J . Phys. Chem. 1989,93, 3436-3439
3436
TABLE 11: Observed Molar Rotation at 300 nm Adjusted for Long-Wavelength CD Contributions; Comparison with Calculated Values (Units of dee cm2 dmol-I)
compound hexopyranosides
[MI%*
methyl-0-D-glucopyranoside methyl-@-D-galactopyranoside methyl-ol-D-mannopyranoside methyl-a-D-glucopyranoside methyl-a-D-galactopyranoside
pentopyranosides methyl-@-D-arabinopyranoside methyl-0-D-xylopyranoside methyl-a-D-xylopyranoside
-310 -30 +760 1490 + 1950
-460 +53 +780 +1530 +2420
-1850 -450 + 1240
-2090 -440 +1310
+
-344 +lo2 +I201 1694 +2220
+
-1469 -287 +I876
least-squares deviation from the observed CD. The KronigKramers transform of the fitted C D was then subtracted from the observed ORD. This procedure gives the background dispersion from higher energy transitions. The results at 300 nm are shown in Table 11. Table I1 indicates that, for the four compounds represented in Figure 1, the long-wavelength CD accounts for the sign of the discrepancy between the calculated and observed ORD. In two
cases the procedure overadjusts and in two cases it underadjusts. For three of the compounds represented in Figure 2, the adjustment (Table 11) is in the correct direction but is much too small to account for the discrepancy. For the fourth compound, methyl-P-D-arabinopyranoside, the adjustment is in the wrong direction. Other factors must be playing a role in these cases (see above). Overall, the agreement between calculated and observed ORD reported here can be taken as support for the theoretical model. The theory, in spite of its necessarily approximate nature, represents a conceptual advance by describing the connection between saccharide molecular structure and optical activity explicitly in terms of the high-energy electronic transitions and vacuum-UV CD features which are intrinsically related. Acknowledgment. This work was supported by USPHS Grant
GM-24862. Registry No. Methyl-@-D-mannopyranoside,22277-65-2; methyl-@methyl-0-D-galactopyranoside, 1824-94-8; D-glucopyranoside,709-50-2;
methyl-a-D-mannopyranoside, 617-04-9;methyl-a-D-glucopyranoside, 97-30-3; methyl-a-D-galactopyranoside, 3396-99-4; methyl-@-D-arabinomethyl-P-o-xylopyranoside, 612-05-5; methyl-apyranoside, 5328-63-2; D-xylopyranoside, 91-09-8.
Solvent Effects on the Soret Absorption Band of Nickel Protoporphyrin I X Dimethyl Ester Containing the Four-Coordinate Metal Orland W. Kolling Chemistry Department, Southwestern College, Winfield, Kansas 67156 (Received: June 24, 1988;
In Final Form: November 10, 1988)
Comparisons were made between observed solvent effects upon the Soret peak for two cases of four-coordinatemetalloporphyrins, nickel protoporphyrin IX dimethyl ester (NiPPDME) and zinc tetraphenylporphyrin (ZnTPP). Solvents included in this study were members of the aromatic polar and apolar, aprotic dipolar and highly dipolar, and hydrogen-bonding classes in the Chastrette-Purcell generalized classification system. With reference to hydrocarbon environments the Soret peak of ZnTPP exhibits a systematic red shift with increasing dipolarity and Lewis basicity of the solvent while that for NiPPDME is blue-shifted. A more exact analysis of the empirical blue shift for the Soret band of MPPDME indicates that the composite solvent effect arises from the variable mixing of so1ute:solvent dipole-dipole orientational and hydrogen-bondinginfluences. Simple distortional contributions predicted by reaction field models for the solvent-induced spectral shift are not detected for the Soret band of NiPPDME.
The synthetic metalloporphyrins containing various transition-metal atoms have been extensively investigated because of their somewhat unique redox behaviors, structural characteristics, and electronic spectra, as well as their mimicry of the biological functions of the very important iron porphyrins. The pronounced Soret absorption peak for the metalloporphyrins is influenced significantly by axial ligation and the solvent environment, just as are the redox potentials for metalloporphyrins measured with For the latter, there appear the DME in nonaqueous to be some qualitiative trends in the shifts in half-wave potentials for a given redox couple which parallel the dielectric constants and/or Gutmann donor numbers (DN) for the Although the limited systematic studies of axial ligand and solvent effects upon the Soret band have not yielded an interpretive consensus, it is clear that those metalloporphyrins which contain a restricted 4-fold coordination of the central metal atom provide ( 1 ) Bottomley, L.; Kadish, K. Inorg. Chem. 1981, 20, 1348. (2) Kadish, K.;Shine, L. Inorg. Chem. 1982, 21, 3623. (3)Kelly, S.;Kadish, K. Inorg. Chem. 1982, 21, 3631. (4) Rillema, D.et al. J . Am. Chem. SOC.1982, 104, 1276. (5)Antipas, A.; Gouterman, M. J . A m . Chem. SOC.1983, 105, 4896. (6)Lexa, D.et al. J . A m . Chem. SOC.1984, 106, 6321. (7)Lexa, D.;Momenteau, M.; Saveant, J.; Xu, F. J . Am. Chem. SOC. 1986, 108, 6937.
0022-3654II 8 9 i12093-3436$01 S O ,I O -
the simplest and most suitable initial models for the analysis of variables to be attributed to the solvent environment. In a major work on the (tetraphenylporphinato)zinc(II) (ZnTPP) case Nappa and Valentine8 concluded that the red shift of the Soret band is due to ligation of a fifth position on Zn(I1) rather than solvation even though the size of the red shift is not generally related to -AH of complex formation as a measure of the strength of that Zn-ligand bond. On the other hand, Vogel and Stahlbush9 reported a semiquantitative correspondence between the Soret red shift and the enthalpy of adduct formation for ZnTPP:donor pairs in cyclohexane. However, such enthalpy comparisons are useful only for those series in which the AS values for adduct formation remain small and relatively constant.I0 Linear free energy relationships between the stability constants of ZnTPP:donor complexes and the pK, of the ligand have been observed by Kadish et al." in dichloromethane as the solvent. Because in some instances the Soret band shifts are electronic responses to ligation effects on the free energy change for the ground state to excited state transition, the variables associated (8) Nappa, M.; Valentine, J. J . Am. Chem. SOC.1978, 100, 5075. (9)Vogel, G.;Stahlbush, J. Inorg. Chem. 1977, 16, 950. (IO)Jones, R.;Staley, R. J . Am. Chem. SOC.1982, 104, 2296. ( 1 1) Kadish, K. et al. Inorg. Chem. 1981, 20, 1274.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 3437
Soret Absorption Band of NiPPDME with the axial ligand in the ZnTPPdonor complex have been more successfully resolved using the linear solvation energy concept of Kamlet and Taft.12 In the general statement of the linear solvation energy relationship (LSER) in eq 1 the most prominent solvent parameters determining the spectral shift, Le., T * , a, and @, are measures of dipolarity, hydrogen-bonding acidity, and basicity, respectively. For the ZnTPP:donor case involving 14 aprotic
AXYZ = AXYZ,
+ s(a*+d6) + aa + b@
TABLE I: Solvents Influencing the Soret Absorption Band of Nickel Protoporphyrin I X Dimethyl Ester (Ni C.N. = 4)
Soret solvent class'
ARA (aromatic apolar) ARP (aromatic polar)
(1)
ligands, it was found that the magnitude of the red shift for the Soret band is predictable from an LSER incorporating only the T * and fi parameters and having a ratio of b / s = 1.2 for the weighting ~0efficients.I~It may be that a correlation between the Soret shift and the Kamlet-Taft parameters is either fortuitous or that ZnTPP is a special case because of a high E / C ratio as a Lewis acid in the Drago-Wayland system. Therefore, it is important to test the linear function in eq 1 with data for other metalloporphyrins containing metallic species having a primary coordination number of four. The case of nickel(I1) protoporphyrin IX dimethyl ester (NiPPDME) was selected for this purpose since nickel is only slightly harder as a Lewis acid than Zn(I1) in porphyrin c ~ m p l e x e s . ' ~ Findsen, Shelnutt, and OndriasI5 have investigated the photodynamic behavior of NiPPDME in a number of nonaqueous solvents using transient Raman spectroscopy. They concluded that both the vibrational characteristics of the nickel *Big electronic state and the a2,, orbital of the porphyrin in the complex are very responsive to the solvent environment. Some data for the solvent shift of the Soret peak of NiPPDME were included in the Findsen, Shelnutt, and Ondrias study; however, the trend in that data as related to the Dimroth-Reichardt ET(30)scale is vague at best with a high degree of scattering and with several instances of seemingly anomalous behavior for solvents which are usually quite normal as media for nonaqueous kinetics and electrochemical processes.15 This report now summarizes the results of a more detailed analysis focused on the solvent itself as a determinant of the shift in the Soret absorption band of NiPPDME. Fifteen solvents ranging from aprotic to hydrogen-bonding liquids are considered and the essential features of the solvent influences are elucidated from the following sources: (a) the Chastrette-Purcell mathematically derived solvent classification system;16 (b) the linear solvation energy relationships using the Kamlet-Taft parameters; and (c) comparisons between the Soret electronic absorptions for the ZnTPP and NiPPDME cases.
Results and Discussion Solvent Groups Influencing the Soret Band. The Chastrette-Purcell general classification scheme places the known solvents into nine discrete groups based upon a nonhierarchical multivariate statistical analysis using eight-dimensional space.16 Six of those solvent classes are represented in the present investigation and the individual members included here are listed in Table I along with the NiPPDME Soret peak value. Only a single member of the electron pair donor (EPD) class is listed and, because it is the one instance of the Ni coordination number (C.N.) being six,l5 piperidine illustrates the very large effect a change in C.N. can have upon the Soret peak position. A similar red shifting of about 28 nm in other C.N. six cases have been observed for the Soret peaks of NiPPDME and NiPP (nickel protoporphyrin) by pyrr01idine.I~ Piperidine was therefore excluded from any subsequent data treatment in this study. It will be noted in Table I that the Soret peak position is essentially constant in the six members from the aromatic apolar (12) Kamlet, M.; Abboud, J.; Abraham, M.; Taft, R. J. Org. Chem. 1983, 48, 2877. (13) Kolling, 0. W. Anal. Chem. 1982, 54, 260. (14) Bain-Ackerman, M.; Lavallee, D. Inorg. Chem. 1979, 18, 3358. (15) Findsen, E.; Shelnutt, J.; Ondrias, M. J . Phys. Chem. 1988, 92, 307. (16) Chastrette, M.; Rajzmann, M.; Chanon,; Purcell, K. J . Am. Chem. SOC.1985, 107, 1-11.
EPD (electron pair donor) AD (aprotic dipolar)
membersb benzene
chlorobenzene carbon tetrachoride' piperidine (C.N. = 6)
nm 402 402 402 402 403 402 430
acetone
396
toluene benzyl alcohol bromobenzene
acetonitrile chloroform dichloromethane nitromethane AHD (aprotic highly dipolar) dimethyl sulfoxide HB (hydrogen bonding) ethanol methanol tetrahydrofuran' water/acetone
395
398 397
396 400
396 395 399
397
'Chastrette-Purcell classifications. For the two cases of CClp and THF, the phenomenological grouping is not coincident with their usual structural classification.16 bSolventsused in the Findsen, Shelnutt, and Ondrias investigation.15 'Reported uncertainty in wavelength values is *I nm.I5
(ARA) and aromatic polar (ARP) classes and that benzyl alcohol is not exceptional in this respect. The absence of any significant shift in the Soret band among these ARA and A R P members tends to rule out any major polarizability influence from these solvents. This was confirmed from two computational tests by the insensitivity of the Soret peak value to (a) changes in the LorenzLorentz function (eq 2) as a measure of solvent polarizability;
f ( n 2 ) = (n2 - 1)/(2n2 + 1) and (b) the solvent inductive component Ld(c,n2)representing a distortional solvation effect by the ARA and ARP members.17 Likewise, there is no evidence for even very weak axial ligation by the aromatic s01vents.l~ By contrast, all of the aprotic dipolar and highly dipolar solvents cause a blue shifting of the Soret peak as do the weak-to-strong hydrogen-bonding solvents. The fact that the magnitudes of the Soret shifts are similar in the HB, AHD, and AD classes strongly suggests that the one common effect from these solvents in the NiPPDME-solvent interaction is most likely a dipolar orientational influence. As a further test of this decision to exclude the distortional component of the solvent effect from the analysis herein, the trend in the solvent inductive component to the reaction field of the excited state as developed by Brady and C a d 7 was examined for all of the nonaromatic solvents. That defining function is given in eq 3 and the corresponding scatter diagram is shown in Figure 1A. The Block-Walker and modified McRae-Bayliss
f'(n2) = 2(n2 - 1)/(2n2 + 1)
(3)
reaction field theories predict that the transition energy of the solute as a free energy change should be linear inf'(n2) if the inductive effect is dominant." The data points in Figure 1A fail to conform to that requirement and indeed the data values for the whole set are again bisected into the two broad clusters of aromatic and nonaromatic solvents. Quantifying the Solvent Influences. In the initial study of the photodynamics of the nickel(I1) porphyrins, Findsen, Shelnutt, and Ondrias15 used the Dimroth-Reichardt ET(30) numbers to represent the combined polar and inductive components associated with the influence by the solvent on the transient Raman and Soret spectral shifts. The general pattern of points for the Soret (Amx) peak as a function of ET(30)for NiPPDME is shown in Figure 1B. The vagueness of that trend does not allow any single linear (17) Brady, J.; Carr, P.J . Phys. Chem. 1985, 89, 5159.
3438
Kolling
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989
TABLE II: Solvent Parameters and Calculated Soret Peak for NXPPDME) at 25 OC
405
t
Kamlet-Taft parameters“
(100
395
--
9 9
dd
4
I
,
,
1
X
9
0.35
0.40
0.45
/
solvent acetone acetonitrile benzene benzyl alcohol bromobenzene carbon tetrachloride chlorobenzene chloroform dichloromethane ethanol
methanol nitromethane tetrahydrofuran toluene water/acetone
a
fl
0.08 0.19 0
0.48 0.31 0.10 0.50 0.06 0 0.07 0 0 0.77 0.62
T*
0.71 0.75 0.59 0.98 0.79 0.28 0.71 0.58 0.82 0.54 0.60 0.85 0.58 0.54 0.75d
0
0 0 0.44 0.30 0.83 0.93 0.22 0 0 0.19d
0.55 0.11
Soret A,, nm exptlb calcd‘ 396 395 402 402 402 402 403 398 397 396 395 396 399 402 397
397 397
402 398 396 397 396 396 399 397
“Parameters reported in summary tables by Kamlet et al.Iz bStated experimental uncertainty is i l . 0 nm.I5 ‘Values computed with eq 4. dBased on the solvatochromic behavior of Phenol Blue for the mixed solvent (X,,, 10
40
ET
50
= 0.264).
h0
pair donacity as p (basicity), and specific solvation by hydrogen bonding as a (hydrogen bond acidity). Values for these solvent Figure 1. The Soret absorption band (as A, in nm) for Ni(PPDME) parameters are listed in Table 11. The Sorer band in the arocorrelated with (A) inductive component of reaction field asf’(n2); and matics is unchanged by the shgr in a* from 0.54(toluene) to 0.98 (9) the ET(30) parameter (Dimroth-Reichardt) of the solvent: ( 0 ) (benzyl alcohol); however, carbon tetrachloride was included with aromatic classes ARA and ARP, (0)all others. the other chloroalkanes, the AD and HB members in the data function to be deduced nor is there any visible clustering of data set for further processing. points beyond the division into aromatic and nonaromatic groups The specific linear solvation energy relationship in eq 4 was noted above. This condition arises in part from the nature of the values into transition deduced after converting the Soret A, ET(30) parameter itself which is known to be a composite solvent energies, E,, (in kcal/mol) (El, = (2.859X 104)/Amax).Derived parameter including dipolarity and inductive components along E,, = 70.65 (&0.02) 1.77a* 0.41~~ ( n = 10;r = 0.979) with some hydrogen-bonding effects. Likewise, the empirical (4) ET(30) numbers for common solvents appear to involve variable uncertainties are s(f0.01); a(f0.03); E,,(calcd) f 0.24kcal/mol mixing of the contributions of the three components within the to f l . O nm). As an optimized regression, the more complex solvents in the AHD and H B c l a s s e ~ . ~ ~ ~ ~ *(corresponding ~~~ function in eq 4 required no bp-term dependency. Including a A viable alternative to these limitations on the ET(30) scale d6-correction term was found to reduce the correlation coefficient is to apply multivariable models for the influence exerted by the significantly ( r = 0.942).The corresponding Soret A,,,(calcd) solvent on the ground-state-excited-state free energy transition. values are listed in Table 11. From a critical analysis of the reaction field models for solvation, Even though the ratio s / a is fairly large (4.3/1)if one drops Brady and C a d 7 concluded that the orientational and distortional the contribution from hydrogen bonding by both the very weak components of solvation must be treated separately and that their and strong HBD solvents, the correlation between the El, and a* composite influence arises from their coupling. Although the deteriorates to become as poor as that with ET(30) in Figure 1B. distortional part can be adequately represented byf’(n2) in eq 3, Thus, solvent dipolarity is clearly the dominant variable associated the better orientational field terms are functions in dielectric with the blue-shifting of the Soret peak of NiPPDME as one constant, Le., f(6,c) orfiO(c,n2)),as derived from modified forms orientational component of the dipole-dipole interaction and yet of the McRae-Bayliss or Block-Walker models.” Such models hydrogen-bond donor behavior by the solvent makes a detectable work best with ARA, ARP, AD, and A H D solvents in which secondary orientational contribution as well. There is no clear refractive index functions become less important as dipolarity evidence for an inductive effect upon the so1vent:NiPPDME inincreases; however, such models are quite unsatisfactory with HB data. teraction within the precision of the A, and HBSA (hydrogen bonding strongly associated) classes where Although some general correspondence between ET(30) and specific solvent effects are operative.16 Therefore, other multiple the intensity ratio for the a and p absorption bands of NiPPDME parameter descriptions of solvation effects upon the Soret peak has been reported,I5 that correlation is very poor and problematic. of NiPPDME must be considered if the behaviors of the broad Those solvent characteristics associated with the Soret band shift range of solvent types in Table I are to be more completely are not necessarily related in a like manner to the C Y / @ band specified. The fact that acetone, acetonitrile, and nitromethane are very weak hydrogen-bond donors even though having dominant intensities of NiPPDME in common solvents. Comparisons between the NiPPDME and ZnTPP Soret Abproperties of the AD class of solvents underscores the conclusion sorptions. The most obvious difference between these 4-fold C.N. that a multiple parameter approach to depicting solvation effects cases is that the Soret peak for NiPPDME is blue-shifted with upon the Soret band is necessary. increasing dipolarity-hydrogen-bonding effects by polar solvents As a type of linear free energy function, the Kamlet-Taft while the Soret peak for ZnTPP is red-shifted by increasing multiple term relationship in eq 1 assumes a simple additivity for dipolarity-Lewis basicity effects from the ~ o l v e n t .For ~ the latter, each of the components contributing to the net solvent effect on the spectral transition energy, Le., orientational component as a* the magnitude of that shift is given byI3 (dipolarity), distortional component d6 (polarizability), electron AE,, = 0.265a* + 0.362p (5)
+
(18) Kamlet, M.; Abboud, J.; Taft, R. Prog. Phys. Org. Chem. 1981, 13, 485.
(19) Fong, C.; Kamlet, M.; Taft, R. J . Org. Chem. 1983, 48, 832.
+
The s / b ratio of 0.73 reflects the dominance of ligation in the composite solvent influence upon ZnTPP in the equilibrium of eq 6; however, the distinction between the NiPPDME and ZnTPP
J . Phys. Chem. 1989, 93, 3439-3444 ZnTPP
+ L:
F=
L:ZnTPP
cases cannot be assigned merely to “noncoordinating” vs “coordinating” solvents since there are at least six AD and AHD solvents in common to the regressions in both eq 4 and eq 5. Clearly, solvent dipolarity has an important role in each instance of the solvent shifting the Soret peak even though the size of that shift is smaller for NiPPDME. On the other hand, for NiPPDME in the presence of strong EPD solvents (i.e., piperidine) in which nickel assumes a higher C.N., the Soret band shows a large red shift not unlike ZnTPP with ligands of comparable ba~icity.~ Just as Nappa and Valentine observed for ZnTPP,* the Soret band for NiPPDME undergoes a greater solvent shift than does the cx band.ls From the Gouterman four-orbital model,20it is the porphyrin alu,a2, e, electronic transition associated with the Soret band which is ultimately responsive to the changes in solvent. In the case of ZnTPP undergoing axial ligation (eq 6) the 4-fold C.N. for the ground-state Zn( 11) expands exclusively to a five-coordinate complex.2’ Thus, the magnitude of the Soret red shift is a measure of the extent to which negative charge is transferred within the complex from the basic atom in the ligand through the metal (Zn) into the porphyrin ringE Since the electronegativity of Ni is a little greater than that for Zn, the magnitude of the Soret peak shift would be expected to be larger for Zn than for Ni in identical porphyrin structures.20 Shelnutt and Ortiz have assigned this influence of metal substitution to a shift in energy for the porphyrin a2, level while the a,, level remains fixed.22 The findings of Spellane et al. indicate that there is a greater resemblance in the orbital spacings between a given metal protoporphyrin and its metal porphyrin than to those spacings in the metal TPP species.23
-
~
~~
~
~
(20) Gouterman, M. et al. J . Chem. Phys. 1973, 59, 676. (21) Kadish, K.; Shiue, L.; Rhodes, R.; Bottomley, L. Znorg. Chem. 1981, 20, 1274. (22) Shelnutt, J.; Ortiz, V. J . Phys. Chem. 1985, 89, 4733. (23) Spellane, P.; Gouterman, M.; Antipas, A.; Kim, S.; Liu, Y. Znorg. Chem. 1980, 19, 386.
3439
In the related C.N. 5 case of vanadyl tetraphenylporphyrin (VOTPP) investigated by Su et al.24the resonance Raman V-0 stretching frequency is responsive to th solvent-induced polarization of the V-O bond. This enhancement of the V-0 stretching mode appears to arise from its resonance with the corresponding Soret electronic transition for the porphyrin. Although the empirical decrease in the V-0 stretching frequency relates linearly to an increase in Gutmann acceptor number (AN) of the solvent, the data set is bisected into the separate AD and H B classes of solvents;24and like the D N numbers, correlations with the A N scale have been shown to lead to ambiguities in the assignment of solvent effects upon electronic spectral shifts. On the other hand, the application of the Kamlet-Taft relationship (eq 1) to the data of Su et al.24now yields the improved regression in eq 7. Again, within the precision of the experimental data,24no d6 B (cm-I) = 1006.3 ( f O . l ) - 9.38 ( f 0 . 0 5 ) ~ *- 14.6 ( f 0 . 0 6 ) ~ ~
(7) (s(ca1cd) S D = f 0 . 5 cm-I; r = 0.988; n = 11) term is needed. From the s / a ratio of 0.64/1 .O in eq 7, it is clear that hydrogen bonding to the oxygen in VOTPP makes a major contribution to the shift in the V-O stretching frequency for both weak and strong hydrogen-bond donor solvents (Le., weak HBD like CH,CN, CH2C12,C H 3 N 0 2 ,CHC1,; strong HBD like the alkanols). At the same time the only remaining solvent influence upon the V-0 stretching band at 1007 cm-’ is a dipole-dipole orientational effect by the ARA, AD, AHD, and HB member solvents.24 In this respect the orientational effects of dipolarity and hydrogen bonding appear to be the principal solvent influences upon the solvent dependency of the Soret shift for those C.N. 4 and C.N. 5 metalloporphyrins where competitive ligation is absent. Registry No. NiPPDME, 15304-70-8; Zn(TPP), 14074-80-7. (24) Su,Y.; Czernuszewicz, R.;Miller, L.; Spiro, T. J . Am. Chem. SOC. 1988, 110, 4150.
Dual Fluorescence of 4-(Dialkylamino)pyrimidines. Twisted Intramolecular Charge Transfer State Formation Favored -by Hydrogen Bond or by Coordination to the Metal Ion Jerzy Herbich,* Zbigniew R. Grabowski,* Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44, 01 -224 Warsaw, Poland
Hanna Wijjtowicz, and Krzysztof Golankiewicz Chemistry Department, Adam Mickiewicz University, Grunwaldzka 6,60-780 Poznai, Poland (Received: June 28, 1988; In Final Form: October 14, 1988)
4-(N,N-Dimethylamino)pyrimidine (CDMAP) does not exhibit any markedly dual luminescenceeven in highly polar (aprotic) solvents, unless the ortho substituent deviates the amino group from coplanarity with the ring. Protic solvents or complexation with Zn2+cause the long-wave fluorescence to appear distinctly. Contrary to 4-DMAP, 4-(N,N-diethylamino)pyrimidine (CDEAP) reveals dual luminescence in sufficiently polar (aprotic) environment. In alcoholic solutions the intensity of the fluorescence is drastically reduced. Fluorescence properties of this group of compounds fit well to the TICT modet. The importance of nonradiative deactivation increases with the proton-donating ability of the solvent. Introduction Electron donor (D)-acceptor (A) molecules linked by a single bond have been widely used for studying structural requirements and kinetics of intramolecular electron transfer processes. Dual fluorescence of a number of dialkylaniline derivatives and high polarity of the lowest excited states of several compounds with mutually orthogonal donor and acceptor orbitals (as well as 0022-3654/89/2093-3439!$01.50/0
thermodynamics and kinetics of the excited-state process) have been explained in terms of the twisted intramolecular charge transfer state (1) Grabowski, Z. R.;Rotkiewicz, K.; Siemiarczuk, A,; Cowley, D. J.; J , Chim, 1979, 3, 443. Baumann, w, (2) Grabowski, Z. R.;Dobkowski, J. Pure Appl. Chem. 1983, 55, 245. (3) Grabowski, Z. R. Acta Phys. Polon. 1987, A71, 743.
0 1989 American Chemical Society
