Dibenzo[a,c]phenazine: A Polarity-Insensitive Hydrogen-Bonding Probe Debarati Dey, † Adity Bose, † Dhananjay Bhattacharyya, ‡ Samita Basu,* ,† Shyam Sundar Maity, § and Sanjib Ghosh § Chemical Sciences DiVision, and Biophysics DiVision, Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Kolkata 700 064, India, and Department of Chemistry, Presidency College, 86/1, College Street, Kolkata 700 073, India ReceiVed: April 25, 2007; In Final Form: August 14, 2007 A derivative of phenazine, dibenzo[a,c]phenazine (DBPZ), can be used as a very good hydrogen-bonding probe unlike its parent phenazine molecule. Steady-state absorption and fluorescence studies reveal that DBPZ is completely insensitive to polarity of the medium. However, DBPZ can form a hydrogen bond very efficiently in its first excited singlet state. The extent of this excited-state hydrogen-bond formation depends both on size and on hydrogen-bond donor ability of the solvents. Time-resolved fluorescence studies and theoretical calculations also suggest that this hydrogen-bond formation is much more favorable in the excited state as compared to the ground state. In the excited state, the electron density is pushed toward the nitrogen atoms from the benzene rings, thereby increasing the dipole moment of the DBPZ molecule. Although the dipole moment of DBPZ increases upon photoexcitation, like other polarity probes, the molecule remains fully insensitive to the polarity of the interacting solvent. This unusual behavior of DBPZ as compared to simple phenazine and other polarity probes is due to the structure of the molecule. Hydrogen atoms at the 1 and 8 positions of DBPZ are sterically interacting with a lone pair of electrons on the proximate nitrogen atoms and make both of the nitrogen atoms inaccessible to solvent molecules. For this reason, DBPZ cannot sense the polarity of the medium. However, DBPZ can only sense solvents, those that have hydrogen with some electropositive nature, that is, the hydrogen-bond donating solvents. Hydrogen being the smallest among all elements can only interact with the lone pair of electrons of nitrogen atoms. Thus, DBPZ can act as a sensor for the hydrogen-bond donating solvents irrespective of their dielectrics. Introduction Hydrogen bonding is one of the fundamental elements of chemical structure and reactivity of water, proteins, and DNA building blocks of life. 1-3 Intermolecular hydrogen bonding is a site-specific local interaction between hydrogen donor and acceptor molecules that plays a dominant role in various forms of molecular recognition processes. The nature of the hydrogen bond in solution is of particular interest, and it has been probed by diverse experimental and theoretical methods. It is evident from the literature that there are quite a large number of phenazine derivatives, 4,5 which show significant differences in their dipole moment in the ground and excited states on photoexcitation. In such cases, the solvent molecules undergo reorganization around the solute molecules according to their own dipole moment and hydrogen-bonding capacity to minimize the total energy of the solute-solvent system. For this dipole- dipole interaction, the emission spectra of those molecules depend very much on the polarity of the solvent employed. Such polarity probes show considerable red shift of the fluorescence maxima in the polar solvents as compared to nonpolar medium. This phenomenon is popularly known as solvation. 6-8 However, it is a very rare observation in the literature that a molecule with a dipole moment higher in the excited state as compared to the ground state remains insensitive to polarity (or dielectric) but sensitive to the hydrogen-bonding capacity of the solvent. In this Article, we would like to report on such a molecule, that is, dibenzo[a,c]phenazine (DBPZ), and its activity as an efficient hydrogen-bonding probe unlike its parent phenazine molecule. Because of the planar structure and rigid framework of the fused ring system, the conformational and configurational relaxation processes are not feasible enough in DBPZ. Only the solvent relaxation process becomes important in its first excited singlet state. Photoexcitation of DBPZ results in significant changes in the spatial charge distribution around its nitrogen atoms like other polarity probes. Yet surprisingly DBPZ remains insensitive to the polarity of the solvents. The structure of the molecule might be responsible for its peculiar behavior. Steady-state absorption and fluorescence studies reveal that DBPZ forms hydrogen bonding with hydroxylic solvents in the excited state. The extent of hydrogen-bond formation depends on the hydrogen-bond donor ability (R) and steric bulk of the solvent. Time-resolved fluorescence studies suggest that the hydrogen-bond formation is much more facilitated in the excited * Corresponding author. Telephone: +91-33-2337-5345. Fax: +91-33- 2337-4637. E-mail: samita.basu@saha.ac.in. † Chemical Sciences Division, Saha Institute of Nuclear Physics. ‡ Biophysics Division, Saha Institute of Nuclear Physics. § Presidency College. CHART 1 10500 J. Phys. Chem. A 2007, 111, 10500-10506 10.1021/jp0731811 CCC: $37.00 © 2007 American Chemical Society Published on Web 09/25/2007