J. Phys. Chem. 1993,97, 6127-6133
6127
SI-SOTransition of N,N-Diethylaniline: Vibrational Assignments and Picosecond Dynamics Robert A. Weersink and Stephen C. Wallace' Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario MSS lA1, Canada Received: November 12, 1992; I n Final Form: March 15, 1993
The multiphoton ionization and laser-induced fluorescence emission of jet-cooled N,N-diethylaniline (DEA) and DEA-dlo are reported. Vibrational analysis for the S l S o transition in DEA and DEA-dlo is presented, with several low-lying modes in the SIassigned in terms of the SOnormal modes. Singlevibroniclevel fluorescence spectra also enable the assignment of several SOvibrational frequencies. Quantum interference effects are observed in the energy-resolved fluorescence decays following picosecond laser excitation of one SI vibrational level in DEA. The observed modulations are explained in terms of a coherent superposition of molecular eigenstates.
1. Introduction
The proposed twisted intramolecular charge transfer (TICT) in molecules such as N,N-(dimethylamino)beenzonitrile (DMABN) presumably involves extensive changes in the geometry of the molecule, particularly with respect to the motion of the dimethylamino group'. Little is known, however, about the vibrational dynamics that correspond to such motions, and in particular, how they are influenced by the electronic structure of the molecule. Studying precursors, such as N,N-dimethylaniline (DMA) and N,N-diethylaniline (DEA), of the TICT molecules can provide background knowledge of the interactions between the various vibrational and torsional modes of the molecules, thereby giving a better understanding of the TICT process. Interest in DEA arises from previous free-jet-expansion investigations of DMABN2 and DMA3, both of which have a large number of low-frequency vibrations, usually attributed to torsion of the dimethylamino group about the N-C,,l bond. Surprisingly, the number of these low-frequency vibrations decreases dramatically in DEA, despite increasing complexity in structure. Such spectral simplicity should facilitate analysis of the torsion and inversion modes of the dialkylaminogroup, making analysisof the much more complexspectra of DMA and DMABN possible. DEA, a molecule with a simple chromophore and large alkane substituent, should be a useful probe of intramolecular dynamics in floppy molecules4,especially with respect to how the torsion and inversion modes of the substituent influence the excited-state dynamics of the molecule. A vibrational analysis of the molecule in both the ground and excited states is needed to make such a study possible. Previous IR and Raman studies of DEA516and DMAS7 have been minimal and so analysis of the vibrational modes in the SO and S1 electronic states of anilineGl0 are also used as a reference in this work. As well, DEA-dlo, in which the ethyl groups of the substituent are deuterated, is also studied to facilitate assignment of thevibrationsof DEA. Excitation of low-energy SIvibrational modes of DEA results in uncongested emission spectra and no anomalies in the fluorescencedecays, implying that the potential energy surfaces are similar for the SOand the SIstates at these energies. At higher vibrational energies, the appearance of quantum beats and a broad continuum in the emission leads to the conclusion that the SO and SI surfaces are considerably different at higher S1 vibrational energies. An analysis of the SIvibrational modes is required to facilitate an understanding of the emission spectra and the observed dynamics. In this paper, the SIdynamics are described using a molecular eigenstate picture. These molecular eigenstates may, in principle, be determined by a normal-mode analysis of the SI state, or approximated by using linear combinations of a complete
zero-order basis set, such as the ground-state normal modes, to describe each eigenstate. At low SIvibrational energy, fluorescence emission spectra show that the excited state can be approximated as linear combinations of just one or a few of the SOmodes. At higher energies, more SOmodes must be used in the description. The observed quantum beats are explained as the time evolution of a coherently prepared superposition of molecular eigenstates. 2. Experiment
Details of the experimental setup have been described elsewherellJ2, and only a brief discussion of the expansion conditions and the excitation sources for the MPI and emission experiments will be given here. The molecule was prepared in a continuous free-jet expansion with He as the carrier gas. The nozzle, heated to about 50 OC, was fitted with either a 100- or 50-pm orifice, and backing pressures were typically 20 or 120 psi, respectively. N,NDiethylaniline (DEA) was purchased from Aldrich with stated purity of 98% and was used without further purification. The sample of DEA-dlo was synthesized by standard techniques. Due to the small amount of available DEA-dlo, only a small number of experiments could be performed on DEA-dlo. The excitation spectra were obtained by one- and two-color resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The excitation and ionization sources were the frequencydoubled outputs of Nd:YAG (Spectra-Physics GCR-3) pumped dye lasers (Quanta-Ray PDL-1). A 2 cm long KDP crystal was used to double the output of Rh640 and DCM dyes (Exciton) to give a tunable range of 330-290 nm. Typical pulse energies used in the experiments were attenuated to about 0.1 mJ for the first photon to avoid saturation effects. The UV pulses crossed 3-5 mm in front of the nozzle, with a spot size of 5-7 mm. Ions produced in the expansion were then deflected into a quadrupole mass spectrometer (VGK300) where they were detected and analyzed. Details of the ion extraction configuration and mass spectrometer are described in a previous publicationll. The time-correlated single photon counting method was used to obtain both the dispersed emission spectra and fluorescence lifetimes12. The second harmonic of a CW mode-locked N d YAG laser (Spectra Physics 3000) synchronously pumped a cavity-dumped dye laser (Spectra Physics 375B dye laser, 344 cavity dumper). R6G was used in the dye laser, yielding 300 mW at 603 nm at a repetition rate of 4 MHz. This output was frequency doubled in a 1-cm LiIOB crystal to generate 10-15 mW in the UV. This excitation beam was focused to a 100-pm waist, which intersected the free-jet expansion 1 mm from the nozzle. Fluorescence was collected perpendicular to both the
0022-365419312097-6127%04.00/0 0 1993 American Chemical Society
Weersink and Wallace
6128 The Journal of Physical Chemistry, Vol. 97, No. 23,1993
laser beam and expansion and focused into the entrance slit of a 0.35-m monochromator (McPherson 270). The dispersed fluorescence was then detected at the monochromator exit slit by a photomultiplier tube (Hamamatsu R1527). For the timeresolved measurements, a microchannel plate (MCP) PMT (Hamamatsu R1564U-02) was used for detection of the fluorescence. Using the MCP, the time response of the system was approximately 70 ps.
3. Symmetry DEA has only one symmetry element, a plane of symmetry perpendicular to the plane of the benzene ring which runs along the long axis of the molecule. It is assumed here that both ethyl chainsin the aminogroup lie out of the benzene plane and interact identically with the chromophore. Consequently, the molecule belongs to the C, point group. Its precursor, aniline, is also of C, symmetry but to distinguish between sublevels arising from inversion, the molecular symmetry group G4 is used, which is isomorphous with the CZ,point group.' Inversion is still possible in DEA, although inversion of the NC2 plane of the substituent through the benzene plane must also be accompanied by a motion of the ethyl groups if the inverted molecule is to be identical to the original conformer. The extent of splitting of vibrational levels due to the multicoordinate motion is unknown, and so to begin the discussion, DEA will initially be characterized as Cb, as was done in the two previous IR and Raman studies of DEASs6. If the assignment of the observed skeletal vibrational bands is not consistent with the use of the CS,point group, then the C, point group will be considered. If the molecule belongs to the CZ,pointgroup, then its molecular vibrationsare labeled as al, a2, bl and bz. If the molecule belongs to the C, point group, the molecular vibrations are labeled as either a' (symmetric with respect to the symmetry plane) or a" (asymmetricwith respect to thesymmetry plane). The electronic transition moment is perpendicular to the symmetry plane and lies in the plane of the ring. The electronic state has Bzsymmetry and therefore the complete electronic transition is symmetric. This electronic state corresponds to the B2" state observed in aniline. Discussionsof the TICT phenomenon generally use Platt's notation for descriptionsof the electronic states. The SIstate in this notation is labeled ILI, while the next highest electronic state is the 'La state, which has A1 symmetry. Because the total transition moment must be symmetric, and because the molecule is prepared in the vibrationless level of the ground electronic state, symmetric vibrational modes will dominate the excitation spectra. Asymmetric modes can be observed only if an even number change in the vibrational quantum numbers occurs, resulting in an overall symmetric mode. 4. Results
A. Excitation Spectrum. The two-color, resonant-enhanced multiphoton ionization spectrumof DEA and DEA-dlo are shown in Figure 1. The SIorigin of DEA appears at 32 294.5 cm-1 while the origin of DEA-&appears at 32 275.4 cm-1, red-shifted from the undeuterated at 19.1 cm-1. Built on the origin of DEA is a 100-cm-1 progression which is also built on the transitions at the 530-, 710-, 890-, 987-, and 1065-cm-1 bands. A corresponding progression of 90 cm-1 is observed in DEA-dlo. The low-energy region of the excitation spectrum of DEA is substantially less congested than those reported for N,Ndimethylaniline(DMA) and its derivatives2.3, despite the larger number of vibrations possible in the diethylamino substituent. Possible assignments for these low-frequency modes include torsion about the N-Carylbond, inversion of the whole substituent through the benzene plane, and torsion of the alkyl chains. In DMA, only methyl rotation occurs, whereas, in DEA, torsion of the ethyl groups as well as the methyl groups on the end of the chains is possible. The inversion potential in the SIstate of DEA is likely to be similar to that found in the SI state of aniline13,
1500.0
1ooo.o
500.0
0.0
ENERGY (cur')
7
5
J
1500.0
lm.o
500.0
0.0
Energy (cm')
Figure 1. Two-colour 1+1 multiphoton ionization spectrum of (a) jetcooled N,N-diethylaniline (DEA);the bands are labeled in cm-l relative totheoriginat 32 294.5cm-I; (b)jet-cooledN~~iethylanilinadlo(DEAdlo). The origin at 32 275.4 cm-l is red-shiftedfrom the origin of DEA by 19.1 cm-l.
a quartic potential, which produces an almost flat-bottomedwell. Such a potential will have anharmonic vibrational spacing. In contrast, torsion potentials will give harmonic spacings between vibrationallevels if the barrier height tointemal rotation is high14. Upon deuteration, the 100-cm-1progression shifts to 90 cm-*and remains harmonic. The three possible torsions within the substituent will have different isotopic shifts in the observed torsional frequencies due to differing effects that deuteration will have on the rotational constant for the torsion motion. In the high-barrier limit, the torsion frequencies, v, and the rotational constant, B, can be expressed in terms of the barrier height ad5
Upon deuteration, the 100-cm-l progression shifts to 90 cm-*, and, assuming identical potentials for both the hydrated and deuterated molecules, eq 1 yields Bd/Bh = 0.81, where the subscripts h and d denote the hydrated and deuterated DEA, respectively. Rotational constantsfor each of the possible torsions were calculated using the reduced amount of inertia and the corresponding ratios of the deuterated and undeuteratedrotational constants were compared. If thevibration is torsion of the whole substituent about the N-C,,, bond, the calculated-frequency of the deuterated rotor is 97% of the calculated frequency for the undeuterated rotor. Likewise, if the observed bands are the result of methyl torsion, then the calculated change in frequency upon deuteration is to 71% of the frequency of the undeuterated rotor, a change that is not observed. It is found that the only torsion consistentwith the observed isotopic shift is torsion of the ethyl chain about the N-C bond of the substituent. Since this progression is built onto each of the skeletal modes, it acts as a signature in identifying which transitions are fundamental skeletal vibrations. As a first approximation, the S1normal modes are assigned in terms of known SOnormal modes of DEA5,'j. These assignments are listed in Table I. This
The Journal of Physical Chemistry, Vol. 97, No. 23, 1993 6129
SlSo Transition of N,N-Diethylaniline TABLE I: Observed Skeletal Vibrational Frequencies of DEA-LO(cur')'
k 0"
symmetry s1
&
267 308
265 313 713 898 987
709 891 981
&b
assignment 15
&C
440 718 858 990 1,035
216 324 713
C,
CZ,
a"
b a1 a1
17b
a' a' a'
12 18a
a' a'
6a 1
983
_.
bl a1
0" + 308 an"
1,065 1,040 1,032 a1 0 These frequencies are accurate to 2 cm-l. These frequencies are obtained from IR and Raman in the condensed phase (ref 5). These frequencies are obtained from IR and Raman in the liquid phase (ref 6).
1
I
4J
0" +
I
e
-lm.o
-m.0
eww
1
Excitation
(e')
530 an" Excitation (1")
0" + 710 an.'
Excitation (1')
E
0.0 0.0
Excitation
-500.0
-1ooO.O
-1500.0
-2000.0
EW~OY (m.') -lm.o
("')
Figure 3. Dispersed fluorescence spectra following excitation to four S1 vibrational levels in DEA. The fluorescence peaks resonant with the laser excitation are labeled with asterisks. Some of the contribution of the resonant peak intensity is scatter for the spectra (b, c, and d). Excitation in all the spectra originates from the vibrationless level in the ground state. The spectra are aligned relative to their respective Au = 0 peak.
TABLE Ik Observed Skeletal Vibrational Frequencies of I
DEA-40 (cm-I)'
--
I
Sl
so
assignment
147 236
152 238 277 67 1 864 989
(lob)
I _ _ _
00
-500.0
-lm.o
-1m.o
664 857 987
enasv (c") Figure 2. Dispersed fluorescence spectra following excitation to the S1 origin of (a) DEA and (b) DEA-40.
description is not completely adequate, however, for vibrations higher than 500 cm-1 (see below). B. Single Vibronic Level Fluorescence. Single vibronic level fluorescence spectra were obtained by exciting the individual vibrational levels in the SI state. The dispersed fluorescence spectrum of the S1 origin of DEA and DEA-dlo are shown in Figure 2, while four other representative spectra of DEA are shown in Figure 3. The strongest transition in each spectrum is the Av = 0 transition, indicating that no major changes occur in the skeletal geometry upon excitation. Built on top of this intense peak are the ground-state modes observed in the origin spectrum. The peak therefore acts as a false origin, as indicated by the relative alignments of the fluorescence spectra in Figure 3. The assignment of the Av = 0 transition to known ground-state vibrational levels determines the assignment of the respective bands in the excited state. For example, excitation of the weak band at 308 cm-1 in the excited state gives the emission spectrum shown in Figure 3b, a spectrum that mimics the origin spectrum and is therefore characteristic of a skeletal normal mode, and not a torsion or inversion level. The transition occurs at 313 cm-l, where the 6a: band is expected to occur, and therefore, the 308-cm-1 level in theexcited stateis assigned to the 6aivibration. The origin fluorescencespectrum exhibits several low-frequency vibrations assigned to torsion about the N-C bond in the
@
15
6a 1 17b 12
C,
C?Ll
a'
bi
a"
bz
a' a' a'
a1 a1 bi
a'
a1
These frequencies are accurate to 2 cm-l.
substituent. As in the excited state of DEA, these vibrations shift in frequency upon deuteration to 90% of the observed frequency in the undeuterated DEA. This torsional progression is also built on each skeletal vibration, and therefore again acts like a signature in the origin spectrum, identifying the skeletal vibrations. This is particularly helpful in distinguishing which bands are skeletal modes and which bands are part of a torsional progression at higher energies. C. Vibrational Assignments. Theobservedvibrations arelisted in Table I for DEA-hlo and Table I1 for DEA-dlo, while the important skeletalmodes are depictedin Figure 4. The numbering of the vibrations is that used by Varsanyi16 for benzene. Since previous IR and Raman studiess.6 have been performed in the liquid phase, discrepancies may arise in the frequencies of the vibrations reported previously and those reported here, particularly for low-frequency modes. Verma et a1.6 also assign frequencies observed in a previous Raman study although these assignments list only the symmetry of the vibrations. Modes 1 and 12 are substantially different in benzene than in substituted benzenes17. In benzene, mode 1 is the ring-breathing mode, while mode 12 has adjacent ring atoms breathing in and out with a 180-degphase difference. Substitution couples1*these two modes so that the motion of the modes are identical except for the phase of adjacent carbon motions. In DEA, however,
6130 The Journal of Physical Chemistry, Vol. 97, No. 23, 1993
Skeletal Modes of DEA
e
R 12
R 18a
17b
Figure 4. Motions of some of the observed normal modes in DEA.
vibration 1 is affected by the size of the substituent, lowering in frequency, while mode 12 remains unchanged. Therefore, the transition observed at 710 cm-I in the origin emission spectrum is assigned as I:, while the transition observed at 987 cm-l is assigned as 12:. This labeling is opposite to some work on anilines and toluenelg, where the substifuent shifted vibration is labeled as 12. These assignments are confirmed by the DEA-dlo results, in which the 1: vibration in the origin emission spectrum appears at 671 cm-1 and the 12: transition is observed at 989 cm-1. With the 1 mode affected by the size of the substituent, it is lowered in frequency upon deuteration, as expected, while the 12 mode remains unchanged in frequency. Since the 6a mode in aniline occurs at 529 cm-1 in the ground state and 492 cm-1 in the excited state, either the 308- or 530-cm-I band in the excitation spectra of DEA could possibly be assigned to the 6aA mode in DEA. Likewise, the 313- and 535-cm-I bands in the origin fluorescence could be assigned to the 6ai mode. As seen in Figure 4, the 6a vibration is essentially a stretch along the C-R bond and hence its frequency is extremely susceptible to substitution effects. GargZo has studied the effect of the substituent on the 6a mode in monosubstituted benzenes and has determined an empirical formula to determine the frequency of the 6a mode for both the ground and excited electronic states:
v’=- lg80 f o r s o
6’
where p is the reduced mass about the C-R stretch. Using these empirical formulas, the calculated frequencies for DEA were u = 324 cm-I in SOand 292 cm-I in SI. This corresponds well with the 3 13- and 308-cm-1 bands in the ground and excited states, respectively, and leads to the attribution of these bands to the 6a mode. In DEA-dlo, the 6a mode appears at 277 cm-1 in the ground electronic state, a drop in frequency that is consistent with the assignment of the mode. Using eq 2, the calculated frequency for the 6a vibration in DEA-dlo is 314 cm-I in So. Based on comparisons with aniline7-8p9 and the IR and Raman data for DEAS, three possibilities exist for the assignment of the band at 535 cm-l in SO,and 530 cm-1 in SI;the inversion mode, twoquantaofthe 15vibration, andonequantaof the 16bvibration. Assignment of these bands as inversion is consistent with 1
Weersink and Wallace characterization of DEA as Cb,while assignment of these bands as 16b is consistent with the use of the C,point group. Either point group can be used if the mode is assigned to the 15 mode since this vibration is an in-plane bending vibration which is not symmetric with respect to the vertical plane of symmetry. The assignment of this band is assisted by the deuteration, which shifts these bands to 478 cm-1 in SO,and 471 cm-I in SI. Both inversion and 15 mode involve motions of the substituent and will be lowered in frequency upon deuteration, whereas 16b will not. The 16b mode is a butterfly motion of the benzene ring along the long axis of the molecule. That the mass of the substituent is not important for this vibration can be confirmed by comparison of the IR data for DMA, DMA-ds, and DEA5. In DMA and DMA-d6, the 16b mode appears at 518 cm-l, while inDEAthe 16bmodeappearsat 525cm-1. Despitethesignificant changes in the mass of the substituent among these molecules, thefrequencyremainsat essentially 52Ocm-I. It can beconcluded from these observations that the 16b mode will not be affected by changes in the mass of the substituent. The large isotope effect observed for this band in DEA therefore negates the 16b mode as a possible assignment of the 530-cm-I band. In both DEA-hlo and DEA-dlo, the second observed overtone in the SO state is exactly twice the frequency of the observed fundamental (Figure 3c). Such harmonicity is inconsistentwith assignment of the DEA-hlo 535-cm-l band as inversion, since the potential energy surface for the inversion mode in SO,a double minimum potential, is unlikely to give harmonic progressions for both the hydrated and deuterated DEA. Attempts were made to fit the SOfrequencies to a double well potential of the form used by Coon et a/.?’ but these attempts were unsuccessful. Although the inversion mode may be important in DEA, it is not observed at 535 cm-I in the ground electronic state of DEA. The 15 mode is essentially a bend along the C,,I-N bond in the plane of the benzene ring. This mode is observed in aniline and is expected to be strongly affected by changes in the mass of the substituent. This vibration will be asymmetric in both the C , and C, point groups, and will therefore be observed only if there is an even numbered change in vibrational quantum number occurs, regardless of the point group used to characterize the molecule. The observed bands at 535 and 530 cm-I in the SOand SI states of DEA, and 477 and 471 cm-l in the SOand SI states of DEA-dlo, are therefore assigned to two quanta of the 15 vibration. The 898-cm-1 band in the emission spectrum of the S1origin is assigned to the 17b mode, although the frequency observed here is slightly larger than that observed in the IR and Raman work. It should be noted that an unassigned band appears in the fluorescencee x c i t a t i ~ nand ~ ~ .MPI ~ ~ spectra of aniline located at 891 cm-I in the SIstate. It may be that the observed band at 898 cm-I in DEA is analogous to this band. In DEA-dlo, the 147-cm-1 band in the SIstate and the 152-cm-1 band in the SOstate are assigned to the 10b mode. Excitation of the 147-cm-l S1band results in an emissionspectrum indicative of a skeletal normal mode. The Au = 0 band in the emission spectrum acts like a false origin, and the low-frequency modes observed in the origin spectrum also appear, built on to the Au = 0 band, indicating that the 147-cm-1 band in the excited state and the 152-cm-l band in the ground state are skeletal normal modes. No analogous bands were found, however, in the undeuterated DEA, leaving the assignmentof thisweak transition ambiguous. The excited state modes can be described in terms of SOnormal modes, but only if their fluorescence spectra mimic the origin fluorescence spectrum24,as in the second spectrum in Figure 3. The lower two spectra in Figure 3 not only exhibit bands that occur in the origin spectrum, but also have a broad underlying congestion which becomes more pronounced at higher excitation energies, indicating a breakdown of our description. Several possible explanations for the existence of this congestion have been tested and subsequently discounted. Varying the concen-
SlSo Transition of N,N-Diethylaniline
tration of DEA in the expansion (via temperature of the sample holder) has no effect on thecongestion, and therefore the possibility of aggregates of DEA is ruled out. Drastically altering the expansion conditions also had no effect on the observed emission spectra. At high S1 vibrational energies, (approximately 500 cm-1) the one-to-one corresondence between the ground- and excited-state modes breaks down and several SOmodes, both symmetric and asymmetric, are needed to describe the excited-state vibrations. Therefore, the number of SOlevelswhich have appreciable FranckCondon overlap with the excited SI modes is magnified. Considering the high number of low-frequency modes in DEA, such as the torsion and inversion modes of the diethylamino substituent, the state density of the SOstate is likely to be quite substantial. Therefore, the simplest explanation of the observed congestion in the fluorescence spectrum is that it originates from a mixed level which has appreciable Franck-Condon overlapwith a large number of levels in the SOstate and in conjunction with a high density of SOlevels, the emission spectrum is so heavily congested that it appears as a continuum. Excitation of the 530-cm-l band gives the fluorescence spectrum shown in Figure 3c. As discussed above, the spectrum shows a strong Av = 0 transition with most of the resolvable bands built on this transition also being prominent in the origin spectrum. However, as well as the aforementioned broad underlying congestion, several other bands at 806,899, and 988 cm-l appear which do not have corresponding bands in the dispersed fluorescence spectrum from the S1 origin, indicating that our description of the 530-cm-l band purely in terms of the SO 15 mode is not correct. Instead, it appears that two or possibly three SOmodes make significant contributionsto the description of the 530-cm-I band. This description isconfirmedby the time-resolved studies discussed below. D. Time-Dependent Studies. Fluorescence lifetimes were determined by monitoring the decays of the undispersed fluorescence resulting from excitation of several SIvibrational levels (Table 111). For some of the excitation bands, fluorescencedecays were measured by monitoring several isolated transitions in the fluorescence spectra. For the higher energy vibrations, this also included measuring the decays of the congested emission. Singleexponential decays were measured following excitation of most of the bands except the 530-cm-l band. No significant variations in the lifetimeswere found between different excitation or emission bands. The congestion has the same lifetime as resolvable transitions and no anomalous rise time effects were observed with the congested emission. Modulated decays were observed in individual fluorescence bands after excitationof the 530-cm-1 S1band. Two sets of beats were measured, differing only in the relative phase and modulation depth of the decays, as seen in Figure 5. Fluorescence bands terminating at the 535-, 647-, 1067-,and 1253-cm-I levels in the ground state showed decays with a single modulation frequency of 3.8 GHz, all having the same phase. Fluorescence decays fromthebandsat 806,899,and988 cm-l alsoshowedamodulation frequency of 3.8 GHz, but these beats were exactly out of phase with the first set of fluorescence bands. Fourier analysis of the decays was complicated by two factors. The quantum beat period was close to the width of the instrument response function (IRF), and the modulations were quickly damped in some of the decys. These complications made it necessary to analyze the data using a convolute and compare technique with the following function for single-frequency modulation:24
z ( t ) a e@ (1
+ Me82' cos ut)
(3) Here rl is the intrinsic fluorescence lifetime and w is the frequency of modulation. M is the modulation depth ranging from 0.0 for unmodulated decays to 1.Ofor completely modulated decays and with a sign depending on the phase. r2 is the damping
The Journal of Physical Chemistry, Vol. 97, No. 23, 1993 6131
TABLE IIk Observed Excitation Bands of DEA-ho displacement from Oo (cm-1)' 71 (IISY assignment 0 39.8 100 165 199 224 254 264 275 300 308 318 363 40 1 412 418 502 530 630 710 729 ai0 89 1 912 987 1065 1084 1150 1182 1251 1348
2.9
0
3.1
G
3.4
2
3.8
3.9
64
3.4 3.1 3.7
15i 1502 1:
3.4 3.4 3.3
$$
1 loto 17b' l$ 1% 184
y74 gbnP 9b0to
The excitation energy of the origin is 32 394.5 cm-l. These lifetimes are accurate to 0.1 IIS. a
time of the modulation envelope which arises from the spread of the frequency components about the central beat frequency w due to rotational level effecW. This modeling procedure was used to determine the frequency,phase, and depth of modulation for the fluorescence decays. The fitting results for two decays are shown in Figure 5. Since the resolvable transitions are on top of an apparent continuum, it is difficult to calculate vibrational coupling coefficients from the measured modulation depth@. The phase and assignments of several bands that exhibit the beating decays are listed in Table IV. As seen in Figure 5, the fit of the in-phase modulated decay of the 530-cm-l band is poor in the first recurrent region. It was speculated that scattered light was contributing to the intensity of the first modulation. Ifscatter were thesourceof theanomalous intensity,varying theconcentrationoftheDEAinthejetexpansion would result in changesin intensity of the first modulation relative to the rest of the decay. However, decreasing the concentration had no effect on the relative intensities of the modulationsin the decay. Also, no scatter appearedin the measurement of the decays at 806 cm-', even though the lower intensity of this band necessitated collection of the data for a longer period of time. It was concluded that the anomalously high intensity of the first recurrence was real. Attempts were made to fit the fast time region of the decay to either another modulation component or to a second fast decaying component. Both modeling attempts failed and the source of the increased intensity of the first modulation is still undetermined. 5. Discussion
As is readily noticed in Tables I and 11, the observed bands that can unambiguously be assigned are all symmetric in the Cb symmetrygroup. The 15 mode is only observed if an even number change in its vibrational quantum number occurs, and hence the observed bands assigned to the 15 mode are symmetrictransitions in either the CZ,or C, symmetry groups. Assignment of those
6132 The Journal of Physical Chemistry, Vol. 97, No. 23, I99'3
Weersink and Wallace described as linear combinations of these zero-order states,
1 0.0
Em = -806
0.5
1.O
cni'
2.0
1.5
Time (ns)
Figure 5. Early portion of the fluorescencedecay curves and the fit after excitation of the 530 cm-1 level in DEA. Decay (a) was obtained by monitoring the fluorescence of the -535-cm-l level in the fluorescence spectrum in Figure Zc, while the lower decay was obtained by monitoring the -806-cm-1 band in the same spectrum. Both curves were fitted with a modulation of 3.87 MHz. The upper decay is +1 phase with a depth of 0.85 with a damping time, r2, of 0.9 ns. The lower decay is -1 phase with a depth of 0.15 with a damping time of 0.7 ns. The instrument response function is shown in decay (a) while the weighted residuals to the fits are shown below each decay.
TABLE Iv: Fluorescence Decpys of Prominent Bands FoUowinfi Excitation of tbe 530-cm-'Band of DEA ~
disdacement from 0 (cm-l) -535 -647 -809 -899 -988 -1067 -1253
assignment 15; 15,ty
modulation phase
+ +
-
15; 15,1:
+ +
modes which are unsymmetric in the Cb group, the 10b and 17b modes, is not certain. Without compelling evidence to suggest the use of either Cb or C, symmetry, DEA should be characterized in an comparable manner to aniline, which has Cb symmetry. It is readily apparent from the intense origin transition that there arenosubstantialchanges in thegeometryof thesubstituent, a result substantially different from DMA3, which has a weak origin and a large number of low-frequency bands. The limited analysis of the low-frequency bands presented here suggests that torsion about the N-C,,, bond is not important for DEA in its SOand S1 state, raising questions about the previous assignments for DMAJ, which attributed the low-frequency bands to just such a torsion. Analogies with the TICT process, however, cannot be madesincethe& stateof DEAdoesnotcorrespondtothecharge transfer state in DMABN. The dispersed fluorescence and quantum beat results can be interpreted in the molecular eigenstate p i c t ~ r e ~ 9 ~ ~In2 6this . picture, quantum beats arise from an initially created vibrational superposition state. In the absence of a normal-mode analysis of the excited state, the true molecular eigenstates, Il), 12), 13), ..., with energies El, E2, E3, ...,may not be directly known. To describe these eigenstates, one starts with a complete basis set of zero-order states, la,) and the molecular eigenstates are then
A convenient, albeit approximate, basis set for the description of the molecular eigenstates is the Sovibrationalmodes observed in the dispersed emission spectra.24 Instead of using the complete basis set of zero-order states, a molecular eigenstate can usually be approximated as a linear combination of only a few members of the basis set. The members of this basis set can be determined from emission spectra since a fluorescencetransition originating from one of these molecular eigenstates and terminating on a particular ground-state vibrational level will be strong only if there is significant Franck-Condon overlap between the excited molecular eigenstate and this ground-state vibrational level. At low SIvibrational energies, only a few of the SOvibrations may be needed to describe the excited-state molecular vibration. Emission from this molecular vibration will therefore be characterized in terms of discrete transitions since the eigenstate will only have significant Franck-Condon overlap with a few of the So vibrations. At higher SIvibrational energies, the prepared state has Franck-Condon overlap with many more vibrations in the ground state, as evident by the increased number of fluorescence transition bands in the emission spectra following excitation of these S1 levels. Consequently, a larger number of zero-order modes are needed to describe an excited molecular eigenstate. If the density of ground-state vibrations is high, then the emission spectrum will appear congested. Depending on the contributions of each zero-order mode to the description of the molecular eigenstate (i.e., the size of the coefficients in the basis set), the emission spectrum may contain both intense, discrete transitions, and congested emission. If the energy difference of two of these eigenstates is within the bandwidth of the excitation source, then excitation of the molecule may create a coherent superposition of states. The observance of quantum beats in the fluorescence decays is then just the result of the time evolution of this initially prepared state. If more than two eigenstates are excited, then the number of modulation frequencies in the fluorescence decays will be n - 1, where n is the number of eigenstates coherently excited. Recently, Brumer and c o - ~ o r k e rhave s ~ ~developed ~~ a method for calculating the normal modes of the St state using semiempirical methods. This procedure eliminates the need to describe the excited-state vibrations in terms of a basis set of zero-order states. Normal-mode calculations usually require a complete set of frequencies as input, a difficult procedure for large polyatomic molecules, especially in the excited state where little information may be available. Their semiempirical method has allowed Brumer et al. to compute the absorption and emission spectra for jet-cooled anthracenez8 and alkylben~enes~'.Features such as quantum beats and congested emission spectra can be explained in terms of the coherent excitation of uncoupled harmonic levelsz9. Without such a normal-mode analysis for the DEA SIstate, the SOnormal modes are used as a basis set for the description of the& bands, a description facilitated by analysis of the emission spectrum originating from each of these excited-state levels. In the case of the 530-cm-l band, several bands appear that are not prominent in the origin emission spectrum. However, these bands, the 806-, 899-, and 988-cm-1 bands, that exhibit negative phase beats, are presently unassignable. The high density of states at these energies leads to severalpossible assignmentsof these bands, but none of these assignments can be confirmed. What is known is that the zero-order mode which couples with the 15 mode at 530 cm-l in the excited state has significant Franck-Condon overlap with all of the emission bands which exhibit out-of-phase quantum beats.
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The Journal of Physical Chemistry, Vol. 97, No. 23, 1993 6133
SI-& Transition of N,N-Diethylaniline
SIstate, the one-to-one correspondence between the normal modes in the SOand the SI states breaks down as witnessed by the
1s
cz; x
appearance of a congestion in the dispersed emission results and by the observation of quantum beats in the energy resolved fluorescence decay measurements resulting from excitation of the SI530-cm-1 band. At higher vibrational energies, the SI vibrations can be characterized by linear combinationsof the ground-state normal modes. Excitation of a coherent superposition of two of these molecular eigenstates has been created with the appearance of quantum beats the manifestation of the time evolution of the initially prepared state.
19 Figure 6. Description of the observed quantum beats in DEA. Two essentially isocnergetic molecular eigenstates are shown with two zeroorder modes contributing to each eigenstate. For the SI53O-ar1 band in DEA the two zero-order modes are the 15; mode and an unknown dark state. Preparation of a coherent superposition of the pair of levels within the depicted laser bandwidth results in the observation of quantum beats. At different times in its evolution, the nonstationarystate will have more Franck-Condon overlap with either a levels in the ground state (emission bands observed at-535,-647,-1067, and-1253 cm-l in the SVLspect" of the SI530-cm-l band), or b levels in the ground state (emission bands observed at -809, -899, and -988 cm-l in the SVL spectrum of the SI 530-cm-1 band).
The appearance of quantum-beats in the time-resolved measurements is illustrated in ,Figure 6, where two essentially isoenergetic molecular eigenstates are shown. Two zero-order modes, the 15; band and an unknown dark state, contribute to the description of each of these eigenstates. The different shaded areas in each of the molecular eigenstates represent the contributions of each of the SOzero-order modes to our description of the molecular eigenstates. The magnitude of the contributions of these two zero-order levels is arbitrary since the actual contributions are not known. However, the 15 mode will contribute substantially more to one of the eigenstates while the dark state will contribute substantially more to the other eigenstate. Coherent excitation of these eigenstates will create a nonstationary state which traverses different regions of the S1 potential energy surface with regular periodicity with the wavepacket describing the time evolution of this state regularly travelling between two different regions of the excited-state potential energy surface. Each region will have significant Franck-Condon overlap with different typesof levels in the ground state. Modulations in the measured fluorescence decays from each of these different types of emission therefore represent the time evolution of the initially prepared state. A large number of other SOvibrational modes can also be included in our basis set to account for the observed congestion in the 530-cm-l fluorescence spectrum. The size of the contribution of these individual zero-order modes is small compared to the contribution of the two previously used zero-order modes. Their total contribution to the description of the eigenstates will, however, be substantial, as indicated by the observation that the eigenstate has significant Franck-Condon overlap with a large number of ground-state levels. Quantum beats were not observed on the congested emission because the many overlapping transitions made it impossible to monitor the fluorescence decay of just a single transition. 6. Conclusion
The vibrational analysis of the SOand S1transition in DEA has been performed through resonance-enhanced multiphoton ionization (REMPI) and single vibronic level fluorescence(SVLF) in combination with selectivedeuteration. At low energies in the S,state. the normal modes can be described in terms of the groundstate normal modes. For vibrational levels above 500 cm'l in the
Acknowledgment. We thank Robert D. Gordon for discussions about the torsional motions in DEA, as well as Prof. M. Lautens for providing the DEA-dlo. This research was supported in part by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Centre of Excellence for Molecular and Interface Dynamics (CEMAID). References and Notes (1) For a review, see: Rettig, W. Angew. Chem., b t . Ed. Engl. 1986, 25, 97 1. (2) (a) Peng, L. W.; Dantus, M.; Zewail, A. H.; Kemnitz, K.; Hocks, J. M.; Eisenthal, K. B. J. Phys. Chem. 1987, 91, 6162. (b) Kobayaahi, T.; Futakami, M.; Kajimoto, 0. Chem. Phys. Lett. 1986,130,63. (c) Gibson, E. M.; Jones,A.C.;Phillips, D.Chem.Phys.Letr. 1987,136,454. (d) Warren, J. A.; Bemstein, E. R.; Seeman, J. I. J. Chem. Phys. 1988,88, 871. (3) (a) Grassian, V. H.; Warren, J. A.; Bemstein, E. R.; Secor, H. V. J. Chem. Phys. 1989,90,3994. (b) Gordon, R. D. J. Chem.Phys. 1990,93, 6908. (c) Bernstein, E. R.; Grassian, V. H.; Warren, J. A. J. Chem. Phys. 1990, 93,6910. (4) Felker, P. M.; Zewail, A. H. Adu. Chem. Phys. 1988, 70, 265. (5) Guichard, V.; Bourkba, V.; Lautie, M. F.; Poizat, 0. Spectrochim. Acta 1989, 45A, 187. (6) Verma, V. N.; Nair, K. P. R.; Rai, D. K. Ind. J. Pure Appl. Phys. 1971, 9, 336. (7) Pemer-Datin, A,; Lebas, J. M.J. Chim. Phys. 1972, 591. (8) Brand, J. C. D.; Williams,D. R.; Cook, T. J. J.Mol. Spectrosc. 1966, 20, 359. (9) Quack, M.; Stockburger, M. J. Mol. Spectrosc. 1972, 43, 87. (10) Chemof, D. A,; Rice, S.A. J. Chem. Phys. 1979, 70, 2511. (11) Hager, J. W.; Wallace, S.C. J. Phys. Chem. 1984,88, 5513. (12) Demmer,D.R.;Leach,G.W.;Outhouse,E.A.;Hager,J.W.;Wallace, S.C. J. Phys. Chem. 1990, 94, 582. (13) Gordon, R. D.; Clark, D.; Crawley, J.; Mitchell, R. Spectrochemica Acta 1984,40A, 657. (14) Lewis, J. D.; Malloy, Jr. T. B.; Chao, T. H.; Laane, J. J. Mol. Struct. 1972, 12, 427. (15) Fateley, W. G.;Hams, R. K.; Miller, F. A.; Witkowski, R. E. Spectrochim. Acta 1965, 21, 231. (16) (a) G. Vananyi, Vibrational Spectra of Benzene Deriuatives; AcademicPress: New York, 1969. (b) G.Varsanyi, VibrationalAssignments for Vibrational Spectra of Seven Hundred Benzene Derivatives; John W h y : New York, 1974. (17) Hopkins, J. B.; Powers, D. E.; Smalley, R. E. J. Chem. Phys. 1980, 72, 5039. (18) Pitzer, K. S.;Scott, D. W. J. Am. Chem. Soc. 1943,65,803. (19) Vasudez, R.; Brand, J. C. D. Chem. Phys. 1979, 37,211. (20) Garg, S.N. Curr. Sci. 1954, 23, 50. (21) Coon, J. B.; Naugle, N. W.; McKenzie, R. D. J. Mol. Spectrosc. 1966,20, 107. (22) Mikami, N.; Hiraya, A.; Fujiwara, I.; Ito, M.Chcm. Phys. Le??. 1980, 74, 53 1. (23) Smith, M.A.; Hagar, J. W.; Wallace, S . C. J. Chem. Phys. 1984, 80, 3097. (24) Bickel, G.A,; Demmer, D. R.; Outhouse, E. A.; Wallace, S.C. J. Chem. Phys. 1989,91,6013. (25) Felker, P. W.; Zewail, A. H. J. Chem. Phys. 1985, 82, 2994. (26) Felker, P. W.; Zewail, A. H. J. Chem. Phys. 1985, 82, 2961. (27) (a) Gruner, D.; Brumer, P. J . Chem. Phys. 1991, 94, 2848. (b) D.; Brumer,P.J. Chem.Phys. 1991,94,2862. (c)chI"r, D. Doctoral Thesis, University of Toronto, 1991. (28) Nguyen, A. H. Master's Thesis, University of Toronto, 1991.
