Excited State Prototropic Activities in 2-Hydroxy 1-Naphthaldehyde Papia Chowdhury, Subhasis Panja, and Sankar Chakravorti* Department of Spectroscopy, Indian Association for the CultiVation of Science, JadaVpur, Kolkata 700032, India ReceiVed: June 28, 2002; In Final Form: October 10, 2002 Excited state proton transfer in 2-hydroxy 1-naphthaldehyde (HNL) has been reported on the basis of steady state absorption and emission, time-resolved emission, and semiempirical quantum chemical calculations. The existence of open and closed ring conformers in the ground state gives rise to different emissions upon excitation in hydroxylic and hydrocarbon solvents, respectively. Formation of the ground state HNL ion in open conformer was observed with addition of base. Excitation of the closed conformer leads to intramolecular proton transfer across the preexisting intramolecular hydrogen bond to give excited zwitterion in hydrocarbon solvents. On excitation of the open ring conformer in hydroxylic solvents, emission from the neutral excited state could be observed for the first time apart from the emission of ionic intermolecular proton transfer species. An increase or decrease in pH results in enhancement or a decrease of emission from the ionic conformer with parallel dwindling or intensification of emission of the neutral species. Phosphorescence in both the solvents arises from the open conformer. Introduction More than 50 years ago, Fo ¨rster 1 introduced the concept of excited state intermolecular proton transfer (ESI er PT) on the basis of some unusual shifts in fluorescence spectrum of some organic molecules. In 1955, Weller 2 found that methyl salycylate exhibited fluorescence with an unusually large stokes shift, which corresponded to an excited state isomer formation via proton transfer. The classic paper 3 of Weller on excited state intramolecular proton transfer (ESI ra PT) of methyl salycilate (MS) spurred intense research on this subject in many laboratories. 4-18 Among them, MS 7-10 and its related com- pounds such as o-hydroxyacetophenone (OHAP) 11-13 and o-hydroxybenzaldehyde (OHBA) 14-17 have been well-studied as prototypes of the molecules showing the ESIPT processes. It is a well-known fact that the properties of the molecules experience a change in the excited states with respect to the ground state due to the change in charge density distribution after excitation. When acid and base moieties of the same molecule become stronger acids or bases in the excited state, proton transfer (PT) may occur rapidly in the excited state to form a tautomer; the signature of that process is emission of strongly stokes-shifted fluorescence following absorption of a UV photon. Kasha 18 first showed that an intramolecular PT is facilitated in the excited state in those molecules where there is a hydrogen bond between the H atom of the donor group and the acceptor of that molecule in the ground state. The importance of this H-bond is such that it is convenient to discuss its nature. Transfer of a proton between two groups of an aromatic molecule causes a large electronic and structural rearrangement, which is associated with significant changes in dipole moments, molecular geometry, and quite large (g1000 cm -1 ) fluorescence shift. The dynamics of such a process could strongly be dependent on the nature of the solvent, namely, with respect to the formation of a hydrogen bond. One of the prime features common to most excited state PTs is their rapidity, on the femtosecond to nanosecond time scale. 19-21 These rapid transfers are commonly attributed to a barrierless process. Also, a number of systems containing more than one H-bond exhibit multiple PTs. 18 ESIPT has a wide range of applications. These include energy/ data storage devices and optical switching, 11,22-24 Raman filters and hard scintillation counters, 25 polymer photostabilizers, 26,27 and triplet quenchers. 28 Other applications center around elec- troluminescent materials with photochemical stability, resistance to thermal degradation, and low self-absorption and light- emitting diode materials. 28 It has been suggested that ESIPTs have the potential for understanding the binding properties of protein, 29 as well as optical probes for biomolecules. 30,31 Because of all of the interesting and useful applications, different groups throughout the world are working with a wide variety of fluorescing acid-base-containing molecules in order to sys- tematize and control their emissive properties. In the present work, we study the emission properties of 2-hydroxy 1-naphthaldehyde (HNL) by steady state and time- resolved emission spectroscopy. HNL is the simplest aromatic molecule with intramolecular hydrogen bonding involving carbonyl groups. 32 The properties of this molecule are interesting in relation to OHBA. Luminescence properties of HNL are rather different from OHBA. HNL shows dual fluorescence, but OHBA shows single fluorescence only in the visible region. So, we hope that a combination of steady state emission spectroscopy with picosecond spectroscopy will provide useful information concerning the structure and dynamics involving the excited states of HNL. In this paper, we first perform quantum semiempirical calculations to see the feasibility of excited state intramolecular proton transfer with respect to the dipole moment change and charge density distribution from the ground to excited state. On the basis of prediction, a detailed investigation of absorption and fluorescence properties of HNL in different environments and in -cyclodextrin (-CD) was carried out. We try to identify the species that are responsible * To whom correspondence should be addressed. E-mail: spsc@ mahendra.iacs.res.in. 83 J. Phys. Chem. A 2003, 107, 83-90 10.1021/jp026404q CCC: $25.00 © 2003 American Chemical Society Published on Web 12/11/2002