90
J . Phys. Chem. 1990, 94, 90-98
Conclusions
We have shown that the inhomogeneous broadening of v4 observed using 30-ps pulse excitation in near resonance with the strong Soret optical transition is due to a nonlinear RR scattering pr-s i n d u d by strong resonant laser fluenes. The observation of the presence of an inhomogeneous distribution, as evidenced by the asymmetric broadening of u4, is not unexpected. Presently, it is not possible to elucidate the origin of the inhomogeneous broadening or its relationship to the functionality of hemoglobin. The results of this study also demonstrate the necessity of addressing field effects when dealing with ultrafast spectroscopic probes ( e 3 0 ps). In particular, heme protein spectra obtained with picosecond pulses should be strongly influenced by Rabi broadening. While Petrich et al.13 attributed the broadening of u A in subpicosecond sDectra (obtained at Dower densities >lolo W/cm*) io thermalization of the heme macrocycle, it is clear from our investigation that Rabi broadening also can contribute to the
line shape of u4 at high (>lo8 W/cm*) power densities. Clearly, Rabi broadening must be considered in any complete description of heme photodynamics during intense ultrashort laser pulses. Future experiments utilizing high peak powers are necessary to determine the extent to which these nonlinear processes play a general role in the spectroscopic behavior of large molecular systems.
Note Added in Proof. A recent study by Shomacker and Champion ( J . Chem. Phys. 1989,90, 5982) has pointed out that the anharmonic exchange theory developed by Harris is not intended for situations where kT 2 hw (w being the frequency of the low-frequency mode coupling to the observed mode). Thus, the use of this approach to estimate temperatures of photopumped hemes is not appropriate. Acknowledgment. This work was performed at AT&T Bell Labs and was supported by the NSF (DMB 8604435).
Theoretical Calculations and Experimental Studies on the Electronic Structures of Hydrazones and Hydrazone Radical Cations: Formaldehyde Hydrazone and Benzaldehyde Dlphenylhydrazones J. Pacansky,* A. D. McLean, and M. D. Miller IBM Almaden Research Center, 650 Harry Road, San Jose, California 951 20-6099 (Received: January 18. 1989; In Final Form: July 7, 1989)
The geometry and electronic structure of the parent formaldehyde hydrazone, benzaldehyde diphenylhydrazone (BDPH), and p(diethy1amino)benzaldehyde diphenylhydrazone (DEH) are studied by using ab initio SCF calculations, electrochemical measurements, and electronic absorption spectroscopy. The computer-optimized geometries for formaldehyde hydrazone indicate that the neutral species is nonplanar while the radical cation is planar. The optimized geometry for BDPH and the experimental geometry for DEH reveal that the structure of the hydrazone group in the neutral systems is planar, or very close to planarity; this change is attributed to an increase in p-r conjugation between the lone pair of electrons on the hydrazone amine nitrogen and phenyl rings. The best representation for the structure of the radical cations of formaldehyde hydrazone and BDPH is one with the unpaired electron residing on the hydrazone amine nitrogen; however, the best description for the structure of the radical cation of DEH is one with the unpaired electron not on the hydrazone amine but on the aniline nitrogen.
Introduction
Molecules containing the hydrazone functional group (1) belong to a very large class of azomethines which are distinguished from
other members of this class (e.g., imines, oximines) by the presence of the two interconnected nitrogen atoms. Hydrazones are readily available due to the facile coupling of hydrazines with carbonyl-containing systems. As a result hydrazones are widely used in synthetic chemistry for the preparation of other compounds and in analytical chemistry for the identification of carbonyl compounds. Industrial uses' are as plasticizers, polymer stabilizers, antioxidants, and polymerization initiators; because of their physiological activity: as herbicides, insecticides, and plant growth stimulants; and, in the pharmaceutical i n d ~ s t r y as , ~ drugs for cancer treatment, schizophrenia, leprosy, and other maladies. ( I ) Sears, J. K.; Darby. J. R. The Technology of Plasticizers; Wiley: New York, 1982. (2) Robinson, B. Chem. Reo. 1963, 63, 373. (3) Massarani. E.; Nardi. D.: Tajana, A,; Degen, L. J . Med. Chem. 1971, 14. 6 3 3 .
0022-3654/90/2094-0090$02.50/0
In this paper we restrict our efforts to understanding the chemistry of hydrazones in copiers and printers. In particular, we examine p(diethy1amino)benzaldehyde diphenylhydrazone, DEH, which is used as a hole-transporting agent in organic layered photocond~ctors.~-~ Since organic layered photoconductors are used extensively in the electrophotographic processes in printers and copiers, there is considerable interest in understanding the nature of transport of positive charges (holes) through these films. In order for facile hole transport through an organic solid to occur (in the presence of an electric field), the hole-transporting agent must at least have a low ionization potential and, according to Murrell,* have overlap between the pertinent molecular orbitals of the neutral and charged systems. Since very little is known about the electronic structure of DEH and its radical ~ a t i o nwe .~ (4) Melz, P. J.; Champ, R. B.; Chang, L. S.; Chiou, C.; Keller, G.S.; Liclican, L. C.: Neiman, R. R.; Shattuck, M. D.; Weiche, W. J. Photogr. Sci. Eng. 1971, 21, 73. (5) Loutfy, R. 0.;Hsiao, C. K.; Kazmaier, P. M. Photogr. Sci. Eng. 1983, 27, 5. (6) Champ, R. B.; Shattuck, M. D. U S . Patent 3,824,099, 1974. (7) Anderson, H. W.; Moore, M . T. US.Patent 4,150,987, 1979. (8) Murrell, J. N. Mol. Phys. 1961, 4 , 205. (9) Pacansky, J.; Coufal, H.;Waltman, R. J.; Cox, R.; Chen, H. Radint. Phys. Chem. 1987, 29, 219.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. I, 1990 91
Hydrazones and Hydrazone Radical Cations TABLE I: Optimized Geometries for Formaldehyde Hydrazone and Formaldehyde Hydrazone Radical Cation"
formaldehyde hydrazone RHF/ RHF/ coordinate RI, 8, R2, 8, R3, 8, R4, 8, R5, 8, R6, 8, a l , deg a2, deg a3,deg a4, deg a5, deg
~ , deg b total energy,
STO-3G
6-31G*
formaldehyde hydrazone+ RHF/ RHF/ STO-3G 6-31G*
1.085 1.074 1.098 1.084 1.098 1.088 1.278 1.25 1 1.306 1.371 1.446 1.440 1.003 1.057 1.037 1.034 0.997 1.053 118.44 115.70 1 18.72 123.42 125.82 124.37 1 14.79 118.41 115.70 108.93 115.82 106.17 110.31 113.55 126.05 32.21 28.27 0.79 -147.13405 -149.02926 -146.921 39
1.013 1.009 1.314 1.277 1.072 1.077 116.71 122.76 117.16 116.87 123.96 0.000 -148.75688
Figure 1. Computer drawing of the structure of the parent hydrazone. The labels are used to define the geometrical parameters listed in Table I.
'H20
H1
hartrees
"See Figure 1 for pertinent atomic labels. (NZN3)(H6N3H7).
bBond-phase angle H31a
undertook an ab initio S C F molecular orbital study to investigate the electronic structure for both systems. An ab initio study on a molecule the size of DEH is itself a challenge; we therefore chose only to obtain a qualitative description for the electron distribution in the neutral and charged systems. Structurally DEH may be considered as either an aniline or a hydrazone derivative. It is known that an increase in electron acceptor character of substituents attached to the hydrazone group is accompanied by a fall in basicity of the amine nitrogen.I0 This, of course, is a direct result of the p a conjugation" between the C=N bond and the lone-pair electrons on the hydrazone amine nitrogen. Since p a conjugation is favored by a planar sp2 amine nitrogen, which in turn greatly affects the ionization potential, we therefore decided initially to study benzaldehyde diphenylhydrazone (BDPH) and DEH to determine where the ionization site was located, the structure of the radical cation, and the salient structural features responsible for p a conjugation. In addition, we present preliminary results for formaldehyde hydrazone CH2=NNH2, the parent hydrazone, to serve as a reference. Computational Details Calculations on DEH and DEH+ were performed with the ~ an STO-3G basis set13 in the computer code A L C H E M Y , ~with ROHF formulism. Total energies were computed for neutral and charged DEH by using coordinates obtained by X-ray crystal diffraction a n a l y ~ i s . ' ~ Ab initio calculations on formaldehyde hydrazone, BDPH, and their radical cations, respectively, were performed by using the vectorized IBM version of the Gaussian 86 programI5 with the STO-3G basis set at the R O H F level of theory. In addition, R O H F calculations were also performed on formaldehyde hydrazone, and its radical cation, using the 6-31 G* basis set. Furthermore, complete geometry optimizations were performed (IO) Pausacker, K. J . Chem. Soc. 1950, 3478. (1 1) Kitaev, Y. P.; Buzykin, B. I.; Troepol'skaya, T. V. Russ. Chem. Reu. 1970, 39, 441. (12) The molecular integrals were calculated by using the MOLECULE program written by J. Almlof. The SCF calculations were performed with the ALCHEMY program written by P. S . Bagus, B. Liu, A. D. McLean, and M. Yoshimine. (1 3) For a description of the basis set and MP2, see: Hehre, W.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab inirio Molecular Orbital Theory; Wiley Interscience: New York, 1986; Chapter 4. (14) Pacansky, J.; Coufal, H. C.; Brown, D. W. J . Phorochem. 1987,37, 293. ( I S ) Frisch, M. J.; Binkley, J. S.;Schlegel, H. B.; Raghavachari, K.; Melius, C. F.; Martin, R. L.; Stewart, J. J. P.; Bobrowicz, F. W.; Rohlfing, C. M.; Kahn, L. R.; DeFrees, D. J.; Seeger, R.; Whiteside, R. A,; Fox, D. J.; Fluder, E. M.; Pople, J. A. Gaussian 8 6 Carnegie-Mellon Quantum Chemistry Publishing Unit: Pittsburgh, PA, 1984.
bH33
Figure 2. Computer drawing of the structure of benzaldehyde diphenylhydrazone. The labels are used to define the geometrical parameters listed in Tables I1 and 111. h
0
Figure 3. Computer drawing of the structure of p-(diethy1amino)benzaldehyde diphenylhydrazone. The labels are used to define the geometrical parameters listed in Table IV.
on the parent hydrazone, BDPH, and their radical cations, respectively. Experimental Section DEH and BDPH were prepared according to literature procedures.16 Electrochemical properties of the photoconductor were investigated with a single compartment cell using Pt, Au, and saturated calomel as the working, counter and reference electrodes, respectively. Solutions for the electrochemical analysis typically contained several micromoles per cubic centimeter hydrazone in 0.1 M tetraethylammonium fluoroborate (TEAFB)/acetonitrile solution. The acetonitrile (Burdick and Jackson, spectrometric grade) was used without further purification. The electrochemical (16) Fieser, F.; Fieser, D. Aduanced Organic Chemistry; Reinhold: New York, 1975.
Pacansky et ai.
92 The Journal of Physical Chemistry, Vol. 94, No. 1, 1990 TABLE 11: Optimized Geometries for Benzaldehyde Diphenylhydrazone" Bond Lengths C2C3 1.083 C2H 1 c 2 cIO 1.083 C3H4 1.083 c3c5 C5H6 1.083 csc7 C8H9 C7C8 1.083 ClOHlI 1.086 C8CIO C12H13 1.079 c7c12 C17H18 C12N14 1.083 C I9H20 C30H3 1 1.081 C21H22 1.083 C32H33 C23H24 1.079 C34H35 C25H26 1.083 C36H37 C28H29 c17c19 1.398 CI 6C25
H 1 C2C3 HIC2CIO H4C3C2 H4C3C5 H6C5C3 H6C5C7 H9C8C7 H9C8C10 HIICIOC8 HIICIOC2 c12c7c5 CI 2C7C8 H 13CI 2C7 N I 5N 14C I 2 CI 6N I5C27
120.12 120.12 120.05 119.85 119.89 119.52 1 19.08 120.47 119.83 1 19.94 119.59 120.54 116.46 117.88 121.29
H 1 C2C3H4 HlC2ClOHll H4C3C5H6 H6C5C7C 12 H9C8C7C I2 HI lCIOC8H9 H13C12C7C5 H I3C12N 14N 1 5 C16N 15N 14C12
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 180.00
(A)
Bond Angles (deg) N 15C16C17 N I 5C16C25 H18C17C16 HI 8C17C19 H20C19Cl7 H20C19C2I H22C21C19 H22C21C23 H24C23C21 H24C23C25 H26C25C23 H26C25C16 H13C12N14 N14N15C27 Torsional Angles (deg) C 17C16N15N I4 C25C16NI 5N I4 H 18C17C16NI 5 H20C19C17H I8 H22C21C19H20 H24C23C21H22 H26C25C23H24 c25c23c21c19 C27N l5C16C 17
1.386 1.388 1.385 1.394 1.396 1.384 1.493 1.287 1.083 1.083 1.083 1.083 1.385
1.398 1.392 1.394 1.394 1.386 1.387 1.387 1.386 1.409 1.438 1.457 1.396 1.385
c21c23 C23C25 C27C28 C27C36 C28C30 C30C32 C32C34 C34C36 N14N15 N15C16 N1x27 C16C17 c19c21
120.37 88.30 1 19.99 1 19.92 119.21 120.03 120.43 120.41 119.92 119.26 120.8 119.89 123.34 122.37
N I 5C27C28 N I 5C27C36 H29C28C27 H29C28C30 H3 1C30C28 H31C30C32 H33C32C30 H33C32C34 H35C34C32 H35C34C36 H37C36C34 H37C36C27 N 14C12C7 N 14N 15C16
120.19 120.18 119.36 120.57 119.85 120.07 119.96 1 19.96 120.08 119.85 120.56 119.36 120.2 116.34
180.00 0.00
C27NISN14C12 C28C27N15N14 H29C28C27N15 H3 IC30C28H29 H33C32C30H3 1 H35C34C32H33 H37C36C27NI 5 C36C27N 15N 14 C34C36C27N15
-0.01 89.99 0.05 -0.01 -0.01 0.01 -0.04 -89 -I 7
-0.01
0.00 0.00 0.00 0.00 0.01 0.02
Bond-Plane Angles (deg) T(
0.0
(N 14N 1 5 ) (C16N 1 x 2 7 ) )
'See Figure 2 for pertinent atomic labels.
10
10
LUMO
LUMO
=
5
0
F-
I 1
-LUI
z
e ?
-
LUMO
F
T O iLi
w
0
P -10
=
HOMO
-5
-10
-HOI -15
-20
Neutral
Radical Cation
RHF
UHF
Figure 4. Molecular orbital energies (units eV) for the neutral and cationic parent hydrazone.
Neutral
Radical Cation
RHF
UHF
Figure 5. Molecular orbital energies (units eV) for the neutral and cationic benzaldehyde diphenylhydrazone.
Hydrazones and Hydrazone Radical Cations
The Journal of Physical Chemistry, Vol. 94, No. 1 , 1990 93
TABLE 111: Optimized Geometries for Benzaldehyde Diphenylhydrazone Radical Cation" Bond Lengths
C2H I C3H4 C5H6 C8H9
ClOHll C12H13 C17H18 C19H20 C2I H22 C23H24 C25H26 C28H29 C 16C25
1.085 1.083 1.084 1.084 1.083 1.084 1.082 1.083 1.085 1 .OS3 1.083 1.084 1.399
(A)
C2C3 c2c10 c3c5 c5c7 C7C8 C8C10 c7c12 C12N14 C30H31 C32H33 C34H35 C36H37 C17C19
1.391 1.392 1.383 1.399 1.400 1.382 1.474 1.311 1.084 1.084 1.084 1.084 1.381
C21C23 C23C25 C27C28 C27C36 C28C30 C30C32 C32C34 C34C36 N14N15 N15C16 NI 5C27 C16C17 C19C21
1.391 1.389 1.389 1.389 1.387 1.390 1.390 1.386 1.441 1.446 1.491 1.400 1.393
Bond Angles (deg)
H IC2C3 H 1 C2C10 H4C3C2 H4C3C5 H6C5C3 H6C5C7 H9C8C7 H9C8C10 HI lClOC8
HI lClOC2
c12c7c5
C12C7C8 H l3C12C7 N 15N 14C12 C 16N I5C27
119.73 119.71 120.15 120.07 120.16 119.86 119.54 120.6 120.04 120.05 118.84 121.29 117.02 1 13.63 120.78
N 15C16C17 NI 5C16C25 H 18C17C16 H 18C17C19 H20C19C17 H20C19C21 H22C21C19 H22C21C23 H24C23C21 H24C23C25 H26C25C23 H26C25C 16 H13C12N14 N14N15C27
1 19.47 119.37 120.58 120.41 119.69 120.16 120.68 118.77 120.18 119.65 121.20 119.85 122.47 124.21
N 15C27C28 N15C27C36 H29C28C27 H29C28C30 H31C30C28 H31C30C32 H33C32C30 H33C32C34 H35C34C32 H35C34C36 H37C36C34 H37C36C27 N 14Cl2C7 N14N15C16
180.19 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.02
C27N15N14CI2 C28C27N 15N14 H29C28C27N15 H31 C30C28H29 H33C32C30H3 1 H35C34C32H33 H37C36C27NI5 C36C27N15N14 C34C36C27N15
1 18.80 118.78 120.54 121.10 119.61 120.25 119.71 119.72 120.25 119.61 121.10 120.53 120.51 115.01
Torsional Angles (deg)
H lC2C3H4 H 1 C2ClOH I I H4C3C5H6 H6C5C7CI2 H9C8C7CI2 H 1 IClOC8H9 H 13C12C7C5 N 15N 14C12C13 C 1 6N 1 5N I 4C1 2
0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.02 1 80.12
C17C16N15N14 C25C16N15Nl4 H 18Cl7C16N15 HZOC19C17H18 H22C21 C 19H20 H24C23C21H22 H26C25C23H24 c25c23c21c19 C27N15C16CI7
0.15 90.06 -0.41 -0.10 0.00
0.02 0.04 -9 -1
Bond-Plane Angles (deg) T(
(N14N 15) (C16N15C27))
0.01
'See Figure 2 for pertinent atomic labels.
measurements were performed on an IBM EC225 Voltammetric Analyzer. UV-visible measurements were carried out on a Perkin-Elmer Lambda Array UV/visible spectrophotometer equipped with a Perkin-Elmer 7700 data station.
Results The results for the geometry optimization for the neutral parent hydrazone and its radical cation are listed in Table I; the geometric parameters are described in Figure 1. The same results for BPDH are listed in Table 11 and the atom labels are described in Figure 2; Table 111 contains the optimized geometry for the BDPH radical cation. The geometric parameters used for DEH are listed in Table IV, and the molecular structure is illustrated in Figure 3. The molecular orbital energies are shown in Figures 4-6 for the neutral and radical cations of the three systems studied in this report. Figures 11 and 12 summarize the results for the Mulliken population analysis of DEH and DEH+. In order to show where charge has been removed upon ionization, the difference in Mulliken population analysis between the neutral and radical cation is presented in Figure 13. Extensive use is made of orbital plots for the molecular orbitals so that a clearer picture may be obtained for the electronic structure of the neutral and radical cations. Figures 14-18 show the HOMOS (highest occupied molecular orbital) of all systems a n d several pertinent SHOMOs (second highest occupied molecular orbital) and THOMOs (third highest occupied molecular orbital). The total energies for DEH, DEH+, and DEH2+, are listed in
LUMO-
5 LUMO
I o P
I1
W
HOMO.
-10
-15
SHOMO
'
Neutral RHF
Radical Cation ROHF
Figure 6. Molecular orbital energies (units eV) for the neutral and cationic p(diethy1amino)benzaldehyde diphenylhydrazone.
Table V; the first and second ionization potentials obtained by subtracting the total energies, respectively, are also given. The values for the computed ionization potentials, 4.16 and 13.2 eV, contain errors which are related to the limited basis set chosen
94 The Journal of Physical Chemistry, Vol. 94, No. 1, I990
Pacansky et al.
TABLE IV: Geometry for p-(Diethy1amino)benzaldehydeDiphenylhydrazone (DEH) Obtained via X-ray Structure A n a l y s i ~ ' ~ " * ~ Bond Lengths (A)
CI IC12
1.385 1.382 1.373 1.356 1.354 1.384 1.376 1.367 1.376
ClC2 C2C3 c3c4 c4c5 C5C6 C6C 1 C7C8 c9cIO
ClOClI
1.357 1.396 1.400 1.443 1.381 1.278 1.449 1.408 1.383
c12c7 NlCl NlC7
NlN2 C13N2 c13c14 C14CI5 C15C16
C16C17 C17C18 C18C19 c14c19 C17N3 N3C22 N3C20
1.396 1.392 1.378 1.379 1.374 1.452 1.470
Bond Angles (deg)
117.6 120.9 121.2 118.9 121.0 121.6 116.6 119.9 121.4 118.7 119.8 120.0 122.8
N2NlCl N2NIC7 CINIC7 N 1 N2C13 C17N3C20 C17N3C22 C20N3C22
NlClC2 NICIC6 C2C 1C6 cI c2c3 C I2C7C8 N2C 13CI4
119.8 119.4 121.5 120.8 120.4 119.6 120.0 119.1 121.4 118.8 120.7 119.9 124.1
c2c3c4 c3c4c5 C4C5C6 ClC6C5 N 1C7C8 NlC7C12 C8C7C 12 C7C8C9 C8C9C10 C9ClOCI 1
CIOC1IC12 C7C12Cll CI3c14c15
CI3c14c19 CI5c14c19 C 14C15C16 C15C16C17
N3C 17Cl6 N3C17C18
C 16C17Cl8 CI7C18C19 CI4Cl9Cl8 N3C20C21 N3C22C23
119.1 116.8 120.5 121.7 121.2 121.3 117.5 120.3 123.0 112.8 113.8
Torsional Angles (deg)
C 1N 1N2C 13
CIN lC7C12
179.4 -6.6 -171.9
C7N I N2C13
N2N lClC2 N2N lClC6 C7N lClC2 C7N lClC6 N2N lC7C8 N2N lC7C12 CIN IC7C8
NIN2C13C14 C20N3C17Cl6 C20N3C17C18 C22N3C17C16 C22N3C17C18
10.1
14.1 -163.8 94.7 -84.8 -91.5
N lClC2C3 NlClC6C5 N 1C7C8C9
88.9 -178.5 -173.6 6.1 -4.1 175.6 -178.7 178.4 179.9
NIC7C12Cll N2C13C14C15 N2C13C14C19 C13C14C15C16 c13c14c19c18 C 15C16C17N3 N3C17C18C19 C 17Cl8C19C14
179.4 -2.3 176.7 179.8 -178.9 179.0 -178.1 -I .4
Bond-Plane Angles (deg) T(
(NlN2)(CINlC7))
3.0
"All CH bond lengths were set to 1.083A for the SCF calculations on DEH and DEH'.
TABLE V Total Energies and Computed Vertical Ionization for DEH and DEH+
DEH DEH' DEH2+
total energy, hartrees
IP, eV
-1035.522493 -1035.369616 -1035.036749
4.16 13.2
for the analysis and neglect of correlation effects. The effect due to the former is best answered by repeating the calculation using larger basis sets; these calculations are in progress and will be subsquently reported. Now we only state that basis set errors will not change the thesis of this report: that the first ionization potential of DEH is indeed very low and use of a more complete basis set will not alter this qualitative picture.
Discussion The len th of the C=N bond in hydrazones varies between 1.27 and 1.35 depending on the substituents R, R', X, and Y.I4s1'
%,
X
R
\
C=N-N,
d
1
Y
The length of the C=N is affected by the presence of the lone pair of electrons on the amine nitrogen which are capable (17) Galigne, P. J.; Falguerettes, J. Acra Crystallogr. 1968, B24, 1523. Hamilton, W.C.; LaPlaca, S. J. Acra Crysrallogr. 1968, B24, 1147.Bjamer, K.;Furberg, S.: Petersen, C. S . Acra Chem. Scand. 1964, 18, 587.
Figure 3 for pertinent atomic labels
of conjugation with A electrons of the hydrazone group, thus leading to an increase in bond length with the extent of A conjugation. The length of the N-N bond is reported to vary between 1.38 and 1.41 A. The length of the N - C bond (Le., between the amine nitrogen and a C atom in X or Y) is close to the value for a C-N single bond, 1.47 A, but decreases when conjugation is present between the hydrazone group and the X or Y group. The C=N-N bond angle is close to 120'. The energy of the C=N bond'* has been reported as 94, 132, and 139.5 kcal/mol. The energy of the N-N bond obviously depends on the nature of the substituents R' and Y but has not been determined for hydrazones. However, it is safe to state that the N-N bond is the weakest bond in the hydrazone group. Hydrazones are a convenient model for studying p-A conjugation. The major condition for interaction between the A electrons of the C=N bond and the lone pair of electrons on the amine nitrogen atom is that the hydrazone group be coplanar, thus permitting appropriate orientation of p and A orbitals. In the absence of steric hindrance, nitrogens attached to aromatic rings prefer planar conformations over gauche or perpendicular conformations. As shown by the calculated structure for the parent CH2= N-NH,, the hydrazone group is nonplanar; that is, the amine group H6N3H7 (see Figure 1) is not coplanar with the C=N double bond. As listed in Table I, T, the angle between the N2-N3 bond and the plane formed by the H6N3H7 atoms, is 28.3". Thus the electronic configuration about the amine nitrogen contains substantial sp3 character, and orientation of the orbital containing (18) Layer, R. W. Chem. Rev. 1963, 63, 489.
Hydrazones and Hydrazone Radical Cations its p electrons is not parallel to the C=N A bond. Substitution of the hydrogens on the amine nitrogen, and one olefinic CH bond with groups that favor p-A interaction, like phenyl rings, produces BDPH. This changes the electronic structure of the hydrazone group so that the pyramidal configuration of the amine group transforms to a planar structure. As a consequence, the hybridization of the amine nitrogen atom approaches sp2,thus increasing the overlapping of p and a orbitals of the hydrazone group. The entire system tends to form a planar geometry: thus, in addition to the p-A interaction, extensive conjugation occurs between the a systems of the phenyl rings and the p a system of the hydrazone group-this is usually referred to as p-x-~ conjugation. The effect of the phenyl is clearly seen by inspection of Table I1 and Figure 2 for the optimized geometry of neutral and the radical cation of BDPH. Comparison of these results to the structure of DEH determined by use of X-ray diffraction analysis (Table I11 and Figure 3) shows that the system also contains an extended a system from the amine nitrogen of the aniline group, N3, to the coplanar phenyl which is attached to the amine nitrogen of the hydrazone functional group, N I . The other phenyl ring attached to N I is perpendicular to the a system and as a result has its molecular orbitals, and hence a system, orthogonal to the extended A system. The pertinent feature of the DEH structure is that the hydrazone group is very close to a planar geometry and hence the amine nitrogen of the hydrazone has a sp2 electronic configuration. After studying the results of the BDPH geometry optimization this last detail is reasonable since we do not expect the addition of a diethylamino group to one end of a large system like BDPH to change the geometry at the other end. Furthermore, the crystal structure verifies this theoretical result allowing us to establish important trends in large molecular systems without lengthy X-ray structure analysis. The energy level diagrams for CH2=N-NH2, CH2=NNH2+,BDPH, BPDH', DEH, and DEH+ are shown in Figures 4-6, offering a qualitative description of the electronic structure for each. First of all, due to differences in correlation energy one must view the relative energies of the molecular orbitals qualitatively. The HOMO, LUMO gaps for the neutral species are much wider than the HOMO, LUMO gaps for the radical cations. Since the gap is approximately equal to the energy required for excitation to the lowest singlet excited state of a neutral system, then this process usually requires more energy than electronic excitation from the ground to first excited state of the radical cation. For example, experimentally the first electronic transition for DEH is centered at 360 nm rendering the material a pale yellow color;I4 the corresponding transition for the cation is predicted to be shifted to the longer wavelength part of the visible spectrum. For DEH+, the electronic transition requiring the lowest energy is promotion of an electron from the doubly occupied SHOMO to the half-filled HOMO. As indicated via the energy level diagram in Figure 6, relative to DEH, this requires much less energy; the radical cation should absorb at longer wavelengths toward the visible part of the electromagnetic spectrum. In order to establish this, the electronic absorption spectrum of DEH' was obtained by use of electrochemical techniques (see Experimental Section). DEH was oxidized at its first oxidation potential at 0.58 V. The electrochemistry was performed in a cell fitted with quartz windows so that electronic absorption spectroscopy could be used to follow the course of the oxidation. A spectrum recorded before and after oxidation is shown in Figure 7 . The band centered in the UV at 360 nm corresponds to the absorption for the first excited electronic state of DEH. The band lying in the visible region of the spectrum is due to DEH'. Thus far the structures for the neutral parent, BDPH, and DEH have revealed that when the hydrazone functional group is planar we have an extended a system and that the planarity is favored by groups that promote p-a overlap attached to the amine nitrogen; the resultant effect is to change the electronic configuration around the amine nitrogen from a pyramidal sp3 to a planar sp2. These concerns do not appear to be as relevant for the structure of the radical cations of hydrazones. The amine nitrogen now
The Journal of Physical Chemistry, Vol. 94, No. I , 1990 95
-
-Max = 0 600 A
-
DEH Radical Cation
-
300
400
500
600
700
800
900
Wavelength (nm)
Figure 7. Electronic absorption spectra for DEH and DEH' in aceto-
nitrile.
Figure 8. Orbital plots for the HOMOs of the neutral and radical cation of the parent hydrazone.
only contains one electron, is charged positively, and in the systems examined exists in an sp2 configuration. This is clearly seen by examination of Table I for the radical cation of the parent hydrazone. The neutral system is nonplanar but upon removal of an electron the system goes to a planar geometry; it reasonable to expect this to occur for nonplanar neutral hydrazones in general. An important aspect of this study is to determine the salient features of electronic charge distribution before and after removal of an electron. Due to the past experience demonstrating the relevance of the energy and electronic distributions of HOMOs on chemical processes, we have concentrated our efforts on the HOMOs and other pertinent molecular orbitals. The best place to start is the parent system; orbital plots for the HOMO of the neutral and radical cation are shown in Figure 8a and Figure 8b, respectively. The HOMO for the neutral system consists of three p orbitals, two of which form the C=N bond and the third contains the lone pair of electrons on the amine nitrogen (note the asymmetry of the lone-pair orbital due to the pyramidalization of the amine nitrogen, N2). Upon ionization all of the p orbitals are perpendicular to the molecular plane, and as indicated by the orbital plot in Figure 8b, the HOMO for the radical cation consists of the C=N bond plus an electron essentially localized in a p orbital on the amine nitrogen. This situation also exists for BDPH, as shown in Figures 9 and 10 for the orbital plots of the HOMOs of the neutral and radical cation, respectively. The orbital plot for the HOMO of the neutral clearly shows the extent of the extended T system from the phenyl ring of the benzaldehyde part of the hydrazone to the phenyl ring of the amine part; it also reveals that the coplanar phenyl ring bonded to the amine nitrogen contributes significantly to the HOMO. Upon ionization the orbital plot for the hydrazone moiety is very similar to that for the parent radical cation, with the exception that some of the charge in the now half-filled molecular orbital is delocalized over the coplanar phenyl attached to the amine nitrogen. However,
96
The Journal of Physical Chemistry, Vol. 94, No. 1 I990
Pacansky et al.
~
0.901
0905 W 0902 e -
7 c36 060407
W
5
1
:-C H \
H
Figure 9. Orbital plot for the HOMO of the benzaldehyde diphenyl-
hydrazone.
U
0.860
08'0
Figure 12. Mulliken population analysis for p-(diethy1amino)benz-
aldehyde diphenylhydrazone radical cation.
U W l L
Figure 10. Orbital plot for the HOMO of the benzaldehyde diphenylhydrazone radical cation.
0011
0.035
0 922
0 925
Figure 13. Difference between the Mulliken population analysis for
neutral p-(diethy1amino)benzaldehyde diphenylhydrazone and its radical cation.
Figure 11. Mulliken population analysis for p(diethy1amino)benz-
aldehyde diphenylhydrazone.
the salient features are still the same; that is, the HOMO for the hydrazone group in this large molecule consists of a C=N a bond and a coplanar p orbital on the amine nitrogen, and for the most part the charge may be considered as localized on or in the immediate vicinity of the hydrazone group. The DEH structure contains two functional groups, a hydrazone and an aniline group. Both characteristically have low ionization potentials and thus may play major roles in the removal of an
electron from DEH; a further consideration is the interaction of these two groups through the extended A system. Initially, we calculated Mulliken population analysis for DEH and DEH+ to determine where the major changes were occurring in the electronic distribution as a result of ionization. The gross Mulliken population analysis for DEH and DEH+ shown in Figures 11-13, and orbital plots contained in Figures 14-1 8 qualitatively describe the electronic distributions before and after the ionization process. The Mulliken population analysis for DEH, compared with the DEH' population analysis, reveals that the aniline nitrogen has lost the most charge as a result of the ionization; other atoms have lost charge but to a much smaller degree. A better way to display the effect of the ionization is shown in Figure 13 by the difference
The Journal of Physical Chemistry, Vol. 94, No. I , 1990 91
Hydrazones and Hydrazone Radical Cations H
‘H
H
\
I
n
/c-c\ \ E-c/ ”
C
I
\
H -
yC
H
\ I
1
H
F-C
H ‘n
H ‘H
Figure 14. Orbital plot for the HOMO of the Mulliken population
analysis for p(diethy1amino)benzaldehyde diphenylhydrazone.
Figure 18. Orbital plot for the THOMO of p-(diethy1amino)benzaldehyde diphenylhydrazone radical cation.
H
Figure 15. Orbital plot for the SHOMO of p-(diethy1amino)benzaldehyde diphenylhydrazone.
I
-0.4
Figure 16. Orbital plot for the HOMO of p(diethy1amino)benzaldehyde diphenylhydrazoneradical cation.
.
I
c-c ;f H’
Figure 17. Orbital plot for the SHOMO of p(diethy1amino)benzaldehyde diphenylhydrazone radical cation.
in the population analyses of DEH and DEH’. Here, the negative sign indicates an increase in electron density after ionization while a positive sign indicates a decrease. The charge on the amine nitrogen N6 of the aniline group has clearly decreased. The atoms N4 and N5 of the hydrazone group decrease in charge by 0.025 and 0.056 of an electron charge while much smaller contributions changes occur on the rest of the A system, for example, the coplanar phenyl ring C24 to C25, and C37, C38, and C42. The scenario presented by the population analysis of DEH and DEH+ is vividly displayed by orbital plots for the HOMOs, SHOMOs, and also the THOMOs. The DEH HOMO, shown in Figure 14, is a direct manifestation of the delocalized A system which extends from the aniline amino group to the coplanar phenyl
I
0
I
0.4 E (Volts)
I
I
0.8
1.2
Figure 19. Cyclic voltammogram for the oxidation-reduction of (a) p-(diethy1amino)benzaldehyde diphenylhydrazone (DEH) and (b) benzaldehyde diphenylhydrazone (BDPH).
ring at the opposite end of the A system. The other phenyl ring (C30-C35)is perpendicular to the extended A system and, if frozen in this position, remains orthogonal and contributes negligibly to any of the HOMOs. The DEH SHOMO, drawn in Figure 15, is also a reflection of the extended A system. In sharp contrast to the DEH HOMO and S H O M O is the relatively localized nature for the orbital plot of DEH+ illustrated in Figures 16 and 17. The single electron in the H O M O is primarily localized on the amine nitrogen of the aniline group with some contribution from the adjacent phenyl ring. The striking feature of the DEH+ HOMO is that the orbitals of the hydrazone functional group are negligibly involved in the wave function. A straightforward conclusion is that the oxidation of DEH is for the most part controlled by the aniline groups and not the hydrazone group and that the charged system should be labeled radical cation of a para-substituted aniline rather than DEH’. The orbital plot for the SHOMO continues the localized trend, but now the orbitals that contribute to the wave function are localized on the opposite end of the radical cation. Experimental confirmation for the theory presented above was obtained by performing electrochemical measurements on BDPH and DEH. Figure 19 shows the results for cyclic voltammetric experiments on both systems. At the top of Figure 19 the results are displayed for DEH; the vertical axis is the current while the horizontal is voltage; thus the trace to the right results when oxidation occurs at the anode; conversely the trace to the left is the result of reduction of the cationic species at the cathode. For DEH the oxidation-reduction is reversible, as discussed in a
J . Phys. Chem. 1990,94, 98-104
98
previous r e p ~ r t .The ~ maximum at the first peak of the anodic current versus voltage occurs at 0.59 V and corresponds to the one-electron oxidation of DEH to DEH'; the second is at 1.04 V and corresponds to the two-electron oxidation of DEH to DEH2+. Only one peak at 1.08 V is found in the electrochemical oxidation of BDPH which corresponds to the one-electron oxidation to BDPH+; furthermore, as indicated by the large difference between the areas under the curves for the anodic current and cathodic current versus voltage, respectively, this one-electron oxidation-reduction process is not reversible. Thus the oxidation of BDPH to its radical cation requires more energy than for DEH and clearly is not the same process as the one-electron oxidation in DEH. In fact, electrochemical studies on aniline derivatives19 show that N,N-dimethylanilines form radical cations, under the same conditions at -0.7 V, and that when good electron-donating (19) Nelson, R . F. In Technique ofElectroorganic Synthesis; Weinberg, N . L., Ed.; Wiley: New York, 1974; Vol. V, p 535.
groups are substituted in the para position of the phenyl in the aniline structure the anodic oxidation potential decreases to -0.5 V. As a consequence, we conclude that the theoretical work discussed above explains and supports the electrochemical results that the oxidation of DEH to DEH' should be viewed as a one-electron oxidation of an aniline derivative; thus, a better picture for the radical cation of DEH is where the unpaired electron is localized on the aniline nitrogen with some delocalization into the aniline phenyl ring. The oxidation of BDPH is best described as a one-electron oxidation out of the hydrazone group. The best description for the structure of the BDPH radical cation is one where the unpaired electron resides on the amine nitrogen of the hydrazone group with some delocalization on the C=N double bond and the phenyl ring attached to the amine nitrogen. Registry No. DEH, 68189-23-1; DEHt, 123701-13-3; DEH2+, 123701-14-4; BDPH, 966-88-1; BDPH (radical cation), 123805-62-9; formaldehyde hydrazone, 6629-9 1-0; formaldehyde hydrazone (radical cation), 123805-61-8.
Photoinduced Intramolecular Charge Transfer and Trans-Cis Isomerization of the DCM Styrene Dye. Picosecond and Nanosecond Laser Spectroscopy, High-Performance Liquid Chromatography, and Nuclear Magnetic Resonance Studies Martine Meyer,? Jean-Claude Mialocq,* and Bruno Perly CEA CENISaclay, IRDI/DESICP/DLPC/SCM/URA 331 CNRS, F-91191 Gif-sur- Yvette CZdex, France (Received: January 23, 1989: In Final Form: June 21. 1989)
The photoexcitation of 4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamino)styryl]-4H-pyran(DCM) induces a large intramolecular charge transfer (ICT) from the dimethylamino electron-donor group to the dicyanomethylene acceptor group. The dramatic effect of the solvent polarity on the absorption and fluorescence spectra on the one hand and the competition between the nonradiative SI So deactivation and trans cis isomerization processes on the other hand has been examined. Our results clearly show that DCM isomerization efficiency is very low in the more polar solvents. The SI So internal conversion may intervene at a torsional angle smaller than 90° before reaching the perpendicular configuration.
-
Introduction The compound 4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamino)styryl]-4H-pyran(DCM) is a very efficient arylidene laser dye.' DCM laser properties are remarkable as regards the weak overlap between its absorption and emission spectra, broad tunability, high conversion efficiency, and low pumping threshold.* The DCM large Stokes shift in poly(methy1 methacrylate) (PMMA) is also valuable in luminescent solar concentrators.) The DCM spectral shift of the absorption and fluorescence spectra with solvent polarity" is related to charge transfer properties in the molecule, which possesses a dimethylamino electron donor chromogen linked to two cyano electron acceptor chromogens by an unsaturated bridge.5 The fact that cyano groups give rise to highly polar merocyanine-type structures is well-kn~wn.~From the DCM Stokes shift values ( v A - vF) in 25 solvents, the Lippert and Mataga theory,6 and a vectorial analysis of the DCM ground-state dipole moment, F~ = 6.1 D, we have estimated the dipole moment of the DCM fluorescent singlet excited state, F~ = 26.3 D.4C A dual fluorescence has been observed by Hsing-kang et al. in polar solvents and attributed to two emitting states in dynamic equilibrium maintained during their decay which was found to be single-exponential and wavelength inde~endent.~" Drake et al. also concluded to a single e ~ p o n e n t i a l .However ~~ these findings were obtained with a single photon counting system and a nanosecond discharge lamp. Recently using a better time 'Also with Quantel SA, F-91941 Les Ulis Ctdex, France. *To whom correspondence should be addressed.
-
-
resolution and picosecond laser excitation, we have concluded that the " i n t r i g ~ i n g "photophysical ~~ picture of DCM should include the trans-cis equilibrium under ambient light.4f We showed indeed that the fluorescence temporal profile could be fitted with a sum of two exponential decays attributable to the trans and cis isomers, (1) Webster, F. G.; McColgin, W. C. US.Patent 3,852,683, Dec 3, 1974. (2) (a) Hammond, P. R. Opr. Commun. 1979,29,331. (b) Hargrove, R. S.; Kan, T. IEEE J . Quantum Electron. 1980, 16, 1108. (c) Marason, E. G. Opt. Commun. 1981,37, 56. (d) Antonov, V. S.; Hohla, K. L. Appl. Phys. 1983,832.9. ( e ) Chen, C. H.; Kramer, S. D. Appl. Opt. 1984, 23, 526. (f) Broyer, M.; Chevaleyre, J.; Delacrttaz, G.; Whte, L. Appl. Phys. 1984, B35, 31. (8) Speiser, S.; Shakkour, N. Appl. Phys. 1985, B38, 191. (h) Taylor, J. R. Opt. Commun. 1986, 57, 117. (3) (a) Batchelder, J. S.; Zewail, A. H.; Cole, T. Appl. Opt. 1981,20, 3733. (b) Drake, J. M.; Lesiecki, M. L.; Sansregret, J.; Thomas, W. R. L. Appl. Opt. 1982, 21, 2945. (c) Sansregret, J.; Drake, J. M.; Thomas, W. R. L.; Lesiecki, M. L. Appl. Opt. 1983,22, 573. (d) Mugnier, J.; Dordet, Y.; Pouget, J.; Le Bris, M. T.; Valeur, B. Sol. Energy Mater. 1987, 15, 65. (4) (a) Hsing-kang, 2.; Ren-Lan, M.; Er Pin, N.; Chu, G. J . Photochem. 1985, 29, 397. (b) Drake, J. M.; Lesiecki, M. L.; Camaioni, D. M. Chem. Phys. Lett. 1985, 113, 530. (c) Meyer, M.; Mialocq, J. C. Opt. Commun. 1987, 64, 264. (d) Meyer, M.; Mialocq, J. C. J. Phys. Colloq. C7. Suppl. N.12 1987.48, 541. (e) Mialocq, J. C.; Meyer, M. Ultrafast Phenomena VI; Springer Series in Chemical Physics 48; Yajima, T., Yoshihara, K., Harris, C. B., Shionoya, S., Eds.; Springer-Verlag: New York, 1988; p 559. (f) Meyer, M.; Mialocq, J. C.; Rougee, M. Chem. Phys. Left. 1988, 150,484. (5) Griffiths, J. Colour and Comtitution oforganic Molecules; Academic Press: New York, 1976; p 140. (6) (a) Lippert, E. Z . Naturforsch., A : Astrophys., Phys., Phys. Chem. 1955, IOa, 541. (b) Lippert, E. Z . Elektrochem. 1957,61,962. (c) Mataga, N.; Kaifu, Y . ; Koizumi, M. Bull. Chem. SOE.Jpn. 1956, 29, 465.
0022-3654/90/2094-0098$02.50/00 1990 American Chemical Society
