J. Phys. Chem. 1991, 95, 9764-9772
9764
Crystallography and AM1 and INDO/S Calculations on Electronic Ground and Excited Singlet States of 2-[p-(Dlmethylamlno)phenyl]-3,3-dlmethyl-3//-lndole. Solvent Effects on the Absorption and Fluorescence Spectra M. LaChapelle, M. Belletete, M. Poulin, N. Godbout, F. LeGrand, A. Héroux, F. Brisse,* and G. Durocher* Département de Chimie, Université de Montréal, C.P. 6128, Succ. A, Montréal, Québec, H3C 3J7 Canada (Received:
April
2, 1991)
Absorption and fluorescence spectra, fluorescence quantum yields (0F), and lifetimes (rF) were obtained for 2-[p-dimethylamino)phenyl]-3,3-dimethyl-3//-indole (1) in a series of «-alkanes and «-alcohols, along with the absorption spectrum in the vapor phase. ÁMPAC and INDO/S calculations as well as crystallographic measurements were also performed on this molecule, using AMPAC for the geometry optimization and INDO/S for electronic transitions calculations and conformational analysis. The structure of 1 was established by single-crystal structure analysis. The refinement converged to a final R value of 0.052 for 1144 observed reflections. The unit cell has dimensions a = 7.978 (4), b 5.980 (2), c = 16.461 (9) Á and ß = 103.09 (4)°, and the space group is P2,. It is shown that AMPAC predicts a final geometry similar to that obtained from the crystallographic investigation, while INDO/S calculates transition energies corresponding to those observed in the absorption spectrum of 1 in the vapor phase. It is also shown that INDO/S calculates ground- and excited-state dipole moments similar to those obtained from spectral shift data. The results also indicate that the dimethylanilino ring (Ph,.) has a certain freedom of rotation in the molecule at room temperature, this motion representing an important deactivation path of the Sj excited state. As the viscosity increases in the «-alkane series, the rotation amplitude of the Phc moiety is reduced, resulting in a decrease of km and consequently an increase of 0F. The knr dependence on a fractional power of the viscosity has shown a friction-limited rate for the rotamers inside the energy barrier at room temperature. On the other hand, 0F remains constant as the «-alcohol viscosity varies because of a more hindered rotation of the Phc ring in protic solvents. It is finally shown that the fluorescence spectra are always larger than the absorption spectra in n-alkanes whereas their widths are almost the same in «-alcohols. The “bandwidth effect” observed on the spectra in «-alkanes is interpreted in terms of a vibronic interaction taking place between the S, and S2 close-lying excited states. On the opposite, such an effect is not observed for 1 in «-alcohols because of the greater stabilization of the more polar Si excited state. This solvent effect increases the Sj-S2 energy gap and should therefore weaken the vibronic coupling between them.
J. Phys. Chem. 1991.95:9764-9772. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/10/19. For personal use only.
-
Introduction
TABLE I: Crystal Data for p-(Dimethylamino)phenyl-3,3,-dimethyl-3//-indole C|8N2H20, fw = 264.369, monoclinic a = 7.978 (4), b = 5.980 (2), c = 16.461 (9) A ß = 103.09 (4)°, V= 764.9 A3, Pcal = 1.148 g cm"3 Z = 2, F2„ µ = 4.86 cm"1, F(000) = 282, T = 20 °C
Para-substituted 3//-indole molecules have been studied in our laboratories for a number of years.1 It has been recognized that the spectroscopic and photophysical properties of these molecules depend on the nature of the environment.5"9 Recently, molecule 1 has been used as a fluorescent probe to study the polarity of microenvironments.10-12 It has been suggested on the basis of experimental results that conformational changes occurring during the relaxation of the first excited singlet state are responsible for the photophysical behavior of molecule 1 in nonpolar and polar environments.13 Moreover, it was suggested that two close-lying excited singlet states through Herzberg-Teller vibronic coupling are playing an important role in the deactivation of the molecule. A unique decision between these two alternatives cannot be given on an experimental basis alone but has to be looked for by quantumchemical arguments. The aim of this paper is to clarify that situation with the help of crystallography and semiempirical calculations (AMPAC and INDO/S methods) along with a more extensive spectroscopic and photophysical study of molecule 1 in a series of «-alkanes and «-alcohols. In order to obtain further insight into the nature of the electronically excited states governing the photophysics of this molecule, we have used the semiempirical AMPAC and INDO/S methods combined with a Cl treatment. The AMPAC method yields reliable results on ground-state geometry and energy values. It is also shown that AMPAC and INDO/S methods are useful tools to obtain a qualitative dynamic picture of the conformational changes of the molecule. This latter image is more accurate than a stiff conformation model, allowing a better understanding of the photophysics of the molecule. 11"4
Ethanol was purified as described elsewhere.1 All the «-alkanes and the other «-alcohols were purchased from Aldrich Chemicals (99+%, anhydrous) and used as received. 2. X-ray Investigation. Single crystals suitable for X-ray diffraction were grown by slow evaporation of a solution of ethanol. The crystal selected was a prism limited by |100), (010j, (001) whose dimensions were 0.11 X 0.18 X 0.38 mm. The crystal was mounted on an Enraf-Nonius CAD-4 diffractometer. The unit cell dimensions were obtained from 25 well-centered reflections in the range 40 < 2 < 50°. Crystal data of interest are given in Table I. The copper radiation, X(Cu Ka) = 1.541 78 Á, was
(1) Belletete, M.; Spectrosc. 1977, 22, (2) Belletete, M.; (3) Belletete, M.; (4) Belletete, M.; (5) Belletete, M.;
(6) Belletete, M.; Lessard, G.; Durocher, G. Can. J. Spectrosc. 1986, 31, 89.
(7) Richer, J.; Lessard, G.; Belletete, M.; Durocher, G. Int. J. Chem. Kinet. 1986, 18, 1163. (8) Belletete, M.; Durocher, G. J. Photochem. Photobiol., A 1988, 44, 275. (9) Belletete, M.; Lessard, G.; Durocher, G. J. Lumin. 1989, 42, 337. (10) Belletete, M.; Durocher, G. J. Colloid Interface Sci. 1990, 134, 289. (11) Belletete, M.; Lachapelle, M.; Durocher, G. J. Phys. Chem. 1990, 94, 7642. (12) Belletete, M.; Lachapelle, M.; Durocher, G. J. Phys. Chem. 1990, 94, 5337. (13) Belletete, M.; Durocher, G. J. Phys. Chem. 1989, 93, 1793. (14) Skrabal, P.; Steiger, J.; Zollinger, H. Helv. Chem. Acta 1975, 58, 800.
Authors to whom correspondence should be addressed.
0022-3654/91/2095-9764S02.50/0
Durocher, G. Can. J. Spectrosc. 1979, 24, 87. Durocher, G. Can. J. Chem. 1982, 60, 2332. Durocher, G. J. Photochem. 1983, 21, 251. Lessard, G.; Richer, J.; Durocher, G. J. Lumin. 1986,
34, 279.
Experimental Section 1. Materials. Synthesis and purification of molecule 1 have been done according to methods published by Skrabal et al.14 *
Scheuer-Lamalle, B.; Baril, L.; Durocher, G. Can. J. 31.
©
1991
American Chemical Society
Ground and Excited States of 3//-Indole Derivative
The Journal
of Physical
Chemistry, Vol. 95, No. 24, 1991
9765
graphite monochromatized. The diffracted intensities were collected, at room temperature, in the mode with a scan width = calculated by (1.00 + 0.14 tan 0)°. The sphere of reflection was explored up to 20max = 140.0° and within the limits given by < 9,0 < A < 7, -20 < / < 20. The orientation of the 0 < crystal was monitored every 200 measurements while the intensities of the six standard reflections were checked every hour. The largest intensity fluctuation during the measurements of 1808 unique reflections was 1.1%. A total of 1144 reflections, for which /0 Í 1.96 (/), were retained for the structure determination and refinement. The intensities were placed on a common scale and corrected for Lorentz and polarization effects. No absorption correction was applied since the absorption coefficient was small. The structure was solved by direct methods using the multan so set of programs. A full-matrix least-squares refinement of the atomic coordinates and isotropic temperature factors of the semiheavy atoms converged to R = 13.4%. After the temperature
TABLE II: Fractional Atomic Coordinates and Their Esd’s (xlO4 for N and C) and Un (A2, xlO3) for the 2-[p-(Dimethylamino)phenyl]-3,3-dimethyl-3//-indole Molecule a V z X atom y Veq
factors were converged from isotropic to anisotropic, the refinement converged to R = 10.6%. At this stage a difference Fourier synthesis revealed the positions of all hydrogen atoms. These were included in the refinement process. However, the isotropic temperature factors on the methyl groups were kept at the value of B = 8.0 Á. The function minimized was |FC|)2. The weights were derived from the counting statistics, w = 1 /a2(F). Convergence of the refinement was reached when the maximum and averaged values of the (displacement/sigma) ratio were 0.25 and 0.01, respectively. The residual electron density fluctuations noted on the final difference Fourier synthesis were -0.18 and +0.23 e Á'3. The agreement indexes were then R = 0.052, R„ = 0.055, and S = 1.648 for 252 parameters.15 The scattering factors for C and N were taken from ref 16 and those for H atoms from ref 17. The programs used here are modified versions of nrc-2, data reduction; nrc-io, bond distances and angles; nrc-22, mean planes;18 fordap, Fourier and Patterson maps (A. Zalkin);
(7) (7) (7) (8) 2571 (8) 5840 (9)
~
multanso, multisolution program;19 nucls, least-squares refinement;20 ORTEP, stereodrawings.21 3, Theoretical Calculations. 3.1. Geometry Optimization. We first use the program model on the VAX of the Université de Montréal, which enables us to draw the molecule, optimize roughly the geometry using the MM2 force field, and finally generate the corresponding Cartesian coordinates.22 A more precise geometry optimization was obtained using the AMI (Austin model 1) Hamiltonian of the AMPAC program which consists of an improved parametrization for the MNDO Hamiltonian.23 3.2. INDO/S Semiempirical Method. The ground-state and transition energies of molecule 1 have been calculated within the framework of the semiempirical all-valence INDO (intermediate neglect of differential overlap) method including configuration interaction (Cl).24·25 We used the INDO/S program sent to us by Professor Michael C. Zerner from the University of Florida. The electron-repulsion integrals were evaluated using the Mataga-Nishimoto formula. Within the Cl scheme all singly excited configurations involving the 12 highest occupied and the 12 lowest unoccupied orbitals were included. This selection was cut using a threshold for the energy of 60000 cm"1 and a threshold for (15) R =
LFol-IFII/Elfol- K
=
EwtiFJ
-
IFI)V(m
-
=
[Iw(|F0|
-
|FC|)2/I>F02]I/2, andS
«)]1/5·
(16) Cromer, D. T.; Mann, J. R. Acta Crystallogr. 1968, A24, 321. (17) Stewart, R. F.; Davidson, E. R.; Simpson, W. T. J. Chem. Phys. 1965,
42. 3175.
(18) Ahmed, F. R.; Hall, S. R.; Pippy, . E.; Huber, C. P. J. Appl. Crystallogr. 1973, 6, 309. (19) Main, P.; Fiske, S. J.; Hull, S. E.; Lessinger, L.; Germain, G.; Declercq, J. P.; Woolfson, . M. MULTANSO. A system of computer programs for the automatic solution of crystal structures from X-ray diffraction data. Universities of York, England and Louvain, Belgium, 1980. (20) Doedens, R. J.; Ibers, J. A. Inorg. Chem. 1967, 6, 204. (21) Johnson, C. K. ORTEP, Report ORNL-3794; Oak Ridge National Laboratory: Oak Ridge, TN, 1965. (22) Tai, J. C.; Allinger, N. L. J. Am. Chem. Soc. 1988, HO, 2050. (23) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. Soc. 1985, 107, 3902. (24) Ridley, J.; Zerner, M. C. Theor. Chim. Acta 1973, 32, 111. (25) Forber, C.; Zerner, M. C. J. Am. Chem. Soc. 1985, 107, 5884.
N(l) N(2) C(2) C(3) C(3A) C(4) C(5) C(6) C(7) C(7A) C(8) C(9) C(10) C(11) C( 12) C(13) C(14) C(15) C(16) C(17)
‘U~
=
11244 6426 9873 9345 10788 11161 12608 13651 13273 11826 8923 9338 8535 7227 6786 7606 9393 7587 6961 5285
(4) (4) (4) (4) (4) (5) (5) (5) (5) (5) (4) (5) (5) (4) (5) (5) (5) (5) (5) (5)
2989 4060 4178 5469 4739 5291
4336 2893 2341 3300
4168 2617 2558 4069 5636 5644 8013 4676
(6) (6) (7) (7) (6) (7) (8) (9) (8) (7) (7) (7) (7)
2575 (2) -1096 (2) 2340 (2) 3048 (2) 3764 (2) 4600 (2) 5114 (2) 4807 (3) 3958 (3) 3448 (2) 1469 (2) 906 (2)
65(1) 72(1) 60(1) 60(1) 59(1)
85 (2)
68 (2)
-265 (2) 297 (2) 1124 (2)
63(1)
2956 (2) 3169 (2)
-1667 (2) -1439 (2)
71
(2)
77 (2) 83 (2) 78 (2)
66(1) 57(1) 67(1) 71
67 77 74 83 89
(2) (1) (2) (2) (2) (2)
[Una,2a2 + U22b*2b2 + Unc*2c*2 + 2Ui2a*b*ab cos
+
2Uncrc*ac cos ß + 2U2ib*c*bc cos a]/3. a, b, c, a, ß, and are the unit cell dimensions and angles; a*b*, and c* are the reciprocal cell parameters. The U¡¡s have been deposited, see ref 29.
mixing of 200 cm"1. The Cl then consists in average of 60 selected single excitations. 4. Instrumentation. 4.1. Absorption and Fluorescence Measurements. The absorption spectra were recorded on a Phillips PU8800 UV/vis spectrophotometer. Corrected fluorescence spectra were measured on a Spex Fluorolog Model 1902. Both types of spectra were then digitized for further manipulations through the use of a Graphpad/IBM-XT system. The fluorescence quantum yield has been measured in «-alkanes by reference to = 0.025 for molecule 1 in the known value of methylcyclohexane.4 For the measurements in «-alcohols, we use the value for molecule 1 in ethanol (4>F = 0.33).13 The theoretical radiative decay rate constant (k'F) has been calculated using the Strickler and Berg relation.4·26 4.2. Lifetime Analysis. Fluorescence lifetimes were measured on a time-correlated single-photon counting system, which uses a synchronously pumped cavity dumped, rhodamine 6G dye laser (Coherent CR-599) pumped by a mode-locked argon ion laser (Innova CR-12) as an excitation source. The laser beam is attenuated by a diaphragm to ensure that the fluorescence count rate is always less than 1% of the cavity-dumper frequency (3.8 MHz). The dye laser beam is frequency doubled by using a KDP “R6G” crystal (Inrad). Single-fluorescence photons are detected by a photomultiplier tube (Phillips 2020Q). The signal is conditioned by an Ortec 584 constant fraction discriminator (CFD) whose output serves as the start signal for an Ortec 467 timeto-pulse height converter (TAC). The stop signal comes from a photodiode (Newport Corp., Model 600 A-2), which monitors the output of the dye laser and is conditioned by an Ortec 473A CFD. This signal is delayed (Ortec 425A) such that the signal appears on the screen of a Tracor Northern TN-7200 multichannel analyzer. The 1024-point histogram is accumulated to at least 10000 counts at its maximum and is transferred to a compatible IBM-XT microcomputer, which is equipped with 640K memory and a 8087 coprocessor.
The lifetimes were obtained by the iterative reconvolution technique (Marquardt algorithm) using the delta function convolution method (DFCM), which gives far better statistical results.27·28 In order to judge the quality of the fit, plots of weighted (26) Strickler, S. J.; Berg, R. A. J. Chem. Phys. 1962, 37, 814. (27) Zuker, M.; Szabo, A. G.; Bramail, L.; Krajcarski, D. T.; Selinger, B. Rev. Sci. Instrum. 1985, 56, 14. (28) Boens, B.; Ameloot, M.; Yamazaki, I.; De Schryer, F. C. Chem. Phys. 1988, 121, 73.
The Journal
9766
Figure
1.
of Physical Chemistry,
Structure of molecule
1
Vol. 95, No. 24, 1991
and atomic numbering.
Figure 2. Packing of the molecules in their unit cell.
TABLE III: Least-Squares Planes in the 3//-Indole Derivative 3//-Indole Plane ( 3 N (1)
C(2) C(3) C(3a) C(4)
=
30.9)
deviation, Á
deviation, A
0.005 (3) 0.009 (4) -0.008 (4) -0.007 (4) 0.007 (4)
0.008 (4) 0.002 (4)
C(5) C(6) C(7) C(7a)
LaChapelle et al.
deviation, A 1.240 (4)
C(14)“ C(15)“
-1.290 (5)
-0.008 (5) -0.009 (5)
equation of the 3//-Í ndole pla ne: 0.6088x + 0.7741y
0.1737z
-
=
5.5379
Phenyl Ring ( 2
9.1) deviation, A
deviation, A
C(8) C(9) C(10)
0.006 (4)
-0.000 (4) -0.006 (4)
=
deviation, A
C(ll)
0.003 (3)
N(2)“
C(12) C( 13)
0.001 (4)
C(16)“ C(17)“
-0.007 (4)
equation of the phenyl ring plane: 0.7441x + 0.6369y 5.9959
0.024 (3) 0.117 (5) 0.230 (5) -
0.20I8z
=
Amino Plane equation of the amino plane: 0.7902x + 0.5991y
-
0.1296z
=
6.1466
“Atoms not included in the least-squares plane calculation.
residuals and the autocorrelation function have been examined along with the reduced 2 value and the Durbin-Watson parameter (DW). Our time resolution being 80 ps, some of the shortest lifetimes have been measured in the Center of Fluorescence Spectroscopy of Professor R. Lakowicz, University of Maryland. Results 1. Crystallographic Investigation. The final positional parameters and their standard deviations are presented in Table II.29 The atomic numbering is indicated in Figure 1. The nine atoms constituting the 3//-indole moiety are coplanar, and so is the phenyl group of atoms. The least-squares planes and the atomic deviations thereof are given in Table III. These two atomic planes are tilted by 11.5° (tilt angle 6) with respect to one another. The two methyl groups at C(3) are nearly symmetrically disposed about the 3//-indole plane: C(14) and C(15) are at 1.240 (4) and 1.290 (5) A, respectively, from that plane. In a phenyl ring the C-C distances average 1.379 A.30 However, in the phenyl ring of this 3//-indole derivative the C(9)-C(10) and C( 12)—C( 13) bond distances are significantly shorter than the other four bonds (Table IV). The corresponding averages are 1.365 and 1.403 A, respectively. Concomitantly, the two endocyclic angles at C(8) and C(11) have values of 114.8 and 115.5° while the other four ring angles average 122.5°. The sum of the three bond angles around N(2) is 358.8°; thus, the coordination about N(2) is that of a flattened pyramid. Furthermore, N(2) is 0.091 (3) A away from the plane formed by C(11), C(16), and C( 17) and the torsion angles around C-
(29) The H atom coordinates, anisotropic temperature factors, leastsquares planes, and the table of observed and calculated structure amplitudes have been deposited as supplementary material. (30) Brisse, F.; Sygusch, J. Acta Crystallogr. 1974, B30, 480.
Figure 3. Variation of the ground-state energy of molecule 1 as a function of the tilt angle of the Phc ring by using the INDO/S semiempirical method.
(11)—N(2) deviate from 0 or 180° by up to 10° (Table V). As far as the packing of the molecules is concerned, one can observe, in the stereopair in Figure 2, that there is no stacking or overlap. This is most likely due to the presence of the two methyl groups at C(3). The molecules form ribbons extending in the a direction. Within a given ribbon, the molecules are oriented in such a way that their long axis is at 70° from the a direction. 2. Theoretical Study. 2.1. Ground-State Calculations. The AMI geometry optimization of 1 has been performed, and the results are reported in Tables IV and V. It can be seen that the AMPAC semiempirical method and the crystallography agree reasonably well with each other. Calculations reproduce the experimental values very well. Both techniques emphasize the geometrical distortion within the Phc ring and in the dimethylamino substituent as well as the tilt angle 6 of the Phc ring with respect to the indole moiety. Except for one bond in the 3//-indole moiety (C(3A)-C(7A)), all calculated bond lengths agreed quite well with the X-ray structure. The root-mean-square deviation for the bond lengths is 0.010 A. The agreement is also very good for the angles with a difference average of 1°. As is well-known though, this agreement stops when one compares the torsional angles (Table V). The disagreement is small for the tilt angle 6(11.5° by X-ray and 4.0° by AMI) but more important for the dimethylamino angle with the phenyl ring (6.2° by X-ray and 15.6° by AMI). It is not unusual that the X-ray data, because of the crystal packing involved, leads to smaller dihedral angles. The classical example is the biphenyl molecule which has a planar structure in the crystal, whereas the dihedral angle is ~45° in the vapor phase.31 More recent examples have also been given recently on the 3-hydroxyflavone where the X-ray structure analysis has been compared with the AMI calculation.32 The INDO/S semiempirical method has been used to calculate the energy of the molecule at various tilt angles 6 of the Phc ring, (31) Almenningen, A.; Bastiansen, O.; Fernholt, L.; Cyvin, B. N.; Cyvin,
S. J.; Samdal, S. J. J. Mol. Struct. 1985, 128, 59. (32) Dick, B. J. Phys. Chem. 1990, 94. 5752.
Ground and Excited States of 3//-Indole Derivative
The Journal
Vol. 95, No. 24, 1991
of Physical Chemistry,
9767
TABLE IV: Bond Distances and Angles in 2-[p(Dimethylamino)phenyl>3,3-dimethyl-3//-indole Obtained from the X-ray Structure Determination and by the AMPAC Method" AMPAC AMPAC X-ray X-ray
N(l)-C(2)
1.288 (5) 1.534 (5) 1.513 (5)
C(2)-C(3) C(3)-C(3a) C(3a)-C(4) C(4)-C(5) C(5)-C(6) C(6)-C(7) C(7)-C(7a) C(3a)-C(7a)
1.381
(5)
1.390 (6) 1.372 (6) 1.401 (6) 1.388 (6)
C(3)-C(15)
107.0 (3) 114.2 (3) 98.9 (3) 131.4 (3) 107.6 (3) 120.9 (4)
C(2)-N(l)-C(7a) N(l)-C(2)-C(3) C(2)-C(3)-C(3a) C(3)-C(3a)-C(4) C(3)-C(3a)-C(7a) C(4)-C(3a)-C(7a) C(3a)-C(4)-C(5) C(4)-C(5)-C(6) C(5)-C(6)-C(7) C(6)-C(7)-C(7a)
(4) (4) (4) (4) (4) (4) (3) (3) 111.0 (3) 109.4 (3)
C(7)-C(7a)-C(3a)
N(l)-C(7a)-C(3a) C(2)—C(3)—C( 14) C(2)—C(3)—C( 15)
C(3a)-C(3)-C(14)
C(8)—C(9)
1.519 1.379 1.403 1.392
C(9)-C(10)
1.462 1.403 1.359 1.402 1.416 1.370
C(10)-C(ll) C( 11)—C( 12)
C(12)-C(I3) C(8)-C(13)
1.391
1.372 1.428 1.432 1.419
1.394 1.433 1.515 1.514
C( 11)—N(2)
108.3 112.9
C(3a)-C(3)-C(15) C(14)-C(3)-C(15)
100.1 131.7
N(l)-C(2)-C(8)
N(2)-C(16) N(2)-C(17)
N(l)-C(7a)
111.5 111.3 120.6 125.2 120.2 125.0 114.8
C(3)-C(2)-C(8) C(2)-C(8)-C(9) C(2)-C(8)-C(13) C(9)-C(8)-C(13)
107.4 120.9 118.3 121.0 121.5 117.9 128.4 120.4 111.3 112.4 111.9 109.4
118.1 121.6 120.3 117.9 126.5 121.3 112.2 114.1
C(7)-C(7a)-N(l)
C(2)-C(8)
1.561
1.401
1.376 (5) 1.530 (6) 1.536 (5)
C(3)-C(!4)
1.323
123.1 122.1 115.5
C(8)—C(9)—C( 10)
C(9)-C(10)-C(l 1) C( 10)—C( 11)—C( 12)
C( 11)—N(2)—C( 16)
121.2 123.4 121.2
C(ll)-N(2)-C(17)
120.1
C(16)-N(2)-C(I7) N(2)-C(l 1)-C(10)
117.5 122.8 121.7
C(11)—C(12)—C(13)
C(12)-C(13)-C(8)
N(2)—C( 11)—C( 12) °
(5) (5) (5) (5) (5) (5) (5) (5) (6) (6) (5)
1.461
(3) (3) (3) (3) (3) (3) (3) (4) (4) (3) (4) (4) (3) (3) (3) (3) (3)
110.2
1.406 1.387 1.417 1.417 1.388 1.401
1.400 1.437 1.439 1.424
112.1
122.9 124.2 119.8 122.7 117.5 121.5 121.0 117.3 120.9 121.8 118.5 118.0 117.3 121.8 120.9
Bond distances are in angstroms and angles in degrees.
TABLE V: Torsional Angles of Interest and Their Estimated Values AMPAC X-ray C (10—C (11)—N (2)—C( 16) -3.7 (5) -5.7 -170.8 (4) -157.5 C(10)-C(l 1)-N(2)-C(17) 175.7 (4) 177.3 C(12)-C(l 1)-N(2)-C(16)
C(12)-C(l 1)-N(2)-C(I7)
8.6 (5)
N(l)-C(2)-C(8)-C(9) N(1)—C(2)—C(8)—C(13)
C(3)-C(2)-C(8)-C(9) C(3)-C(2)-C(8)-C(13)
25.5
TABLE VI: Electronic Transition Data Obtained by the INDO/S Semiempirical Method for Molecule electronic transitions S,
—
S2
—S0
S3
S4
— —
S0
S0 S0
Ss-S0 S6-S„ S,
—
S0
S,-S0 S,
—
S0
"The long molecular axis
E,
cm'1
f
058
1.0797
33517 34721
0.0186 0.0376 0.0046 0.0349 0.0098 0.1128 0.2230 0.2739
31
35 837 39 826 42 809 44 583 45 888
46815 is
MO/character 52 53 54 52 52 55 52 52 56
—
51
—
51
—
51
—
47 50
—
kk* irir* rr* * kk* kk*
—
51
—
49 kk* 48 kk*
— —
X-ray 9.6 (5) -169.0 (4) -168.0 (4)
51
kk*
13.3 (6) 1
AMPAC 3.6
-176.9 -171.1 4.3
at the AMPAC Optimized Geometry
localization deloc loc (Phc) loc (In) deloc deloc deloc deloc deloc deloc
transition dipole axis"
y X
y Z
µ,* D 13.2 5.6 6.1 5.1
X
5.9 5.3 6.9 7.8
xy
8.1
y y xy
y. 6The ground-state dipole moment (µ0) has been calculated to be 4.6 D.
while the geometry of the remaining part of the molecule is kept unchanged and fixed by the AMI method. Figure 3 shows the results of the calculations. In fact, one can see that there is a broad range of angles around 9=15° with only shallow energy variations. The angle will thus fluctuate over a wide range which includes both X-ray observation and AMI prediction. From Figure 3 one can see though that all conformations with £30° can be ruled out with certainty since the barrier height at room temperature is 25 meV. A thermal equilibrium between all other conformations (f-'
-
1)
=
1.37
(±0.02)
-
0.79 (±0.05) log
(6)
This equation predicts a nearly total annihilation of the emission (0F «= 0) at low enough viscosity. It is therefore possible to use this 3//-indole as a probe to determine the viscosity of nonpolar environments. For polar environments, results in acetonitrile show that 4>f also depends on the polarity of the medium so that molecule 1 cannot probe the viscosity of these polar environments. This empirical results as described by eq 6 can be rationalized in view of the dependence of the nonradiative (knr) rate on a fractional power of the viscosity. This is a quite common phenomenon of reactions in viscous environments. It was found in reactions in biological systems40 and in intermolecular41 and in-
Ground and Excited States of 3/f-Indole Derivative
The Journal
of Physical Chemistry,
WAVENUMBER
Vol. 95, No. 24, 1991
9771
(103 cm'1!
m
3 II
Figure 6. Modified absorption spectrum t(v)/v (right-hand solid curve) and modified fluorescence spectrum F(y)/P* *S.***(left-hand broken curve) and its reflection F(2f0 p)/(2p0 P)3 (right-hand dot curve) for molecule 1 in various solvents at room temperature: 1, in «-pentane; 2, in «-hexadecane; 3, in MeOH; 4, in 1-hexanol. *3
-
-
log
n
Figure 7. Log-log plot of the fluorescence quantum yield of molecule 1 at room temperature as a function of the solvent viscosity in a «-alkane series.
tramolecular42 electron-transfer reactions as well as in isomerization reactions.43 In the presence of a large-amplitude motion, various models based on very different assumptions44 can account for the phenomenological dependence such as in
4r
=
Clf*
(7)
where C is a proportionality constant. It is now obvious that if eq 7 is operative, the fluorescence quantum yield which is defined as =
4/(4
+
4r)
(8)
can be related to the viscosity as
4'1
-
1
=
(C/4)if*
(9)
(40) Beece, D.; Eisenstein, L.; Frauenfelder, H.; Good, D.; Marden, M. C.; Reinich, L.; Reynolds, A. H.; Sorensen, L. B.; Yue, K. T. Biochemistry 1980 19, 5147; Photochem. Photobiol. 1981, 33, 517. (41) Huppert, D.; Ittah, V.; Kosower, E. M. Chem. Phys. Lett. 1988, 15, 144.
(42) Finckh, P.; Heitele, H.; Michel-Beyerle, . 138,
E. Chem. Phys. 1989,
1.
(43) Velsko, S. P.; Fleming, G. R. J. Chem. Phys. 1982, 76, 3553. Velsko, S. P.; Waldeck, D. H.; Fleming, G. R. J. Chem. Phys. 1983, 78, 249. Zeglinsky, D. M.; Waldeck, D. H. J. Phys. Chem. 1988, 92, 692. (44) Villaeys, A. A.; Boeglin, A.; Lin, S. H. J. Chem. Phys. 1985,82, 4044. Bagchi, B.; Oxtoby, D. W. J. Chem. Phys. 1983, 78, 2735. Gegiou, D.; Muszkat, K. A.; Fisher, E. J. Am. Chem. Soc. 1968, 90, 12.
This shows when one compares eqs 6 and 9 that C/4 = 23.4 and x = 0.79 and since 4 >s independent of the viscosity and equals 4.2 X 10® s'1 (see Table IX), the proportionality constant Cis equal to 9.8 X 109 cP0·79 s'1. Exponents x varied from 0.2 to nearly 1 for various reactions in the literature.40"44 It is sometimes related to the volume of the rotating moiety and the volume of solvent it must displace. In n-alcohols, the knr values are much smaller than the one measured in n-alkanes. Consequently, the Phc ring torsion seems to be hindered in protic solvents possibly due to the hydrogenbonded complex formed in these solvents as shown above. In that respect it has been recognized that molecule 1 gives rise to strong hydrogen-bonding interactions with ethanol.® Here, we can also conclude that the hydrogen-bonding interaction involved the N(l) atom of the indolic moiety. Indeed, Table VIII shows clearly that the absorption band of 1 is red-shifted in alcoholic media as compared to nonpolar media when one would expect a blue-shifted absorption with protonation at the N(2) center of the aniline type nitrogen lone pair.34 The hindrance of the Phc ring torsion in the first electronic excited singlet state would also explain the independence of km with viscosity in that series of alcohols. We have also performed a geometry optimization of the ground electronic state of the protonated 3//-indole (1) at the N(l) position of the indolic moiety using AMPAC. The following torsional angles:
C(10)C(11)N(2)C(16), C(10)C(11)N(2)C(17), and N(1)C(2)C(8)C(9) see their values change from -5.7°, -157.5°, and 3.6° in the unprotonated compound (see Table V) to -0.2°, -179.8°, and 2.6°, respectively, in the protonated species. Concomitantly, the bond of the pz atomic orbitals and interatomic distance of the C(2)-C(8) bond go from 0.324 and 1.461 Á in the neutral species to 0.520 and 1.431 Á in the protonated cation. These calculations clearly show that protonation (hydrogen-bonding interaction) at the N(l) center increases the conjugation throughout the molecular frame in the ground electronic state, giving rise to much more hindered phenyl ring torsional movement (higher potential barrier for rotation). On the other hand, preliminary results by INDO-S calculations of the protonated 3íf-indole (1) at the N(l) atomic center have also clearly shown the stronger stabilization energy of the first singlet excited state as compared to the ground state (¿sESrS¡ = 24845 cm"1) in comparison with the unprotonated compound (A£'So_S| = 31 270 cm"1). This explains the red shift experimentally observed when hydrogen bonding occurs at the N(l) atomic center of molecule (1). > kF in aprotic or protic polar solvents. Table IX shows that Since µ3| > (Table VI), the solute-solvent interactions should
4
9772
The Journal
of Physical Chemistry,
Vol. 95, No. 24, 1991
be stronger in the first excited singlet state which should affect the nuclear configurations of the Sj state much more than that of the ground state. Considering that k'F > kF this suggests that the nuclear configuration of 1 in polar aprotic or protic media would be a little more twisted in the S, state compared to the ground state.13 4. Effect of Close-Lying Excited States on the Fluorescence Spectra. From the mirror-image relationship of 1 in «-alkanes (see Figure 6) it is obvious that the fluorescence band is larger than the absorption spectrum. In light of the previous section, this behavior cannot be interpreted in terms of conformational changes occurring during the relaxation of the S, state. On the other hand, the INDO/S calculations have shown that the energy gap between the first two excited states (AEs^) is relatively small (~ 2500 cm'1 2*at = 0°; see Table VII) and decreases with the tilt angle of the Phc ring. Thus, these two excited states may interact with each other through vibronic coupling. This behavior has been observed in many compounds with about the same energy gap between these first and second excited singlet states.39,45"48 It then appears that the fluorescence of 1 arises from a mixed state (following the Herzberg-Teller vibronic coupling mechanism) rather than from two distinct excited singlet states for the following reasons: (1) the fluorescence spectrum of 1 is rigorously the same whatever the excitation wavelength used, (2) the shape of the excitation spectrum does not vary with the fluorescence wavelength, and (3) the fluorescence lifetime decay shows a singleexponential fit and is not affected by the variation of the fluorescence wavelength. Figure 6 shows that the bandwidths of the absorption and fluorescence spectra of 1 in «-alcohols are nearly similar, and this is also true in very polar aprotic solvents like acetonitrile. This can be explained by the different polarity of the S, and S2 states. It has been shown above that the dipole moment of the state is much larger than that of the S2 state (Table VI). Thus, the first excited singlet state should be more stabilized by dipole-dipole interactions in polar environments and by hydrogen-bonding interactions in protic solvents. This should increase the energy gap between the S, and S2 states and then contribute to weaken the vibronic coupling between these states. The mirror-image relationship agrees with that statement since the bandwidth of the fluorescence spectrum of 1 is considerably reduced in «-alcohols (Figure 6). In that respect it is important to note that the 3Hindole with a N(CH3)2 group in the para position of the Phc ring and an N02 substituent in the para position of the indole moiety shows a perfect mirror-image symmetry in methylcyclohexane.4 For that molecule it has been recently calculated by INDO/S that = AEsi-Sj 5000 cm"1,49 which is larger by a factor of 2 as compared to the energy gap obtained for 1. Moreover, preliminary results by INDO/S calculations of the protonated 3//-indole (1) at the N(l) atomic center have shown that the energy gap between the S, and S2 states dramatically increases to about 7400 cm'1. It thus seems reasonable to think that, in «-alcohols, the fluoresence arises from the “pure” S, state of the hydrogen-bonded complex.
(45) Easterly, C. E.; Christophorov, L. G.; Blaunstein, R. P.; Carter, J. Chem. Phys. Lett. 1970, 6, 579. (46) Easterly, C. E.; Christophorov, L. G.; Carter, J. G. J. Chem. Soc., Faraday Trans. 2 1973, 69, 471. (47) Easterly, C. E.; Christophorov, C. G. J. Chem. Soc., Faraday Trans. 2 1974, 70, 267. (48) Wermuth, G.; Rettig, W. J. Phys. Chem. 1984, 88, 2729. (49) Unpublished results.
LaChapelle et al. Concluding Remarks In this paper, we first thoroughly characterized the ground-state conformation and geometry of 2-[p-dimethylamino)phenyl]-3,3dimethyl-3Z/-indole (1). Both crystallographic measurements and AMI geometry optimization showed very good agreement with each other. It was shown that the solid-state structure of 1, which was determined by X-ray diffraction, represents one of the most stable conformations available to the molecule in the gas phase and in solution at room temperature as obtained by AMI and INDO/S calculations and gas-phase and solution absorption and fluorescence spectra. We have shown that the Phc ring is twisted (dihedral angle ) to some extent with the indole moiety in the solid state. Moreover, this ring can líbrate from about -30° to +30° within the kT energy barrier at room temperature in the gas phase. AMPAC (AMI) and INDO/S calculations have also shown that the dihedral angle of 1 in the first excited singlet state is similar to that in the ground state. This has been confirmed experimentally by the fact that both the radiative fluorescence decay rate constant (kF) and the theoretical radiative fluorescence decay rate constant (kF) are the same in the nonpolar solvents. We have also shown the excellent agreement that exists between the calculated (INDO/S) and experimental values of the ground and first excited state dipole moments of molecule 1. It has been shown that the nonradiative fluorescence decay rate constant (knr) entirely controls the excited-state dynamics of molecule 1 in solution. A km dependent decay rate on a fractional power of the viscosity in nonpolar solvents showed a friction-limited rate for the rotamers inside the energy barrier at room temperature. On the other hand, increasing the dipole-dipole interactions in polar environments or hydrogen-bonding complexation in protic environments profoundly influences the first excited state geometry and stabilizes its electronic energy. The geometry becomes more planar such that the Phc ring torsion is hindered, giving rise to much lower knT values. This was the reason why this molecule has been used before as a good fluorescence probe of the interfacial micropolarity in inverted micelles. Furthermore, one might propose on the basis of INDO/S calculations on the protonated 3//-indole that, in these hydrogen-bonded environments, the energy gap between the first and second excited singlet states (AESl^) increases so that the vibronic coupling between these states would be much reduced in the fluorescence spectra. Finally, this paper has shown that the AMPAC method yields reliable results on ground-state geometry and energy values for this molecule. It has also shown that AMPAC and INDO/S methods coupled together are useful tools to obtain a qualitative dynamic picture of the conformational changes of the molecule in various excited states, allowing for a better understanding of its photophysics.
Acknowledgment. We gratefully acknowledge the financial assistance of the Natural Sciences and Engineering Research Council of Canada and the Fonds FCAR (Quebec) in the form of grants. We also acknowledge Mr. Adrian Popowicz for the synthesis and crystallization of molecule 1. Registry No.
1,
4203-57-0.
Supplementary Material Available: Tables containing positional coordinates and anisotropic temperature factors for the 3//-indole derivative (2 pages); table listing observed and calculated structure amplitudes (4 pages). Ordering information is given on any current masthead page.
