Dey Debarati, Bose Adity, Bhattacharyya Dhananjay, Basu Samita, Maity Shyam Sundar, Ghosh Sanjib
Chemical Sciences Division, and Biophysics Division, Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Kolkata 700 064, India.
J Phys Chem A. 2007 Oct 25;111(42):10500-6. doi: 10.1021/jp0731811. Epub 2007 Sep 25.
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.
与母体吩嗪分子不同，吩嗪的衍生物二苯并[a,c]吩嗪（DBPZ）可作为一种非常好的氢键探针。稳态吸收和荧光研究表明，DBPZ对介质的极性完全不敏感。然而，DBPZ在其第一激发单重态下能非常有效地形成氢键。这种激发态氢键形成的程度既取决于溶剂的大小，也取决于溶剂的氢键供体能力。时间分辨荧光研究和理论计算还表明，与基态相比，这种氢键形成在激发态下更有利。在激发态下，电子密度从苯环推向氮原子，从而增加了DBPZ分子的偶极矩。尽管DBPZ的偶极矩在光激发时会增加，但与其他极性探针一样，该分子对相互作用溶剂的极性仍然完全不敏感。与简单吩嗪和其他极性探针相比，DBPZ的这种不寻常行为是由于其分子结构。DBPZ 1位和8位的氢原子与相邻氮原子上的孤对电子存在空间相互作用，使得两个氮原子都无法与溶剂分子接触。因此，DBPZ无法感知介质的极性。然而，DBPZ只能感知那些具有带部分正电性质氢的溶剂，即氢键供体溶剂。氢是所有元素中最小的，只能与氮原子的孤对电子相互作用。因此，无论其介电常数如何，DBPZ都可以作为氢键供体溶剂的传感器。
