Laser studies of pyridylindoles in supersonic jets

17 December 1999

Chemical Physics Letters 315 Ž1999. 87–94 www.elsevier.nlrlocatercplett

Laser studies of pyridylindoles in supersonic jets Y. Nosenko a , Y. Stepanenko a , F. Wu b, R.P. Thummel b, A. Mordzinski a

a,)

Institute of Physical Chemistry, Polish Academy of Sciences, 44 Kasprzaka, 01-224 Warsaw, Poland b Department of Chemistry, UniÕersity of Houston, Houston, TX 77204-5641, USA Received 23 September 1999

Abstract One- and two-photon spectroscopy has been used to study jet-cooled 2-Ž2X-pyridyl.indole ŽPyIn-0. and 3,3X-dimethylene2-Ž2X-pyridyl.indole ŽPyIn-2.. The laser-induced fluorescence and resonance-enhanced photoionisation spectra of PyIn-2 are congested due to the flexibility of the ethylene chain. Single vibronic level fluorescence spectra allow identification of equivalent modes in the ground and electronically excited states. Deuteration at nitrogen reveals a 4 cmy1 red shift of the origin. Ab initio calculations were conducted at the TD B3LYPr6-31GŽd,p. level of theory. We were able to reproduce the energies of pure electronic transitions both in PyIn-0 and PyIn-2. The properties of pyridylindoles in supersonic jets are compared with those for indole and 2-phenylindole. q 1999 Elsevier Science B.V. All rights reserved.

1. Introduction Over the past three decades, supersonic jets have offered an attractive source of isolated cold molecules in the gas phase w1x. Among many molecular systems and their clusters studied in jets, indole and its various derivatives have a special importance w2–13x. One reason for studying indoles under jet conditions is to elucidate properties that make them useful as a fluorescence probe in biological systems. In particular, the indole moiety of tryptophan makes a dominant contribution to the near-UV absorption and fluorescence of most proteins. Considerable attention has been given to the electronic origin of L b and L a states. An inversion of the L a and L b states may depend on environmental factors, thus creating a probe of solvent polarity. It has been established that ) Corresponding author. Fax: q48-3912-0238; e-mail: [email protected]

the lowest excited state in indole has L b character and the L a state lies at least 1500 cmy1 higher w8x. However, in some isolated indole systems and solvent clusters studied by the Wallace group w11,12x, the proximity of L b and L a states has been observed. Relatively strong mode mixing occurs in the S 1 –S 0 spectra of 2,3-dimethylindole, mediated by vibronic coupling between L a and L b states. Indole properties have been used to assess the polarity of the environment of tryptophan residues in polypeptide chains. The highly resolved electronic spectra of tryptophan have been studied in jet expansion w14–18x and also recently in cold helium droplets w19x. In this Letter, we focus our attention on a new group of indole derivatives, namely pyridylindoles. These bi-functional systems can simultaneously act as both H-bond donor and acceptor. For a cyclic complex with a protic solvent molecule, the cooperative excited-state proton transfer of two or more protons has been postulated w20x. Here, we report the

0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 1 1 9 6 - 3

Y. Nosenko et al.r Chemical Physics Letters 315 (1999) 87–94

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Scheme 1.

laser-induced fluorescence excitation ŽFE. spectra, dispersed fluorescence ŽDF., and one-colour, twophoton ionisation ŽR2PI. of two pyridylindoles: 2-Ž2X-pyridyl.indole ŽPyIn-0. and 3,3X-dimethylene-2Ž2X-pyridyl.indole ŽPyIn-2. Žsee Scheme 1.. Measurements of isolated molecules seeded in supersonic jets and studied in the vicinity of their S1 –S 0 transition proceed studies of solvent clusters of pyridylindoles.

ergy did not exceed 50 mJ. A frequency-doubled DCM dye laser or p-terphenyl dye laser was operated at wavelengths in the range of 317–344 nm. The fluorescence signals were detected directly via a Hamamatsu R2949 photo-multiplier or dispersed in Spectra Pro 275 ŽActon Research. with LN CCDcamera ŽPrinceton Instruments.. R2PI spectra were obtained by the 1 q 1 photoionisation technique. For the excitationrionisation steps, the laser power was maintained at 100 mJ. Ion signals were detected via a differentially pumped, 0.6 m linear time-of-flight ŽTOF. mass spectrometer. Signal processing for R2PI was performed with a set of microchannel plates ŽGalileo. and a two-channel 300 MHz digital oscilloscope ŽLeCroy 9310. with a dedicated personal computer. Each experimental point was averaged at least 8 times. Ab initio calculations, conducted using the GAUSSIAN 98 w24x package at TD B3LYPr6-31GŽd,p. and CISr6-31GŽd,p. levels of theory, were made to predict the electronic transition energies of indole, 2phenylindole and selected pyridylindoles.

3. Results and discussion 2. Experimental PyIn-0 and PyIn-2 were obtained as reported previously w21x. They were typically heated to 1208C to obtain sufficient vapour pressure, and the samples were seeded in He Žstagnation pressure, 3 bar. or Ne Žstagnation pressure, 1 bar. and expanded through a modified high-temperature 600–800 mm pulsed nozzle ŽGeneral Valve Series 9.. The width of the valve pulse was typically 500 ms. In a series of subsequent experiments, the samples were heated to 2308C. Deuteration of PyIn-2 was accomplished by the addition of CH 3 OD ŽGlaser AG Basel; isotopic purity, 98%. to the carrier gas. The partial vapour pressure of the solvent was controlled by maintaining its temperature within "18C. Three kinds of measurements were carried out: laser-induced FE, DF and R2PI. Supersonic jet systems with optical and mass detection have been described elsewhere w22,23x. Fluorescence was excited 6–12 mm downstream from the nozzle with a narrow-band Ž0.3 cmy1 . dye laser pumped by a LPX 105 i ŽLambda Physik. excimer laser. The laser en-

The complete active space ŽCAS. SCF method has been used for analysis of the electronic spectrum of indole w25x. The geometry of phenylindole ŽPI., PyIn-0 and PyIn-2 in the ground state were optimised at the B3LYP level of theory, using 6-31GŽd,p. basis sets. The results obtained for PI give the dihedral angle between the indole and phenyl moieties 278, what is in good agreement Ž248. with the ab initio Hartree– FockrSTO-3G calculations by Sinclar et al. w26x. For PyIn-0 and PyIn-2, the ab initio calculations predict a near co-planar equilibrium structure in the ground state, with the dihedral angle between the indole and pyridyl Ž28 and 88, respectively.. For PyIn-0, the cis Žsyn.-conformer is predicted to be more stable by 18 kJrmol at this level of theory. Comparison of the calculated electronic transition energies using the TD B3LYP and CIS methods with the same basis set is shown in Table 1. The calculated transition energies are compared with the experimental results obtained from FE spectra of jetcooled pyridylindoles.

Y. Nosenko et al.r Chemical Physics Letters 315 (1999) 87–94 Table 1 Comparison among calculated and experimental excitation energies Žcmy1 . for the PI, PyIn-0 and PyIn-2 Molecule

Exp. S1

PI PyIn-0 Žsyn. PyIn-2

31 356 30 308 29 211

§ S rcm

y1

0

TD DFT

CIS

32 840 30 560 29 900

40 470 38 920 37 540

In the case of PyIn-0 and PyIn-2, the TD DFT results give reasonably good agreement with experimental data. The comparison is not as successful as in the case of PI. This may be due to the large geometry change upon electronic excitation w26x; presumably more adequate calculations require reoptimisation of the equilibrium geometry in the excited state. The comparison is also not successful with the ab initio results obtained using the CIS

89

Table 2 Transition energies in the FE spectrum of PyIn-0 relative to the origin at 30 307.7 cmy1 . The assignments of the main vibronic transitions are also given D n rcmy1

Assignment

Relative intensity

0.0 5.7 17.2 98.2 102.9 162.9 196.5 260.7 273.5 320.6 324.4 329.4 336.5 357.2 371.9 419.0 433.3 483.2 492.0 499.2 532.3 534.5 536.3 551.1 578.2 580.2 582.0 589.4 596.9 631.8 633.2 641.2 643.6

0 00 – – A00 – B10 A20 A10 B10 C 10 D01 B 02 – – A20 B10 A10 C 10 A10 D10 B10 C 10 B10 D10 – E 10 A10 B10 C 10 F01 G 10 – – H 10 A10 B10 D10

1.

0.47 0.15

0.08 0.91

0.32 0.74 0.40

0.38

A10 E10 A10 F01 A10 G01 D 02

method. The excitation energies are about 1 eV above the experimental values. Laser-induced FE spectra of PyIn-0 and PyIn-2 in supersonic jets are shown in Fig. 1. In both cases, many detailed narrow features have been detected. We assign the strongest bands at 30 308 " 1 and 29 211 " 1 cmy1 as the origin of the S 1 S 0 transition. The FE spectra were recorded with up to 650 cmy1 excess energy. The FE spectrum of PyIn-0 shows a relatively sparse structure in the region between 0 and 300 cmy1 , followed by rich sequential bands over the

§

Fig. 1. Laser-induced FE spectra of PyIn-0 and PyIn-2 in supersonic free jets. The assignment of the bands Žsee Table 2 and 3. are indicated.

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Table 3 Transition energies and corresponding assignments of prominent vibronic bands in the FE spectrum of PyIn-2 relative to the origin at 29 211.6 cmy1 y1

D n rcm

Assignment

Relative intensity

0.0 51.2 96.3 102.3 119.2 141.1 148.1 153.9 167.2 170.8 192.8 198.9 204.5 215.9 217.9 222.4 237.3 244.2 250.1 263.2 268.2 272.8 276.4 286.3 289.2 295.2 297.5 308.1 311.7 314.3 319.2 323.9 327.7 333.3 336.8 340.6 348.7 352.1 356.4 359.2 365.1 372.7 375.4 378.9 383.6 387.9 395.7 403.6 411.1 416.5 424.2

0 00 A10 B10 A20 C 10 D01 A10 B10 A30 E 10 A10 C 10 B 02 , A10 D10 A20 B10 A40 B10 C 10 A10 E10 A20 C 10 B10 D10 , C 02 A20 D10 , A10 B 20 A30 B10 B10 E10 A20 E10 , A10 B10 C 10 A30 C 10 F01 C 10 E10 B 30 , A10 B10 D10 A20 B 20 , A30 D10 G 10 D10 E10 A10 C 10 D10 , B 20 C 10 A10 B10 E10 A30 E10 A40 C 10 A10 F01 E 20 , B 20 D10 A10 C 10 E10 , B10 C 02 A10 B 30 A10 G01 H 10

0.95 0.87 0.78

A10 D10 E10 , B 20 E10 A20 B10 E10 B10 F01 J01 A20 F01 B10 C 10 E10 A10 B10 C 02 , A20 C 10 E10 C 10 F01 A10 H 10 , B10 D10 E10 A10 B 20 E 20 C 10 G01 , D10 F01 A10 B10 F01

0.61 0.30

0.59

0.66

0.19

0.33

0.85

Table 3 Žcontinued. D n rcmy1

Assignment

427.0 433.2 439.2 448.4 454.6 469.5 473.0 476.7 479.2 493.9 520.6 524.7 537.6 569.3 620.0

A10 J01 A10 B10 C 10 E10 , B 20 C 02 A20 B10 C 02 , A10 C 10 E10 B10 H 10 A20 H 10 B 20 F01 , C 10 H 10 B10 J01

Relative intensity

A20 J01 A10 E10 F01 , D10 H 10 , C 10 J01 E10 H 10 A10 B10 J01 K 10 A10 K 10

1.0

next 300–350 cmy1 . All major transitions observed in the excitation spectrum of PyIn-0 and PyIn-2 are listed in Tables 2 and 3. In both cases, the most prominent progression is built on the 0–0 transition. In the case of PyIn-0, another harmonic progression is observed in the FE spectrum, up to an excess energy of 700 cmy1 . This suggests a deep single minimum potential along a low-frequency coordinate in the excited state. Sinclair et al. w26x have found a different FE spectrum for the 2-phenylindole ŽPI.. Their spectrum of PI exhibits a relatively weak origin and is dominated by strong torsional active 64 cmy1 modes involving a long progression of up to the 15 modes. This behavior was explained in terms of a significantly different geometry along the torsional coordinate in the two ŽS 0 , S 1 . electronic states w26x. The FE spectrum of PyIn-2 in the range 0–650 cmy1 is shown in Fig. 1. One expects that the spectrum of PyIn-2 would become simpler, due to the absence of the low-frequency torsional modes of the pyridine ring. Moreover, PyIn-0 may exist in both the cis Žsyn.- and trans Žanti.-conformation of the pyridine and indole rings. In PyIn-2, the dimethylene bridge prohibits formation of the trans Žanti.rotamer. Surprisingly, in the case of PyIn-2, we observe many low-frequency modes and a relatively congested vibrational structure. This behaviour may be explained by a low inversion barrier to rotation about the 2,2X bond, due to considerable flexibility of the dimethylene bridge w27–29x.

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The role of synranti-rotamers has been studied in more detail. The FE spectra of PyIn-0 and PyIn-2 were measured at temperatures between 120 and 2308C. At higher temperatures, the material remaining in the nozzle shows some symptoms of thermal decomposition. Additional bands have been observed. In principle, they can originate from the thermally populated anti-form in the ground state. However, it seems that the observed bands rather correspond to ‘hot’ bands of the syn-rotamer due to insufficient cooling in jet conditions. Optimisation of the cooling procedure by increasing the laser–nozzle distance or by changing the time delay between the valve opening and the laser pulse causes the new high-temperature bands to disappear. By selective excitation of the ‘hot’ bands, we were also able to detect dispersed fluorescence. The single vibroniclevel fluorescence spectrum is practically identical with that obtained by excitation of the origin. This

Fig. 3. Effect of deuteration on laser-induced FE spectra of jet-cooled PyIn-2.

finding can be treated as further evidence that pyridylindoles exist predominantly in the syn-form. In order to achieve more insight into the groundstate potential surface of pyidylindoles, the DF spectra have been measured. As shown in Fig. 2, the DF spectra are produced by the selective excitation of the most prominent excited state vibrational features. The main vibrational progression is built on the true origin. Weaker fluorescence is observed in the low-energy region representing transitions to excited vibrational levels of the ground state. The DF spectrum is dominated by many active low-frequency modes. The relatively high intensities of the lines reflect the allowable character of the S 1 S 0 transitions and small geometry changes in the two lowest singlet states. The DF spectrum obtained by excitation into different vibronic levels gives a one-to-one correspondence between equivalent modes in the ground and

™

Fig. 2. DF spectra of jet-cooled PyIn-2 obtained with excitation of different vibrational levels.

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electronically excited singlet state ŽFig. 2.. Comparison of the equivalent modes in the two electronic states shows that the frequencies in the S 1 excited state decrease by about 10% with respect to the S 0 state. However, from the band frequencies observed in the excited state, one particular mode, namely a 51 cmy1 vibration, exhibits a different behaviour. Single vibrational-level fluorescence obtained from the 51 cmy1 mode gives a resonance enhancement of 110 cmy1 in the ground state. By selective excitation of its overtones Ž102 and 153 cmy1 ., we find enhancement of 54 and 158 cmy1 and 112 and 214 cmy1 vibrational modes in the ground state. Definitely, one can treat this observation as evidence that upon S 1 S 0 electronic excitation a large geometry change along this particular coordinate takes place. If one applies the calculation of Sinclair et al. w26x, the 110 cmy1 vibration Žcorresponding to mode Õ 70 s 104 cmy1 in PI. can be tentatively assigned as the C5C-pyridyl bending mode.

§

Fig. 5. Resonance-enhanced photoionisation spectra of PyIn-0 in helium and neon. FE spectrum of PyIn-0 in helium is shown for comparison.

Fig. 4. R2PI spectra of PyIn-0 and PyIn-2. The FE spectra of PyIn-0 and PyIn-2 are shown for comparison.

Next, the PyIn-2 molecule seeded in helium enriched with some CH 3 OD vapour was studied. Fig. 3 shows the changes in the laser-induced FE spectrum of the isolated PyIn-2 molecule produced by increasing of methanol–d 1 partial pressure. We observe the appearance of new absorption bands, red-shifted by 4 cmy1 . Independent experiments with CH 3 OH under the same conditions do not show any significant spectral changes in the FE spectrum of the isolated molecule. Thus, we believe that the transition at 29 207 cmy1 corresponds to the origin of the N–D deuterated PyIn-2. Hager et al. has studied 2,3-dimethylindole deuterated at nitrogen w8x. Their results show changes by a few wavenumbers in the higher-energy part of the origin band. In addition, for the deuterated sample, they reported some line narrowing, due to the reduction of the radiationless transition by isotope substitution. On the other hand, we have recently studied the effect of NH ND substitution for por-

™

Y. Nosenko et al.r Chemical Physics Letters 315 (1999) 87–94

phycene in supersonic free jets w30x. In this case, the origin of the deuterated species is strongly red-shifted by about 24 cmy1 . Indoles, in particular in the condensed phase at low temperature, exhibit a tendency to aggregate. Thus, we have studied PyIn-0 and PyIn-2 by means of R2PI and mass-selective detection. Indeed, at higher temperatures, we have noticed, in the TOF spectrum, the formation of ions corresponding to a PyIn-2 dimer. If the temperature of the sample is decreased to 1208C, no dimer ions were detected. Under our experimental conditions, we also do not observe any products of thermal decomposition. R2PI spectra of PyIn-0 and PyIn-2 seeded in helium are shown in Fig. 4 and compared with the corresponding portion of the FE spectra. In general, mass-resolved studies confirm that all the observed transitions belong to monomers of PyIn-0 and PyIn-2. However, the R2PI spectra show some line broadening and the appearance of ‘hot’ andror sequence bands. This observation suggests poorer rotational cooling in our TOF system as compared with results obtained with the laser-induced fluorescence spectrometer. We have obtained much better results when the carrier gas Ž3 bar of helium. was replaced by neon at a pressure of 1 bar ŽFig. 5..

atures, the formation of a dimer in the gas phase was observed. In a forthcoming paper w31x, optical and mass-resolved studies of microclusters of selected pyridylindoles with different solvents will be presented. Acknowledgements This work has been supported by the Polish Committee of Scientific Research. R.T. and F.W. would like to thank the Robert A. Welch Foundation ŽE-621. and the National Sciences Foundation ŽCHE9714998. and US–Polish M. Sklodowska-Curie Fund ŽNo. 97-305. for financial support. The authors would like to thank Professor A.L. Sobolewski for helpful discussion. References w1x w2x w3x w4x w5x w6x w7x w8x

4. Conclusions w9x

Optical FE and DF as well as mass-selected R2PI ionisation spectra of samples of PyIn-0 and PyIn-2 entrained in a helium jet expansion have been studied. The vibronic structure of their S 1 S 0 transition in the region 0–700 cmy1 is quite distinct allowing identification of the main spectral features. We have concluded that PyIn-0 exists predominantly in the syn-form in the gas phase. In the case of PyIn-2, the FE spectrum is quite congested; lowfrequency progressions observed in the excitation spectrum is attributed to flexibility of the dimethylene bridge. Deuteration of PyIn-2 at nitrogen reveals a red shift of the FE spectrum. The R2PI spectra of both pyridylindoles seeded in helium and neon reproduced the main spectral features of the FE spectra. For PyIn-2 at higher temper-

§

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w10x w11x w12x w13x w14x w15x w16x w17x w18x w19x

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