J . Phys. Chem. 1991, 95, 3491-3497 with our experimental value of 0.044 is therefore undoubtedly fortuitous, but clearly the calculation tends to support a much lower yield than the 0.18-0.19 values usually employed. A new absolute determination of the quantum yield is strongly suggested by these studies.
3491
Acknowledgment. This research was supported in part by the U S . Department of Energy, Division of Chemical Science, Office of Basic Energy Science. Registry No. Benzene, 71-43-2.
"Twisted" Intramolecular Charge-Transfer States in Supercooled Molecules: Structural Effects and Clustering with Polar Molecules Jerzy Herbich,*" Francisca PBrez Salgado, Rudolf P. H. Rettschnick,* Laboratory of Physical Chemistry, University of Amsterdam, Nieuwe Achtergracht 127, 1018 WS Amsterdam, The Netherlands
Zbigniew R. Grabowski, Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44, 01 -244 Warsaw, Poland
and Hanna W6jtowicz Chemistry Department, Adam Mickiewicz University, Crunwaldzka 6, 60-780 Poznan, Poland (Received: February 1, 1990; In Final Form: November 28, 1990)
This paper presents a study of laser-induced fluorescence and excitation spectra of jet-cooled 4-(dimethylamino)benzonitriles and 4-(dialkylamino)pyrimidinesand their complexes with small polar molecules (methanol and acetonitrile). Various ground-state forms (monomers, dimers, and larger clusters) have been observed. The bare molecules (except 111) do not exhibit any distinct long-wave fluorescence that can be assigned to a twisted intramolecular charge-transfer (TICT) state. Microsolvation of pretwisted compounds by small polar molecules gives rise to a long-wave fluorescence, which is interpreted as emission from a TICT state.
Introduction Numerous electron donor-acceptor molecules (D-A), with D and A linked by a single bond, react upon excitation with the formation of a highly polar state with a mutually perpendicular conformation of the D+ and A- subunits (Figure 1). The existence of such a 'twisted" intramolecular charge-transfer (TICT) state seems to be well proved by spectroscopic, thermodynamic, and quantum chemical evidence.I** Their prospective role in photochemistry, photobiology, and photochemical applications is also ~ignalled.~ Most investigations have been carried out in liquid solutions. The role of the solvent in the formation of the TICT state is one of the most important questions, and many recent investigations have focused on this problem.IAH Steady-state and timeresolved studies on a picosecond time scale indicate that these molecules interact with polar media: the energies of initial, intermediate, and final states can be dramatically affected by the solvent. The kinetics are affected by the solvent as well. The compound most widely used for studying this process is p(dimethy1amino)benzonitrile (I) along with its derivatives, e.g., I1 and 111 (Figure 2). Their dual fluorescence in solution fits well to the TICT model. The 'blue" emission (b) is assigned to the quasi-planar locally excited B* state, and the 'red" emission (a) to the A*(TICT) state (Figure 1). Varma et a1.6 claim exciplex formation to be responsible for the long-wave emission. The evidence contradicting such an interpretation was supplied by Suppan,' who-along with other investigators-found as a rule the solvation to be due to nonspecific, general (polar solvent-polar solute) interactions. Cazeau-Dubroca et aI.* ascribe the long-wave fluorescence to the hydrogen-bonded complexes of I with ubiquitous traces of water that are supposed to be present in the ground state. These com-
plexes are hydrogen bonded at the amino group, which is said to be already 'pretwisted" in the ground state. Evidence against this assignment was presented in several paper^.^,'^ The photophysical properties of 4-(dia1kylamino)pyrimidines (IV-VII) (Figure 2) have been investigated in solution.s The TICT-state formation is favored by hydrogen bonding or metal ion coordination at one of the pyrimidine ring nitrogen atoms. It is to be emphasized that this H-bonded complex formation does not support the claims of Cazeau-Dubroca: (i) the hydrogen bond (1) (a) Grabowski, Z. R.; Rotkiewicz, K.; Siemiarczuk, A.; Cowley, D. J.; Baumann, W. N o w . J. Chim. 1979.3,443. (b) Grabowski, Z. R.; Dobkowski, J. Pure Appl. Chem. 1983, 55, 245. (c) RulliCe, C.; Grabowski, Z. R.; Dobkowski, J. Chem. Phys. Le??.1987,137,408. (d) Rotkiewicz, K. Specrrochim. Acra 1986,42A, 575. (2) Lippert, E.; Rettig, W.; Bonazic-Kouttcky, V.; Heisel, F.; Mieht, J. A. Photophysics of Internal Twisting. In: Ado. Chem. Phys. 1987, 68, 1. (3) (a) Rettig, W. Angew. Chem., Inf. Ed. Engl. 1986, 25, 971. (b) Cowley, D. J. Nurure 1986,319, 14. (c) Stolarczyk, L.; Piela, L. Chem. Phys. 1984, 85, 451. (d) Rettig, W. Appl. Phys. 1988, B45, 145. (4) (a) Wang, Y., McAuliffe, M.; Novak, F.; Ewnthal, K. B. J . Phys. Chem. 1981,85,3376. (b) Wang, Y.; Eisenthal, K. B. J. Phys. Chem. 1982, 77,6076. (c) Hicks, J. M.; Vandersall, M. T.; Babarogic, Z.; Eisenthal, K. B. Chem. Phys. Le??.1985, 116, 18. (d) Hicks, J. M.; Vandersall, M. T.; Sitzmann, E. V.; Eisenthal, K. B. Chem. Phys. Le??.1987, 135, 413. ( 5 ) Herbich, J.; Grabowski, Z. R.; W6jtowicz, H.; Golankiewicz. K. J . Phys. Chem. 1989, 93, 3439. (6) (a) Visser, R. J.; Weisenborn, P. C. M.; Konijnenberg, J.; Huizer, B. H.; Varma, C. A. G. 0. J. fhotochem. 1986, 32, 217. (b) Visser, R. J.; Weisenborn, P. C. M.; Varma, C. A. G. 0.; de Haas, M. P.; Warman, J. M. Chem. Phys. Leu. 1984, 104, 38. (c) Visscr, R. J.; Varma, C. A. G. 0.; Konijnenberg. J.; Bergwerf, P. J. J . Chem. Soc.,Furuduy Trans. 2 1983,79, 347. (7) Suppan, P. Chem. Phys. Le??.,1986,128, 160. (8) (a) Cazeau-Dubroca, C.; Ait-Lyazidi, S.; Cambou, P.; Peirigua, A,; Cazeau, Ph.; Pesquer, M. J. Phys. Chem. 1989,93,2347. (b) Cazeau-Dub
roca, C.; Ait-Lyazidi, S.;Nouchi, G.; Peirigua, A.; Cazeau, Ph. Now. J. Chim. 1986, IO, 337.
(9) Pilloud, D.; Suppan, P.; van Haelst, L. Chem. Phys. Len. 1987, 137,
'Present address: Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44, 01 -244 Warsaw, Poland.
130.
(IO) RuIliEre, C.; Dobkowski, J.; Grabowski, Z. R. J . Phys. Chem., in press.
0022-365419 1 12095-349 1~-S02.50lO .~, . 0 1991 American Chemical Society I
I - -
~
~
~~
3492 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991
m-
P
c
0
- 1
tf-90
Figure 1. Schematic model for TICT formation along the reaction path (see text).
c
C
N
N
C I N
II
111
111
I
V
IV Et,,Et
Et.
N
Et
s. 1
N
N y " 3
L-' N
VI
Herbich et al.
VI1
Figure 2. Formulas of the studied compounds: 4-(dimethylamino) benzonitrile (I) and 4-(dimethylamino)pyrimidine(IV) and their derivatives.
is not at the donor (-NR2) but on the acceptor moiety (pyrimidine ring); (ii) it does not correspond to any "pretwist" of the amino group; on the contrary, we can infers it would be more coplanar with the ring in the ground state than the bare molecule. Laser-induced fluorescence (LIF) studies and two-color timeof-flight spectroscopy in combination with the free-jet-expansion technique provide information about structural changes in the excited state. A few results concerning studies in a supersonic jet have been recently reported. For isolated IIIIIa a red-shifted emission was found, which was also observed in a thermalized vapor"b*c and in nonpolar solvents. Isolated 9,9'-bianthrylI2 exhibits primary fluorescence, but microsolvation by probably just one molecule of acetone gives rise to a red-shifted emission. Complexes of higher stoichiometry further stabilize t h e CT state. Complexation of I with various polar solvents does not yield any evidence of TICT emission.I3 Comparison with the behavior (11) (a) Kobayashi, T.; Futakami, M.;Kajimoto, 0. Chem. Phys. Lett. 1987, 141, 450. (b) Rotkiewicz, K.; Rubaszewska, W. J. Lumin. 1982, 27, 221. (c) Bischof, H.; Baumann, W.; Detzer, N.; Rotkiewicz, K. Chem. Phys. Lett. IWS, 116, 180. (12) Kajimoto, 0.;Yamasaki, K.; Arita, K.; Hara, K. Chem. Phys. Lett. 1986, 125, 184. (13) (a) Kobayashi, T.; Futakami, M.; Kajimoto, 0. Chem. Phys. Lett. 1986,130,63. (b) Gibson. E.M.,Jones, A. C.; Phillips, D. Chem. Phys. Lett. 1987, 136, 454. (c) Gibson, E. M.;Jones, A. C.; Taylor, A. G.; Bouwman, W. G.; Phillips, D.; Sandell, J. J . Phys. Chem. 1988, 92, 5449. (d) Peng, L.
W.; Dantus, M.; Zewail, A. H.; Kemnitz, K.; Hicks, J. M.; Eisenthal, K. B. J . Phys. Chem. 1987,91,6162. (e) Warren, J. A.; Bernstein, E.R.; Seeman, J. 1. J . Chem. Phys. 1988, 88, 871. (f) Grassian, V. H.; Warren, J. A,; Bernstein, E. R.; Secor, H. V. J. Phys. Chem. 1989, 90, 3994.
Figure 3. Supersonic jet apparatus. SHG: second harmonic generation. P: prism. BS: beam splitter. RM: radiometer. REC: chart recorder. L1: quartz lens,f= 330 mm. L2: combination of two quartz lenses, f = 110 mm and f = 120 mm. LR: "solvent" reservoir. V: valve. S: oven containing cell with sample. N: nozzle. 0:laser beam crossing supersonic jet. PUMP: pumping system. M I : mirror. M: monochromator. PM: photomultiplier. BOXCAR: boxcar integrator. PC: computer. LB: light shielding box.
of 111 seems to confirm that the charge transfer can be promoted by the degree of deviation of the amino group from coplanarity in the ground state. August et al.14aclaim to have found TICT emission in 44dialky1amino)benzoic esters, but this is refuted by Howell et who assign the red-shifted emission to excimeric emission from dimers. Their assignment is based on experiments performed with different concentrations of the compounds; no experiments were performed with polar solvents in the jet, but in solution TICT emission was found in polar surroundings. In the present study we report upon vapor-phase and free-jet experiments of compounds I-VII. These experiments have been performed to study the conditions that enable the occurrence of charge transfer in the excited state of supercooled molecules and isolated small clusters. The conformation of the ground-state molecule, being quasi-planar or pretwisted, and the use of different polar solvent molecules as complexing partners have been our main variables in studying the TICT phenomenon.
Experimental Section Materials: 4-(Dimethy1amino)benzonitrile (I, Aldrich, 98%) was used without further purification. Its ortho-methylated derivatives (I1 and I11 were supplied by Dr.K. Rotkiewicz) and the 4-(dialky1amino)pyrimidines (IV-VII) were synthesized and purified as described p r e v i o u ~ l y . ~ Spectroscopic J~ grade solvents (Merck) were used. The compounds were analyzed by mass spectrometry and NMR spectroscopy before and after being heated in the injector of the supersonic jet apparatus. Neither chemical nor photochemical reaction products were found. Vapor-phase experiments: The luminescence and excitation spectra in a thermalized vapor (temperature range 298-450 K) were recorded by means of a SPEX 1608 double spectrometer and SPEX DMl B spectroscopy coordinator. Absorption spectra were measured with a Cary 14 instrument. All compounds, after heating up to 135 OC, did not show any fluorescent decomposition products upon excitation a t X > 265 nm. Supersonic beam investigations: A schematic diagram of the apparatus for measuring the fluorescence excitation and dispersed fluorescence spectra is shown in Figure 3. The expansion chamber was evacuated by a Roots pump (500 m3/h), backed by a rotary fore pump (Edwards E2M40,40 m3/h). A continuous supersonic jet was produced by expanding helium through the nozzle of a (14) (a) August, J.; Palmer, T. F.; Simons, J. P.; Jouvet, Ch.; Rettig, W.
Chem. Phys. Lett. 1988, 145, 273. (b) Howell, R.; Jones, A. C.; Taylor, A. G.; Phillips, D. Chem. Phys. Lett. 1989, 163, 282. ( 1 5) Rotkiewicz, K.; Grabowski, Z. R.; Krbwczynski, A.; Kiihnle, W. J . Lumin. 1976. 12/13, 877.
I
'Twisted" Intramolecular CT States in Supercooled Molecules Et*
30
The Journal of Physical Chemistry, Vol. 95, No. 9, I991 3493
,Et
4 / 3 0 ,
300 nm
302
wrvsmth
A Am-!
20
?o L
m
-
l
Figure 4. LIF excitation (a, b) and dispersed fluorescence (c) spectra of VII. The higher energy part of the excitation spectra corresponds to the monomer, the lower energy part to the dimer. The intensity scales for both regions (308-310 and 299-304 nm) are identical. (a) The temperature of the sample, Twmplc = 60 OC. (b) Tumple= 90 OC. (c) Fluorescence spectra, resulting from excitation in the bands indicated by an asterisk.
heated injector. The nozzle diameter was 100 pm. The background pressure in the expansion chamber was 0.1 mbar at 3 bar of helium pressure. The compounds were heated to a temperature of 50-90 OC in a glass vessel, situated inside the injector close to the nozzle. The temperature of the nozzle was kept 10 O C higher to prevent condensation. The complexing solvent was introduced into the expansion chamber by passing the carrier gas over the solvent first, which was kept in a reservoir. The partial pressure of the solvent was varied by changing the temperature of the reservoir. A pulsed dye laser (Lumonics Hyper-DYE-300) was used, pumped at 308 nm (XeCI) with an excimer laser (Lumonics Hyper-Ex-460) operating at 80 Hz. The dye laser output was frequency doubled (Hypertrack-1 000) with KDP crystals. The spectral bandwidth was 0.15 cm-I at a pulse duration of 9 ns. The intensity of the UV beam was monitored by a calibrated EG&G radiometer and was between 10 and 100 mW. A hollow cathode discharge lamp with neon was used for calibration of the wavelength of the dye laser. Measurements were performed at distances between 10 and 40 nozzle diameters downstream of the nozzle. Laser-induced fluorescence (LIF) spectra were recorded from 270 to 325 nm. Dispersed fluorescence from the supersonic jet was collected with right-angle geometry by a lens system, which imaged onto the slit of a grating monochromator (Zeiss M20, used at a spectral resolution of 10-25 nm). To avoid reflecting stray light, the injector was blackened. The signals from the photomultiplier (EM1 S20,9558QA) were fed into a gated integrator (SRS 250), which was connected to a chart recorder. The emission spectra were corrected for the wavelength dependence of the photomultiplier and the monochromator. The LIF excitation spectra were corrected for the intensity of the UV beam, unless mentioned otherwise. The measured peak positions were determined with an accuracy of 0.3 cm-'. Results and Discussion Vapor Phase. Experiments in the vapor phase show the absence of TICT luminescence of all studied compounds, except for 111. The absorption and fluorescence spectra in a vapor, recorded upon excitation between 260 and 300 nm, are similar to those in non-
polar solvent^.^-^^ Spectral positions (and bandwidths) are summarized in Table I. Bare Molecules and Sey-Complexes in the Jet. LIF excitation spectra of I1 and IV-VI1 in the supersonic jet were recorded in the wavelength range 270-325 nm. For each molecule (except V) emission of the monomeric form was detected. For the pyrimidines, IV-VII, also dimers and larger clusters were found, the latter being associated with the underlying continua in the excitation spectra. The assignment of monomers and dimers is based on the intensity ratios of intense peaks (relative peak heights in the excitation spectra) at different concentrations of the chromophore. The concentration was varied by changing the temperature of the injection system. At low concentrations of the chromophore where the dimer concentration is small with respect to the monomer concentration, the peaks ascribed to the dimer change their intensity approximately quadratically with respect to those ascribed to the monomer when the temperature of the injector is varied. As an example, the excitation spectra of both monomer and dimer of VI1 are shown in Figure 4 for a low and high dimer concentration, respectively. We have measured the rotational contours of several intense peaks in the excitation spectra. It was found that the width of the peaks assigned to the dimers is smaller than the width of the corresponding monomer peaks, as it should be in view of the larger moments of inertia. All monomer spectra become severely congested at about 500 cm-' above the origin. At lower frequencies many vibronic transitions can be observed, which is characteristic for large-amplitude motions. Similar patterns have been analyzed for alkyltoluenes," f l u o r ~ t o l u e n e s , ' ~m-cresol,'* ~ ' ~ ~ * ~ toluidines,'" and (16) Rotkiewicz, K.; Grellmann, K. H.; Grabowski, 2.R. Chem. Phys. 19, 315. (17) (a) Breen, P. J.; Warren, J. A.; Bernstein, E. R.;Seeman, J. I. J . Chem. Phys. 1987, 87, 1917. (b) Breen, P. J.; Warren, J. A.; Bernstein, E. R.; Seeman, J. I. J. Chem. Phys. 1987, 87, 1927. (c) Breen, P. J.; Bernstein, E. R.; Seeman, J. 1. J. Chem. Phys. 1987,87, 3269. (18) Moss, D. B.; Parmenter, C. S.;Ewing, G. E. J. Chem. Phys. 1987. 86, 51. (19) (a) Okuyama, K.;Mikami, N.; Ito, M. J . Phys. Chem. 1985, 89, 5617. (b) Ito, M. J . Phys. Chem. 1987,91, 517. (c) Mizuno, H.;Okuyama. K.; Ebata, T.; Ito, M. J . Phys. Chem. 1987, 91, 5589. (d) Okuyama, K.; Mikami, N.; Ito, M.Laser Chem. 1987, 7, 197. h i t . 1973,
3494 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991
Herbich et al.
TABLE I: Positions of the Fluorescence, Excitation, and Absorption Maxima in a Vapor and in Liquid n-Heptane ( A V ~ , ~Half-Width : of the Fluorescence Band) vapor phase ( T = 348 K) n-heptane (T= 273 K)
fluorescence, cm-l
excitation, cm-’
II
29 900 (1200)
35 100 (1200)
lllb
26 000 (1300)
33 600 (1200)
IV
30000 (k200)
33 800-34 350
V
29 750 (i200)
34 350 (i300)
VI
30 350 (i200)
34 500 (1300)
VI1
28 800 (i200)
34 250 (1300)
compd
absorption,” cm-l
35 700 (i200) 41 650 (i200) -35000 (sh) 40 800 (f200) 35 700 (1200) 41 250 (1200) 35 700 (1200) 40 150 (f200)
fluorescence AvIl2, cm-I
absorption,” cm-l
27 200 (f200) 5500 25 700 (f200) 5300 30 500 (1200) 4600 29 400 (i200) 4900 30 200 (i200) 4250 27 400 (i200) 5400
35600 (1100) 44400 (f200) 33 000 (1200) 43 500 (1200) 35250 (1100) 41 000 (1100) 35 700 (1250) 39700 (i100) 34 700 (k300) 40300 (1100) 35000 (i300) 39500 (i100)
#The position of the maxima of the first and second absorption bands. bThe compound shows TICT emission in the vapor phase.llbvc TABLE II: Electronic Origin (v,
compd I II IV
V VI VI1
in cm-’) of Various Species in the Jet
monomer 32 246.3 32651.7 33 023.8 32971.2 32950.8
dimer
complexes with methanol I:n (n > 1)
1:l
a
32 409
C
d
b d
32 333.7 32436.9 32 357.9 32268.1
33491.5 d 33 278.0 d
33 686.5 d 32 808.6 d
32505.5
32640.5
a No definite origin can be presented for the dimer or for larger self complexes; the excitation spectrum is structureless in the investigated region of 320-280 nm.Iu *Fluorescencefrom these complexes appears to be too weak to be observed (cf. ref 13c). CTheexcitation spectrum exhibits a very weak structureless background in the region of 315-290 nm (31 700-34500 cm-I). The corresponding fluorescence is somewhat different from that of monomer. It is not certain that the emission originates from a dimer of compound I1 since the dependence on the concentration was not investigated. dExcitation spectra are structureless. The excitation spectra were measured over the range 32 250-34 500 cm-I.
(dialkylamino)benzonitrileslkfin terms of internal rotation of the methyl groups, internal rotation (torsion) of the dialkylamino group, and inversion. The precise nature of the vibrations active in the studied compounds is not yet clear enough, but it is most probably associated with inversion and torsion of the dialkylamino group. The lowest energy transitions of the spectra of dimer and monomer are assigned to the 000 transition, in spite of their weakness. The relative intensity of the first peaks in each region does not change upon variation of the cooling conditions, from which we exclude the possibility of hot bands. We think that the low intensity of the first vibronic band is due to a displacement of the position of the dialkylamino group along the torsional coordinate in the excited state relative to that in the ground state. For compound I, which is similar to the compounds IV-VI1 with respect to the torsional motion of the dialkylamino group, it has been shown by Grassian et al.”‘ that the low intensity of the 0; band can be ascribed to a change of equilibrium position of this torsional coordinate upon electronic excitation by about 30-40’. The positions of the 0; transitions of monomers and dimers are collected in Table 11. The vibrational structure of the excitation spectra of I1 and VI1 is rich compared with that of the related compounds I and VI. We do not think that this difference is to be attributed completely to the vibrational motions of the extra methyl group since this methyl group, in close proximity to the dialkylamino group, causes a distortion of the potential energy surface. The steric repulsion produced by the methyl group causes a “pretwist” of the molecule in the ground state and a lowering of the barrier to torsion in the excited electronic state (see Figure 5 ) . The resulting reduction of the frequency of the torsional mode will probably contribute to the enhanced complexity of the vibrational structure in the excitation spectra of 11 and VII. Substitution of another methyl group in the aromatic ring leads to an increase in the twist angle of the ground-state equilibrium conformation and a further reduction, or even suppression, of the potential barrier in the upper electronic state. This must be the
Figure 5. Schematic representation of the potential energy of ground and excited states as a function of the twist angle of the dialkylaminogroup with respect to the aromatic ring. The left-hand side illustrates the situation for the (quasi-)planar compounds I, IV, and VI, and the right-hand side shows the influence of the steric repulsion caused by the ortho-methyl group in the compounds 11, V, and VII. It has been assumed that the steric repulsion (represented by the shaded area) is similar in both electronic states.
reason that the dispersed emission from Ill is located at markedly longer wavelengths than those from I and 11. The fluorescence from isolated molecules 111, measured in the hot vapor phase and in a supersonic expansion, was assigned as TICT state emission.” The LIF excitation spectrum is diffuse, even under jet-cooled conditions. A possible explanation of the diffuseness is the broadening of individual lines as a result of a short lifetime of the optically excited state caused by rapid vibrational redistribution (IVR) preceding the TICT state formation. We have not observed any separate peaks in the emission spectra of the investigated compounds because of the used monochromator
'Twisted" Intramolecular CT States in Supercooled Molecules
The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 3495
b
>
c 5
310
f
C
\exc -270 nm
308
308
20
25
30
h 302
ml
wmebngth Inm)
20
25
30 x 103
wavenumber km-7
Figure 6. Effect of increasing the excess excitation energy on the dis-
persed emission of compound 11. A broadening of the band and a red shift in the position of the maximum are observed when exciting at higher energy.
bandwidths (10-25 nm). The dispersed fluorescence of the monomers, recorded upon excitation in the $ region, has its maximum intensity at 340-345 nm and is attributed to the primary excited state B*. The fluorescence spectra of the dimers are surprisingly narrow. The maximum intensity of the dimer emission is situated at 330-335 nm. It should be stressed that the emission maxima have no direct relation to the 0 : positions, since the shape of the fluorescence bands is different for monomers and dimers. For the larger clusters, the fluorescence spectra are broader than those of the monomeric form; their maximum is centered at about 355 nm. We have examined the shape and position of the fluorescence spectrum of I1 as a function of the excitation energy. The molecule was excited up to 4500 cm-l above the 0 : transition. As can be seen in Figure 6, the maximum shifts to the red and the half-width of the spectrum markedly increases with excess excitation energy. However, no distinct shoulder is found in the expected region for TICT emission. In this respect the behavior of I1 is comparable with that of I. No TICT emission was observed from isolated molecules I upon excitation up to 3400 cm-I above the electronic origin.'3d Increasing the excitation energy of isolated molecules usually gives rise to a red shift and broadening of the dispersed emission. Generally, the vibrational frequencies are smaller in the excited electronic state, and consequently the emission bands shift to the red with increasing excess energy. The diffuseness of the spectrum is due to the diversity of these frequency shifts since, as a result of IVR, the emission originates from a number of different vibronic levels. The red shift and broadening of the emission from 11, observed with increasing excess energy, is relatively strong in comparison with the magnitude of these effects for molecules of similar size. Such a different behavior can occur if the emitting vibronic states contain vibrational modes with strongly reduced frequencies in the upper electronic state. The torsional mode of the dialkylamino group could act as such a vibration. If the emitting vibronic states would not be produced directly by optical excitation, they might be formed by IVR. Comparison of I1 with 111 where the TICT fluorescence is evident leads us to the conclusion that the TICT state is not markedly populated in IT. The excess energy might be insufficient, or the IVR processes are too slow to cross the barrier to the TICT state during the lifetime of the excited state.
Figure 7. LIF excitation spectra (left side) and dispersed fluorescence spectra of VII-(CH,CN), complexes. The excitation position is indicated by an asterisk: (a) I:l complex, pCHCN = 0.1% of the stagnation pres= 0.56; (c) higher complexes, ~ C H , C N = sure; (b) 1:2 complex, pCH,CN 0.5%. The peaks belong to the bare molecule.
Microsolvation Effects. Solutesolvent complexes of different stoichiometry were produced by varying the partial pressure of the solvent over a wide range. The solvent partial pressure was varied from about 0.05% to 3% of the helium flow passing through the solvent reservoir. Spectra of some of the complexes of VI1 with acetonitrile are presented in Figure 7 . The LIF excitation spectrum shows the presence of two complexes of which the first (Le., lowest energy) peaks are red shifted by 595 (1:l complex) and 720 cm-l (probably a 1:2complex) with respect to the 0; band of the bare molecule (located at 32950.8 cm-' ?* 303.40 nm). The assignment of the stoichiometry is based on the influence of the acetonitrile concentration on the relative intensities. The band system located around 308 nm is the first new feature that appears in the excitation spectrum upon addition of small amounts of acetonitrile to the carrier gas, and therefore we assign it to a 1:l complex. When the acetonitrile concentration is raised, a second band system appears around 310 nm. Its intensity increases at the expense of the first mentioned band system upon a further enhancement of the acetonitrile concentration. This behavior suggests that the peaks near 310 nm belong to a 1:2 complex. No other (structured) band systems were observed in the LIF excitation spectra of VI1 complexed with acetonitrile. From the large red shift we can deduce that the complexation energy is much higher for the excited state than for the ground state. The strong red shifts indicate a stabilization of the highly polar excited state. It is in agreement with INDO/S calculations showing that the primary excited state is more polar than the ground state.s The observed low-frequency band structure of both complexes is completely different from that of the bare molecule. This indicates that the diethylamino moiety has been significantly perturbed by complexation. The spectral positions of both complexes indicate that the second acetonitrile molecule provides an increased stabilization of the excited state. This increased stabilization is not clearly demonstrated by the fluorescence spectra because of the low resolution of the detection. Fluorescence recorded upon excitation of either of these complexes consists of one band with a maximum at 330 nm and a fwhm of 5000 cm-l, corresponding to emission from the primary excited state. A long-wave tail is observed, however with low intensity. If the background underlying the excitation spectra of both complexes is excited, a broad and strongly red-shifted emission band is observed with its maximum at about 400 nm (see the emission band displayed in Figure 7c). The intensity of this background increases with the acetonitrile concentration, which indicates that the background belongs to larger complexes (i.e., VII-(CH3CN), with n > 2). The emission spectrum recorded upon excitation
3496 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 Et,
Herbich et al.
,Et
Y
n
n
f
/
I
methono1 pressure (&or)
A 20
25
Joxdcm-'
20
25
30~10~cm-'
20
25
30x1o3cm-'
Figure 8. Three parts of the LIF excitation spectrum of various complexes of VI with methanol. The partial pressure of methanol is 0.1% (I) and I % (11) of the stagnation pressure. The electronic origins of the various complexes are indicated by A-D. In the central part of the LIF
excitation spectrum an underlying continuum is observed, which is assigned to higher complexes. (The LIF excitation spectra are not corrected for dye laser intensity.) The intensity scales for the regions A-B, C, and D are identical. Three typical emission spectra are shown in 111. The excitation position is indicated by an asterisk in I or 11.
in the investigated region (306.5-298 nm) is broad and its maximum intensity is located between 395 and 410 nm, indicating a charge-transfer emission. The spectral position of the emission maximum depends slightly on the excitation wavelength, indicating that the continuum in the excitation spectrum is caused by the presence of different complexes with overlapping spectra. The only distinct features that appear in the excitation spectrum of this red-shifted emission band belong to the monomer (Figure 7c and Figure 4). The intensities of the monomer peaks decrease in favor of the intensity of the continuum upon increasing the concentration of acetonitrile. The Occurrence of structured as well as structureless excitation spectra of the complexes VII-(CH3CN), can be explained in the following way: The B* state is strongly stabilized by solvation with 1 and 2 molecules of acetonitrile. The TICT state, being more polar than the B* state, will be stabilized even to a greater extent. The addition of another acetonitrile molecule will lower the barrier still more, so that the TICT state can be formed and red-shifted emission is observed. The absence of any vibrational structure in the excitation spectra of the larger complexes ( n > 2) may be caused by the short lifetime of the excited Franck-Condon state, due to a rapid energy flow toward the twist motion. A critical number of solvent molecules seems to be necessary to allow the process to occur within the lifetime of the excited state. Also a protic solvent (CH30H) was used to investigate the complex formation with the compounds I1 and IV-VII. We will consider planar and nonplanar molecules successively. The LIF excitation spectra of complexes of the planar compounds IV and VI with C H 3 0 H show a rich vibrational structure. The excitation spectrum of VI complexed with methanol (Figure 8) exhibits four different regions, all of them giving rise to b fluorescence. The spectral origins are either blue (D) or red shifted (A-C) with respect to the monomer. The peak intensities were measured at different partial pressures of methanol; in the case of complex C the data include the intensity of the underlying continuum. Several peaks in region D belong to the bare molecule, and consequently only a few peaks in this spectral region can be used as a measure of the relative concentration of complex D. Figure 9 shows the relative intensities plotted versus the partial pressure of methanol. The fluorescence quantum yields of the various species are unknown, and therefore the intensities are normalized at an arbitrarily chosen value of the methanol pressure. The relative intensities behave differently upon changes of the methanol concentration, indicating that complexes of different stoichiometry and/or different conformers are formed. The peaks in region D are the first new features that appear in the spectrum
Normalized intensities in the LIF excitation spectra of different complexes of VI with methanol (see Figure 8) as a function of the partial pressure of methanol. The emission was detected at 340 nm. F l g u r e 9.
upon addition of methanol to the helium flow, and that is why we assign the complex peaks in region D to a 1:l complex. Initially, the peak intensities belonging to this complex increase linearly with the methanol concentration. A further growth of the complex concentration seems to be hampered by the formation of the other complexes that probably contain more than one methanol molecule. At high methanol concentrations a continuous background emerges red shifted with respect to the monomer (most evidently in region C). This continuum is probably due to complexes of VI with several CHJOH molecules. The intensity maximum of the complex emission bands displayed in Figure 8 is blue shifted by about 10 nm from that of the bare molecule. This is no indication of the spectral positions of the 01 bands since the emission band of the bare molecule is broader than those of the complexes. All complexes exhibit a frequency structure dissimilar to that of the bare molecule: different Franck-Condon envelopes and smaller frequency spacings between the vibronic bands. For all complexes the first vibronic band, which is probably the 0 : transition, is quite intense. This suggests that in the various complexes the displacement of the equilibrium conformation along the torsional coordinate, upon electronic excitation, is less than in the bare molecule. The LIF excitation spectrum of IV, complexed with methanol, shows two electronic origins that are both strongly blue shifted with respect to the bare molecule (see Table 11). The band system with the smallest blue shift (+468 cm-I) is assigned to a 1:l complex for reasons similar to those for the case of the complex VI-CH30H. The emission from both complexes originates from the primary excited state. Contrary to the spectra of the complexes of IV and VI, the LIF excitation spectra of the complexes of the nonplanar compounds 11, V, and VI1 with methanol are completely structureless. Excitation of these complexes results in a red-shifted charge-transfer emission, as suggested by Rettig.3 As an example, Figure 10 shows the dispersed emission obtained from a supersonic expansion of VII, seeded in helium containing different amounts of methanol. The optical excitation was achieved at 302.36 nm, in a vibronic band of the bare molecule and in the underlying continuum, belonging to the complexes with methanol. The relative intensity of this continuum increases with increasing methanol concentration, and the relative contributions of both the molecule and the complexes with methanol are reflected in the emission spectra, represented in Figure 10. It is not probable that the absence of any vibrational structure in the excitation spectra of the methanol complexes would be due to the Occurrence of many ground-state complexes with different geometries. The spectra of the lower complexes with acetonitrile as well as the complexes of IV and VI with methanol exhibit a nicely resolved vibrational structure (cf. Figures 7 and 8). Most likely the lack of any vibrational structure in the spectra of these methanol complexes is due to the absence of a Franck-Condon
"Twisted" Intramolecular CT States in Supercooled Molecules
The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 3497 TABLE 111: R"ce (+) or Absence (-) of a Distinct LongWave (Presumably TICT') Fluorescence in Isolated Molecules and Complexes with Methanol (in the Jet) isolated complexes with molecule methanol remarks a, b, also ref 13
-
-
a
-
+
a
-
+
a, b
Me
II N
e
4
MI
V
20
25
30
103,d Figure 10. Dispersed fluorescence spectra of VI1 and complexes = 302.36 nm, cf. Figure 4): (a) no methanol added Vll-(CH30H), (X, to the helium flow: (b) partial pressure of methanol pCH,oH = 0.1% of the stagnation pressure; (c) PCH,OH = 0.6% (d) PCH,OH = 3%. minimum at the hypersurface of the excited state. A disappearance of the barrier or a rapid IVR seems to produce the structurally very different charge-transfer (TICT) state. Hydrogen bonding in the pyrimidine ring is expected to contribute to the stabilization of the charge-transfer state. We must conclude that this is indeed what happens: the addition of just one methanol molecule is sufficient to accelerate the TICT state formation.
Conclusions Several studies') have been performed concerning the properties of jet-cooled 4-(dimethylamino)benzonitrile (molecule I) and its isolated solutesolvent complexes. The absence of any long-wave (TICT) fluorescence from the isolated complexes indicates that the necessary condition for the appearance of that emission is not the definite chemical nature of the complex (exciplex,6 H-bonded complex*). The present study suggests that the height of the potential barrier to torsion of the amino group with respect to the aromatic plane is an essential criterion for the formation of a TICT state in this type of compound. The barrier is higher in the 'planar" molecules I, IV, and VI than in the corresponding nonplanar molecules 11, 111, V, and VII. Varying the excitation energy of I1 and comparing the emission spectra with those reported for I and 111 lead us to the conclusion that (in compound 11) either the barrier to torsion is higher than 4500 cm-' or the vibrational redistribution is too slow to populate the TICT state sufficiently within the lifetime of the excited state. The effect of microsolvation by C H 3 0 H and CH3CN was examined. A qualitative summary of the results is given in Table 111. The planar and nonplanar compounds behave differently with respect to microsolvation. Complexation of the planar molecules does not give rise to emission from a stabilized (20) (a) Lim, E. C . J . Phys. Chem. 1986, 90. 6770. (b) Hiraya, A.; Achiba, Y.;Kimura, K.; Lim, E. C. J . Chem. Phys. 1984, 8 / , 3345.
+
also in a vapor, ref 11
+
measured only in a vapor, ref 3(a)
I11 MI
M/
OPresent work. bAlso complexes with acetonitrile; see text. charge-transfer state upon electronic excitation. On the other hand, complexation of the nonpolar molecules with the protic solvent methanol, even with a single solvent molecule (n = I), leads to the formation of a TICT state upon electronic excitation. The effect of an aprotic, polar solvent (acetonitrile) was studied for compound VII. Complexes of VI1 with 1 and (probably) 2 acetonitrile molecules exhibit structured excitation spectra, while no TICT state emission was observed. However, if the number of solvent molecules in the complexes is increased, optical excitation gives rise to emission from a stabilized charge-transfer state. Although the minimum value of n is unknown, a critical number of acetonitrile molecules seems to be necessary to produce a TICT state. The excitation spectra of the complexes emitting TICT fluorescence are not structured. The lack of any vibrational structure indicates the absence of a Franck-Condon minimum at the potential energy surface of the excited state. Rapid IVR seems to produce the conformationally different TICT state.
Acknowledgment. We express our thanks to Mr. G. Jansen, Mr. P. Hinrich, and Ing. D. Bebelaar for technical help and to Dr. K. Rotkiewicz and Prof. W. Rettig for stimulating discussions. Professors J. W. Verhoeven, N. M. M. Nibbering, and H. Cerfontain are kindly acknowledged for their help in the vapor-phase measurements, mass spectrometry, and N M R analysis, respectively. This work is supported by the Netherlands Foundation for Chemical Research (SON). The Netherlands Organization for Scientific Research (NWO) is kindly acknowledged for financial aid to J. Herbich and F. PErez Salgado. This work is partly done within the Polish research programs CPBP 01.19 and R P 11.1 3.
