9832
J. Phys. Chem. 1992,96, 9832-9839
Photophyslcs of a Sterically Crowded Tertiary-Saturated Amine: Triisopropylamkre Arthur M. Halpem* and B. R. Ramaehandran Department of Chemistry, Indiana State University, Terre Haute, Indiana 47809 (Received: June 4, 1992; In Final Form: July 30, 1992)
The spectroscopic and photophysical properties of triisopropylamine (TIPA), a sterically overcrowded amine, are reported in the vapor phase and in n-hexane and tetrahydrofuran solution. On the basis of available experimental data and several semiempirical and ab initio quantum mechanical calculations, it is concluded that TIPA is planar or nearly planar in the ground state. Inversion barriers of NH3, trimethylamine, and TIPA are examined with the aid of computational methods. In the vapor phase, Em and E,, are 36 550 and 38 OOO cm-I, respectively, for the SO SItransition. The zero-pressureextrapolated fluorescence lifetime and quantum efficiency are 57.8 ns and 0.79, respectively. Its properties are compared with other tri-C,-substituted amines, such as tri-n-propylamine (TNPA) and the cage amine, l-azabicyclo[3.3.3]undecane (ABCU). As compared with TNPA, the So SIabsorption of TIPA is red shifted and its Stokes shift is smaller. The cage structure of ABCU results in the S, SItransitions being very vertical. Multielectron configuration interaction calculations using MNDO are used to determine the E,,,, and E,,, values of the amines; the results are compared with experiment.
-
--
Introduction One of the dominant characteristics of the spectroscopy and photophysics of saturated amines, a broad class of compounds that encompasses ammonia and primary, secondary, and tertiary amines, is that in the absence of overriding structural constraints, the ground electronic state is pyramidal, whereas the excited state is planar.' On the orbital level, this situation can be viewed in t e r m of the promotion of an electron from the nN orbital, which is oriented along the C3 molecular axis away from the N atom substituents, to the 3sN orbital, which, being spherical, disposes the substituents bonded to the N atom to adopt a planar, or nearly planar, arrangement with respect to the N a t o p The well-known Franckendon profde of the X & . transition in ammonia2 and the analogous transition in trimethylamine (TMA)3 are classic examples of transitions between potential energy surfaces whose minima are significantly displaced from each other along a particular degree of freedom, which, in the case of the amines, is principally the C-N-C bending motion. Thus, these transitions are characterized by very weakly allowed origins and by Franck-Condon maxima that lie at significantly higher energies (6400 and 5400 cm-' for ammonia and TMA, respectively) from the origin. In the saturated amines, only the tertiary amines possess significant fluorescence quantum efficiencies (Le., qf> lo4)> For these compounds, the observed Stokes shifts are very large, as would be expected for transitions between widely displaced potential energy surfaces. Thus, for TMA, the Stokes shift is ca. 8150 cm-' (the separation between the maxima of the X A transition^).^ It is significant, however, that for TMA, and presumably other acyclic tertiary amines, the absorption spectrum is dominated by the NC3 bending mode, while the fluorescence spectrum is characterized by vibrational motion based on C-N stretching activity. For this reason, the absorption and fluorescence of TMA, particularly, and other homologous amines reveal a striking absence of mirror-image symmetry. As an example, for TMA, more than 20 members of a progression in 375 cm-l (the upper state out-of-plane bending mode) can be discerned in the absorption spectrum; in contrast, 3 members of a progression in ca. 1450 cm-', corresponding to the N-C stretching and methyl torsional activity, are observed in the fluorescence ~ p e c t r u m . ~ When the three alkyl substituents are bonded to the N atom in such a way that the motion of the N atom along the C3 axis is restricted or constrained, there are, as expected, both spectroscopic and photophysical consequences. A significant example of this structural effect is manifest by the cage amine, l-azabicyclo[2.2.2]octane (ABCO), in which the N atom is located at the bridgehead position. Because of the constraint imposed by the three ethylene bridges, the N atom cannot become planar with
-
-
respect to the three a-C atoms, an arrangement that characterizes the potential minimum of the electronically excited state in a flexible amine. The ground-state C-N-C bond angle in ABCO is close to Accordingly, the absorption and fluorescence spectra of ABCO do not reveal the anomalies seen in the case of TMA.6 Another important consequence of the structural constraints present in ABCO is the fact that it forms a highly emissive (excited dimer) excimer, both in the vapor phase' and in nonpolar solution.* The ability of ABCO to achieve a stable head-to-head excimer structure has been ascribed to its ability to form a 'threeelectron bond" between the N atoms? The close proximity of the N atoms required for this net associative interaction is facilitated in ABCO via the reduction of steric hindrance that is present in those amines that have planar excited states. An example of the other extreme in structural constraint in tertiary amines OCCUA when the N atom is forced to adopt a planar configuration with respect to its three substituents. This is the case that is desxibed here. We report a study of the spectroscopy and photophysics of triisopmpylamine (TIPA), a sterically crowded amine in which the N atom is significantly flattened with respect to the three bonded C atoms.I0 We compare these properties with two homologous tertiary amines, a bicyclic cage amine, l-azabicyclo[3.3.3]undecane (ABCU), in which the three trimethylene substituents constrain the N-bridgehead atom to be nearly planar with respect to the three bonded C atoms," and the straight-chain isomer of TIPA, tri-n-propylamine (TNPA), in which the three n-propyl substituents do not present extraordinary structural constraints on the N atom. Because of its unusual structure, TIPA provides the unique opportunity to examine in detail the consequence of N atom sp2 hybridization on the spectroscopy and photophysics of saturated amines. TIPA thus represents a molecule that, because of severe steric crowding,~osseapesthe transition-state structure with respect to the out-of-plane motion that figures so prominently in the spectroscopy and photophysics of trivalent nitrogen compounds. Furthermore, TIPA and ABCO form an interesting contrasting pair in terms of the nature of their highest-filled MOs, TIPA's being 2 b and ABCOs being nN. Thus, from this study, we will be able to perform direct measurements of the energetics and transition probabilities of an amine in a region of the potential energy surface that is usually inaccessible. Experimentnl Section
Materials. Triisopropylamine (TIPA) was prepared from diisopropylamine by the three-step synthesis described by Bock et
0022-3654/92/2096-9832$03.00/00 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 9833
Photophysics of Triisopropylamine Pr\ iPr
NH,+CI'
KCN
iPr \
CHaCHO
/ CN
b
L
/
i Pr
/N-CH
CHa
I I -
CH$lgC 1
iPr iP r
'
\ /N
-
iPr
Diisopropylamine (60 g; 0.59 mol) (Aldrich) was neutralized with 50 mL of concentrated HCI, and the resulting amine hydrochloride was dissolved in a minimum amount of water. That solution was treated dropwise with a saturated aqueous solution of KCN (40g; 0.62 mol), followed by a slow addition of acetaldehyde (26 g; 0.59 mol) (Aldrich) at 273 K under an Ar atmosphere. The reaction mixture was stirred at 273 K for 4 h and was then allowed to stand overnight, after which it separated into two liquid phases. The denser (aqueous) phase was treated with K2C03and was then extracted with ether. The lighter (organic) phase, which was combined with other extracts, was dried with anhydrous Na2S04and was fractionally distilled at ca. 15 Torr. Under this condition, the intermediate, a-(diisopropy1amino)propionitrile (I), was distilled as a colorless liquid at 70-75 OC. The middle fraction was kept for further use (35 g; 39%). A crystalline solid, which was not identified, was distilled in the final fractions. Fifty milliliters of an ether solution of I (31 g; 0.2 mol) was added dropwise to a solution of CH3MgCl in ether (prepared by bubbling anhydrous CH3Cl (Aldrich) into ether that contained 16 g of Mg and 0.2 mL of 1.2-dibromoethane) under an Ar atmosphere; the resultant mixture was refluxed for 4.0 h. The Grignard complex was decomposed with ice water, was made basic with a concentrated solution of KOH, and was extracted with ether. The ether extracts, dried with anhydrous Na2S04, were collected and fractionally distilled under reduced pressure (15 Torr; Ar atmosphere). TIPA (14 g; 49%) was distilled as a colorless liquid at 38-39 OC (lit. 47 OC/14 Torr). TIPA was redistilled immediately prior to use. Its identity and purity were ascertained through its GC/MS, IR, and UV spectra. The GC showed one sharp peak. IR (neat) 2960.0, 2933.9, and 2868.3 (C-H and N-H stretches); 1466.0 and 1458.3 and 1394.6 and 1361.8 (C-H symmetric and CH3 antisymmetric bending); and 1215.2 and 1155.4 cm-I (C-N stretch). 'H NMR (neat, TMS) 6 0.8-1.0 (d, 18 H, CH3) and 2.6-3.4 (sept, 3 H, CH). MS m / e 143 (M'), 127 (M' - CH3- H), 100 (M' - i-Pr), 85 (M' - i-Pr - CH3), 70 (M' - i-Pr - 2CH3), 57-58 (M+ - 2i-Pr). The TIPA vapor pressure was measured between 273.0 and 296.0 K. Over this range, the following relation described the data: In P = 17.00 - 4367/T where P is in Torr. Trimethylamine (Aldrich) was used without further purification. Triethylamine (Aldrich), tri-n-propylamine (Aldrich), and tri-nbutylamine (Aldrich) were dried over KOH and purified by trap-to-trap distillation. 1-Bromopropane (Eastman), used as a fluorescence quencher, and n-hexane (Burdick & Jackson, UV grade), and THF (Burdick & Jackson, UV grade) were used without further treatment. The vapor pressure of the amines was determined using an MKS pressure transducer (Baratron Model 77). The amine was kept in a side arm, which was immersed in a constant-temperature bath. Absorption spectra were obtained with a Varian Cary 5 spectrophotometer. Fluorescence excitation and emission spectra were run using a Spex Fluorlog I1 spectrofluorometer. The correction function that compensated for the emission monochromator-photomultiplier tube combination was obtained as follows. Radiation from a 6 6 W dc D2 lamp was scattered from the sample holder into the analyzing optical path; with this arrangement, the excitation and emission monochromators were
scanned synchronously between 240 and 500 nm. Subsequently, the output of the lamp was analyzed by scanning the excitation monochmtor between 240 and 500 nm; the signal recorded in this scan was produced by a "flat" photodiode quantum counter. The instrumental correction function was then obtained as the quotient of the synchronous scan to the excitation scan. Hence, "raw" fluorescence spectra were multiplied by this correction function to furnish the corrected spectrum. Using the same procedure, a 150-W dc Xe lamp was used to generate the correction function between 350 and 800 nm. The two correction functions were truncated at 420 nm, normalized, and smoothly joined at that point. The c o d o n function thus produced, C(X), was used between 250 and 800 nm. To convert the responsecorrected spectrum from energy per wavelength interval to energy per wavenumber interval, it was then multiplied by A*. Hence, the corrected fluorescence spectra reported here were generated as follows:
F ( t ) = X2C(Xxf(X) where F ( t ) is the corrected fluorescence spectrum (displayed in cm-') and f(X) is the instrumental response function. Corrected fluorescence excitation spectra were obtained by recording the ratio of the fluormcence signal to that produced by a photodiode, which viewed a fraction of the excitation radiation. Solutions were degassed through six freeze-pumpthaw cycles, or deaerated via bubbling with dry N2 for 5 min prior to measurements. Fluorescence quantum efficiencies of the amines were determined using triethylamine (vaporI2 or a 1.5 X lo-" M n-hexane sol~tion'~) as the standard. In the former case, qr)- = 0.98 and kq = 6.5 X lo9 M-I s-'. In the latter case, q f ) d = 0.69 and kq = 6.0 X lo9 M-' s-l. In addition, another reference used was a 5.0 X lo-' M n-hexane solution of toluene; for this sample, qf = 0.17.13 The absorbances of the samples in these experiments were determined in the fluorometer cavity under actual band-pass conditions by measuring the relative intensities of the excitation beam passing through a sample vs an n-hexane blank. Fluorescence lifetimes were determined by the timecorrelated single-photon method. The instrument has been described elsewhere. Radiation of the required wavelength was isolated from a pulsed D2 flash lamp (0.5 atm; 30 kHz) by a Bausch & Lomb monochromator (3.2-nm band-pass); fluorescence was viewed at right angles through a Corion interference filter (10-Aband-pass) centered at the appropriate wavelength. The detector was an Amperex 56DUVP/03 photomultiplieroperating with the photocathode at ground potential, the anode being capacitively coupled to the fast discriminator. Pulse-processing hardware has been previously described.I4 Decay curves were analyzed using a nonlinear least-squaresoptimization program based on a reconvolution algorithm described by Frye et a1.ls IR spectra were acquired on a Midac M2000 FTIR spectrophotometer operating at 2-cm-' resolution. Calculations were performed on an IBM RS/6000 Model 320 workstation. Ab initio calculations on TIPA at the 6-31G** level were camed out using Gaussian 92 on a Cray Y-MPJ832 through the Pittsburgh Supercomputing Center.
Results and Discussion The synthesis and properties of TIPA have been recently described by Bock et al. in the context of work dealing with sterically overcrowded molecules.'o The very interesting question arises as to whether TIPA is planar or not, Le., whether the repulsions between the three methyl groups located at each of the ends of the isopropyl substituents effectively counterbalanceeach other, thus constraining the N-C3 atoms to adopt a planar or quasiplanar arrangement. Gas-phase electron diffraction data interpreted by Bock et a1.I0 indicate that at 298 K, the molecule is nearly planar (Le., slightly bent), with an average C-N-C bond angle of 119.2'. Amrdingly, they report that the isopropyl groups are slightly tilted from the C3axis, making a H-C-N-C dihedral angle of 5.0'. Bock et a1.I0 report the results of a few computational attempts to provide
9834
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992
Halpern and Ramachandran
TABLE I: Structud Properties of NHk “MA, .ad TIPA. Compuisons betweea Cdculated V d w .ad Experimeatrl h t r NH3
exptl MNDO AM 1 PM3 STO-3G 3-2 1 G 6-31G 6-31G** D95**
LH-N-H 106.67 105.25 109.09 108.03 104.16 112.39 116.13 107.32 108.19
TMA
rNM/A
r/D
1.0124 1 .OO72 0.9978 0.9995 1.0325 1 .W26 0.9913 1. m 9 1.0011
1.47 1.753 1.846 1.551 1.876 1.752 1.382 1.839 1.806
Vo 2049 4052 1484 3494 3839 562 130 1936 1717
LC-N-C 110.6 116.00 112.97 112.33 110.29 113.03 114.23 111.96 111.57
TIPA
rNr/A
u/D
Vn
1.451 1.4644 1.445 1 1.4795 1.4853 1.4641 1.4525 1.4453 1.4482
0.61 0.75 1.02 1.15 1.13 0.87 0.82 0.74 0.87
2900 738 1609 2928 3591 1906 1570 2770 3053
LC-N-C 119.2 119.99 116.31 116.29 120.00 119.98 120.00 119.01
rNriA
V,
~~~~~
1.460 1.4743 1.4569 1.4979 1.4710 1.4632 1.4616 1.4562
2 820 680
0 0 0 50
‘Dipole moment, D. bInversionbarrier, cm-I. CReference18. “Reference 33. ‘Reference 34. ’ref (10). support of whether the TIPA ground-state potential energy surface is characterized by a single-minimum potential or a doubleminimum potential with respect to the NC3 frame-isopropyl torsional motion. They showed that although MM3 and AM1 calculations predict a bent structure (C-N-C angles of 117.9 and 116.3’, respectively), MNDO indicates a planar configuration (120.0°).10 We have confirmed the results of those calculations. Structure of TIPA. In view of the fact that any computational method of gaining further insight about the structure of TIPA is required to deal with a quasi-planar, trivalent, neutral nitrogen compound, we thought it would be useful to examine several semiempirical (SE) and ab initio (AI) approaches to this very interesting problem. The pursuit of the optimized structure of TIPA leads naturally to the question of how well these computational methods deal with the N-inversion potential surface in amines. This matter was recognized by Bock et al., who reported the (AMI) inversion barrier for TIPA to be 9.6 kcal mol-1 (803 cm-I).Io Thus, an attempt to describe the potential surface of TIPA provides the opportunity to compare the results of a variety of computational methods with respect to a well-established computational benchmark, i.e., the inversion barrier in NH3 in the context of a reasonably large molecule whose equilibrium structure is quite far from the pyramidal configuration that is typical of tertiary amines, such as tri-n-propylamine. There are various criteria that can be used to evaluate the quality of the potential energy surface that is obtained in a computational method. In the application discussed here, we will examine those properties that bear on the shape of the surface that describes the out-of-plane bending motion of the N atom in ammonia, trimethylamine (TMA), and TIPA. Some of these structural, physical, and spectroscopic characteristics are the inversion barrier, the C-N-C bond angle, the C-N bond length, the C-N stretching frequencies, and the out-of-plane bending frequency (or frequencies). We have employed several standard SE and AI computational methods; these utilities are becoming increasingly more accessible and valuable tools in predicting molecular structure and properties. The SE approaches that we used, MNDO, AM 1, and PM3, were applied via MOPAC version 6.0.16 The AI methods employed H F calculations using the following basis sets: STO-3G, 3-21G, 6-31G**, and D95**. Ab initio calculations were performed using Gaussian 90.” The 6-31G** calculations on TIPA were run under Gaussian 92. We present the primary results of these calculations in Table I, which contains the C(H)-N-C(H) bond angles, C(H)-N bond lengths, dipole moments, and inversion barriers of NH3, TMA, and TIPA. Experimental data are included where available. First, we focus on the results concerning the inversion barrier. Ammonia. The three SE methods poorly capture the barrier height in NH3; MNDO and PM3 overestimate the experimental value of 2049 cm-l by ca. 2000 and 1440 mi1,respectively. The AM1 method underdetermines the barrier by ca. 560 cm-I. Of the four basis sets used in AI calculations, one can see in Table I that only those containing polarization functions with the larger basis sets, i.e., 6-31G** and D95**, capture the NH3 barrier height, i.e., 1936 and 1717 cm-I, respectively. Charge density and structural predictions vary with these methods considerably; for example, all approaches (except for 6-31G) overestimate the dipole moment of NH,; all methods
(except for MNDO and STO-3G) overestimate the H-N-H bond angles. Despite the success of the 6-31G** and D95** basis sets in predicting the barrier height, they do not provide consistently superior results with respect to the structural and dipole properties of NH3 as compared with the SE methods. Trimethyhmine. With respect to TMA, all the computational methods used here indicate that the most stable pyramidal form is the one in which three C-H bonds are anti to the N atom nonbonding electron pair. Of the three SE methods used, only PM3 accounts reasonably well for the TMA inversion barrier. Despite the ability of the PM3 Hamiltonian to replicate the barrier height, it overestimates the TMA dipole moment. MNDO, on the other hand, which is closest in reproducing the TMA dipole moment, grossly underestimates the barrier height. Of the AI methods used here, the 6-31G** and D95** basis sets perform best with respect to calculations of the turrier height and the dipole moment. The results for 6-31G** are somewhat better than those for the D95** basis set. It should be noted that in the calculations of the barrier height of TMA, the energy of the planar form was determined by o p timizing the structure in which the three C atoms were constrained to be coplanar with the N atom. In the optimization process, the orientations of the methyl groups retained the same rotational conformational structure as in the pyramidal form; i.e., each one had a C-H that made a dihedral angle of 180’ with res+ to the N atom lone pair orbital (i.e., the C3axis). This arrangement does not, however, represent the globally minimized planar structure; it retains a small dipole moment (e.g., 0.27 D from 6-31G**). The nonpolar planar structure, in which each methyl group has a C-H bond forming a 90’ dihedral angle with respect to the C3axis, is lower in energy by about 300 cm-’ relative to the planar polar form (with the 6-31G** basis set). With regard to the energetics of the rotameric forms of the planar structure of TMA, the SE methods give varied predictions. MNDO shows the nonpolar form to be more stable than the polar structure by 138 cm-I, whereas AM1 indicates the polar form to be more stable by 116 cm-’.The conclusionsregarding the relative stability of the polar and nonpolar planar forms of TMA are consistent with the calculations reported by Eades et al.I9, who, using a minimum basis set MO method at the PRDDO level, found the nonplanar rotamer to be more stable by 168 cm-l relative to the all-anti configuration. Triisopropyl8miae. It appears that the Hartree-Fock AI method us* the 6-31G** or D95** basis set would provide a reliable * ‘ng whether TIPA is planar or bent; we used the tool for 6-31G** basis set in these calculations. From the results in Table I, one can see that the SE approaches return different predictions. MNDO indicates a planar geometry, while AM1 and PM3 both predict somewhat bent structures, the former giving an inversion barrier of ca. 820 cm-I and the latter a barrier of ca. 660 cm-I. The MNDO and AM1 results are consistent with the report published by Bock et al. All of the basis sets used in the AI approach, ST03G, 3-21G, 6-31G**, predict planar, or in the case of 6-31G**, very nearly planar TIPA structures. The SE results should be interpreted in the context that MNDO, AM1, and PM3 overestimate the C-N-C bond angle in saturated tertiary amines; for example, the MNDO C-N-C bond angle in TMA is 116.0O0 (see Table I). In the case of TIPA, MNDO and PM3 predict
The Journal of Physical Chemistry, Vol. 96, No. 24. 1992 9835
Photophysics of Triisopropylamine an increase. in the C-N-C bond angle of ca. 4.0° relative to TMA. In the case of AM1, this increase is 3.2'. A vibrational analysis of TIPA was performed using the MNDO Hamiltonian in MOPAC. In order to interpret the results from that calculation, the eigenvectors from the .out file were used in conjunction with KGNGRAF20 to create visualized atomic displacements and animated motion of the vibrational modes. Using this utility, the vibrational modes corresponding principally to (1) the out-of-plane bending and (2) the N - C stretching activities of TIPA were identified. The most prominant out-of-place bending mode corresponds in this calculation to a frequency of 481 cm-I, with the involvement also of modes in 77.2 (with isopropyl torsion) and 103.7 cm-' (with CH3 torsion). All of these modes have A symmetry. C-N stretching modes, all of which are doubly degenerate, are represented by frequencies of 1087, 1367, and 1534 cm-I. These motions are accompanied by methyl rocking or (N)-C-H bending activity. The optimized structure of TIPA, obtained by the Hartree-Fock AI method at the 6-31G** level, is shown below.
It is evident from the structure of TIPA that the extension of the three isopropyl moieties above and below the NC3 plane sterically shields the N atom from close approach along the C3 molecular axis. h o t h e r significant consequence of the flattened NC3 structure in TIPA is that the ionization potential (IP) is lowered relative to a pyramidal structure; this is consistent with ionization from an N2 -type MO. The experimental vertical IP is reported by Bock et al.lg to be 7.18 eV, which compares with 7.92 eV for TNPAaZ1 The IP of ABCU, which also possesses a flattened NC3 structure (C-N-C bond angles are 115.0-115.9°),11 is also lower than that of TNPA, although the resolved vibrational structure that is present in the ABCU photoelectron spectrum precludes a facile vertical assignment. The adiabatic IP, however, of ABCU is reported to be 7.05 eVSZ1This lowering of the ionization energy is expected to have a direct effect on the energetics of the lowest energy (singlet) electronic transition in TIPA. Because of the Rydberg nature of the electronic transitions in the saturated amines, there is a correlation between the state energies of an amine and its ionization potential. This relationship has been demonstrated for amines in several cases.22 TIPA. Absorption and Fluorescence. Vapor Phase. The gas-phase electronic absorption spectrum of TIPA is presented in Figure 1. Two transitions are evident, one with a maximum at ca. 38 OOO cm-l and another at 46 100 cm-'. By analogy with the assignments proposed for other saturated tertiary amines, the lowest-lying transition at 38 OOO c m - I is assigned as the nN 3sN Rydberg transition, where nN 2 h . Likewise, the more intense band at 46 100 cm-l is assigned as the nN 3pp, Rydberg transition. The feature seen at ca. 5OOOO cm-I possibly corresponds to the onset of the higher energy nN 4sN Rydberg transition. A closer analysis of the 38000-cm-' transition using a second-derivative technique allows the maxima of three very diffuse bandheads to be assigned at 36 630,37 880, and 39 120 cm-' . The separation of these features corresponds to a vibrational frequency
-
-
-
-
WAVENUMBERS / cm-I
Figure 1. (A) Absorption (-), (FE) corrected fluorescence excitation and (F) corrected fluorescence (- -) spectra of TIPA in the vapor phase at 298 K. (-e),
-
of ca. 1250 cm-', which likely corresponds to the excited-state C-N stretching mode. The gas-phase (corrected) fluorescence spectrum, also depicted in Figure 1, reveals a somewhat more well-defined vibrational structure. Two features display maxima, the most intense one being at 36 127 cm-I and the other at 35063 cm-'; there is a shoulder at ca. 33 800 cm-'.This spacings implicate a ground-state vibration of ca. 1163 cm-'. This vibrational mode probably correlates with the 1250-cm-' vibration assigned for the upper state. The IR spectrum of TIPA shows two poorly resolved bands at 1215.2 and 1224.9 anL1.From the vibrational analysis discussed above, this frequency might be assigned to the N - C stretching (and CH3 rocking or i-Pr torsional) modes obtained from the MNDO vibrational analysis (1087 or 1367 cm-'). Figure 1 also shows the gas-phase (corrected) fluorescence excitation spectrum obtained at 5.0 Torr of TIPA and at 295 K. Under these conditions, the fluorescence efficiency of the higher TIPA electronic state is greatly suppressed, and the So S1profie is nearly isolated from the higher-lying absorption. This characteristic of amine photophysics has been noted before with rapect to trimeth~lamine.~~ From the fluorescence and fluorescence excitation spectra of TIPA shown in Figure 1, it can be seen that reasonably good mirror-image symmetry is displayed; furthermore, from the intersection of these (maximum-normalized) spectra, the 0-0 gap between the So and SIelectronic-state surfaces can be estimated to be ca. 36 550 cm-'. The Stokes shift of TIPA is ca. 5400 cm-I, which indicates that the So and SIpotential surfaces are considerably displaced from each other with respect to the coordinate represented in the absorption and emission spectra (Le., the N-C stretching-CH3 rocking mode). By normalizing the fluorescence excitation spectrum with the quantitative absorption spectrum of TIPA at the lower energy side of the maximum (ca. 37 800 cm-'), we were able to obtain the oscillator strength, f, of the TIPA So -+ SItransition with diminished contribution from its stronger, higher-lying transition. The value off thus obtained is 0.013.24 By using the quantitative absorption and the corrected fluorescence spectra, the radiative rate constant can be calculated by an expression that attempts to account for the frequency dependence of the radiation-induced component of the transition according to B i r k ~ .That ~ ~ approach leads to a calculated radiative rate constant of 8.86 X lo6 s-l. The fluorescence lifetime of vapor-phase TIPA, excited at 262 nm and analyzed at 296 nm, is (self) pressure dependent. The self-quenching constant of this process is 1.91 X 1O'O M-I PI, and the zero-pressure-extrapolated lifetime is 57.8 ns. TIPA fluorescence self-quenching thus takes place with an efficiency of ca. 0.1 relative to the gas kinetic rate. We measured the fluorescence quantum efficiency of TIPA by using triethylamine (TEA) vapor as a standard.12 By using the reported ZercFprrssure fluorescence efficiency and self-quenching rate constant for TEA, we determined the zero-pressure-limiting value of the TIPA +
9836 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992
Halpern and Ramachandran
TABLE II: Spectroscopic and Photophysical Data for TIPA, TNPA, and ABCU amine
Elm0
rr'
36 550 37 OOO
E"," 38 OOO 40 500
f b
TIPA (vapor) TIPA (n-hexane) TIPA (THF) TNPA (vapor) TNPA (n-hexane)
0.013 0.023
38 300
43 380
0.054
57.8 46.5 45.4 56.1 32.7 91.0
ABCU'
35 932
O I n cm-'. bOscillator strength. CInns.
0.0077
4r 0.79 0.37 0.07
kwd 1.9 x 10'0 1.1 x 108
0.049 1.o
3.2
X
lo8
kr(expy 1.37 0.796 0.15
0.76 1.1
kr(ca1)' 0.886
0.67
M-I s-I. CInlo7 s-l. 'Reference 6b.
A2
WAVENUMBERS / cm-1
WAVENUMBERS / cm-1
Figure 2. ( A i ) Absorption (-)
and (Fl) corrected fluorescence (-) spectra of TIPA. (A2) Absorption (---) and (F2) corrected fluorescence (---) spectra of TNPA. Spectra are in n-hexane solution at 298 K.
Figure 3. (Al) Absorption (-) and (Fl) corrected fluorescence (-) spectra of TIPA in n-hexane solution at 298 K. (A2) Absorption (---) and (F2) corrected fluorescence (-- -) spectra of TIPA in THF solution at 298 K.
lo7
fluorescence efficiency to be 0.79. These data yield 1.37 X s-' for the observed radiative rate constant for TIPA. This value
is larger than that calculated from ref 25 by a factor of ca. 1.54. These spectroscopic and photophysical results are summarized in Table 11. Coaderrped Phase. The absorption and fluorescence spectra of TIPA were obtained in a nonpolar as well as a polar solvent, n-hexane and tetrahydrofuran, respectively. Figure 2 portrays the absorption and fluorescence spectra of TIPA and TNPA in n-hexane; the comparison between the spectra of these amines will be discussed below. In n-hexane solution, the diffuse vibrational structure observed for TIPA in the vapor phase (see Figure 1) is almost totally obliterated, as is typical of saturated amines that possess vaporphase vibronic structure.26 The maxima of the So SIand So S2transitions are blue shifted relative to the vapor phase, the former by ca. 2500 cm-I and the latter by ca. 3900 an-';see Figure 1. The larger blue shift seen with the higher energy transition might be associated with the higher diffusivity, hence enhanced solvent interaction, of the S2 excited Rydberg state relative to that of SI.The maximum of the So SItransition is at 40 500 cm-', and the oscillator strength of that transition is estimated to be ca. 0.023. The 0-0 gap between the relaxed So and SIstates in n-hexane is estimated from the intersection of the maximum-normalized (corrected) fluorescence and absorption spectra; this value is ca. 37000 cm-I, which compares with 36 550 cm-I for the vapor phase. The maximum of the TIPA fluorescence spectrum in n-hexane solution, however, is nearly unchanged relative to the vapor phase, being red shifted by < 100 cm-I; see Figure 1. It is interesting to compare the photophysical properties of TIPA in n-hexane solution with those in the vapor phase. As with the case of other tertiary amines, both the fluorescence lifetime and quantum efficiency (extrapolated to zero concentration) decrease Thus,for TIPA, in n-hexane, relative to the vapor-phase ~alues.2~ these values are 7 = 46.5 ns and qf = 0.37,respectively. The fluorescence quantum efficiency was measured using triethylamine in n-hexane as a standard (see Experimental Section). It is noteworthy that the TIPA fluorescence self-quenching rate con-
-
-
-
stant in n-hexane is 1.1 X lo8 M-' s-I, which corresponds to an efficiency of
H +
S& 0 .ad Verticd rad Vduea CllCllLted from MNDO MECI (4,l) Using MOPAC Version
6.0"
I
I
m IW
z
H
U
u W B
TABLE Uk E-td
TMA TNPA TIPA ABCU
37550 38300 36600 35932
35620 35404 33344 39285
43 480 43 380 38 OOO
38 947 39 243 36 679 39 550
All energies in cm-' .
l
P~
0 ,
2~
42000*
40000-X
w
* 32000
34000
36000
&
38000
38000-
0
\
WAVENUMBERS / cm-1
u
Figure 5. (A) Absorption (-) and (F) corrected fluorescence ( - - - ) spectra of ABCU vapor at 298 K.
is also reflected in the several other higher-lying transitions, is in strong contrast with the Franck-Condon profiles of the acyclic amines, TIPA and TNPA. In the latter two cases, the spectroscopic reorganization energies, i.e., the difference between ,,Y and vm are substantial, namely, 1650 and 5480 cm-l as compared with
