Main Research Interests
- Organic molecules and coordination compounds employed in molecular recognition processes, with special attention to stereochemical issues
- Structural studies of organic molecules, polymers, biomolecules, biomimetic systems and supramolecular adducts of biological, pharmacological or technological interest
- Development of new methods for the stereochemical determination (configuration and conformation) by means of spectroscopic techniques such as NMR, ECD, VCD and CPL
- Modelling of organic molecules with molecular mechanics, semi-empirical and ab initio methods to support their structural investigations.
- Non-empirical calculations of electronic and chiroptical properties of organic and coordination compounds
Recent Projects and Collaborations
- Study of the chiroptical properties and aggregation modes of chiral organic conjugate polymers (with Lorenzo Di Bari and Francesco Zinna)
- Assignment of absolute configuration of natural products such as bio-active fungal secondary metabolites by means of ECD techinques, including a solid-state CD-TDDFT method (in collaboration with Antonio Evidente, Napoli; Jin-Ming Gao, Yangling; Zeping Xie, Yantai; Hidayat Hussain, Nizwa)
- Structural characterization of biomimetic systems (with Gaetano Angelici)
- Structural elucidation of coordination compounds (with Lorenzo Di Bari and Francesco Zinna, and in collaboration with Christoph Janiak, Düsserdolf, and Mohammed Enamullah, Jahangirnagar)
- Development of new methods for the interpretation and simulation of chiroptical properties, including the treatment of exciton-coupled systems with strong electric/magnetic coupling giving apparent exception to the exciton chirality method (with Benedetta Mennucci, Pisa, Fabrizio Santoro, CNR-ICCOM Pisa, and Torsten Bruhn, Würzburg)
- Study of chiral organo-gels (in collaboration with Stefano Cicchi, Firenze)
- Interpretation of chiroptical properties of rhodopsin and analogue proteins (in collaboration with Robert W. Woody, Colorado State University, and Koji Nakanishi and Nina Berova, Columbia University)
Conjugated organic polymers such as PPV poly(phenylene vinylene)s, PPE poly(phenylene ethynylens)s and PT poly(thiophene)s are “materials with unique properties, not otherwise observable” (Alan Heeger, 2000 Nobel in Chemistry). They are used for several applications such as photovoltaics, LEDs, sensors, field-effect transistors, and so on.
The efficiency of these materials depends strongly on their organization at various hierarchical levels.
At supramolecular level, chirality represents a unique possibility to control the geometry of the aggregates.
Oblique orientation between polymer chains offers the best compromise between emission efficiency and exciton migration (see here). Electronic CD (ECD) and circularly polarized luminescence (CPL) are ideal techniques for charaterizing thin films of these materials, to study their aggregation modes and multiple aggregation pathways. See our recent review on Chemical Reviews.
In collaboration with various partners from three PRIN projects (2007, 2009 and 2012), we have been designing and investigating chiral PPEs in solution and in the solid state, mainly by means of electronic CD. Papers: #1 #2 #3 #4 #5 We have also theoretically investigated the very peculiar vibronic ECD spectra of chiral polythiophenes in their aggregate states. Papers: #1 #2
Recently, we have applied synchrotron light source (Diamond, UK) to obtain a CD mapping of polymer thin films with sub-mm resolution. This new technique is called CDi (CD-imaging). Papers #1 #2
We have also reported several examples of non-reciprocal chiroptical materials, e.g. whose thin films exhibit mirror-image CD and CPL spectra from the two faces. Papers #1 #2 #3 #4
Absolute configuration assignment of natural products
Most natural products such as secondary metabolites of fungi are chiral and contain multiple stereogenic elements. While relatively few of them are crystalline, almost all contain at least one chromophore and provide measurable ECD spectra. Nowadays, the simulation of ECD spectra with TDDFT methods represents a practical and reliable means for assigning absolute configurations. See a review on the absolute configuration assignment of natural products of fungal metabolites and a tutorial on good computational practice in TDDFT/ECD calculations.
A specific application is the so-called “solid state CD/TDDFT method”, where one employs the X-ray structure as input for ECD calculations. It has been summarized in a review for the thematic Chirality issue “Advances in Chiroptical Methods“, and a chapter in the 2-volume book “Comprehensive Chiroptical Spectroscopy” edited by Wiley. More examples in recent papers: #1 #2 #3 #4 #5
Chirality seems intimately connected with the capability of organic compounds
to form gels in organic solvents. Some typical chiral organic scaffolds like
trans-3,4-diaminopyrrolidine consistently provide stable gels when appended
with long alkyl chains and H-bonding groups such as carbamate and urea.
In these gels, the helical structure at the micrometer scale may be studied
by microscopy (AFM and TEM), but only the use of CD may provide
information about the supramolecular chirality at the molecular scale.
Papers #1 #2 #3 #4 #5
Interpretation of the CD spectrum of rhodopsin and analog proteins
Rhodopsin is the retina pigment responsible for vision. Retinal is the photoreactive chromophore involved in light absorption. Rhodopsin shows a distinctive CD spectrum which has been deeply investigated, especially using conformationally-locked retinal analogs.
In collaboration with Prof. Koji Nakanishi and Nina Berova (Columbia, NY), non-empirical methods of CD analysis have been employed to study the stereochemistry of retinoids. Papers #1 #2
In collaboration with Prof. Robert Woody (CSU, Fort Collins), we have addressed by means of TDDFT calculations the long-debated question of the origin of CD signals of rhodopsin (intrinsic chirality of retinal vs. coupling with protein groups), bacteriorhodopsin (exction coupling vs. different mechanisms), and a new channel rhodopsin. Papers #1 #2 #3
Electric/Magnetic coupling in exciton-coupled systems
The exciton chirality method is still one of the most popular approaches for interpreting ECD spectra of compounds with many chromophores, including natural products and supramolecular systems.
In most exciton chirality applications the role of intrinsic magnetic moments is neglected because typical chromophores like benzoates have null or negligible transition magnetic moments. However this is not the case for other important chromophores such as BODIPY or phenanthrenes. Here, only by the correct inclusion of magnetic moments (using the routine EXAT) we could reproduce correctly exciton-coupled ECD spectra. A similar phenomenon occurs for the exciton-coupled vibrations in VCD spectra. Papers #1 #2 #3
BODIPY DYEmers are also capable of emitting circularly polarized light (CPL) with a high degree of polarization (glum = 0.004). An almost unique feature of these compounds is that the emitting state is the low-lying exciton-coupled level, i.e. it is directly involved in the exciton coupling between the strong electric-dipole allowed transitions of BODIPY rings. Paper #1