Non-radiative decay pathways of the first excited electronic states of 1:1 hydrogen bonded complexes of 7-azaindole with phenol and formamide IACS Moitrayee Mukherjee, Anamika Mukhopadhyay, Tapas Chakraborty Physical Chemistry Department Indian Association for the Cultivation of Science Jadavpur, Kolkata , West Bengal, India

 In doubly hydrogen-bonded dimeric structure, tautomerization is the primary non-radiative decay channel in the excited electronic state Wavelength(nm) Fluorescence H N N H N N N N H H N N 7-azaindole: A prototype of DNA bases from photophysical viewpoints H N N Acceptor Donor  E lectronic excitation results in reorganization of charge distribution leading to enhanced acidity of pyrrolic N–H group and basicity of the Pyridinic nitrogen Energy Reaction Co-ordinate Normal form Tautomeric form

N N H O O H C R N N H O H R N H N O O H C R O N N H H R Catalytic tautomerzation in complexes with alcohols and carboxylic acid  Tautomerization by carboxylic acid catalyst is very efficient, and the dimer shows no local emission following UV excitation in hydrocarbon liquids.  In alcohols, tautomerization is slow, exhibits primary kinetic isotope effects as well as a classical Arrhenius type activation barrier that has been associated to formation of specific cyclic structure with an alcohol molecule.

Goal of our studies H N N O N H H C H N N H N H O H H C  Photophysics of the mixed dimer between 7-azaindole and formamide, particularly the following tautomeric conversion in the excited state  The specific question that we have addressed is whether the relaxation of the locally excited state to a tautomeric state could still be the major non-radiative decay pathway.  Significance: Here the tautomerization of 7AI leads to a simultaneous tautomerization of FM, and the system mimics the base pairs more realistically.

Energy Reaction Co-ordinate  10 kcal/mol FM Energetics of tautomeric conversion of 7AI---FM mixed dimer Energy Reaction Co-ordinate  6.5 kcal/mol (7AI) 2 In case of (7AI) 2, electronic excitation is delocalized over the entire dimer, and tautomerization in S 1 is exothermic Energy Reaction Co-ordinate 7AI-FM complex ? Excitation energy is localized only on 7 AI moiety. The question is, whether the tautomerization in S 1 of the mixed dimer could effectively be an exothermic process

 A 1:1 mixed dimeric complex is synthesized in a supersonic jet expansion and the laser-induced fluorescence excitation and dispersed fluorescence spectra following excitation to several single vibronic levels in S 1 are measured.  Photophysical studies, i.e., absorption, emission and fluorescence excitation spectra are also performed by dissolving the two solutes in hydrocarbon solvents, and the spectra were recorded both at room temperature and 77 K hydrocarbon glass. Measurement schemes:

    178   170 (32335 cm-1) cm cm -1  cm -1 FE Spectrum of (7-AI) 2 probing visible fluorescence Wavenumber(cm -1 ) Intensity FE Spectrum of 7AI-FM FE Spectrum of (7-AI) 2  Beyond the displayed energy range the spectrum shows no distinct vibronic feature  7AI-FM complex absorbs in the same spectral region of the 7AI homodimer LIF of 7AI-FM complex in a Supersonic Jet Expansion cm -1 7AI-FM S0S0 S1S cm -1 7AI 2296cm -1  No vibronic feature appears in the spectrum when a UV cutoff/visible pass filter is placed in front of the PMT detector

DF spectrum of 7AI … FM; band DF spectrum of 7AI monomer; band  200  400  600  800  1000 Wavenumber(cm- 1 ) LIF of 7AI-FM complex in a Supersonic Jet Expansion

Wavenumber(cm- 1 )  200  400  600  800   Analysis of the vibrational features of both the spectra reveals that emission originates from the level excited, and no relaxed fluorescence is detected  The absence of visible fluorescence confirm that the excited state tautomerization does not occur on excitation to S 1 origin region of the mixed dimer in a cold supersonic jet expansion cm -1 band LIF of 7AI-FM complex in a Supersonic Jet Expansion Dispersed Fluorescence Spectra cm -1 band

 The spectral change signify either a general solvation shift, or formation of the 1:1 complex in the ground state  312nm Abs. Wavelength(nm)  7AI(10 -5 M)  7AI(10 -5 M) +FM(10 -3 M)  7AI(10 -5 M)  7AI(10 -5 M) +FM(10 -3 M)  7AI(10 -5 M) +FM(5  M) e x = 295nm     10 7 Wavelength(nm) 7AI-FM complex in a hydrocarbon solution at room temperature  The visible fluorescence is the distinct signature of 7-AI tautomerization by FM at room temperature

 7AI(10 -5 M)  7AI(10 -5 M) +FM(10 -2 M)  7AI(10 -5 M) +FM(10 -1 M) 312nm em = 500nm Wavelength(nm) 7AI-FM complex in a hydrocarbon solution at room temperature Fluorescence excitation spectra for detection of tautomer fluorescence Wavelength(nm) Normalization shows that the detailed features of the pure and mixed dimer spectra appear different even in the solution phase The first absorption band of the mixed dimer appears at ~312 nm

7AI-FM heterodimer shows no local emission in ultraviolet indicating that tautomerization is very efficient 7AI-FM complex in a hydrocarbon solution at room temperature ex = 312nm Wavelength(nm)  7AI(10 -5 M)  7AI(10 -5 M) +FM(10 -2 M)  7AI(10 -5 M) +FM(10 -1 M)

 Some of the low frequency inter- and intramolecular vibrational modes significant for the tautomerization process are populated to higher levels at room temperature, which assist in crossing the energy barrier.  Thermal motion of the solvent molecules allows orientational relaxation, internal rotation between two dimer moieties and quick exchange between the normal and tautomeric forms of FM in the complex. H N N N H H C H O N N H H N H C H O  In room temperature liquids a small quantity of the tautomeric form of FM exists in equilibrium with the normal form, and such distribution could promote the tautomeric conversion. Origin of photophysical distinctions in cold jet and hydrocarbon soutions

Photophysics of 7AI-FM complex in a rigid hydrocarbon glass at 77K  7AI(10 -5 M)(300K)  7AI(10 -5 M) +FM(5 × M) (300K) ­­ 7AI(10 -5 M)(77K) ­­ 7AI(10 -5 M) +FM(5 × M) (77K) ex = 295nm  In the cold rigid matrix the complex shows only local UV fluorescence  Rigidity of the medium inhibits tautomerization ex = 312nm Wavelength(nm)

Summary:  Photophysical studies of the 1:1 complex of 7AI and FM are performed in Supersonic jet expansion, in hydrocarbon solution at room temperature and in a hydrocarbon glass at 77K.  In hydrocarbon solution tautomerization has been found be the dominant nonradiative decay channel of the complex from the locally excited state (S 1 ). However, the process does not occur in the cold jet expansion on excitation to S 1 origin region of the 7AI moiety of the complex. On the other hand, the process shows restricted behavior in the cold hydrocarbon glass, and the complex shows red-shifted local fluorescence.  The observations indicate that no straightforward correlation of this type of a photophysical process occurring in a cold supersonic jet and liquids at room temperature can be established.

Acknowledgement Co-worker: Dr. Montu K Hazra Mr. Amit K Samanta Mr. Prasenjit Pandey Mr. Biman Bandyopadhyay Financial Support: Department of Science and Technology, Government of India