) 28920 28920 26320 26740 a Emission maximum of the exciplex in cm-l. The intensity of the fluorescence is too weak to determine the emission maximum.
25 20
-
I5
I
,?'E
./a
@ c y -
2
4
6
i
1'0
ri
I
-
1'4
1'8
2b'
r'8
Eo 01J) 70
-
Box) x)-
4030-
0
2
4
6
8
10
12
14
16
18
20
Eo 01J)
Figure 2. Dependence of the amplitude of the optoacoustic signal of
2-hydroxybenzophenone (-) and P2NM (- - -) upon the incident laser energy EO: (a, top) isooctane and (b, bottom) acetonitrile.
TABLE VI. Experimentally Obtained Values of a for PZNM, P3NM, PCNZNM, and PBr2NM in Solvents of Different Polarity isooctane tetrahydrofuran acetonitrile 0.81 f 0.04 0.99 f 0.05 P2NM 0.65 f 0.03 0.65 f 0.03 0.77 f 0.04 P3NM 0.50 f 0.03 0.80 f 0.04 0.94 f 0.05 PCN2NM 0.65 f 0.03 0.78 f 0.04 0.88 f 0.05 PBr2NM 0.70 f 0.03 Discussion Properties of the Exciplex. The emission spectra and their solvent dependence suggest that the broad structureless emission spectrum is due to an exciplex. A more quantitative analysis of this dependence suggests6J0~2~that for P2NM, PBr2NM, and PCN2NM the exciplex dipole moment and hence the electronic nature of the exciplex do not depend upon the solvent polarity and that the wave function of theexciplex has a major contribution from the charge-transfer state. This would, however, not exclude the competition between exciplex formation and the formation of a nonfluorescent solvent separated ion pair in polar solvents.25 The emission maximum of the intramolecular exciplexes of P2NM, 2-(pfluorophenyl)-N,.N-dimethylaminoethane(PF2NM),
TABLE VII: Calculated Values of B for LIOAS Experiments Using P2NM, P3NM, PCNZNM, and PBr2NM in Solvents of Different Polarity isooctane tetrahydrofuran acetonitrile P2NM 0.71 f 0.05 0.97 f 0.08 1.02 f 0.07 P3NM 0.64 f 0.05 0.71 0.05 0.49 f 0.04 PCN2NM 0.66 f 0.05 0.79 f 0.06 0.93 f 0.06 PBr2NM' 0.92 0.07 0.98 f 0.06 1.02 f 0.07 PBr2NMb 0.38 f 0.12 0.65 i 0.13 0.85 f 0.12 Assuming that no triplet decay occurs within the integration time of the detector. Assuming that 45% of the triplets has decayed within the integration time of the detector. ~~
~
TABLE VIE Calculated Values for the Rate Constants of Intersystem Crossing and Internal Conversion of the Exciplex Decay ktcm s-'
kIscs,
P2NM (isooctane) 4.3 x 107 1.8 x 107 P2NM (tetrahydrofuran) 2.5 X 106 8.1 x 107 P2NM (acetonitrile) 2.4 X lo8
